Spiraxsarco-b14-condensate Recovery

Block 14 Condensate Recovery

Introduction to Condensate Recovery Module 14.1

Module 14.1 Introduction to Condensate Recovery

The Steam and Condensate Loop

14.1.1

Introduction to Condensate Recovery Module 14.1

Block 14 Condensate Recovery

Introduction to Condensate Recovery Steam is usually generated for one of two reasons: o

To produce electrical power, for example in power stations or co-generation plants.

o

To supply heat for heating and process systems.

When a kilogram of steam condenses completely, a kilogram of condensate is formed at the same pressure and temperature (Figure 14.1.1). An efficient steam system will reuse this condensate. Failure to reclaim and reuse condensate makes no financial, technical or environmental sense.

1 kg steam

Condensate

1 kg condensate

Fig. 14.1.1 1 kg of steam condenses completely to 1 kg of condensate

Saturated steam used for heating gives up its latent heat (enthalpy of evaporation), which is a large proportion of the total heat it contains. The remainder of the heat in the steam is retained in the condensate as sensible heat (enthalpy of water) (Figure 14.1.2).

Total heat Steam

Latent heat used in heating the process

Sensible heat Condensate

Fig. 14.1.2 After giving up its latent heat to heat the process, steam turns to water containing only sensible heat

As well as having heat content, the condensate is basically distilled water, which is ideal for use as boiler feedwater. An efficient steam system will collect this condensate and either return it to a deaerator, a boiler feedtank, or use it in another process. Only when there is a real risk of contamination should condensate not be returned to the boiler. Even then, it may be possible to collect the condensate and use it as hot process water or pass it through a heat exchanger where its heat content can be recovered before discharging the water mass to drain. Condensate is discharged from steam plant and equipment through steam traps from a higher to a lower pressure. As a result of this drop in pressure, some of the condensate will re-evaporate into ‘flash steam’. The proportion of steam that will ‘flash off’ in this way is determined by the amount of heat that can be held in the steam and condensate. A flash steam amount of 10% to 15% by mass is typical (see Module 2.2). However, the percentage volumetric change can be considerably more. Condensate at 7 bar g will lose about 13% of its mass when flashing to atmospheric pressure, but the steam produced will require a space some 200 times larger than the condensate from which it was formed. This can have the effect of choking undersized trap discharge lines, and must be taken into account when sizing these lines.

14.1.2

The Steam and Condensate Loop

Block 14 Condensate Recovery

Introduction to Condensate Recovery Module 14.1

Example 14.1.1 Calculating the amount of flash steam from condensate

Hot condensate at 7 bar g has a heat content of about 721 kJ / kg. When it is released to atmospheric pressure (0 bar g), each kilogram of water can only retain about 419 kJ of heat. The excess energy in each kilogram of the condensate is therefore 721 – 419 = 302 kJ. This excess energy is available to evaporate some of the condensate into steam, the amount evaporated being determined by the proportion of excess heat to the amount of heat required to evaporate water at the lower pressure, which in this example, is the enthalpy of evaporation at atmospheric pressure, 2 258 kJ / kg.  7KHUHIRUHLQWKLVH[DPSOHWKHSHUFHQWDJHRIIODVKVWHDPHYDSRUDWHG  [  )ODVKVWHDPHYDSRUDWHG  The subject of flash steam is examined in greater depth in Module 2.2, ‘What is steam?’ A simple graph (Figure 14.1.3) is used in this Module to calculate the proportion of flash steam. Example: Proportion of flash steam using Figure 14.1.3: Pressure on the trap = 4 bar g Flash steam pressure = 0 bar g % Flash steam = 10% The amount of flash steam in the pipe is the most important factor when sizing trap discharge lines. Flash steam pressure bar g

15

rg 0 ba

ar g

0.5 b

ar g

1.0 b

ar g ar g 1.5 b

2 .5 b

13

2.0 b

ar g

14

12 11

Pressure on traps bar

10 9 8 7 6 5 4

Atmospheric pressure

3 2 1 0

0

0.02

0.06

0.10 0.14 10% kg Flash steam/kg condensate

0.18

0.22

Fig. 14.1.3 Quantity of Flash Steam Graph The Steam and Condensate Loop

14.1.3

Introduction to Condensate Recovery Module 14.1

Block 14 Condensate Recovery

Steam produced in a boiler by the process of adding heat to the water is often referred to as live steam. The terms live steam and flash steam are only used to differentiate their origin. Whether steam is produced in a boiler or from the natural process of flashing, it has exactly the same potential for giving up heat, and each is used successfully for this purpose. The flash steam generated from condensate can contain up to half of the total energy of the condensate. An efficient steam system will recover and use flash steam. Condensate and flash steam discharged to waste means more make-up water, more fuel, and increased running costs. This Module will look at two essential areas – condensate management and flash steam recovery. Some of the apparent problem areas will be outlined and practical solutions proposed. Note: The term ‘trap’ is used to denote a steam-trapping device, which could be a steam trap, a pump-trap, or a pump and trap combination. The ability of any trap to pass condensate relies upon the pressure difference across it, whereas a pumping trap or a pump-trap combination will be able to pass condensate irrespective of operational pressure differences (subject to design pressure ratings).

Condensate return

An effective condensate recovery system, collecting the hot condensate from the steam using equipment and returning it to the boiler feed system, can pay for itself in a remarkably short period of time. Figure 14.1.4 shows a simple steam and condensate circuit, with condensate returning to the boiler feedtank. Pan

Pan Process vessels

Steam

Space heating system

Steam Condensate

Make-up water

Vat

Vat Condensate

Steam Feedtank

Boiler Feedpump Fig. 14.1.4 A typical steam and condensate circuit

Why return condensate and reuse it? Financial reasons

Condensate is a valuable resource and even the recovery of small quantities is often economically justifiable. The discharge from a single steam trap is often worth recovering. Un-recovered condensate must be replaced in the boiler house by cold make-up water with additional costs of water treatment and fuel to heat the water from a lower temperature.

Water charges

Any condensate not returned needs to be replaced by make-up water, incurring further water charges from the local water supplier. 14.1.4

The Steam and Condensate Loop

Block 14 Condensate Recovery

Introduction to Condensate Recovery Module 14.1

Effluent restrictions

In the UK for example, water above 43°C cannot be returned to the public sewer by law, because it is detrimental to the environment and may damage earthenware pipes. Condensate above this temperature must be cooled before it is discharged, which may incur extra energy costs. Similar restrictions apply in most countries, and effluent charges and fines may be imposed by water suppliers for non-compliance.

Maximising boiler output

Colder boiler feedwater will reduce the steaming rate of the boiler. The lower the feedwater temperature, the more heat, and thus fuel needed to heat the water, thereby leaving less heat to raise steam.

Boiler feedwater quality

Condensate is distilled water, which contains almost no total dissolved solids (TDS). Boilers need to be blown down to reduce their concentration of dissolved solids in the boiler water. Returning more condensate to the feedtank reduces the need for blowdown and thus reduces the energy lost from the boiler.

Summary of reasons for condensate recovery: o

Water charges are reduced.

o

Effluent charges and possible cooling costs are reduced.

o

Fuel costs are reduced.

o

More steam can be produced from the boiler.

o

Boiler blowdown is reduced - less energy is lost from the boiler.

o

Chemical treatment of raw make-up water is reduced.

Figure 14.1.5 compares the amount of energy in a kilogram of steam and condensate at the same pressure. The percentage of energy in condensate to that in steam can vary from 18% at 1 bar g to 30% at 14 bar g; clearly the liquid condensate is worth reclaiming. Specific enthalpy (kJ / kg)

3000 Total energy in steam

2500 2000 1500 1000

Total energy in condensate

500 0

0

2

4

8 6 Pressure bar g

10

12

14

Fig. 14.1.5 Heat content of steam and condensate at the same pressures

The following example (Example 14.1.2) demonstrates the financial value of returning condensate.

Example 14.1.2

A boiler produces: 10 000 kg /h of steam 24 hours /day, 7 days/week and 50 weeks/year (8 400 hours / year). Raw make-up water is at 10°C. Currently all condensate is discharged to waste at 90°C. Raw water costs £0.61 / m3, and effluent costs are £0.45 / m3 The boiler is 85% efficient, and uses gas on an interruptible tariff charged at £0.01 / kWh (£2.77/GJ).

The Steam and Condensate Loop

14.1.5

Introduction to Condensate Recovery Module 14.1

Block 14 Condensate Recovery

Determine the annual value of returning the condensate Part 1 - Determine the fuel cost Each kilogram of condensate not returned to the boiler feedtank must be replaced by 1 kg of cold make-up water (10°C) that must be heated to the condensate temperature of 90°C. (DT = 80°C). Calculate the heat required to increase the temperature of 1 kg of cold make-up water by 80°C, by using Equation 2.1.4. 4

PFS ∆7

Equation 2.1.4

Where: Q = Quantity of energy (kJ) m = Mass of the substance (kg) cp = Specific heat capacity of the substance (kJ /kg °C ) DT = Temperature rise of the substance (°C) m is unity; DT is the difference between the cold water make-up and the temperature of returned condensate; cp is the specific heat of water at 4.19 kJ / kg °C. 1 kg x 4.19 kJ / kg °C x 80°C = 335 kJ / kg Basing the calculations on an average evaporation rate of 10 000 kg / h, for a plant in operation 8 400 h / year, the energy required to replace the heat in the make-up water is: 10 000 kg / h x 335 kJ / kg x 8 400 h / year = 28 140 GJ / year If the average boiler efficiency is 85%, the energy supplied to heat the make-up water is:  *- \HDU   *- \HDU 

With a fuel cost of £2.77 / GJ, the value of the energy in the condensate is: Annual fuel cost = 33 106 GJ / year x £2.77 / GJ = £91 704 Part 2 - Determine the water cost Water is sold by volume, and the density of water at normal ambient temperature is about 1 000 kg / m3. The total amount of water required in one year replacing non-returned condensate is therefore:

K[NJ  K   Pó \HDU NJ Pó If water costs are £0.61 per m³, the annual water cost is: Annual water cost = 84 000 m3 / year x £0.61 / m3 = £51 240 Part 3 - Determine the effluent cost The condensate that was not recovered would have to be discharged to waste, and may also be charged by the water authority. Total amount of water to waste in one year also equals 84 000 m³ If effluent costs are £0.45 per m³, the annual effluent cost is: Annual effluent cost = 84 000 m3 / year x £0.45 / m3 = £37 800

14.1.6

The Steam and Condensate Loop

Block 14 Condensate Recovery

Introduction to Condensate Recovery Module 14.1

Part 4 - Total value of condensate The total annual value of 10 000 kg / h of condensate lost to waste is shown in Table 14.1.1: Table 14.1.1 The potential value of returning condensate in Example 14.1.2 Fuel savings = £ 91 704 Water savings = £ 51 240 Effluent savings = £ 37 800 Total value = £ 180 744

On this basis, it follows that for each 1% of condensate returned per 10 000 kg / h evaporated as in Example 14.1.2, a saving of 1% of each of the values shown in Table 14.1.1 would be possible.

Example 14.1.3

If it were decided to invest £50 000 in a project to return 80% of the condensate in a similar plant to Example 14.1.2, but where the total evaporation rate were only 5 000 kg / h, the savings and simple payback term would be:   6DYLQJV …[ [     6DYLQJV

… \HDU …  … \HDU

3D\EDFN



3D\EDFN

\HDUZHHNV

This sample calculation does not include a value for savings due to correct TDS control and reduced blowdown, which will further reduce water losses and boiler chemical costs. These can vary substantially from location to location, but should always be considered in the final analysis. Clearly, when assessing condensate management for a specific project, such savings must be determined and included. TDS control and water treatment have already been discussed in Block 3. The routines outlined in Examples 14.1.2 and 14.1.3 may be developed to form the basis of a forced path calculation to assign a monetary value to projects intended to improve condensate recovery. Equation 14.1.1 can be used to calculate the fuel savings per year: )XHOVDYLQJV \HDU 

;$%&' ( 

Equation 14.1.1

Where: X = Expected improvement in condensate return expressed as a percentage between 1 and 100 A = Cost of fuel to provide 1 GJ of energy: If gas on an interruptible tariff costs £0.01/kWh (1 kWh = 3.6 MJ) … &RVWRI*-RIHQHUJ\  [ … 0Similarly, if oil has a calorific value of 42 MJ / l, and costs £0.15 / l … &RVWRI*-RIHQHUJ\  [ … 0B = Energy required per kilogram of make-up water to reach condensate temperature (kJ/kg). This is determined by Q in Equation 2.1.4 (Q = m cp DT) C = Average evaporation rate (kg / h) D= Operational hours per year (h / year) E = Boiler efficiency (%)

The Steam and Condensate Loop

14.1.7

Introduction to Condensate Recovery Module 14.1

Block 14 Condensate Recovery

Savings in water costs can be determined using Equation 14.1.2: ;&'  6DYLQJVLQZDWHUFRVWV \HDU    [&RVWRIZDWHU P    

Equation 14.1.2

Savings in effluent costs can be determined using Equation 14.1.3: ;&'  6DYLQJVLQHIIOXHQWFRVWV \HDU    [&RVWRIHIIOXHQW P    

Equation 14.1.3

Where: X = Expected improvement in condensate return expressed as a percentage between 1 and 100 C = Average evaporation rate (kg / h) D= Operational hours per year (h / year)

Example 14.1.2

A major condensate management project costing £70 000 expects to recover an additional 35% of the condensate produced at a plant. The average boiler steaming rate is 15 000 kg / h, and the plant operates for 8 000 h / year. The fuel used is gas on a firm tariff of £0.011 / kWh, and the boiler efficiency is estimated as 80%. Make-up water temperature is 10°C and insulated condensate return lines ensure that condensate will arrive back at the boiler house at 95°C. Consider the water costs to be £0.70 / m3 and the total effluent costs to be £0.45 / m3. o

Determine the payback period for the project.

Part 1 - Determine the fuel savings Use Equation 14.1.1: )XHOVDYLQJV \HDU 

;$%&' ( 

Equation 14.1.1

Where: X = Expected improvement in condensate return = 35% … $ &RVWRISURYLGLQJ*-RIHQHUJ\  [ … 0B = Energy required per kilogram of make-up water to reach condensate temperature (kJ/kg). This is determined by Q in Equation 2.1.4 (Q = m cp DT) Q = m x cp x DT Q = 1 x 4.19 x (95°C - 10°C) Q = 356.15 kJ / kg B = Q in Equation 2.1.4 = 356.15kJ / kg C = Average evaporation rate = 15 000 kg / h D = Steaming hours per year = 8 000 h E = Boiler efficiency = 80% Substituting the values for X, A, B, C, D, and E into Equation 14.1.1 )XHOVDYLQJV \HDU

…

[[[ [ [

)XHOVDYLQJV  \HDU … 

14.1.8

The Steam and Condensate Loop

Block 14 Condensate Recovery

Introduction to Condensate Recovery Module 14.1

Part 2 - Determine the water and effluent savings Use Equation 14.1.2 to calculate the savings in water costs / year: ;&'  6DYLQJVLQZDWHUFRVWV \HDU    [&RVWRIZDWHU P   

Equation 14.1.2

Substituting values into Equation 14.1.2:

6DYLQJVLQZDWHUFRVWV \HDU 6DYLQJVLQZDWHUFRVWV  \HDU

  

[ [    [… P  

… 

Use Equation 14.1.2 to calculate the savings in effluent costs / year: ;&'  6DYLQJVLQHIIOXHQWFRVWV \HDU    [&RVWRIHIIOXHQW P    

Equation 14.1.3

Substituting values into Equation 14.1.3:

 [ [   [… P     

6DYLQJVLQHIIOXHQWFRVWV \HDU 6DYLQJVLQHIIOXHQWFRVWV  \HDU

… 

Total water and effluent savings / year = £29 400 + £18 900 Total water and effluent savings / year = £48 300 Part 3 - Determine the payback period Total savings = Fuel savings + Water and effluent savings Total savings = £57 122 + £ 48 300 Total savings = £105 422 / year 6LPSOHSD\EDFN\HDUV



&RVWRISURMHFW $QQXDOVDYLQJV

6LPSOHSD\EDFN\HDUV



…  … 

6LPSOHSD\EDFN\HDUV \HDUZHHNV

The Steam and Condensate Loop

14.1.9

Introduction to Condensate Recovery Module 14.1

Block 14 Condensate Recovery

Questions 1. When 10 kg of steam condenses at 0 bar g, how much condensate is produced? a| 10 kg

¨

b| 1.5 kg

¨

c| 10% of the mass of the steam

¨

d| 10% of the volume of the steam

¨

2. 10 kg of steam condenses at 14 bar g. What proportion of the total heat in the steam is held in the condensate? a| 5%

¨

b| 10%

¨

c| 20%

¨

d| 30%

¨

3. A boiler produces 1 000 kg / h of steam at 7 bar g, but none of the condensate is recovered. Approximately at what rate is energy being wasted ? (Steam tables are required). a| 20 kW

¨

b| 40 kW

¨

c| 200 kW

¨

d| 1 000 kW

¨

4. If, in Question 3, it is proposed that 50% of the wasted condensate is to be returned to the boiler feedtank at 90°C, and the fuel cost is £3 / GJ, the cold water make-up temperature is 15°C, the water make-up temperature is 15°C, and the water/effluent costs are £0.8 / m³, what are the potential total annual condensate savings if the boiler steams at 85% efficiency for 4 000 hours per year? a| £1 500

¨

b| £2 218

¨

c| £10 100

¨

d| £500

¨

5. If in Question 4, the cost of this project were £2 000, what would be the simple payback term? a| 3 weeks

¨

b| 33 weeks

¨

c| 18 months

¨

d| 47 weeks

¨

Answers

1: a, 2: d, 3: c, 4: b, 5: d

14.1.10

The Steam and Condensate Loop

Block 14 Condensate Recovery

Layout of Condensate Return Lines Module 14.2

Module 14.2 Layout of Condensate Return Lines

The Steam and Condensate Loop

14.2.1

Layout of Condensate Return Lines Module 14.2

Block 14 Condensate Recovery

Layout of Condensate Return Lines No single set of recommendations can cover the layout of condensate pipework. Much depends on the application pressure, the steam trap characteristics, the position of the condensate return main relative to the plant, and the pressure in the condensate return main. For this reason it is best to start by considering what has to be achieved, and to design a layout which will ensure that basic good practice is met. The prime objectives are that: o

o

Condensate must not be allowed to accumulate in the plant, unless the steam using apparatus is specifically designed to operate in this way. Generally apparatus is designed to operate non-flooded, and where this is the case, accumulated condensate will inhibit performance, and encourage the corrosion of pipes, fittings and equipment. Condensate must not be allowed to accumulate in the steam main. Here it can be picked up by high velocity steam, leading to erosion and waterhammer in the pipework.

