Spiraxsarco-b10-steam Distribution

Introduction to Steam Distribution Module 10.1

Block 10 Steam Distribution

Module 10.1 Introduction to Steam Distribution

The Steam and Condensate Loop

10.1.1

Introduction to Steam Distribution Module 10.1

Block 10 Steam Distribution

Introduction to Steam Distribution The steam distribution system is the essential link between the steam generator and the steam user. This Module will look at methods of carrying steam from a central source to the point of use. The central source might be a boiler house or the discharge from a co-generation plant. The boilers may burn primary fuel, or be waste heat boilers using exhaust gases from high temperature processes, engines or even incinerators. Whatever the source, an efficient steam distribution system is essential if steam of the right quality and pressure is to be supplied, in the right quantity, to the steam using equipment. Installation and maintenance of the steam system are important issues, and must be considered at the design stage.

Steam system basics From the outset, an understanding of the basic steam circuit, or ‘steam and condensate loop’ is required – see Figure 10.1.1. As steam condenses in a process, flow is induced in the supply pipe. Condensate has a very small volume compared to the steam, and this causes a pressure drop, which causes the steam to flow through the pipes. Steam

Space heating system

Steam Pan

Pan Condensate Process vessel

Steam Condensate

Steam

Condensate

Make-up water Feedpump

Feedtank

Condensate

Fig. 10.1.1 A typical basic steam circuit

The steam generated in the boiler must be conveyed through pipework to the point where its heat energy is required. Initially there will be one or more main pipes, or ‘steam mains’, which carry steam from the boiler in the general direction of the steam using plant. Smaller branch pipes can then carry the steam to the individual pieces of equipment. When the boiler main isolating valve (commonly called the ‘crown’ valve) is opened, steam immediately passes from the boiler into and along the steam mains to the points at lower pressure. The pipework is initially cooler than the steam, so heat is transferred from the steam to the pipe. The air surrounding the pipes is also cooler than the steam, so the pipework will begin to transfer heat to the air. Steam on contact with the cooler pipes will begin to condense immediately. On start-up of the system, the condensing rate will be at its maximum, as this is the time where there is maximum temperature difference between the steam and the pipework. This condensing rate is commonly called the ‘starting load’. Once the pipework has warmed up, the temperature difference between the steam and pipework is minimal, but some condensation will occur as the pipework still continues to transfer heat to the surrounding air. This condensing rate is commonly called the ‘running load’. 10.1.2

The Steam and Condensate Loop

Introduction to Steam Distribution Module 10.1

Block 10 Steam Distribution

The resulting condensation (condensate) falls to the bottom of the pipe and is carried along by the steam flow and assisted by gravity, due to the gradient in the steam main that should be arranged to fall in the direction of steam flow. The condensate will then have to be drained from various strategic points in the steam main. When the valve on the steam pipe serving an item of steam using plant is opened, steam flowing from the distribution system enters the plant and again comes into contact with cooler surfaces. The steam then transfers its energy in warming up the equipment and product (starting load), and, when up to temperature, continues to transfer heat to the process (running load). There is now a continuous supply of steam from the boiler to satisfy the connected load and to maintain this supply more steam must be generated. In order to do this, more water (and fuel to heat this water) is supplied to the boiler to make up for that water which has previously been evaporated into steam. The condensate formed in both the steam distribution pipework and in the process equipment is a convenient supply of useable hot boiler feedwater. Although it is important to remove this condensate from the steam space, it is a valuable commodity and should not be allowed to run to waste. Returning all condensate to the boiler feedtank closes the basic steam loop, and should be practised wherever practical. The return of condensate to the boiler is discussed further in Block 13, ‘Condensate Removal’, and Block 14,’Condensate Management’.

The working pressure

The distribution pressure of steam is influenced by a number of factors, but is limited by: o

The maximum safe working pressure of the boiler.

o

The minimum pressure required at the plant.

As steam passes through the distribution pipework, it will inevitably lose pressure due to: o

Frictional resistance within the pipework (detailed in Module 10.2).

o

Condensation within the pipework as heat is transferred to the environment.

Therefore allowance should be made for this pressure loss when deciding upon the initial distribution pressure. A kilogram of steam at a higher pressure occupies less volume than at a lower pressure. It follows that, if steam is generated in the boiler at a high pressure and also distributed at a high pressure, the size of the distribution mains will be smaller than that for a low-pressure system for the same heat load. Figure 10.1.2 illustrates this point. Specific volume m³/kg

2.0 1.5 1.0 0.5 0

0

2

4

6 8 10 12 14 Pressure bar g Fig. 10.1.2 Dry saturated steam - pressure /specific volume relationship

Generating and distributing steam at higher pressure offers three important advantages: o

o

o

The thermal storage capacity of the boiler is increased, helping it to cope more efficiently with fluctuating loads, minimising the risk of producing wet and dirty steam. Smaller bore steam mains are required, resulting in lower capital cost, for materials such as pipes, flanges, supports, insulation and labour. Smaller bore steam mains cost less to insulate.