The subject of condensate piping will divide naturally into four basic types where the requirements and considerations of each will differ. These four basic types are defined and illustrated in Figure 14.2.1.

Steam main

Drain line to trap

Steam flow

Discharge line from trap Common return line

Condensate flow Type of condensate line Drain line to trap Discharge line from trap Common return line Pumped return line (not shown)

Condensate line is sized to carry the folllowing: Condensate Flash steam Flash steam Condensate

Fig. 14.2.1 A steam main trap set discharging condensate into a common return line

14.2.2

The Steam and Condensate Loop

Block 14 Condensate Recovery

Layout of Condensate Return Lines Module 14.2

Drain lines to steam traps In the drain line, the condensate and any incondensable gases must flow from the drain outlet of the plant to the steam trap. In a properly sized drain line, the plant being drained and the body of the steam trap are virtually at the same pressure and, because of this, condensate does not flash in this line. Gravity is the driving force and is relied upon to induce flow along the pipe. For this reason, it makes sense for the trap to be situated below the outlet of the plant being drained, and the trap discharge pipe to terminate below the trap. (An exception to this is the tank heating coils discussed in Module 2.10). The type of steam trap used (thermostatic, thermodynamic or mechanical) can affect the piping layout.

Thermostatic steam traps

Thermostatic traps will cool condensate below saturation temperature before discharging. This effectively waterlogs the drain line, often allowing condensate to back-up and flood the plant. There are some applications where the sub-cooling of condensate has significant advantages and is encouraged. Less flash steam is produced in the trap discharge line, and the introduction of condensate into the condensate main is gentler. Thermostatic traps discharging via open-ended pipework will waste less energy than mechanical traps because more of the sensible heat in the waterlogged condensate imparts its heat to the process; a typical example is that of a steam tracer line. Thermostatic traps should not be used to drain steam mains or heat exchangers, unless proper consideration is given to a longer and / or larger drain line to act as a reservoir and dissipate heat to atmosphere. The extra length (or larger diameter) of drain line required to do this is usually impractical, as shown in Example 14.2.1.

Example 14.2.1

A 30 kW air heater is to be fitted with a DN15 thermostatic steam trap, which releases condensate at 13°C below saturation temperature. The normal working pressure is 3 bar g, the ambient temperature is 15°C, and the heat loss from the drain line to the environment is estimated to be 20 W / m2 °C. Determine the minimum required length of 15 mm drain line to the thermostatic trap. From steam tables, at 3 bar g: Saturation temperature of steam = 144°C Trap discharge temperature = 144 - 13°C = 131°C Enthalpy of evaporation (hfg) = 2 133.24 kJ / kg Equation 2.8.1 can be used to calculate the steam flow from the heat load:

6WHDPIORZUDWHNJ K =

/RDGLQN:[ KIJ DWRSHUDWLQJSUHVVXUH

6WHDPIORZUDWHNJ K =

+HDWORDGN: [VK KIJ DWRSHUDWLQJSUHVVXUHN-NJ

6WHDPIORZUDWHNJ  K =

[ 

Equation 2.8.1

Steam flowrate = 50.6 kg / h (= 0.014 1 kg / s)

The Steam and Condensate Loop

14.2.3

Layout of Condensate Return Lines Module 14.2

Block 14 Condensate Recovery

As the trap discharges at 131°C, the drain line has to emit enough heat such that the condensate at the heater outlet is at saturation temperature, and that condensate will not back-up into the heater. The required heat loss from the drain line can be calculated from Equation 2.6.5. 

FS ∆7

Equation 2.6.5

Where: Q = Mean heat transfer rate (kW) m = Mean secondary fluid flowrate (kg /s) cp = Specific heat capacity of the secondary fluid (kJ / kg K) or (kJ / kg °C) = 4.19 for water DT = Temperature rise of the secondary fluid (K or °C)

DT in Equation 2.6.5 is the required temperature drop along the drain line of 13°C.



NJ V[N- NJ ƒ&[ƒ&



N:

This heat loss will be achieved from the mean condensate temperature along the drain line.

  ƒ&  The surface area of the drain line to provide the required heat loss can be calculated using Equation 2.5.3. 0HDQFRQGHQVDWHWHPSHUDWXUHLQWKHGUDLQOLQH 



8$∆7

Equation 2.5.3

Where: Q = Heat transferred per unit time (W ( J /s)) U = Overall heat transfer coefficient (W/m² K or W/m² °C) A = Heat transfer area (m²) DT = Temperature difference between the primary and secondary fluid (K or °C) Note: Q will be a mean heat transfer rate (QM) if DT is a mean temperature difference (DTLM or DTAM). DT in Equation 2.5.3 is the difference between the mean condensate temperature and the ambient temperature = 137.5°C - 15°C = 122.5°C



N:

8

:P ƒ&

From Equation 2.5.3: 0.768 x 103 watts = 20 watts / m2 °C x A x 122.5°C Therefore, A = 0.313 m2 The length of pipe required to provide this surface area can be calculated using information from Table 2.10.3. Table 2.10.3 Nominal surface areas of steel pipes per metre length Nominal bore mm 15 20 25 32 40 Surface area (m²/m) 0.067 0.085 0.106 0.134 0.152

50 0.189

65 0.239

80 0.279

100 0.358

The surface area of 15 mm pipe = 0.067 m2 / m 7KHUHIRUHWKHOHQJWKRIGUDLQOLQH 0LQLPXPOHQJWKRIGUDLQOLQH

14.2.4

P P P  PIRU([DPSOH The Steam and Condensate Loop

Block 14 Condensate Recovery

Layout of Condensate Return Lines Module 14.2

This length of pipe (4.7 m) is probably impractical in the field. Two alternatives remain. One is to increase the diameter of the drain line, which is still usually impractical; the other is much simpler, to fit the correct trap for this type of application; a float-thermostatic trap which discharges condensate at steam temperature and hence requires no cooling leg. Should a thermostatic trap be considered essential, and fitted no more than 2 metres away from the heater outlet, it would be necessary to calculate the required diameter of drain line. The heat loss required from the pipe remains the same, along with the total surface area of the pipe, but the surface area per metre length must increase. P P 7KHVXUIDFHDUHDUHTXLUHG PHWUHOHQJWK P  P 7KHVXUIDFHDUHDUHTXLUHG PHWUHOHQJWK



From Table 2.10.3, it can be seen that the minimum sized pipe to give this area per metre is a 50 mm pipe, which, again, may be construed as being impractical and expensive to fabricate. The moral of this is that it is usually easier and cheaper to select the correct trap for the job, than have the wrong type of trap and fabricate a solution around it.

Thermodynamic steam traps

Traps that discharge intermittently, such as thermodynamic traps, will accumulate condensate between discharges. However, they are extremely robust, will tolerate freezing ambient temperatures and have a relatively small outer surface area, meaning that heat loss to the environment is minimised. They are not suitable for discharging condensate into flooded return lines, as will be explained later in this Block.

Mechanical steam traps

Mechanical steam traps with a continuous discharge characteristic, for example float-thermostatic traps, often prove to be the best option, and have the additional advantage of being able to vent air. Most float traps are available in two basic flow configurations, either horizontal or vertical flow through the trap. Some inverted bucket traps have bottom inlet and top outlet connections. Clearly, the trap connections will affect the path of connecting pipework. The drain line should be kept to a minimum length, ideally less than 2 metres. Long drain lines from the plant to the steam trap can fill with steam and prevent condensate reaching the trap. This effect is termed steam locking. To minimise this risk, drain lines should be kept short (see Figure 14.2.2). In situations where long drain lines are unavoidable, the steam locking problem may be overcome using float traps with steam lock release devices. The problem of steam locking should be tackled by fitting the correct length of pipe in the first place, if possible.

✓

✗

Fig. 14.2.2 Keep drain lines short

The detailed arrangements for trapping steam-using plant and steam mains drainage are different as is explained in the following paragraphs.

The Steam and Condensate Loop

14.2.5

Layout of Condensate Return Lines Module 14.2

Block 14 Condensate Recovery

With steam-using plant, the pipe from the condensate connection should fall vertically for about 10 pipe diameters to the steam trap. Assuming a correctly sized ball float trap is installed, this will ensure that surges of condensate do not accumulate in the bottom of the plant with its attendant risks of corrosion and waterhammer. It will also provide a small amount of static head to help remove condensate during start-up when the steam pressure might be very low. The pipework should then run horizontally, with a fall in the direction of flow to ensure that condensate flows freely (see Figure 14.2.3).

Steam main

Steam

Air heater battery

Slight fall in the direction of flow

➤ ➤

10 D

➤

➤

Condensate

D

Fig. 14.2.3 Ideal arrangement when draining a steam plant

With steam mains drainage, provided drain pockets are installed as recommended in Module 10.3, then the drain line between the pocket and the steam trap may be horizontal. If the drain pocket is not as deep as the recommendation, then the steam trap should be fitted an equivalent distance below it (see Figure 14.2.4).

Steam main

D

Steam

d d2

Drain pocket Float trap

Check valve

Strainer Sight glass

Condensate Main diameter D Up to 100 mm 125 mm - 200 mm 250 mm and above

Pocket diameter d1 d1 = D d1 = 100 mm d1 = D/2

Pocket depth d2 Minimum d2 = 100 mm Minimum d2 = 150 mm Minimum d2 = D

Fig. 14.2.4 Ideal arrangement when draining a steam main

14.2.6

The Steam and Condensate Loop

Block 14 Condensate Recovery

Layout of Condensate Return Lines Module 14.2

Discharge lines from traps These pipes will carry condensate, incondensable gases, and flash steam from the trap to the condensate return system (Figure 14.2.5). Flash steam is formed as the condensate is discharged from the high-pressure space before the steam trap to the lower pressure space of the condensate return system. (Flash steam is discussed briefly in Module 14.1, and in more detail in Module 2.2). These lines should also fall in the direction of flow to maintain free flow of condensate. On shorter lines, the fall should be discernible by sight. On longer lines, the fall should be about 1:70, that is, 100 mm every 7 metres.

Condensate

High pressure drain line Float trap

Isolating valve

Check valve

Low pressure discharge line

Condensate and flash steam

Fig. 14.2.5 Trap discharge lines pass condensate, flash and incondensibles

Discharging into flooded return lines

Discharging traps into flooded return lines is not recommended, especially with blast action traps (thermodynamic or inverted bucket types), which remove condensate at saturation temperature. Good examples of flooded condensate mains are pumped return lines and rising condensate lines. They often follow the same route as steam lines, and it is tempting to simply connect mains drainage steam trap discharge lines into them. However, the high volume of flash steam released into long flooded lines will violently push the water along the pipe, causing waterhammer, noise and, in time, mechanical failure of the pipe.

Common return lines Where condensate from more than one trap flows to the same collecting point such as a vented receiver, it is usual to run a common line into which individual trap discharge lines are connected. Provided the layouts as featured in Figures 14.2.6/7/8 and 10 are observed, and the pipework is adequately sized as indicated in Module 14.3, this is not a problem.

Blast discharge traps

If blast discharge traps (thermodynamic or inverted bucket types) are used, the reactionary forces and velocities can be high. Swept tees will help to reduce mechanical stress and erosion at the point where the discharge line joins the common return line (see Figure 14.2.6).

Steam

Steam main

Swept tee Common return line Condensate Fig. 14.2.6 A swept tee connection The Steam and Condensate Loop

14.2.7

Layout of Condensate Return Lines Module 14.2

Block 14 Condensate Recovery

Continuous discharge traps

If, for some reason, swept tees cannot be used, a float-thermostatic trap with its continuous discharge action is a better option (Figure 14.2.7). The flooded line will absorb the dissipated energy from the (relatively small) continuous flow from the float-thermostatic trap, more easily. It the pressure difference between the steam and condensate mains is very high, then a diffuser will help to cushion the discharge, reducing both erosion and noise. Diffuser

Condensate in flooded line

Condensate

Condensate Steam

Steam main Float-thermostatic trap

Fig. 14.2.7 Float trap with a diffuser into a flooded line

Another alternative is to use a thermostatic trap that holds back condensate until it cools below the steam saturation temperature; this reduces the amount of flash steam formed (Figure 14.2.8). To avoid waterlogging the steam main, the use of a generous collecting pocket on the main, plus a cooling leg of 2 to 3 m of unlagged pipe to the trap is essential. The cooling leg stores condensate while it is cooling to the discharge temperature. If there is any danger of waterlogging the steam main, thermostatic traps should not be used.

Diffuser Condensate

Steam

Steam main

Condensate in flooded line

Condensate Balanced pressure thermostatic trap

Thermostatic trap set with cooling leg Fig. 14.2.8 Balanced pressure thermostatic trap with cooling leg into a flooded line

Temperature controlled plant with steam traps draining into flooded lines

Processes using temperature control provide an example where the supply steam pressure is throttled across a control valve. The effect of this is to reduce steam trap capacity to a point where the condensate flow can stop completely, and the system is said to have stalled. The subject of stall is discussed in greater depth in Block 13. Stall occurs as a result of insufficient steam pressure to purge the steam plant of condensate, and is more likely when the plant has a high turndown from full-load to part load. 14.2.8

The Steam and Condensate Loop

Block 14 Condensate Recovery

Layout of Condensate Return Lines Module 14.2

Not all temperature controlled systems will stall, but the backpressure caused by the condensate system could have an adverse effect on the performance of the trap. This in turn, might impair the heat transfer capability of the process (Figure 14.2.9). Condensate drain lines should, therefore, be configured so that condensate cannot flood the main into which they are draining as depicted in Figure 14.2.10.

Steam Heat exchanger

✗

Lifting common line causing backpressure and flooding

Condensate from others

Temperature control may cause low condensate pressure in the drain line Steam trap

Flooded common line Fig. 14.2.9 Discharge from steam traps on temperature controlled equipment into flooded lines should be avoided if possible

Vacuum breaker Steam Heat exchanger

✓

Condensate from others

Temperature control may cause low condensate pressure in the drain line

Slope 1:70 ➤ ➤

Steam trap

Non-flooded common line

Condensate draining down to a vented receiver

Falling common line allowing condensate to drain freely.

Fig. 14.2.10 Condensate discharging freely via a falling common line

Discharge lines at different pressures

Condensate from more than one temperature controlled process may join a common line, as long as this line is: o Designed to slope in the direction of flow to a collection point. o Sized to cater for the cumulative effects of any flash steam from each of the branch lines at full-load. The concept of connecting the discharges from traps at different pressures is sometimes misunderstood. If the branch lines and the common line are correctly sized, the pressures downstream of each trap will be virtually the same. However, if these lines are undersized, the flow of condensate and flash steam will be restricted, due to a build up of backpressure caused by an increased resistance to flow within the pipe. Condensate flowing from traps draining the lower pressure systems will tend to be the more restricted. Each part of the discharge piping system should be sized to carry any flash steam present at acceptable steam velocities. The discharge from a high-pressure trap will not interfere with that from a low-pressure trap if the discharge lines and common line are properly sized and sloped in the direction of flow. Module 14.3, ‘Sizing of condensate return lines’ gives further details. The Steam and Condensate Loop

14.2.9

Layout of Condensate Return Lines Module 14.2

Block 14 Condensate Recovery

Pumped return lines Flash steam may, at some point, be separated from the condensate and used in a recovery system, or simply vented to atmosphere from a suitable receiver (Figure 14.2.11). The residual hot condensate from the latter can be pumped on to a suitable collecting tank such as a boiler feedtank. When the pump is served from a vented receiver, the pumped return line will be fully flooded with condensate at temperatures below 100°C, which means flash steam is less likely to occur in the line. Vent

Steam

Steam

Condensate pumped to boiler High level feedtank condensate Condensate main receiver

Steam

MFP Pump Fig. 14.2.11 Condensate recovery from a vented receiver

Flow in a pumped return line is intermittent, as the pump starts and stops according to its needs. The pump discharge rate will be higher than the rate at which condensate enters the pump. It is, therefore, the pump discharge rate which determines the size of the pump discharge line, and not the rate at which condensate enters the pump. The pumping of condensate is discussed in further detail in Module 14.4, ‘Pumping condensate from vented receivers’.

14.2.10

The Steam and Condensate Loop

Block 14 Condensate Recovery

Layout of Condensate Return Lines Module 14.2

Questions 1. How many different basic types of condensate lines are there? a| One

¨

b| Two

¨

c| Three

¨

d| Four

¨

2. Why are thermostatic traps not recommended for draining steam mains? a| They tend to waterlog the drain line

¨

b| They tend to waterlog the process

¨

c| Long drain lines are necessary to cool the condensate

¨

d| All of the above

¨

3. When might a thermostatic trap be used to drain a steam main? a| When it is fitted to a correctly sized drain pocket

¨

b| When the difference in pressure between the steam and condensate is high

¨

c| When it is fitted with a cooling leg and draining into a flooded main

¨

d| Never

¨

4. When are thermodynamic traps not recommended for draining steam mains? a| They are not intended to drain steam mains

¨

b| When draining into flooded condensate lines

¨

c| When fitted outside and there is a danger of freezing

¨

d| When fitted to large drain pockets

¨

5. What will a trap discharge line normally carry that a drain line does not? a| The weight of the trap

¨

b| Live steam

¨

c| A mixture of live steam and condensate

¨

d| A mixture of flash steam and condensate

¨

6. Upon which criterion is a pump discharge line sized? a| The condensate discharge rate from the pump

¨

b| The pump filling rate

¨

c| The size of the pump outlet

¨

d| The height of the process above the top of the pump

¨

Answers

1: d, 2: d, 3: c, 4: b, 5: d, 6: a The Steam and Condensate Loop

14.2.11

Block 14 Condensate Recovery

14.2.12

Layout of Condensate Return Lines Module 14.2

The Steam and Condensate Loop

Block 14 Condensate Recovery

Sizing Condensate Return Lines Module 14.3

Module 14.3 Sizing Condensate Return Lines

The Steam and Condensate Loop

14.3.1

Sizing Condensate Return Lines Module 14.3

Block 14 Condensate Recovery

Sizing Condensate Lines The four main types of condensate line, as mentioned in Module 14.2, are shown in Table 14.3.1: Table 14.3.1 The four basic types of condensate line Type of condensate line Drain lines to trap Discharge lines from traps Common return lines Pumped return lines

Condensate line is sized to carry the following Condensate Flash steam Flash steam Condensate

Sizing of all condensate lines is a function of: o

Pressure - The difference in pressure between one end of the pipe and the other. This pressure difference may either promote flow, or cause some of the condensate to flash to steam.

o

Quantity - The amount of condensate to be handled.

o

Condition - Is the condensate predominately liquid or flash steam?

With the exception of pumped return lines which will be discussed in Module 14.4, the other three main types of condensate line and their sizing, will be covered in this Module.

Sizing drain lines to traps

It should not be assumed that the drain line (and trap) should be the same size as the plant outlet connection. The plant may operate at a number of different operating pressures and flowrates, especially when it is temperature controlled. However, once the trap has been correctly sized, it is usually the case that the drain line will be the same size as the trap inlet connection, (see Figure 14.3.1).

Plant

DN20 outlet

✗

Plant

DN20 outlet

20 mm pipe

✓

25 mm pipe

DN25 trap

Fig. 14.3.1 The drain line should not be sized on the plant connection

Regarding the conditions inside the drain line, as there is no significant pressure drop between the plant and the trap, no flash steam is present in the pipe, and it can be sized to carry condensate only. When sizing the drain line, the following will need consideration: o

The condensing rate of the equipment being drained during full-load.

o

The condensing rate of the equipment at start-up. At plant start-up, the condensing rate can be up to three times the running load – this is where the temperature difference between the steam and colder product is at its maximum. The drain line, trap, and discharge line also have to carry the air that is displaced by the incoming steam during this time.

The sizing routine for the steam trap will have to consider both of these variables, however, in general: o

For steam mains drainage, the condensate load for each drain trap is typically 1% of the steam capacity of the main based on drain points at 50 m intervals, and with good insulation. For most drain points, sizing the trap to pass twice the running load at the working pressure (minus any backpressure) will allow it to cope with the start-up load.

14.3.2

The Steam and Condensate Loop

Block 14 Condensate Recovery

o

o

Sizing Condensate Return Lines Module 14.3

On constant steam pressure processes such as presses, ironers, unit heaters, radiant panels and boiling pans, sizing the traps on approximately twice the running load at the working pressure (less any backpressure) will provide sufficient capacity to cope with the start-up load. On temperature controlled applications, the steam pressure, the plant turndown, the set temperature and steam trap location need to be considered in detail, and the trap needs to be sized to cater for both the full and minimum load conditions. If these conditions are not known it is recommended that the steam trap be sized on 3 x the running load at the running differential pressure. This should satisfy the start-up condition and provide proper drainage at minimum loads. When the trap is sized in this way, it will also cater for the start-up load. Consequently, if the drain line to the trap is sized on the trap size, it will never be undersized.

For practical purposes, where the drain line is less than 10 m, it can be the same pipe size as the steam trap selected for the application. Drain lines less than 10 m long can also be checked against Appendix 14.3.1 and a pipe size should be selected which results in a pressure loss at maximum flowrate of not more than 200 Pa per metre length, and a velocity not greater than 1.5 m / s. Table 14.3.2 is an extract from Appendix 14.3.1. On longer drain lines (over 10 m), the pressure loss at maximum flowrate should not be more than 100 Pa /m, and a velocity not greater than 1 m / s. Table 14.3.2 Flow of water in heavy steel pipes Flowrate Capacity kg / h Pipe size Ø 15 mm 20 mm 25 mm 32 mm 40 mm 50 mm 65 mm 80 mm 100 mm Pa / m mbar / m <0.15 m / s 0.15 m / s 0.3 m / s 90.0 0.900 173 403 745 1 627 2 488 4 716 9 612 14 940 30 240 92.5 0.925 176 407 756 1 652 2 524 4 788 9 756 15 156 30 672 95.0 0.950 176 414 767 1 678 2 560 4 860 9 900 15 372 31 104 97.5 0.975 180 421 778 1 699 2 596 4 932 10 044 15 552 31 500 1.0 m / s 100.0 1.000 184 425 788 1 724 2 632 5 004 10 152 15 768 31 932 120.0 1.200 202 472 871 1 897 2 898 5 508 11 196 17 352 35 100 140.0 1.400 220 511 943 2 059 3 143 5 976 12 132 18 792 38 160 160.0 1.600 234 547 1 015 2 210 3 373 6 408 12 996 20 160 40 680 180.0 1.800 252 583 1 080 2 354 3 589 6 804 13 824 21 420 43 200 200.0 2.000 266 619 1 141 2 488 3 780 7 200 14 580 22 644 45 720 220.0 2.200 281 652 1 202 2 617 3 996 7 560 15 336 23 760 47 880 240.0 2.400 288 680 1 256 2 740 4 176 7 920 16 056 24 876 50 400 1.5 m / s 260.0 2.600 306 713 1 310 2 855 4 356 8 244 16 740 25 920 52 200 280.0 2.800 317 742 1 364 2 970 4 536 8 568 17 388 26 928 54 360 300.0 3.000 331 767 1 415 3 078 4 680 8 892 18 000 27 900 56 160

Example 14.3.1

An item of plant, using steam at constant pressure, condenses 470 kg of steam an hour at fullload. The pipework between the plant item and the steam trap has an equivalent length of 2 m. Determine the size of pipe to be used. Revised load allowing for start-up = 470 kg / h x 2 = 940 kg / h. As the pipe length is less than 10 metres, the maximum allowable pressure drop is 200 Pa /m. Using Table 14.3.1, by looking across from 200 Pa /m it can be seen that a 25 mm pipe has a capacity of 1 141 kg / h, and would therefore be suitable for the expected starting load of 940 kg /h. Checking further up the 25 mm column, it can be seen that a flowrate of 940 kg / h will incur an actual pressure drop of just less than 140 Pa /m flowing through a 25 mm pipe.

The Steam and Condensate Loop

14.3.3

Sizing Condensate Return Lines Module 14.3

Block 14 Condensate Recovery

Sizing discharge lines from traps

The section of pipeline downstream of the trap will carry both condensate and flash steam at the same pressure and temperature. This is referred to as two-phase flow, and the mixture of liquid and vapour will have the characteristics of both steam and water in proportion to how much of each is present. Consider the following example.

Example 14.3.2

An item of plant uses steam at a constant 4 bar g pressure. A mechanical steam trap is fitted, and condensate at saturation temperature is discharged into a condensate main working at 0.5 bar g. Determine the proportions by mass, and by volume, of water and steam in the condensate main. Part 1 - Determine the proportions by mass From steam tables: At 4.0 bar g hf = 640.7 kJ / kg At 0.5 bar g hf = 464.1 kJ / kg hfg = 2 225.6 kJ / kg Equation 2.2.5 is used to determine the proportion of flash steam:

3URSRUWLRQRIIODVKVWHDP =

KI DW3 KI DW3 KIJ DW3

Equation 2.2.5

Where: P1 = Initial pressure P2 = Final pressure hf = Specific liquid enthalpy (kJ /kg) hfg = Specific enthalpy of evaporation (kJ /kg)

3URSRUWLRQRIIODVKVWHDP =

  [    

Clearly, if 7.9% is flashing to steam, the remaining 100 – 7.9 = 92.1% of the initial mass flow will remain as water. Part 2 - Determine the proportions by volume Based on an initial mass of 1 kg of condensate discharged at 4 bar g saturation temperature, the mass of flash steam is 0.079 kg and the mass of condensate is 0.921 kg (established from Part 1). Water: The density of saturated water at 0.5 bar g is 950 kg / m3,  DQGWKHYROXPHRFFXSLHGE\NJ   P  Steam: From steam tables, specific volume (vg) of steam at 0.5 bar g = 1.15 m3 / kg The volume occupied by the steam is 0.079 kg x 1.15 m3 / kg = 0.091 m3 The total volume occupied by the steam and condensate mixture is: 0.001 m3 (water) + 0.091 m3 (steam) = 0.092 m3 By proportion (%):   7KHZDWHURFFXSLHV  [ VSDFH     [ VSDFH 7KHVWHDPRFFXSLHV    From this, it follows that the two-phase fluid in the trap discharge line will have much more in common with steam than water, and it is sensible to size on reasonable steam velocities rather than use the relatively small volume of condensate as the basis for calculation. If lines are undersized, the flash steam velocity and backpressure will increase, which can cause waterhammer, reduce the trap capacity, and flood the process. 14.3.4

The Steam and Condensate Loop

Block 14 Condensate Recovery

Sizing Condensate Return Lines Module 14.3

Steam lines are sized with attention to maximum velocities. Dry saturated steam should travel no faster than 40 m /s. Wet steam should travel somewhat slower (15 to 20 m /s) as it carries moisture which can otherwise have an erosive and damaging effect on fittings and valves. Trap discharge lines can be regarded as steam lines carrying very wet steam, and should be sized on similarly low velocities. Condensate discharge lines from traps are notoriously more difficult to size than steam lines due to the two-phase flow characteristic. In practice, it is impossible (and often unnecessary) to determine the exact condition of the fluid inside the pipe. Although the amount of flash steam produced (see Figure 14.3.2) is related to the pressure difference across the trap, other factors will also have an effect. Flash steam pressure bar g

15

rg 0 ba

ar g

0.5 b

ar g

1.0 b

ar g ar g 1.5 b

2 .5 b

13

2.0 b

ar g

14

12 11

Pressure on traps bar

10 9 8 7 6 5 4

Atmospheric pressure

3 2 1 0

0

0.02

0.06

0.10 0.14 10% kg Flash steam / kg condensate

0.18

0.22

Fig. 14.3.2 Quantity of flash steam graph

Factors having a bearing on two-phase flow inside a pipe, include: o

o

If the condensate on the upstream side of the trap is cooler than the saturation temperature (for example: a thermostatic steam trap is used), the amount of flash steam after the trap is reduced. This can reduce the size of the line required. If the line slopes down from the trap to its termination, the slope will have an effect on the flow of condensate, but to what magnitude, and how can this be quantified?

The Steam and Condensate Loop

14.3.5

Sizing Condensate Return Lines Module 14.3

Block 14 Condensate Recovery

o

o

o

o

On longer lines, radiation losses from the line may condense some of the flash steam, reducing its volume and velocity, and there may be a case for reducing the line size. But at what point should it be reduced and by how much? If the discharge line lifts up to an overhead return line, there will be times when the lifting line will be full of cool condensate, and times when flash steam from the trap may evaporate some or all of this condensate. Should the rising discharge line be sized on flash steam velocity or the quantity of condensate? Most processes operate some way below their full-load condition for most of their running cycle, which reduces flash steam for most of the time. The question therefore arises: is there a need for the system to be sized on the full-load condition, if the equipment permanently runs at a lower running load? On temperature controlled plant, the pressure differential across the trap will itself change depending on the heat load. This will affect the amount of flash steam produced in the line.

Recommendations on trap discharge lines

Because of the number of variables, an exact calculation of line size would be complex and probably inaccurate. Experience has shown that if trap discharge lines are sized on flash steam velocities of 15 to 20 m / s, and certain recommendations are adhered to, few problems will arise. Recommendations: 1. Correctly sized trap discharge lines which slope in the direction of flow and are open-ended or vented at a receiver, will be non-flooded and allow flash steam to pass unhindered above the condensate (Figure 14.3.3). A minimum slope of 1 in 70 (150 mm drop every 10 m) is recommended. A simple visual check will usually confirm if the line is sloping - if no slope is apparent it is not sloping enough! Vent

Process

Easy passage for flash steam

Steam Pumped condensate

Easy passage for condensate

Vented receiver

1:70 slope = 150 mm per 10 m run

Pump Fig. 14.3.3 Discharge line sloping 1:70 in the direction of flow

2. If it is unavoidable, non-pumped rising lines (Figure 14.3.4) should be kept as short as possible and fitted with a non-return valve to stop condensate falling back down to the trap. Risers should discharge into the top of overhead return lines. This stops condensate draining back into the riser from the return main after the trap has discharged, to assist the easy passage of flash steam up the riser. 14.3.6

The Steam and Condensate Loop

Block 14 Condensate Recovery

Sizing Condensate Return Lines Module 14.3

Vent

Condensate from others 1:70 slope = 150 mm per 10 m run

Common return line

Pumped condensate

Non pumped rising line Process Steam

Flash steam has to pass through the condensate

Vented receiver

Pump

Fig. 14.3.4 Keep rising lines short and connect to the top of return lines

It is sensible to consider using a slightly larger riser, which will produce a lower flash steam velocity. This will reduce the risk of waterhammer and noise caused by steam trying to force a path through the liquid condensate in the riser. Important: A rising line should only be used where the process steam pressure is guaranteed to be higher than the condensate backpressure at the trap outlet. If not, the process will waterlog unless a pumping trap or pump-trap combination is used to provide proper drainage against the backpressure. 3. Common return lines should also slope down and be non-flooded (Figure 14.3.4). To avoid flash steam occurring in long return lines, hot condensate from trap discharge lines should drain into vented receivers (or flash vessels where appropriate), from where it can be pumped on to its final destination, via a flooded line at a lower temperature. Condensate pumping is dealt with in more detail in Module 14.4.

The condensate pipe sizing chart

The condensate pipe sizing chart (Figure 14.3.5) can be used to size any type of condensate line, including: o o

Drain lines containing no flash steam. Lines consisting of two-phase flow, such as trap discharge lines, which are selected according to the pressures either side of the trap.

The chart (Figure 14.3.5): o

o

o

o

Works around acceptable flash steam velocities of 15 - 20 m/ s, according to the pipe size and the proportion of flash steam formed. Can be used with condensate temperatures lower than the steam saturation temperature, as will be the case when using thermostatic steam traps. Is used to size trap discharge lines on full-load conditions. It is not necessary to consider any oversizing factors for start-up load or the removal of non-condensable gases. May also be used to estimate sizes for pumped lines containing condensate below 100°C. This will be discussed in Module 14.4.

The Steam and Condensate Loop

14.3.7

Sizing Condensate Return Lines Module 14.3

Block 14 Condensate Recovery

500

100000

Condensate pipe size mm 400 350 300

250

200 150 100

50000

80 65

10000

50

5000

40 32

2000

25

1000

5

20 15

500

Condensate pipe size mm

Codensate flowrate kg/h

20000

10

200 100

6

50 20 10 1 3

180 160 140 120 100

50 Steam system pressure bar g

200

2

20

40 30 20

2

10 5 2 1 0.5 0

4

10

1

5

3

2 1 0.5 0

Fig. 14.3.5 Condensate pipe sizing chart

Condensate system pressure bar g

Steam temperature °C

250

4

Using the condensate pipe sizing chart (Also available in Appendix 14.3.2)

Establish the point where the steam and condensate pressures meet (lower part of the chart, Figure 14.3.5). From this point, move vertically up to the upper chart to meet the required condensate rate. If the discharge line is falling (non-flooded) and the selection is on or between lines, choose the lower line size. If the discharge line is rising, and therefore likely to be flooded, choose the upper line size. Note: The reasoning employed for the sizing of a steam trap is different to that used for a discharge line, and it is perfectly normal for a trap discharge line to be sized different to the trap it is serving. However, when the trap is correctly sized, the usual ancillary equipment associated with a steam trap station, such as isolation valves, strainer, trap testing chamber, and check valve, can be the same size as the trapping device selected, whatever the discharge line size. 14.3.8

The Steam and Condensate Loop

Block 14 Condensate Recovery

Sizing Condensate Return Lines Module 14.3

Example 14.3.3 1 on the chart (Figure 14.3.6)

A steam trap passing a full-load of 1 000 kg / h at 6 bar g saturated steam pressure through a falling discharge line down to a flash vessel at 1.7 bar g. As the discharge line is non-flooded, the lower figure of 25 mm is selected from the chart (Figure 14.3.4). 6 bar g High pressure steam Shell and tube heat exchanger

Low pressure steam Float trap set

1.7 bar g

Discharge line being sized Pipeline size selected by use of the chart, Figure 14.3.5, is Ø25 mm

Flash vessel

Condensate Fig. 14.3.6 A non-flooded pressurised trap discharge line (refer to Example 14.3.3)

Example 14.3.4 2 on the chart (Figure 14.3.7)

A steam trap passing a full-load of 1 000 kg / h at 18 bar g saturated steam pressure through a discharge line rising 5 m up to a pressurised condensate return line at 3.5 bar g. Add the 0.5 bar static pressure (5 m head) to the 3.5 bar condensate pressure to give 4 bar g backpressure. As the discharge line is rising and thus flooded, the upper figure of 32 mm is selected from the chart, (Figure 14.3.4). 18 bar g

3.5 bar g

High pressure steam Air vent

5 m (0.5 bar g static pressure)

Float trap SA control valve acting as an air vent and condensate drain on start-up

Discharge line being sized Pipeline size selected by use of the chart, Figure 14.3.5, is Ø32 mm

Fig. 14.3.7 A flooded trap discharge line (refer to Example 14.3.4) The Steam and Condensate Loop

14.3.9

Sizing Condensate Return Lines Module 14.3

Block 14 Condensate Recovery

Example 14.3.5 3 on the chart (Figure 14.3.8)

A steam trap passing a full-load of 200 kg / h at 2 bar g saturated steam pressure through a sloping discharge line falling down to a vented condensate receiver at atmospheric pressure (0 bar g). As the line is non-flooded, the lower figure of 20 mm is selected from the chart, (Figure 14.3.4).

2 bar g High pressure steam

Plate heat exchanger

Discharge line being sized Pipeline size selected by use of the chart, Figure 14.3.5, is Ø20 mm Vent

To high level condensate return line

Fig. 14.3.8 A non-flooded vented trap discharge line (refer to Example 14.3.5)

Example 14.3.6 4 on the chart (Figure 14.3.9) A pump-trap passing a full-load of 200 kg / h at 4 bar g saturated steam space pressure through a discharge line rising 5 m up to a non-flooded condensate return line at atmospheric pressure. The 5 m static pressure contributes the total backpressure of 0.5 bar g. As the trap discharge line is rising, the upper figure of 25 mm is selected from the chart, (Figure 14.3.4). Discharge line being sized Pipeline size selected by use of the chart, Figure 14.3.5, is Ø25 mm 4 bar g High pressure steam 5 m (0.5 bar g at static pressure)

Air flow

Fig. 14.3.9 A flooded trap discharge line (refer to Example 14.3.6)

14.3.10

The Steam and Condensate Loop

Block 14 Condensate Recovery

Sizing Condensate Return Lines Module 14.3

Example 14.3.7 5 on the chart (Figure 14.3.10)

Consider a condensate load of 200 kg / h to a receiver and pump. The pump discharge rate for this mechanical type pump is taken as six times the filling rate, hence, the condensate rate taken for this example is 6 x 200 = 1 200 kg/ h. Because the condensate will have lost its flash steam content to atmosphere via the receiver vent, the pump will only be pumping liquid condensate. In this instance, it is only necessary to use the top part of the chart in Figure 14.3.5. As the line from the pump is rising, the upper figure of 25 mm is chosen. Note: If the pumped line were longer than 100 m, the next larger size must be taken, which for this example would be 32 mm. A useful tip for lines of 100 m or less is to choose a discharge pipe which is the same size as the pump. For further details refer to Module 14.1 ‘Pumping condensate from vented receivers’. Vent

Sloping non-flooded return line

Discharge line being sized pipeline size selected by use of the chart, Figure 14.3.5, is Ø25 mm

Condensate in (200 kg / h)

Pumped condensate out (1 200 kg / h)

Fig. 14.3.10 A discharge line from the condensate pump (refer to Example 14.3.7)

The Steam and Condensate Loop

14.3.11

Sizing Condensate Return Lines Module 14.3

Block 14 Condensate Recovery

Common return lines - falling lines

It is sometimes necessary to connect several trap discharge lines from separate processes into a common return line. Problems will not occur if the following considerations are met: o

o

The common line is not flooded and slopes in the direction of flow to an open end or a vented receiver, or a flash vessel if the conditions allow. The common line is sized on the cumulative sizes of the branch lines, and the branch lines are sized from Figure 14.3.5.

Example 14.3.8

Figure 14.3.11 shows three heat exchangers, each separately controlled and operating at the same time. The condensate loads shown are full loads and occur with 3 bar g in the steam space. The common line slopes down to the flash vessel at 1.5 bar g, situated in the same plant room. Condensate in the flash vessel falls via a float trap down to a vented receiver, from where it is pumped directly to the boiler house. The trap discharge lines are sized on full-load with steam pressure at 3 bar g and condensate pressure of 1.5 bar g, and as each is not flooded, the lower line sizes are picked from the graph. Determine the condensate line sizes for the falling discharge lines and common lines. HE1

HE2

HE3

3 bar g

3 bar g

3 bar g

Full-load 750 kg / h

Full-load 750 kg / h 1” FT14HC

1 Ø20 mm

Flash steam

Full-load 375 kg / h 2 1” FT14HC

Ø20 mm

Ø20 mm

1” FT14

Ø28 mm

3 Ø15 mm

1.5 bar g

Ø32 mm

To receiver

Fig. 14.3.11 Refer to Example 14.3.8

Using Appendix 14.3.2, Condensate pipe sizing chart: Line 1 picked as 20 mm, 2 picked as 20 mm, 3 picked as 15 mm The bore of the common line connecting two discharge lines can be found by calculating the square root of the sum of the squares of the bores of the two discharge lines, as shown below: Common line for 1 + 2 , = Ö 20² + 20² = 28 mm : Pick a DN25 pipe (see note below) Common line for ( 1 + 2 )+ 3 = Ö 28² + 15² = 32 mm : Pick a DN32 pipe Note: The theoretical dimension of 28 mm for the common line 1 + 2 does not exist as a nominal bore in commercial pipe sizes. The internal diameters of pipes can be larger or smaller than the nominal bore depending on the pipe schedule. For example, for a DIN 2448 steel pipe, the internal diameter for a 25 mm pipe is about 28.5 mm, while that for a 25 mm Schedule 40 pipe is about 26.6 mm. Where the calculated bore is not much greater than the nominal bore, it is practical to choose the next lower size pipe. In this instance, a nominal bore 25 mm pipe may be selected. If, however, the calculated bore is not near the nominal bore, then the next larger nominal bore pipe should be selected. Common sense should be applied. 14.3.12

The Steam and Condensate Loop

Block 14 Condensate Recovery

Sizing Condensate Return Lines Module 14.3

Common return lines - rising lines

It is sometimes unavoidable for condensate discharge and common lines to rise at some point between the trap and the point of final termination. When this is the case, each discharge line is sized by moving up to the next size on the chart, as previously discussed in this Module.

Example 14.3.9

Figure 14.3.12 shows the same three heat exchangers as in Example 14.3.8. However, in this instance, the common line rises 15 m and terminates in an overhead nonflooded condensate return main, giving the same backpressure of 1.5 bar as in Example 14.3.8. Each of the discharge lines is sized as a rising line. Determine the condensate line sizes for the discharge lines and common lines. 1.5 bar g

HE1

HE2

3 bar g

HE3

3 bar g

Full-load 750 kg / h

Full-load 375 kg / h

Full-load 750 kg / h 1 1” FT14HC

Ø25 mm

15 m

3 bar g

2 1” FT14HC

Ø25 mm

1” FT14

Ø25 mm

Ø40 mm Fig. 14.3.12 Refer to Example 14.3.9

3 Ø20 mm Ø50 mm

Using Appendix 14.3.2, Condensate pipe sizing chart: Line 1 picked as 25 mm, 2 picked as 25 mm, 3 picked as 20 mm Because the common line is rising, it can be seen that each of the discharge lines is a size larger than in Example 14.3.8 even though the backpressure is the same at 1.5 bar g. The bore of the common line connecting two discharge lines can be found by calculating the square root of the sum of the squares of the bores of the two discharge lines, as shown below: Common line for 1 + 2 ,

= Ö 25² + 25² = 36 mm : Pick a DN40 pipe

Common line for ( 1 + 2 )+ 3 = Ö 36² + 20² = 42 mm : Pick a DN50 pipe Note: For rising lines, the chosen nominal bore pipe should always be larger than the calculated bore.

The Steam and Condensate Loop

14.3.13

Sizing Condensate Return Lines Module 14.3

Block 14 Condensate Recovery

Example 14.3.10 - Falling common line

Calculating the common line sizes for the application shown in Fig. 14.3.12 which falls to a final termination point:

Ø15 mm A

Line A B C D E F G H J K L

Ø40 mm B

Ø20 mm

Ø25 mm D

F

H

Ø32 mm K

C

E

G

J

L

?

?

?

?

?

Falling line to termination

Pipeline diameter (mm) 15 40

Commercial pipe size selected (DN)

Ö 40²+15² = 43* 25

40*

Ö 25²+43² = 50 20

50

Ö 20²+50² = 54 25

65

Ö 25²+54² = 60 32

65

Ö 32²+60² = 68*

65*

Fig. 14.3.13

14.3.14

Ø25 mm

*Close to nominal bore size

The Steam and Condensate Loop

Block 14 Condensate Recovery

Sizing Condensate Return Lines Module 14.3

Example 14.3.11 - Rising common line

Calculating the common line sizes for the application shown in Fig. 14.3.14 which rises to a final termination point: Note that the steam loads are the same as Example 14.3.10, but the discharge lines are one size larger due to the rising common line.

Ø20 mm A

Ø50 mm B

Line A B C D E F G H J K L

Ø25 mm

Ø32 mm D

F

Ø32 mm H

Ø40 mm

Rising line to termination

K

C

E

G

J

L

?

?

?

?

?

Pipeline diameter (mm) 20 50

Commercial pipe size selected (DN)

Ö 50²+20² = 54* 32

50*

Ö 32²+54² = 63 25

65

Ö 25²+63² = 68* 32

65*

Ö 32²+68² = 75 40

80

Ö 40²+75² = 85*

80*

Fig. 14.3.14

*Close to nominal bore size

The procedure shown in Examples 14.3.10 and 14.3.11 can be simplified by using Appendix 14.3.3. For example, where pipes A and B (20 mm and 50 mm) join, the minimum required pipe diameter is shown as 54 mm. Clearly, the user would fit the next largest size of commercial pipe available, unless the calculated bore is close to a nominal bore size pipe.

The Steam and Condensate Loop

14.3.15

Block 14 Condensate Recovery

Sizing Condensate Return Lines Module 14.3

Appendix 14.3.1 Flow of water in heavy steel pipes Flowrate kg / h Pipe size Ø 15 mm 20 mm 25 mm 32 mm 40 mm 50 mm Pa / m mbar / m <0.15 m / s 0.15 m / s 10.0 0.100 50 119 223 490 756 1 447 12.5 0.125 58 133 252 554 853 1 634 15.0 0.150 65 151 277 616 943 1 807 17.5 0.175 68 162 302 670 1 026 1 966 20.0 0.200 76 176 328 720 1 105 2 113 22.5 0.225 79 187 349 770 1 177 2 254 25.0 0.250 83 198 371 814 1 249 2 387 27.5 0.275 90 209 389 857 1 314 2 513 30.0 0.300 94 220 410 900 1 379 2 632 32.5 0.325 97 230 428 940 1 440 2 747 35.0 0.350 101 241 446 979 1 498 2 858 37.5 0.375 104 248 464 1 015 1 555 2 966 40.0 0.400 112 259 479 1 051 1 609 3 071 42.5 0.425 115 266 497 1 087 1 663 3 175 45.0 0.450 119 277 511 1 123 1 717 3 272 47.5 0.475 122 284 526 1 156 1 768 3 370 50.0 0.500 126 292 540 1 188 1 814 3 463 52.5 0.525 130 299 558 1 220 1 865 3 553 55.0 0.550 130 306 572 1 249 1 912 3 636 57.5 0.575 133 317 583 1 282 1 958 3 744 60.0 0.600 137 324 598 1 310 2 002 3 816 62.5 0.625 140 331 612 1 339 2 048 3 888 65.0 0.650 144 338 626 1 368 2 092 3 996 67.5 0.675 148 346 637 1 397 2 131 4 068 70.0 0.700 151 353 652 1 422 2 174 4 140 72.5 0.725 151 356 662 1 451 2 218 4 212 75.0 0.750 155 364 677 1 476 2 257 4 284 77.5 0.775 158 371 688 1 505 2 297 4 356 80.0 0.800 162 378 698 1 530 2 336 4 464 82.5 0.825 166 385 709 1 555 2 372 4 536 85.0 0.850 166 389 724 1 580 2 412 4 608 87.5 0.875 169 396 734 1 606 2 448 4 680 90.0 0.900 173 403 745 1 627 2 488 4 716 92.5 0.925 176 407 756 1 652 2 524 4 788 95.0 0.950 176 414 767 1 678 2 560 4 860 97.5 0.975 180 421 778 1 699 2 596 4 932 100.0 1.000 184 425 788 1 724 2 632 5 004 120.0 1.200 202 472 871 1 897 2 898 5 508 140.0 1.400 220 511 943 2 059 3 143 5 976 160.0 1.600 234 547 1 015 2 210 3 373 6 408 180.0 1.800 252 583 1 080 2 354 3 589 6 804 200.0 2.000 266 619 1 141 2 488 3 780 7 200 220.0 2.200 281 652 1 202 2 617 3 996 7 560 240.0 2.400 288 680 1 256 2 740 4 176 7 920 260.0 2.600 306 713 1 310 2 855 4 356 8 244 280.0 2.800 317 742 1 364 2 970 4 536 8 568 300.0 3.000 331 767 1 415 3 078 4 680 8 892

14.3.16

65 mm 80 mm 100 mm 0.3 m / s 2 966 4 644 9 432 3 348 5 220 10 656 3 708 5 760 11 736 4 032 6 264 12 744 4 320 6 732 13 680 4 608 7 164 14 580 4 860 7 596 15 408 5 112 7 992 16 200 5 364 8 352 16 956 5 616 8 712 17 712 5 832 9 072 18 432 6 048 9 396 19 116 6 264 9 720 19 764 6 480 10 044 20 412 6 660 10 368 21 024 6 876 10 656 21 636 7 056 10 944 22 212 7 236 11 232 22 788 7 416 11 520 23 364 7 596 11 808 23 904 7 776 12 060 24 444 7 920 12 312 24 984 8 100 12 600 25 488 8 280 12 852 25 992 8 424 13 068 26 496 8 568 13 320 27 000 8 748 13 572 27 468 8 892 13 788 27 972 9 036 14 040 28 440 9 180 14 256 28 872 9 324 14 472 29 340 9 468 14 724 29 772 9 612 14 940 30 240 9 756 15 156 30 672 9 900 15 372 31 104 10 044 15 552 31 500 10 152 15 768 31 932 11 196 17 352 35 100 12 132 18 792 38 160 12 996 20 160 40 680 13 824 21 420 43 200 14 580 22 644 45 720 15 336 23 760 47 880 16 056 24 876 50 400 16 740 25 920 52 200 17 388 26 928 54 360 18 000 27 900 56 160

0.5 m/s

1 m/s

1.5 m/s

The Steam and Condensate Loop

Block 14 Condensate Recovery

Sizing Condensate Return Lines Module 14.3

Appendix 14.3.2 Condensate pipe sizing chart 100000 50000

Condensate pipe size mm 400 350 300

250

200 150 100 80 65

10000

50

5000

40

2000 1000 500 200 100

32 25 20 15

Condensate pipe size mm

Codensate flowrate kg/h

20000

500

10 6

50 20 10

180 160 140 120 100

Steam system pressure bar g

200

50 20 10 5 2 1 0.5 0

The Steam and Condensate Loop

40 30 20 10 5 2 1 0.5 0

Condensate system pressure bar g

Steam temperature °C

250

14.3.17

Sizing Condensate Return Lines Module 14.3

Block 14 Condensate Recovery

Appendix 14.3.3 Common pipe sizing table D1 = Connecting branch size (N.B.) D2 = Common pipe size D2 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

14.3.18

15 21 22 23 23 24 25 26 27 27 28 29 30 31 32 33 34 34 35 36 37 38 39 40 41 42 43 44 45 46 46 47 48 49 50 51 52 53 54 55 56 57 58 59

D1 - Connecting branch size (NB) 20 25 32 40 50 65 80 25 29 35 43 52 67 81 26 30 36 43 52 67 82 26 30 36 43 53 67 82 27 31 37 44 53 67 82 28 31 37 44 53 68 82 28 32 38 45 54 68 82 29 33 38 45 54 68 83 30 33 39 46 55 69 83 30 34 39 46 55 69 83 31 35 40 47 55 69 84 32 35 41 47 56 70 84 33 36 41 48 56 70 84 34 37 42 48 57 70 84 34 38 43 49 57 71 85 35 38 43 49 58 71 85 36 39 44 50 58 72 85 37 40 45 51 59 72 86 38 41 45 51 59 72 86 39 41 46 52 60 73 87 39 42 47 52 60 73 87 40 43 47 53 61 74 87 41 44 48 54 62 74 88 42 45 49 54 62 75 88 43 45 50 55 63 75 89 44 46 50 56 63 76 89 45 47 51 57 64 76 89 46 48 52 57 65 77 90 47 49 53 58 65 77 90 47 50 54 59 66 78 91 48 51 54 59 67 78 91 49 51 55 60 67 79 92 50 52 56 61 68 80 92 51 53 57 62 69 80 93 52 54 58 62 69 81 93 53 55 59 63 70 81 94 54 56 59 64 71 82 94 55 57 60 65 71 83 95 56 58 61 66 72 83 95 57 59 62 66 73 84 96 58 60 63 67 74 85 97 59 60 64 68 74 85 97 59 61 64 69 75 86 98 60 62 65 70 76 86 98

100 101 101 101 102 102 102 102 102 103 103 103 103 104 104 104 104 105 105 105 106 106 106 107 107 107 108 108 108 109 109 110 110 110 111 111 112 112 113 113 114 114 115 115

D2

15 58 60 59 61 60 62 61 63 62 64 63 65 64 66 65 67 66 68 67 69 68 70 69 71 70 72 71 73 72 74 73 75 74 76 75 76 76 77 77 78 78 79 79 80 80 81 81 82 82 83 83 84 84 85 85 86 86 87 87 88 88 89 89 90 90 91 91 92 92 93 93 94 94 95 95 96 96 97 97 98 98 99 99 100 100 101

D1 - Connecting branch size (NB) 20 25 32 40 50 65 80 61 63 66 70 77 87 99 62 64 67 71 77 88 99 63 65 68 72 78 88 100 64 66 69 73 79 89 101 65 67 70 74 80 90 101 66 68 71 75 80 91 102 67 69 72 75 81 91 102 68 70 72 76 82 92 103 69 71 73 77 83 93 104 70 72 74 78 84 93 104 71 72 75 79 84 94 105 72 73 76 80 85 95 106 73 74 77 81 86 96 106 74 75 78 81 87 96 107 75 76 79 82 88 97 108 76 77 80 83 88 98 108 77 78 81 84 89 98 109 78 79 82 85 90 99 110 79 80 82 86 91 100 110 80 81 83 87 92 101 111 81 82 84 88 93 102 112 81 83 85 89 93 102 112 82 84 86 89 94 103 113 83 85 87 90 95 104 114 84 86 88 91 96 105 115 85 87 89 92 97 105 115 86 88 90 93 98 106 116 87 89 91 94 99 107 117 88 90 92 95 99 108 117 89 91 93 96 100 109 118 90 91 94 97 101 109 119 91 92 95 98 102 110 120 92 93 96 98 103 111 120 93 94 96 99 104 112 121 94 95 97 100 105 113 122 95 96 98 101 106 113 123 96 97 99 102 106 114 123 97 98 100 103 107 115 124 98 99 101 104 108 116 125 99 100 102 105 109 117 126 100 101 103 106 110 118 127 101 102 104 107 111 118 127 102 103 105 108 112 119 128

100 116 116 117 117 118 118 119 119 120 120 121 121 122 123 123 124 124 125 126 126 127 127 128 129 129 130 131 131 132 133 133 134 135 135 136 137 137 138 139 139 140 141 141

The Steam and Condensate Loop

Block 14 Condensate Recovery

Sizing Condensate Return Lines Module 14.3

Questions 1. As a simple rule, what can condensate drain lines be sized on? a| The plant condensate outlet connection

¨

b| The plant steam inlet connection

¨

c| The trap inlet connection with the correct sized trap

¨

d| It is unimportant to size drain lines correctly

¨

2. For steam mains and constant pressure processes, how is start load estimated? a| Twice the running load at the rated pressure

¨

b| Three times the running load at a third of the rated pressure

¨

c| Ten times the running load at half the rated pressure

¨

d| The running load at twice the rated pressure

¨

3. On which pressure loss should drain lines be sized? a| 100 Pa / m

¨

b| They need only be sized on velocity

¨

c| 200 Pa / m

¨

d| 200 Pa / m for lines less than 10 m and 100 Pa / m for lines over 10 m

¨

4. What is the major factor that influences the size of the trap discharge lines? a| The size of the trap

¨

b| The size of the drain line

¨

c| The amount of flash steam produced in the discharge line

¨

d| The amount of condensate flowing

¨

5. Using Appendix 14.3.1, which size of drain line 1.5 m long should be chosen for a constant pressure process with a maximum running load of 450 kg / h? a| 20 mm

¨

b| 32 mm

¨

c| 25 mm

¨

d| 15 mm

¨

6. Three discharge lines 25 mm, 50 mm, 65 mm are to branch into a common line discharging into a vented receiver. What should be the nominal size of the common line into the receiver? a| 100 mm

¨

b| 80 mm

¨

c| 65 mm

¨

d| 50 mm

¨

Answers

1: c, 2: a, 3: d, 4: c, 5: a, 6: a The Steam and Condensate Loop

14.3.19

Block 14 Condensate Recovery

14.3.20

Sizing Condensate Return Lines Module 14.3

The Steam and Condensate Loop

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

Module 14.4 Pumping Condensate from Vented Receivers

The Steam and Condensate Loop

14.4.1

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

Pumping Condensate from Vented Receivers The justification for returning condensate has already been made and, often, this will entail lifting condensate by a pump into the boiler feedtank. Before looking at the types of pump available for returning condensate, it may be helpful to discuss some basic pumping terminology.

Pumping terminology Vapour pressure - This term is used to define the pressure corresponding to the temperature at which a liquid changes into vapour. In other words, it is the pressure at which a liquid will boil. o

At 100°C, water will boil at atmospheric pressure.

o

At 170°C, water will boil at a pressure of 7 bar g.

o

At 90°C, water will boil at a pressure of 0.7 bar a.

The vapour pressure is a very important consideration when pumping condensate. Condensate is usually formed at a temperature close to its boiling point, which may cause difficulties where a centrifugal pump is concerned. This is because centrifugal pumps have an area of lower pressure at the centre, or eye, of the impeller. This produces the suction effect, which draws the liquid into the pump. Although the drop in pressure is small, if the condensate is already very close to its vapour pressure, a proportion of the liquid will flash to steam in the form of small bubbles. These steam bubbles occupy a significantly greater volume than the equivalent mass of water, and have a high ratio of surface area to mass. As the bubbles travel through the impeller passageways towards its outer edge, they experience increasing pressure. At some point during this journey, the vapour pressure is exceeded, and the steam bubbles implode with considerable force. This is termed ‘cavitation’ and the implosions are both noisy and destructive. The noise is similar to gravel being shovelled and the implosions will, in time, damage the pump internals. For this reason, it is recommended that condensate be pumped by electrical pumps specifically built for the task, and that condensate temperatures in atmospheric systems do not exceed 98°C. Some pumps will have limits as low as 94°C or 96°C, depending on the design of the pump, the speed of rotation and the height of the receiver above the pump. Head (h) - Head is a term used to describe the potential energy of a fluid at a given point. There are several ways that head can be measured: pressure head, static head and friction head. Pressure head and static head are essentially the same thing, but tend to be measured in different units. Pressure head is measured in pressure units such as pascal or bar g; whilst static head is referred to in terms of height, usually in metres (or metres head). For water, a static head of 10 metres is approximately equivalent to a pressure head of 1 bar g (see Figure 14.4.1). Pressure head (hp) - Pressure head is the fluid pressure at the point in question. For example: A pump is required to discharge water against a static head of 30 metres, which approximately equals a pressure head of 3 bar g. The pump fills from a static head of 1 metre, which equals a pressure head of 0.1 bar g. (See Figure 14.4.2). Static head (hs) - Static head is the equivalent vertical height of fluid above a datum. The following example explains the measure of static head. Example: the pump inlet in Figure 14.4.2 is subjected to a static head (known as the suction or filling head) of 1 m, and discharges against a static head (known as the static delivery head) of 30 m. Note that in this case, the water being pumped is above the pump inlet (this situation is called a flooded suction).

14.4.2

The Steam and Condensate Loop

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

10 m

1m

0.1 bar g

1 bar g

Fig. 14.4.1 Pressure of water in terms of head Collecting tank

Header tank

Static delivery head 30 m

Filling head Static suction head 1 m

Fig. 14.4.2 Suction, delivery, and filling heads

Net static head - This depends upon whether the pump is a centrifugal type pump or a positive displacement, mechanical type pump. With an electrical centrifugal pump (Figure 14.4.3), the pressure exerted by the suction head is always present in the pump. The net static head, against which the pump has to work, is the difference between the suction head and the delivery head. Collecting tank

Net static head 29 m Static delivery head 30 m

Header tank

Static suction head 1 m Pump inlet Fig. 14.4.3 Net static head for an electrical pump The Steam and Condensate Loop

14.4.3

Pumping Condensate from Vented Receivers Module 14.4

Block 14 Condensate Recovery

With a mechanical displacement pump (Figure 14.4.4), the suction head only provides the energy to fill the pump during the filling cycle. It is not present in the pump body during pumping and has no effect on the delivery head against which the pump has to operate. The net static head is simply the delivery head. Collecting tank

Static delivery head 30 m

Header tank Filling head Static suction head 1 m

Fig. 14.4.4 Net static head for a mechanical pump equals static delivery head

Friction head (hf) - The friction head (or head loss to friction) is more accurately defined as the energy required to move the fluid through the pipe. This is discussed in further detail in Module 10.2, ‘Pipes and pipe sizing’. Pressure loss can be calculated using the procedures shown in Block 4, ‘Flowmetering’ and Block 10, ‘Steam distribution’, but is more usually found from tables that correlate liquid flowrate, pipe diameter and velocity. To be precise, the resistance to flow encountered by the various pipeline fittings must also be taken into account. Tables are available to calculate the equivalent length of straight pipe exerted by various pipe fittings. This extra ‘equivalent length’ for pipe fittings is then added to the actual pipe length to give a ‘total equivalent length’. However, in practice, if the pipe is correctly sized, it is unusual for the pipe fittings to represent more than an additional 10% of the actual pipe length. A general rule, which can be applied, is: Total equivalent length (le ) = Actual length + 10% In most cases, the Steam Plant Engineer will be designing a system with a proprietary manufactured pump arrangement, which has appropriate factors built in. Bearing this in mind, the figure of 10% will be used in this Block as the equivalent length for calculating pressure loss due to friction. This pressure loss due to friction is greatly dependent on the velocity of the water in the pipe. In simple terms, the pressure loss due to friction increases by a factor proportional to the square of the velocity. Tables are available which give head loss per metre of pipe for various flowrates and pipe diameters. Table 14.4.1 Flow of water in black steel pipes (kg / h) Pressure drop Pipe size (mm) Pa / m mbar / m 15 20 25 32 40 50 100 1.00 184 425 788 1 724 2 632 5 004 114 1.14 194 450 845 1 832 2 790 5 366 118 1.18 198 457 857 1 890 2 830 5 443

14.4.4

65 10 152 10 841 11 022

80 15 768 16 828 17 055

100 31 932 34 247 34 746

The Steam and Condensate Loop

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

Example 14.4.1

The 50 mm discharge pipework on a pumped condensate line rises vertically for 29 metres to a vented tank. The line is 150 m long and the pumping rate is 5 000 kg / h of water. What is: (A) the pressure head loss due to friction (the friction head), and (B) the total delivery head?

A - Calculate the pressure head loss due to friction (the friction head) Total equivalent length (le) = 150 + 10 % = 165 metres From Table 14.4.1, it can be seen that a 50 mm pipe carrying 5 004 kg / h of water will experience a pressure drop of 1.0 mbar / m. The flowrate in this example is marginally less, and, although a more accurate estimate could be obtained by interpolation, take the pressure drop as 1 mbar / m. Pressure head loss due to friction is therefore: 165 metres x 1 mbar / m = 165 mbar (0.165 bar) Taking 1 bar to be equivalent to 10 metres of water head the equivalent friction head loss in terms of metres is: 0.165 bar x 10m / bar = 1.65 metres.

B - The total delivery head

Total delivery head (hd) - The total delivery head hd against which the pump needs to operate is the sum of three components as can be seen in Equation 14.4.1:

7RWDOGHOLYHU\KHDGKG  KV KI KS

Equation 14.4.1

Total filling head

Condensate movement

Where: hd = Total delivery head hs = Pressure required to raise the water to the desired level (static head) hf = Pressure required to move the water through the pipes (friction head) hp = Pressure in the condensate system (zero in this example as the condensate tank is vented to atmosphere).

Total discharge head

Fig. 14.4.5 Net static head for a mechanical pump equals static delivery head

From the information above: Total delivery head (hd) required = static head + equivalent loss in static head due to friction hd = 29 m + 1.65 m hd = 30.65 metre The Steam and Condensate Loop

14.4.5

Pumping Condensate from Vented Receivers Module 14.4

Block 14 Condensate Recovery

Electrical centrifugal condensate pumps Pump operation

Liquid entering the pump is directed into the centre, or eye, of the rotating impeller vanes. The liquid will then gain velocity as it travels towards the outside of the impeller.

Pump application

The electrical pump is well suited to applications where large volumes of liquid need to be transported. Electrical pumps are usually built into a unit, often referred to as a condensate recovery unit (CRU). A CRU will usually include: o

A receiver.

o

A control system operated by probes or floats.

o

One or two pumps.

The instantaneous flow from the CRU can be up to 1.5 times greater than the rate at which condensate returns to the receiver. It is this pumping rate that must be considered when calculating the friction loss in the discharge line. On twin pump units, a cascade control system may also be employed which allows either pump to be selected as the lead pump and the other as a stand-by pump to provide back-up if the condensate returning to the unit is greater than one pump can handle. This control arrangement also provides back-up in the case of the one pump failing to operate; the condensate level in the tank will increase and bring the stand-by pump into operation. Cascade type units usually pump at a rate of 1.1 times the return rate to the receiver, allowing a smaller discharge line to be considered. It is very important to follow the manufacturer’s literature regarding the discharge pumping rate. Failure to do so could result in undersizing the pump discharge pipework. Vent Condensate inlet Condensate receiver Level sensor

Overflow with ‘U’ seal

Centrifugal pump

Condensate discharge

Centrifugal pump Fig. 14.4.6 A typical electrical condensate recovery unit (CRU)

Sizing an electrical condensate recovery unit

To size an electric condensate recovery unit, it is necessary to know: o o

o o

14.4.6

The amount of condensate reaching the receiver at running load. The temperature of the condensate. This must not exceed the manufacturer’s specified ratings to avoid cavitation, however, manufacturers usually have different impellers to suit different temperature ranges, for example, 90°C, 94°C and 98°C. The total discharge head the pump has to pump against - To be determined from the site conditions. The pump discharge rate in order to size the return pipework - It is necessary to read the manufacturer’s data properly to determine this. The Steam and Condensate Loop

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

Example 14.4.2 Sizing discharge pipework for an electric condensate recovery unit Where: Temperature of condensate Condensate to be handled Static lift (hs) Length of pipework Condensate backpressure

= 94°C = 1 000 kg / h = 30 m = 150 m = friction losses only (hf)

An initial selection of a condensate recovery unit can be made by using the manufacturer’s sizing chart (an example of which is shown in Figure 14.4.7). From the chart, CRU1 should be the initial choice subject to frictional losses in the delivery pipework.

Pump delivery head in metres

CRU1

CRU2

CRU3

Condensate to be handled at 94°C kg / h Fig. 14.4.7 A typical electrical condensate recovery unit (CRU) sizing chart (see Example 14.4.2)

From the chart in Figure 14.4.7, it can be seen that CRU1 is actually rated to handle 2 000 kg / h of condensate against a maximum delivery head of 35 m. However, on CRUs with pumps that work intermittently, in order to be able to handle the rated amount of condensate, the pump has to actually move the condensate at some higher flowrate during the time it is pumping. It is important to know this to be able to size the discharge pipe correctly. Consider that the manufacturer’s data shows that the CRU will actually pump at a rate of 1.5 times the amount of condensate being handled as shown on the sizing chart i.e.: Actual pumping rate = 1.5 x 2 000 kg / h = 3 000 kg / h It is this figure, 3 000 kg / h, that must be used to size the discharge pipework. It is now possible to calculate the optimum size for the return line. Actual length of pipework = 150 m Equivalent length of pipework = 150 m + 10% = 165 m The Steam and Condensate Loop

14.4.7

Pumping Condensate from Vented Receivers Module 14.4

Block 14 Condensate Recovery

Estimating the friction loss in the pipe (hf)

To size a pumped discharge line it is usually a good idea to begin the friction loss calculation with an arbitrary pressure drop of between 100 and 200 Pa / m From the pressure drop Table 14.4.2 (extract shown below), it can be seen that, for a flowrate of 3 000 kg / h, and for a pressure drop of between 100 and 200 Pa/m, a 40 mm discharge pipe will suffice. Extract from Table 14.4.2 Flowrate kg / h Pipe size Ø 15 mm 20 mm 25 mm 32 mm 40 mm 50 mm Pa / m mbar / m <0.15 m / s 0.15 m / s 100.0 1.000 184 425 788 1 724 2 632 5 004 120.0 1.200 202 472 871 1 897 2 898 5 508 140.0 1.400 220 511 943 2 059 3 143 5 976 160.0 1.600 234 547 1 015 2 210 3 373 6 408 180.0 1.800 252 583 1 080 2 354 3 589 6 804 200.0 2.000 266 619 1 141 2 488 3 780 7 200

65 mm 80 mm 100 mm 0.3 m / s 10 152 15 768 31 932 11 196 17 352 35 100 12 132 18 792 38 160 12 996 20 160 40 680 13 824 21 420 43 200 1.5 14 580 22 644 45 720 m / s

It can be interpolated from Table 14.4.2 that a flowrate of 3 000 kg / h will correspond to a pressure drop of 128 Pa / m, for 40 mm pipework, The head loss to friction can now be calculated for 40 mm pipework. Head loss to friction (hf) = 128 Pa / m x 165 m hf = 21 000 Pa hf = Approximately 2.1 metres

Establishing the total delivery head

The total delivery head against which the pump has to discharge is therefore h s + h f = h d, where: hs = static lift of 30 m (given) hf = 2.1 metres hd = 30 m + 2.1 m = 32.1 metres The delivery head of 32.1 metres needs to be checked against the CRU manufacturer’s sizing chart to confirm that the unit can pump against this amount of head. It can be seen from Figure 14.4.7 that this CRU can actually pump against a 35 metre head. Had the design head of 35 metres been exceeded, then the options are to re-calculate using a larger pipe, or to select a CRU with a greater lifting capacity.

An alternative way to size the delivery pipework

With an actual static head (hs) of 30 m, and a CRU design head of 35 m, a 5 m head is available for pipe friction losses (hf). It might be possible to install a smaller diameter pipe and have a larger friction loss. However, the designer must weigh this initial cost saving against the extra running power (and hence cost) required to pump against a larger head. Velocity also needs to be checked against a typical maximum of about 3 m / s allowable for pumped water at temperatures below 100°C. Table 14.4.2 will show that, if the next lower sized pipe (32 mm) were chosen, the unit friction loss (hf) to pass 3 000 kg / h is interpolated to be 286 Pa / m, and the velocity is about 1 m / s, which is below 3 m / s and therefore suitable for the application. hf is 286 Pa / m x 165 m Therefore, total delivery head (hd) hd hd

= = = =

47 190 Pa (or 4.72 m) hs + h f 30 + 4.72 m 34.72 m

The conclusion is that a 32 mm pipe could be used, as the CRU1 pump can handle up to 35 m total delivery head. However, from a practical viewpoint, it might not be reasonable to design a system to operate so close to its limits, and that, in this instance, 40 mm pipe would probably be the better solution. 14.4.8

The Steam and Condensate Loop

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

Table 14.4.2 A section of a typical friction loss table for fully flooded pipelines (flowrates in kg / h) Flowrate kg / h Pipe size Ø 15 mm 20 mm 25 mm 32 mm 40 mm 50 mm 65 mm 80 mm 100 mm Pa / m mbar / m <0.15 m / s 0.15 m / s 0.3 m / s 10.0 0.100 50 119 223 490 756 1 447 2 966 4 644 9 432 12.5 0.125 58 133 252 554 853 1 634 3 348 5 220 10 656 15.0 0.150 65 151 277 616 943 1 807 3 708 5 760 11 736 17.5 0.175 68 162 302 670 1 026 1 966 4 032 6 264 12 744 20.0 0.200 76 176 328 720 1 105 2 113 4 320 6 732 13 680 22.5 0.225 79 187 349 770 1 177 2 254 4 608 7 164 14 580 25.0 0.250 83 198 371 814 1 249 2 387 4 860 7 596 15 408 27.5 0.275 90 209 389 857 1 314 2 513 5 112 7 992 16 200 30.0 0.300 94 220 410 900 1 379 2 632 5 364 8 352 16 956 32.5 0.325 97 230 428 940 1 440 2 747 5 616 8 712 17 712 35.0 0.350 101 241 446 979 1 498 2 858 5 832 9 072 18 432 37.5 0.375 104 248 464 1 015 1 555 2 966 6 048 9 396 19 116 40.0 0.400 112 259 479 1 051 1 609 3 071 6 264 9 720 19 764 42.5 0.425 115 266 497 1 087 1 663 3 175 6 480 10 044 20 412 45.0 0.450 119 277 511 1 123 1 717 3 272 6 660 10 368 21 024 47.5 0.475 122 284 526 1 156 1 768 3 370 6 876 10 656 21 636 50.0 0.500 126 292 540 1 188 1 814 3 463 7 056 10 944 22 212 52.5 0.525 130 299 558 1 220 1 865 3 553 7 236 11 232 22 788 55.0 0.550 130 306 572 1 249 1 912 3 636 7 416 11 520 23 364 57.5 0.575 133 317 583 1 282 1 958 3 744 7 596 11 808 23 904 60.0 0.600 137 324 598 1 310 2 002 3 816 7 776 12 060 24 444 62.5 0.625 140 331 612 1 339 2 048 3 888 7 920 12 312 24 984 65.0 0.650 144 338 626 1 368 2 092 3 996 8 100 12 600 25 488 67.5 0.675 148 346 637 1 397 2 131 4 068 8 280 12 852 25 992 70.0 0.700 151 353 652 1 422 2 174 4 140 8 424 13 068 26 496 72.5 0.725 151 356 662 1 451 2 218 4 212 8 568 13 320 27 000 75.0 0.750 155 364 677 1 476 2 257 4 284 8 748 13 572 27 468 77.5 0.775 158 371 688 1 505 2 297 4 356 8 892 13 788 27 972 80.0 0.800 162 378 698 1 530 2 336 4 464 9 036 14 040 28 440 82.5 0.825 166 385 709 1 555 2 372 4 536 9 180 14 256 28 872 85.0 0.850 166 389 724 1 580 2 412 4 608 9 324 14 472 29 340 87.5 0.875 169 396 734 1 606 2 448 4 680 9 468 14 724 29 772 90.0 0.900 173 403 745 1 627 2 488 4 716 9 612 14 940 30 240 92.5 0.925 176 407 756 1 652 2 524 4 788 9 756 15 156 30 672 95.0 0.950 176 414 767 1 678 2 560 4 860 9 900 15 372 31 104 97.5 0.975 180 421 778 1 699 2 596 4 932 10 044 15 552 31 500 100.0 1.000 184 425 788 1 724 2 632 5 004 10 152 15 768 31 932 120.0 1.200 202 472 871 1 897 2 898 5 508 11 196 17 352 35 100 140.0 1.400 220 511 943 2 059 3 143 5 976 12 132 18 792 38 160 160.0 1.600 234 547 1 015 2 210 3 373 6 408 12 996 20 160 40 680 180.0 1.800 252 583 1 080 2 354 3 589 6 804 13 824 21 420 43 200 200.0 2.000 266 619 1 141 2 488 3 780 7 200 14 580 22 644 45 720 220.0 2.200 281 652 1 202 2 617 3 996 7 560 15 336 23 760 47 880 240.0 2.400 288 680 1 256 2 740 4 176 7 920 16 056 24 876 50 400 260.0 2.600 306 713 1 310 2 855 4 356 8 244 16 740 25 920 52 200 280.0 2.800 317 742 1 364 2 970 4 536 8 568 17 388 26 928 54 360 300.0 3.000 331 767 1 415 3 078 4 680 8 892 18 000 27 900 56 160

The Steam and Condensate Loop

0.5 m/s

1 m/s

1.5 m/s

14.4.9

Pumping Condensate from Vented Receivers Module 14.4

Block 14 Condensate Recovery

Mechanical (positive displacement) condensate pumps Pump operation

A mechanical pump consists of a body shell, into which condensate flows by gravity. The body contains a float mechanism, which operates a set of changeover valves. Condensate is allowed to flow into the body, which raises the float. When the float reaches a certain level, it triggers a vent valve to close, and an inlet valve to open, to allow steam to enter and pressurise the body to push out the condensate. The condensate level and the float both fall to a preset point, at which the steam inlet valve shuts and the vent valve re-opens, allowing the pump body to refill with condensate. Check valves are fitted to the pump inlet and discharge ports to ensure correct directional flow through the pump. The cyclic action of the pump means that a receiver is required to store condensate while the pump is discharging (see Figure 14.4.8). Condensate in

Motive steam

Vent

Receiver Condensate out

Pump

Fig. 14.4.8 A typical mechanical condensate recovery unit (CRU)

Pump application

Generally, mechanical pumps handle smaller amounts of condensate than electrical pumps. They are however, particularly valuable in situations where: o

High condensate temperatures will cause cavitation in electrical pumps.

o

Condensate is in vacuum.

o

Plant room space is at a premium.

o

Low maintenance is an issue.

o

The environment is hazardous, humid or wet.

o

Electrical supplies are not at hand.

o

Condensate has to be removed from individual items of temperature controlled equipment, which may be subject to stall conditions (see Block 13 ‘Condensate Removal’, for further details).

As with electrically driven pumps, positive displacement mechanical pumps are sometimes, but not always, specified as packaged condensate recovery units. A mechanical condensate recovery unit will comprise a condensate receiver and the pump unit. No additional control system is required as the pump is fully automatic and only operates when needed. This means that the pump is self-regulating. With mechanical pumps, the pump cycles as the receiver fills and empties. The instantaneous flowrate while the pump is discharging can often be up to six times the filling rate and it is this instantaneous discharge flowrate, which must be used to calculate the size of the discharge pipe. Always refer to the pump manufacturer for data on sizing the pump and discharge line. A typical mechanical pump sizing chart is shown in Figure 14.4.10. 14.4.10

The Steam and Condensate Loop

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

Sizing a mechanical condensate pump

To size a mechanical condensate pump, the following information is required: o o

The maximum condensate flowrate reaching the receiver. The motive pressure of steam or air available to drive the pump. The selection of steam or air will depend on the application and site circumstances.

o

The filling head available between the receiver and pump.

o

The total delivery head of the condensate system.

The method of sizing mechanical pumps varies from manufacturer to manufacturer, and is usually based on empirical data, which are translated into factors and nomographs. The following example gives a typical method for sizing a mechanical pump. (The pipe length is less than 100 m consequently friction loss is ignored):

Example 14.4.3 How to size a mechanical condensate pump Where:

Condensate handling load = 2 100 kg / h Steam pressure available for operating pump = 5.2 bar g Vertical lift from pump to return piping = 9.2 m Pressure in the return piping (piping friction negligible) = 1.7 bar g Available filling head on the pump = 0.3 m

Condensate manifold

1.7 bar g return main pressure

Vent

9.2 m lift

Total plant condensate 2 100 kg / h Reservoir * Note: Steam supply to pump not shown

* Pump

Filling head 0.3 m 5.2 bar g operating pressure

Fig. 14.4.9 Sizing a mechanical condensate recovery unit (see Example 14.4.3)

Calculate the total backpressure (delivery head hd), against which the condensate must be pumped: Total backpressure (hd) = lift (hs) + condensate pressure (hp) Note: The friction loss is neglected because the pipeline is shorter than 100 m. Condensate lift (hs) Condensate pressure (hp) Total delivery head (hd) Total delivery head (hd)

= = = =

9.2 m 1.7 bar g = 17 m head 9.2 m + 17 m 26 m

With reference to the sizing chart shown in Figure 14.4.10: a DN50 pump at 5.2 bar g motive pressure will pump 2 600 kg / h against a 26 m head. A DN50 pump will thus be an adequate choice for this example, where the condensate handling load is 2 100 kg / h. The Steam and Condensate Loop

14.4.11

Pumping Condensate from Vented Receivers Module 14.4

Block 14 Condensate Recovery

Sizing the discharge pipework for a mechanical condensate pump

The discharge pipe from a mechanical pump can usually be taken to be the same size as the pump outlet when it is below 100 m long. The frictional resistance of the pipe is relatively small compared to the backpressure caused by the lift and condensate return pressure, and can usually be disregarded. For discharge pipes longer than 100 m, the general rule would be to select one pipe size larger than the pump outlet check valve, but for such longer lines, the size should be checked as shown in Example 14.4.4

Delivery lines longer than 100 metres

On delivery lines over 100 m, and / or where the condensate flow is near the pump capacity, it is advisable to check the pipe size to ensure that the total friction loss (including inertia loss) does not exceed the pump’s capability. Inertia loss is explained in Example 14.4.4 Consider the same condensate pumping requirement as in Example 14.4.3 but with a delivery line 250 metres long.

Example 14.4.4 Sizing a delivery line 250 m long (refer to Figure 14.4.10):

For a DN50 pump, with 5.2 bar g motive steam and 26 m delivery head, the maximum pump capacity = 2 600 kg / h. From Figure 14.4.10, the following can be determined: The actual condensate flowrate into pump = 2 100 kg / h. Maximum backpressure permissible at 2 100 kg / h = 32 m Therefore, maximum frictional resistance allowable = 32 - 26 m Maximum frictional resistance allowable = 6 m (approximately 60 000 Pa)

4 m lift

10 m lift

30 m lift

40 m lift

50 m lift

80 m lift

13

26 metres lift

20 m lift

32

14

Example 14.4.4

11 10

Motive pressure bar g

9

Example 14.4.3

12

8 7 6

5.2

5

4 3 2 1 0

1 000

2 000 2 100

3 000 2 600

4 000

5 000

DN50 size capacities kg / h Note: The pump is sized on the filling rate Fig. 14.4.10 Mechanical condensate recovery unit sizing chart - DN50 pump

14.4.12

The Steam and Condensate Loop

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

The effect of inertia loss on pump delivery lines longer than 100 metres.

On lines over 100 m, a considerable volume of liquid will be held within the pump discharge pipe. The sudden acceleration of this mass of liquid at the start of the pump discharge can absorb some part of the pump energy and result in a large amount of waterhammer and noise. This needs to be considered within the calculation by reducing the allowable friction loss of 60 000 Pa in Example 14.4.4 by 50%, thus: Total allowable friction loss = 50% × 60 000 Pa = 30 000 Pa Consider delivery pipe length to be 250 m + 10% for additional fittings = 275 m Consequently, maximum frictional resistance allowable / metre =

3D P

Maximum frictional resistance » 109 Pa / m For this type of pump the delivery flowrate is taken as 6 times the filling rate = 6 × 2 100 kg / h Therefore, the delivery rate of condensate from the pump = 12 600 kg / h

Total allowable friction loss

With a frictional resistance of 109 Pa / m, Table 14.4.2 reveals that an 80 mm pipe (minimum) is required to give an acceptable flowrate of 12600 kg / h. In fact, Table 14.4.2 indicates that an 80 mm pipe will pass 16 480 kg / h with a frictional resistance of 109 Pa / m. By rising up the ’80 mm column’ in the table, it can be seen that, by interpolation, the flowrate of 12 600 kg / h actually induces a frictional loss of 65 Pa / m in an 80 mm pipe.

Fully loaded pumps and longer lines

In Example 14.4.4, Figure 14.4.10 shows that the maximum pump filling rate with a motive pressure of 5.2 bar g and a delivery head of 26 metres is 2 600 kg /h. Had the filling rate been close to this maximum, (perhaps 2 500 kg / h), then less delivery head would have been available for friction loss. For the same size DN50 pump, this would mean a larger delivery pipeline as shown in Example 14.4.5

The Steam and Condensate Loop

14.4.13

Pumping Condensate from Vented Receivers Module 14.4

Block 14 Condensate Recovery

Example 14.4.5 Consider the same DN50 pump as described in Example 14.4.4, but having a condensate filling rate of 2 500 kg / h. Now determine the size of the delivery pipeline.

2 500 3 000

4 m lift

30 m lift

2 000

10 m lift

40 m lift

50 m lift

80 m lift

13

20 m lift

27 metres lift

14

12 11 10

Motive pressure bar g

9 8 7 6

5.2 5

4 3 2 1 0

1 000

4 000

5 000

DN50 size capacities kg / h Fig. 14.4.11 Mechanical condensate recovery unit sizing chart (DN50 pump)

Sizing on a filling rate of 2 500 kg / h, and a steam pressure of 5.2 bar, referring to Figure 14.4.11, for the DN50 pump, it can be seen that a condensate filling rate of 2 500 kg / h equates to a maximum backpressure of about 27 m, so in this instance: With an actual delivery head of 26 m: Available head left for friction losses = 27 - 26 m Available head left for friction losses = 1 m The conversion tables in the Engineering Support Centre reveal that a head of 1 metre is equivalent to 9 806.65 Pa. For an equivalent length line of 275 m: 3D The frictional resistance allowable = P = 35.7 Pa / m Minus allowance of 50% for inertia loss = 50% × 35.7 Pa / m Maximum frictional resistance allowable = 18 Pa / m As before, the discharge pipework has to be sized on the instantaneous flowrate from the pump outlet, which is taken as 6 × the filling rate. In this instance, the pipe would have been sized on 6 × 2 500 kg / h = 15 000 kg / h with a friction loss of 18 Pa / m. Table 14.4.2 shows that this would require a pipe larger than 100 mm (actually 125 mm) to allow the pump to operate within its capability. Although the system would certainly work with this arrangement, it is probably more economical to consider a larger pump in conjunction with a smaller pipeline. 14.4.14

The Steam and Condensate Loop

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

Considerations of a larger pump and smaller pipeline

Consider the same pumping conditions as Example 14.4.4, but with a larger DN80 pump. As a larger unit can pump against a higher delivery head, a smaller delivery line can be used

10 m lift

20 m lift

40 m lift

50 m lift

80 m lift

13

26 m

30 m lift

35 m

14

12 11 10

8 7 6

4 m lift

Motive pressure bar g

9

5.2 5 4 3 2 1 0

1 000

2 000

2 500

3 000

4 000

5 000

6 000

DN80 x DN50 size capacities kg / h Fig. 14.4.12 Mechanical condensate recovery unit sizing chart (DN80 pump)

Figure 14.4.12 shows that a DN80 pump under the same conditions of 5.2 bar g motive steam and 2 500 kg / h flowrate would allow a maximum delivery head of 35 m. From Example 14.4.4, the actual delivery head = 26 m At a filling rate of 2 500 kg / h, maximum allowed = 35 m Head available for friction loss = 35 m - 26 m = 9 metres The conversion tables in the Engineering Support Centre reveal that a head of 9 m is equivalent to 88 259.9 Pa. 3D Therefore 88 259.9 Pa over 275 m and including inertia loss = 50% × P Maximum frictional resistance allowable = 160 Pa / m The delivery pipe is again sized to carry 6 x 2 500 kg / h = 15 000 kg / h of condensate. By interpolation, Table 14.4.2 shows that an 80 mm pipe will accommodate 20 160 kg / h with a friction loss of 160 Pa / m, flowing at about 1 m / s. In this instance, the larger DN80 pump will comfortably allow a pipe two sizes smaller than that for the smaller pump, and with a velocity of about 1 m / s, which is within recommendations. The 80 mm pipe is therefore suitable for the DN80 pump. Note: The DN80 pump would cost about 10% more than the DN50 pump, but the extra cost would be justified by the difference in installation costs on long delivery lines; which in this instance would mean the difference in cost between a 80 mm and 125 mm pipe; installation, fittings, and insulation. The Steam and Condensate Loop

14.4.15

Pumping Condensate from Vented Receivers Module 14.4

Block 14 Condensate Recovery

Condensate velocities

Equation 14.4.2 can be used to check the condensate velocity. &RQGHQVDWHYHORFLW\PV   &RQGHQVDWHIORZUDWHNJ  K [&RQGHQVDWHVSHFLILFYROXPHP NJ

Equation 14.4.2

SLSHERUH [ PP ] [ 

&RQGHQVDWHYHORFLW\PV  

NJ K[P  NJ PP  [ 

In Equation 14.4.2, the specific volume of water is taken to be 0.001 m3 / kg. This value varies slightly with temperature but not enough to make any significant difference on condensate lines. The condensate velocity can be checked for the 80 mm pipework in Example 14.4.4. The pumping rate = 15 000 kg / h Condensate specific volume = 0.001 m³ / kg Pipe bore = 80 mm &RQGHQVDWHYHORFLW\ 

[   [

Condensate velocity = 0.83 m / s From Table 14.4.3 the maximum velocity for an 80 mm bore pipe is 1.8 m / s. Table 14.4.3 Maximum recommended velocities for pipe bores (based on a maximum friction loss of 450 Pa/ m) Pipe bore, mm 15 20 25 32 40 50 65 80 100 Velocity, m/s 0.62 0.8 1.0 1.23 1.27 1.5 1.8 1.84 2.4

Best practice for long delivery lines

The momentum of the moving contents of a long delivery line may keep the water in motion for some time after a mechanical pump has completed its discharge stroke. When the water in the discharge pipe comes to rest, the backpressure in the line will attempt to reverse the initial flow of water, back towards the outlet check valve. The result is noise and pipe movement due to waterhammer, which can be both alarming and serious. Installing another check valve in the discharge pipe one pipe length from the pump will usually alleviate the problem. Line over 100 m

Mechanical pump

Additional check valve 1 pipe length from pump

Fig. 14.4.13 An additional check valve 1 pipe length from the pump body to reduce the effect of backflow

14.4.16

The Steam and Condensate Loop

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

If there is any choice, it is always best to lift immediately after the pump to a height allowing a gravity fall to the end of the line (Figure 14.4.14). If the fall is enough to overcome the frictional resistance of the pipe (Table 14.4.4), then the only backpressure onto the pump is that formed by the initial lift. A vacuum breaker can be installed at the top of the lift not only to assist the flow along the falling line but also to prevent any tendency for backflow at the end of the stroke. Should the falling line have to fall anywhere along its length to overcome an obstruction, then an automatic air vent fitted at the highest point will reduce air locking and assist flow around the obstruction, see Figure 14.4.14. Automatic air vent

Vacuum breaker

fall fall due to obstruction

Mechanical pump

Fig. 14.4.14 Best choice - lift after the pump Table 14.4.4 Pipefall to overcome frictional losses Pipe size (DN mm) 40 50 65 Litres of water per hour 25 mm in 15 m 48 140 303 580 907 1 950 3 538 25 mm in 10 m 59 177 381 694 1 134 2 449 4 445 25 mm in 8 m 69 204 442 800 1 310 2 834 5 148 25 mm in 6 m 79 231 503 907 1 487 3 220 5 851 25 mm in 5 m 86 256 553 1 007 1 642 3 551 6 441 25 mm in 4 m 93 279 598 1 093 1 778 3 878 7 030 25 mm in 3 m 113 338 730 1 329 2 168 4 672 8 527 25 mm in 2 m 140 419 907 1 655 2 694 5 851 10 614 25 mm in 1.75 m* 152 454 984 1 793 2 923 6 327 11 498 25 mm in 1.5 m 165 490 1 061 1 932 3 152 6 804 12 383 25 mm in 1 m 206 612 1 324 2 404 3 923 8 482 15 422 *A fall of 25 mm in 1.75 m is equivalent to a fall of 1:70. Pipefall needed to overcome pipe friction

15

20

The Steam and Condensate Loop

25

32

80

100

125

150

5 806 7 257 8 391 9 525 10 568 11 521 13 925 17 327 18 756 20 185 25 174

12 610 15 680 18 159 20 638 22 770 24 811 30 073 37 421 40 573 43 726 54 431

22 906 28 576 33 089 37 602 41 821 45 994 54 073 68 039 73 708 79 378 99 019

37 284 46 492 53 862 61 223 67 538 73 571 89 356 111 128 120 426 129 725 161 476

14.4.17

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

Alternatively, any question of backpressure caused by the horizontal run can be entirely eliminated by an arrangement as in Figure 14.4.15 in which the pump simply lifts into a vented break tank. The pipe from the tank should fall in accordance with Table 14.4.4. Vent

Break tank

Condensate

Mechanical pump

Condensate Fig. 14.4.15 Alternative choice - lift after the pump to a break tank

Vented pumps, pumping traps and pump-trap installations

Discharge lines from pumps vented to atmosphere are sized on the discharge rate of the pump. Condensate passing through pumping traps and pump-trap combinations in closed loop applications will often be at higher pressures and temperatures and flash steam will be formed in the discharge line. Because of this, discharge lines from pumping traps and pump-trap combinations are sized on the trapping condition at full-load and not the pumping condition, as the line has to be sized to cater for flash steam. Sizing on flash steam will ensure the line is also able to cope with the pumping condition.

14.4.18

The Steam and Condensate Loop

Block 14 Condensate Recovery

Pumping Condensate from Vented Receivers Module 14.4

Questions 1. For pumping condensate, what is the total delivery head? a| Pressure required to raise the condensate to the required level

¨

b| Pressure required to move the condensate through the pipes

¨

c| Pressure in the condensate system

¨

d| All of the above

¨

2. What is the important factor to consider when sizing a pump discharge line? a| The pump filling rate

¨

b| The pump discharge rate

¨

c| The size of the pump discharge connection

¨

d| The size of the pump inlet connection

¨

3. For a mechanical pump, what is the net static head? a| The static delivery head

¨

b| The static delivery head less the filling head

¨

c| The static delivery head less the static suction head

¨

d| All of the above

¨

4. As a general rule, what equivalent length is added to pipe length to account for pipe fittings? a| 5%

¨

b| 10%

¨

c| 15%

¨

d| 20%

¨

5. What is a good arbitrary pressure drop to choose to initially size a pumped delivery line? a| 10 to 20 Pa / m

¨

b| 50 to 100 Pa / m

¨

c| 500 to 1 000 Pa / m

¨

d| 100 to 200 Pa / m

¨

6. In Figure 14.4.7, what is the maximum capacity of a CRU3 pumping unit against a 15 metre delivery head? a| 2 000 kg / h

¨

b| 100 kg / h

¨

c| 500 kg / h

¨

d| 1 400 kg / h

¨

Answers

1: d, 2: b, 3: a, 4:b, 5: d, 6: d The Steam and Condensate Loop

14.4.19

Block 14 Condensate Recovery

14.4.20

Pumping Condensate from Vented Receivers Module 14.4

The Steam and Condensate Loop

Block 14 Condensate Recovery

Lifting Condensate and Contaminated Condensate Module 14.5

Module 14.5 Lifting Condensate and Contaminated Condensate

The Steam and Condensate Loop

14.5.1

Lifting Condensate and Contaminated Condensate Module 14.5

Block 14 Condensate Recovery

Lifting Condensate and Contaminated Condensate Lifting condensate from a steam main

It is sometimes necessary to lift condensate from a steam trap to a higher level condensate return line (Figure 14.5.1). The condensate will rise up the lifting pipework when the steam pressure upstream of the trap is higher than the pressure downstream of the trap. The pressure downstream of the trap is generally called backpressure, and is made up of any pressure existing in the condensate line plus the static lift caused by condensate in the rising pipework. The upstream pressure will vary between start-up conditions, when it is at its lowest, and running conditions, when it is at its highest. Backpressure is related to lift by using the following approximate conversion: 1 metre lift in pipework = 1 m head static pressure @ 0.1 bar backpressure If a head of 5 m produces a backpressure of 0.5 bar, then this reduces the differential pressure available to push condensate through the trap; although under running conditions the reduction in trap capacity is likely to be significant only where low upstream pressures are used. In steam mains at start-up, the steam pressure is likely to be very low, and it is common for water to back-up before the trap, which can lead to waterhammer in the space being drained. To alleviate this problem at start-up, a liquid expansion trap, fitted as shown in Figure 14.5.1, will discharge any cold condensate formed at this time to waste. As the steam main is warmed, the condensate temperature rises, causing the liquid expansion trap to close. At the same time, the steam pressure rises, forcing the hot condensate through the ‘working’ drain trap to the return line.

High level condensate return

Steam flow

Steam main

Trap

Liquid expansion trap Drain to waste Fig. 14.5.1 Use of a liquid expansion trap

The discharge line from the trap to the overhead return line, preferably discharges into the top of the main rather than simply feed to the underside, as shown in Figure 14.5.1. This assists operation, because although the riser is probably full of water at start-up, it sometimes contains little more than flash steam once hot condensate under pressure passes through. If the discharge line were fitted to the bottom of the return line, it would fill with condensate after each discharge and increase the tendency for waterhammer and noise. 14.5.2

The Steam and Condensate Loop

Block 14 Condensate Recovery

Lifting Condensate and Contaminated Condensate Module 14.5

It is also recommended that a check valve be fitted after any steam trap from where condensate is lifted, preventing condensate from falling back towards the trap. The above general recommendations apply not just to traps lifting condensate from steam mains, but also to traps draining any type of process running at a constant steam pressure. Temperature controlled processes will often run with low steam pressures. Rising condensate discharge lines should be avoided at all costs, unless automatic pump-traps are used.

Contaminated condensate

Occasionally, condensate is discharged from sources where it might have become contaminated by corrosive process liquids. This is unsuitable for boiler feedwater because of the dangers of foaming, scaling, and corrosion which it can cause in the boiler and distribution pipes. However, although contaminated, the condensate still carries the same useful heat as clean condensate which could be recovered if proper contamination detection equipment were employed. Such equipment detects changes in condensate conductivity. When a change from the desired conductivity occurs then this may mean that the condensate is contaminated. A controller signals a dump valve to open, allowing the condensate to flow to drain. In some countries, continuous monitoring of condensate is a legal requirement. Controller

Dump valve Check valve creating a small resistance to promote flow through the sensor Condensate in

Condensate out

Sensor

Contaminated condensate to waste

Drain Fig. 14.5.2 Condensate contamination detection equipment

The Steam and Condensate Loop

14.5.3

Lifting Condensate and Contaminated Condensate Module 14.5

Block 14 Condensate Recovery

Questions 1. Approximately how much backpressure will 15 m head of water produce? a| 0.15 bar

¨

b| 1.5 bar

¨

c| 15 bar

¨

d| 15 000 Pa

¨

2. What type of steam trap can assist in draining steam mains at start-up? a| Thermodynamic type

¨

b| Float-thermostatic type

¨

c| Thermostatic type

¨

d| Liquid expansion type

¨

3. Why is it sensible to dump contaminated condensate? a| It can corrode steam boilers and distribution pipework

¨

b| It can cause scale in steam boilers and distribution pipework

¨

c| It can cause the boiler water to foam and create carryover

¨

d| All of the above

¨

4. Why is it good practice to run a trap discharge line into the top of any condensate return main? a| It is cheaper

¨

b| It removes the backpressure

¨

c| It helps to keep the rising line free of residual condensate

¨

d| It removes the static lift

¨

Answers

1: b 2: d, 3: d, 4: c

14.5.4

The Steam and Condensate Loop

Block 14 Condensate Recovery

Flash Steam Module 14.6

Module 14.6 Flash Steam

The Steam and Condensate Loop

14.6.1

Flash Steam Module 14.6

Block 14 Condensate Recovery

Flash Steam The formation of flash has already been discussed in Module 2.2, ‘What is steam’, and a major flash steam application has been covered in Module 3.13, ‘Heat recovery from boiler blowdown’. This Module will provide a brief reminder of these earlier Modules; discuss how flash steam is formed, and focus on how flash steam can be used effectively to improve steam plant efficiency.

What is flash steam and why should it be used?

‘Flash steam’ is released from hot condensate when its pressure is reduced. Even water at an ambient room temperature of 20°C would boil if its pressure were lowered far enough. It may be worth noting that water at 170°C will boil at any pressure below 6.9 bar g. The steam released by the flashing process is as useful as steam released from a steam boiler. As an example, when steam is taken from a boiler and the boiler pressure drops, some of the water content of the boiler will flash off to supplement the ‘live’ steam produced by the heat from the boiler fuel. Because both types of steam are produced in the boiler, it is impossible to differentiate between them. Only when flashing takes place at relatively low pressure, such as at the discharge side of steam traps, is the term flash steam widely used. Unfortunately, this usage has led to the erroneous conclusion that flash steam is in some way less valuable than so-called live steam. In any steam system seeking to maximise efficiency, flash steam will be separated from the condensate, and used to supplement any low pressure heating application. Every kilogram of flash steam used in this way is a kilogram of steam that does not need to be supplied by the boiler. It is also a kilogram of steam not vented to atmosphere, from where it would otherwise be lost. The reasons for the recovery of flash steam are just as compelling, both economically and environmentally, as the reasons for recovering condensate.

How much flash steam is available?

If use is to be made of flash steam, it is helpful to know how much of it will be available. The quantity is readily determined by calculation, or can be read from simple tables or charts. Example 14.6.1 - Consider the jacketed vessel shown in Figure 14.6.1 The condensate enters the steam trap as saturated water, at a gauge pressure of 7 bar g and a temperature of 170°C. The specific amount of heat in the condensate at this pressure is 721 kJ / kg. After passing through the steam trap, the pressure in the condensate return line is 0 bar g. At this pressure, the maximum amount of heat each kilogram of condensate can hold is 419 kJ and the maximum temperature is 100°C. There is an excess of 302 kJ of heat which evaporates some of the condensate into steam. The quantity of steam is calculated in the following text. Ball valve Air vent Constant pressure steam at 7 bar g

Condensate at 7 bar g hf = 721 kJ / kg Condensate at 0 bar g hf = 419 kJ / kg

Steam at 7 bar g

Excess heat at 0 bar g = 721 - 419 kJ / kg = 302 kJ / kg Fig. 14.6.1 Excess heat in condensate produces flash steam

14.6.2

The Steam and Condensate Loop

Block 14 Condensate Recovery

Flash Steam Module 14.6

The heat needed to produce 1 kg of saturated steam from water at the same temperature, at 0 bar gauge, is 2 257 kJ. An amount of 302 kJ can therefore evaporate:

N   NJRIVWHDPSHUNJRIFRQGHQVDWH NFrom each kilogram of condensate in this example, the proportion of flash steam generated therefore equals 13.4% of the initial mass of condensate. If the equipment using steam at 7 bar g were condensing 250 kg / h, then the amount of flash steam released by the condensate at 0 bar g would be: 0.134 x 250 kg / h of condensate = 33.5 kg / h of flash steam Alternatively, the chart in Figure 14.6.2 can be read directly for the moderate and low pressures encountered in many plants. The example shown in Figure 14.6.1 is depicted in Figure 14.6.2 and shows that 0.134 kg of flash steam is produced per kg of condensate passing through the trap. 15

0 ba rg

ar g 0 .5 b

ar g

1 .0 b

ar g ar g

1 .5 b

2.5 b

13

2.0 b

ar g

14

12 11

Pressure on traps bar g

10 9 8 7 6 5 4 3 2 1 0

0

0.02

0.06

0.10

0.14

0.18

0.22

0.134 (See Example 14.6.1) kg Flash per kg condensate Fig. 14.6.2 Flash steam graph

The Steam and Condensate Loop

14.6.3

Flash Steam Module 14.6

Block 14 Condensate Recovery

Sub-cooled condensate

If the steam trap is of a thermostatic type, the discharged condensate is sub-cooled below saturation temperature. The heat in the cooler condensate will be slightly less, and the amount of flash steam produced would be less. If the trap in Example 14.6.1 discharged condensate at 15°C below the steam saturation temperature, then the available heat in the condensate would be less. Example 14.6.2 Consider condensate discharging at 7 bar g and with 15°C of subcooling Temperature of saturated condensate at 7 bar g = 170°C Amount of sub cooling = 15°C Temperature of sub-cooled condensate at 7 bar g = 155°C From steam tables: Amount of heat in condensate at 155°C = 654 kJ / kg At 0 bar g, saturated condensate can only hold = 419 kJ / kg Surplus heat in saturated condensate at 0 bar g = 235 kJ / kg Heat in steam at 0 bar g = 2 257 kJ / kg Proportion of flash steam



N- NJ N- NJ

Proportion of flash steam from the condensate = 0.104 (10.4%) Therefore, in this example, condensate discharging at a temperature lower than the saturation temperature has reduced the proportion of flash steam from 13.4% to 10.4%.

Pressurised condensate Example 14.6.3 Consider the condensate in Example 14.6.1 discharging to a flash vessel pressurised at 1 bar g If the return line were connected to a vessel at a pressure of 1 bar g, then it could be seen from steam tables that the maximum heat in the condensate at the trap discharge would be 505 kJ / kg and the enthalpy of evaporation at 1 bar g would be 2 201 kJ / kg. The proportion of the condensate flashing off at 1 bar g can then be calculated as follows: Heat in condensate at 7 bar g = 721 kJ / kg At 1 bar g saturated condensate can only hold = 505 kJ / kg Surplus heat in saturated condensate at 1 bar g = 216 kJ / kg Heat in steam at 1 bar g = 2 201 kJ / kg Proportion of flash steam



N- NJ  N- NJ

Proportion of flash steam from the condensate = 0.098 (9.8%) In this example, if the equipment using steam at 7 bar g were condensing 250 kg / h of steam, then the amount of flash steam released by the condensate at 1 bar g would be 0.098 x 250 kg / h = 24.5 kg / h of flash steam. Therefore, the amount of flash steam produced can depend on the type of steam trap used, the steam pressure before the trap, and the condensate pressure after the trap.

14.6.4

The Steam and Condensate Loop

Block 14 Condensate Recovery

Flash Steam Module 14.6

The flash steam recovery vessel (flash vessel)

Flash vessels are used to separate flash steam from condensate. Figure 14.6.3 shows a typical flash vessel constructed in compliance with the European Pressure Equipment Directive 97/23/EC. After condensate and flash steam enter the flash vessel, the condensate falls by gravity to the base of the vessel, from where it is drained, via a float trap, usually to a vented receiver from where it can be pumped. The flash steam in the vessel is piped from the top of the vessel to any appropriate low pressure steam equipment. Flash steam out

Condensate in

Condensate out Fig. 14.6.3 A typical flash vessel constructed to European standards

Sizing flash steam recovery vessels To size a flash vessel, the following information is required: o

The steam pressure before the steam trap(s) supplying the vessel.

o

The total condensate flowrate into the flash vessel.

o

The flash steam pressure in the flash vessel.

Using this information, together with a flash vessel sizing chart (see Figure 14.6.4), the size of the vessel can be determined. Example 14.6.4 demonstrates flash vessel sizing, using a chart.

The Steam and Condensate Loop

14.6.5

Flash Steam Module 14.6

Block 14 Condensate Recovery

Example 14.6.4 Determine the size of a flash vessel to suit the following conditions: The pressure onto the steam traps is 12 bar g with a total condensate flow of 2 500 kg / h. The flash steam from the vessel is to be supplied to equipment using low pressure steam at 1 bar g. Method: 1. From the ‘Pressure on steam traps’ axis at 12 bar g, move horizontally to the 1 bar g flash steam pressure curve at point A. 2. Drop down vertically to the condensate flowrate level of 2 500 kg / h, point B, and follow the curved line to point C. 3. Move right from point C to meet the 1 bar g flash line at point D. 4. Move upwards to the flash vessel size and select the vessel. For this example, an FV8 flash vessel would be selected. Flash steam pressure bar g 7 65 4 3 2 1

20

0.5 0.2

Pressure on steam traps bar g

18 16

0

14 12

Example

A

10 8

Flash vessel size 6

8

6 8 10 12 14 16 18 20% 0 0.2 0.5 1 1.5 2 3 4 5 7

250 300 400 500 1 000 2 000 3 000 4 000 5 000

C

D

Flash steam pressure bar g

Condensate or blowdown flowrate kg /h

FV

FV

12

15

18

0 2 4

FV

FV

4

FV

6

B

10 000 15 000 20 000 30 000 Fig. 14.6.4 Flash vessel sizing chart

Requirements for successful flash steam applications

If full use is to be made of flash steam, some basic requirements must be satisfied: o

o

14.6.6

It is essential to have a continual supply of sufficient condensate from applications operating at higher pressures, to ensure that enough flash steam can be released for economic recovery. The steam traps and the equipment they are draining must be able to function satisfactorily against the backpressure applied by the flash system. The Steam and Condensate Loop

Block 14 Condensate Recovery

o

o

o

o

Flash Steam Module 14.6

Care must be taken when attempting flash steam recovery with condensate from temperature controlled equipment. At less than full-load, the steam space pressure will be lowered by the closing action of the steam control valve. If the steam pressure in the equipment approaches or falls below the specified flash steam pressure, the overall amount of flash steam formed will be marginal, and one must question whether recovery is worthwhile in this instance. It is important that there is a demand for low pressure flash steam that either equals or exceeds the flash steam being produced. Any deficit of flash steam can be made up by live steam from a pressure reducing valve. If the supply of flash steam exceeds its demand, surplus pressure will be created in the flash steam distribution system, which will then have to be vented to waste through a surplussing valve. It is possible to utilise the flash steam from condensate on a space heating installation - but savings will only be achieved during the heating season. When heating is not required, the recovery system becomes ineffective. Wherever possible, the best arrangement is to use flash steam from process condensate to supply process loads - and flash steam from heating condensate to supply heating loads. Supply and demand are then more likely to remain in-step. It is preferable to actually use the flash steam close to the high pressure condensate source. Relatively large diameter pipes are used for low pressure steam, to reduce pressure loss and velocity, which can mean costly installation if the flash steam has to be piped any distance.

Control of flash steam pressure

Another consideration is a method of controlling the pressure of the flash steam. In some cases, flash pressure will find its own level and nothing more needs to be done. When supply and demand are always in-step, and particularly if the low pressure steam is used on the same equipment producing the high pressure condensate, it is only neccessary to pipe the flash steam to the low pressure plant without any other control. Figure 14.6.5 shows the application of flash steam recovery to a multi-bank air heater battery, which is supplying high temperature air to a process. Condensate from the high pressure sections is taken to the flash vessel, from where the low pressure flash steam is used, to preheat the cold air entering the battery via the frost coil (preheater). The surface area of the preheater section, and the relatively low temperature of the incoming air, will mean that the low pressure flash steam is readily condensed. Temperature control valve High pressure steam supply Flash steam

Air flow

High pressure traps Flash vessel bypass line Flash vessel

Low pressure condensate

Fig. 14.6.5 Flash steam recovery on a multi-bank air heater battery The Steam and Condensate Loop

14.6.7

Flash Steam Module 14.6

Block 14 Condensate Recovery

Depending on operating temperatures, the flash steam will condense at some low pressure, perhaps even sub-atmospheric. If site conditions and layout permit, the flash vessel and the steam trap draining the preheater should be located far enough below the preheater condensate outlet to give enough hydrostatic head to push the condensate through the trap. If this is not possible, pumping traps can be used to drain both the preheater coil and the flash vessel. Steam condensing in the preheater at sub-atmospheric pressure will generally mean that a vacuum breaker is required on the flash steam supply to the preheater. This will prevent the pressure in the battery becoming sub-atmospheric, thereby assisting condensate flow to the trap. Drainage from the preheater trap is induced by gravity flow. Figure 14.6.6 shows an application where the flash steam system is kept at a specified constant pressure by steam fed from a reducing valve. This ensures a reliable source of steam to the low pressure system if there is a lack of flash steam to meet the load.

Typical applications for flash steam Flash steam supply and demand in-step

This gives maximum utilisation of the available flash steam. The air heater battery discussed in Figure 14.6.5 is one such system, but similar arrangements are practical with many other applications such as space heating installations using either radiant panels, or unit heaters. Figure 14.6.6 depicts a system where a number of heaters are supplied with high pressure steam. The condensate from approximately 90% of the heaters is collected and taken to a flash recovery vessel. This supplies low pressure steam to the remaining 10% of the heaters. With this system, the total heat output of the system is marginally reduced, as 10% of the heaters are operating at a lower steam pressure. However, it is rare to find an installation that does not have a sufficient margin of output above the normal load to accept this small reduction. Sometimes a problem arises where the use of available flash steam may require more than one heater but less than two. It would be better in this case to connect two heaters to the flash steam supply, rather than vent the excess flash steam off to waste. Two heaters together will usually pull the flash pressure down to a lower level, even to sub-atmospheric levels. To cope with this, the supply of flash steam can be supplemented with live steam from a pressure reducing valve. Pressure reducing valve set High pressure steam supply

High pressure heaters

Low pressure heaters

Low pressure traps

High pressure traps Flash vessel bypass line

Flash vessel

Trap set Fig. 14.6.6 Flash steam supply and demand in step

Low pressure condensate

Another example where supply and demand are ‘in step’ is the steam heated hot water storage calorifier. Some of these incorporate a second coil, fitted close to the bottom of the vessel adjacent to where the cold feedwater enters. Condensate and flash steam from the trap on the primary coil is passed directly to the secondary coil. Here, any flash steam produced by the drop in pressure across the trap is condensed, while giving up its heat to the feedwater. A typical arrangement is shown in Figure 14.6.7. 14.6.8

The Steam and Condensate Loop

Block 14 Condensate Recovery

Flash Steam Module 14.6

Hot water out

Steam

Primary coil trapset

Primary coil Secondary coil acting as a flash cooler Return water in

Low temperature condensate

Fig. 14.6.7 Secondary flash steam coil in a storage calorifier

Another example of this idea is shown in Figure 14.6.8. Here, a normal steam-to-water calorifier drains condensate through a float trap to a smaller shell-and-tube heat exchanger (called a flash condenser), in which the flash steam is condensed to sub-cooled condensate. The unit is fitted such that the secondary flow pipework is in series with both calorifier and condenser. This enables the secondary return water to be preheated by the condenser, thereby reducing the demand for live steam in the first instance. If the condensate in the flash condenser is likely to be sub-atmospheric, a mechanical pump is required to lift the condensate to any higher return line. The motive steam exhausting from the pump is itself condensed in the flash condenser. The pumping of the condensate is then achieved at virtually no cost. Consideration must be given to the pump filling head in that it needs to be greater than the pressure drop across the flash condenser tubes under full-load conditions. A minimum head of 600 mm will usually achieve this.

Secondary flow Steam Heating calorifier Temperature control Steam trap

* Balance line

Air vent

Secondary flow path Shell-and-tube heat exchanger (flash condenser)

*

Secondary return

Receiver

Condensate return Motive steam

Filling head > 600 mm Pump

Fig. 14.6.8 Packaged calorifier and flash condenser unit The Steam and Condensate Loop

14.6.9

Flash Steam Module 14.6

Block 14 Condensate Recovery

Flash steam supply and demand not in-step

The arrangement in Figure 14.6.9 is an example of flash steam recovery where the supply and demand are not always ‘in-step’. Condensate from three jacketed pans and a drain pocket releases flash steam, but it can only be used to augment the supply of steam to the space heating installation. This is quite satisfactory during the heating season, as long as the heating load exceeds the availability of flash steam. During the summer season the heating equipment will not be in use, and even during spring and autumn the heating load may not be able to use all the available flash steam. The arrangement is not ideal, although it is quite possible for the steam savings made during the winter to justify the cost of the flash steam recovery equipment. Sometimes, surplus flash steam must be vented to atmosphere, and, as indicated, a surplussing valve is more suitable for this purpose than a safety valve, which usually has a ‘pop’ or ‘on / off’ action and a seat arrangement designed for infrequent operation. The surplussing valve will be set so that it begins to open slightly above the normal pressure in the system. When the heating load falls and the pressure in the system begins to increase, the pressure reducing valve supplying the make-up steam closes down. A further increase of pressure, perhaps of 0.15 to 0.2 bar, is then allowed before the surplussing valve begins to open to release the excess flash steam. A safety valve may still be required if the surplussing valve fails. It must be set to open at a pressure between the surplussing valve set pressure and the system design pressure. It is usually convenient to fit the safety valve onto the flash vessel. Occasionally, during summer conditions it may be preferable to bypass the flash system with a manual valve (not shown in Figure 14.6.9). The condensate and its associated flash steam will then pass directly to a condensate receiver, where the flash steam will be vented to atmosphere. Pressure reducing valve

Surplussing valve

Low pressure steam

Steam Flash steam

Condensate

Medium pressure steam

Condensate Condensate Flash vessel

Condensate

Condensate Fig. 14.6.9 Flash steam supply and demand not in-step

14.6.10

The Steam and Condensate Loop

Block 14 Condensate Recovery

Flash Steam Module 14.6

Boiler blowdown heat recovery applications

Continuous blowdown of boiler water is necessary to control the level of TDS (Total Dissolved Solids) within the boiler. Continuous blowdown lends itself to the recovery of the heat content of the blowdown water and can enable considerable savings to be made. Boiler blowdown contains massive quantities of heat, which can easily be recovered as flash steam. After it passes through the blowdown control valve, the lower pressure water flows to a flash vessel. At this point, the flash steam is free from contamination and is separated from the condensate, and can be used to heat the boiler feedtank (see Figure 14.6.10). The residual condensate draining from the flash vessel can be passed through a plate heat exchanger in order to reclaim as much heat as possible before it is dumped to waste. Up to 80% of the total heat contained in boiler continuous bowdown can be reclaimed in this way. Cold water

Level controller

Make-up tank

Condensate

Boiler feedtank Steam supply to injector Flash vessel

Steam Blowdown valve

Float trap

Boiler

Heat exchanger Feedpump

Drain Fig. 14.6.10 Typical heat recovery from boiler blowdown

The Steam and Condensate Loop

14.6.11

Flash Steam Module 14.6

Block 14 Condensate Recovery

Spray condensing

Finally, consideration should be given to those cases where flash steam is unavoidably generated at low pressure, but where no suitable load is available which can make use of it. Rather than simply discharge the flash steam to waste, the arrangement in Figure 14.6.11 can often be adopted. This arrangement can be useful where the condensate receiver vent cannot be piped to outside, and where the presence of flash steam would be detrimental if left to discharge in a plant room. A lightweight stainless steel chamber is fitted to the receiver tank vent. Cold water is sprayed into the chamber in sufficient quantities to just condense the flash steam. The flow of cooling water is controlled by a simple self-acting temperature control, adjusted so that minimal amounts of flash steam appear from the vent. The process will use roughly 6 kilograms of cooling water per kilogram of flash steam condensed. If the cooling water is of boiler feed quality, then the warmed water is added to the condensate in the receiver and re-used. This will continue to make water savings throughout the year. If the cooling water is not suitable for recovery, the spray pipework can be installed as shown by the dotted arrangement. The cooling water and condensed flash will then fall to waste.

Vented to atmosphere

Water in Self-acting temperature control

Alternative arrangement

Condensate

Condensate receiver Condensed water to waste Overflow with ‘U’ seal Pumped condensate Centrifugal pump Fig 14.6.11 Flash steam condensing and water saving by spray

14.6.12

The Steam and Condensate Loop

Block 14 Condensate Recovery

Flash Steam Module 14.6

Questions 1. What is the difference between live steam and flash steam? a| Live steam is made from water, flash steam is made from condensate

¨

b| Live steam is always hotter than flash steam

¨

c| Live steam is made by adding heat to water, flash steam is made from heat already contained in water

¨

d| Live steam is always at a higher pressure than flash steam

¨

2. What percentage of flash steam is made from condensate at 10 bar g passing into a flash vessel at 0.5 bar g? a| 12%

¨

b| 13%

¨

c| 14%

¨

d| 5%

¨

3. What is the effect on the production of flash steam from sub-saturated condensate? a| The flash steam produced is less than that with saturated condensate

¨

b| The flash steam produced is more than that with saturated condensate

¨

c| There is no effect at all

¨

d| Live steam is always at a higher pressure than flash steam

¨

4. With reference to Example 14.6.1, what would be the proportion of flash steam produced if the flash pressure were 2.5 bar g? a| 3%

¨

b| 6%

¨

c| 8%

¨

d| 10%

¨

5. In a steam system, the trap pressure is 15 bar g, the flash pressure is 0.5 bar g, and the condensate flowrate is 1300 kg / h. Which flash vessel is required? a| FV6

¨

b| FV8

¨

c| FV12

¨

d| FV16

¨

6. What is used to top-up the flash pressure? a| A safety valve

¨

b| A larger condensate flow

¨

c| A pressure surplussing valve

¨

d| A pressure reducing valve

¨

Answers

1: c 2: c, 3: a, 4: b, 5: b, 6: d The Steam and Condensate Loop

14.6.13

Block 14 Condensate Recovery

14.6.14

Flash Steam Module 14.6

The Steam and Condensate Loop