The Steam and Condensate Loop

10.1.3

Introduction to Steam Distribution Module 10.1

Block 10 Steam Distribution

Having distributed at a high pressure, it will be necessary to reduce the steam pressure to each zone or point of use in the system in order to correspond with the maximum pressure required by the application. Local pressure reduction to suit individual plant will also result in drier steam at the point of use. (Module 2.3 provides an explanation of this). Note: It is sometimes thought that running a steam boiler at a lower pressure than its rated pressure will save fuel. This logic is based on more fuel being needed to raise steam to a higher pressure. Whilst there is an element of truth in this logic, it should be remembered that it is the connected load, and not the boiler output, which determines the rate at which energy is used. The same amount of energy is used by the load whether the boiler raises steam at 4 bar g, 10 bar g or 100 bar g. Standing losses, flue losses, and running losses are increased by operating at higher pressures, but these losses are reduced by insulation and proper condensate return systems. These losses are marginal when compared to the benefits of distributing steam at high pressure.

Pressure reduction

The common method for reducing pressure at the point where steam is to be used is to use a pressure reducing valve, similar to the one shown in the pressure reducing station Figure 10.1.3. Safety valve

Pressure reducing valve Separator Steam

Steam Strainer

Trap set

Condensate Fig. 10.1.3 Typical pressure reducing valve station

A separator is installed upstream of the reducing valve to remove entrained water from incoming wet steam, thereby ensuring high quality steam to pass through the reducing valve. This is discussed in more detail in Module 9.3 and Module 12.5. Plant downstream of the pressure reducing valve is protected by a safety valve. If the pressure reducing valve fails, the downstream pressure may rise above the maximum allowable working pressure of the steam using equipment. This, in turn, may permanently damage the equipment, and, more importantly, constitute a danger to personnel. With a safety valve fitted, any excess pressure is vented through the valve, and will prevent this from happening (safety valves are discussed in Block 9). Other components included in the pressure reducing valve station are: o

The primary isolating valve - To shut the system down for maintenance.

o

The primary pressure gauge - To monitor the integrity of supply.

o

The strainer - To keep the system clean.

o

The secondary pressure gauge - To set and monitor the downstream pressure.

o

10.1.4

The secondary isolating valve - To assist in setting the downstream pressure on no- load conditions.

The Steam and Condensate Loop

Introduction to Steam Distribution Module 10.1

Block 10 Steam Distribution

Questions 1.

Distributing steam at high pressure, instead of low pressure, will have the following effect.

a | Heat losses from the pipes will be less. b | A lower storage capacity in the high pressure pipes. c | High pressure small bore steam pipes cost less to install and insulate. d | The steam pipes will be smaller creating wet steam. 2.

A steam pressure reducing valve is fitted to:

a | Prevent the pressure at the plant exceeding its safe working pressure. b | Help dry the steam supply to the plant. c | Reduce the flash steam losses as condensate passes through the plant steam traps. d | Supply the plant with steam at the designed temperature and pressure. 3.

¨ ¨ ¨ ¨

¨ ¨ ¨ ¨

The start-up condensate load of a steam main is generally greater than the running load because:

a | The pipework and fittings are cold, so steam is required to heat it up to steam temperature.

¨

b | The steam space within the pipework has to be charged with steam to the desired running pressure.

¨

c | The boiler crown valve or stop valve is opened very slowly and initially there is insufficient pressure to discharge condensate through the steam traps.

¨

d | On initial opening of the crown valve, the steam distribution pressure will be low and the enthalpy of evaporation of low pressure steam is greater than at high pressure ¨ so a greater mass of steam will be condensed. 4.

The pressure at which steam is supplied to the plant should be dictated by:

a | The boiler operating pressure. b | The steam distribution pressure. c | The maximum allowable safe working pressure of the plant. d | The plant design pressure and temperature. 5.

Which of the following results in pressure losses in distribution pipework?

a | Sizing the pipes on low pressure instead of high pressure. b | Frictional resistance within and heat loss from the pipe and fittings. c | Sizing the pipes on start-up load of the plant. d | Large steam users. 6.

¨ ¨ ¨ ¨

¨ ¨ ¨ ¨

The steam pipe after a pressure reducing valve is likely to be:

a | Smaller than the upstream pipe because of the smaller volume of low pressure steam. ¨ b | The same size as the connection to the plant.

¨

c | Larger than the upstream pipe because the volume of the low pressure steam is greater.

¨

d | The same size as the upstream pipe because the flowrate through each pipe is the same.

¨

Answers

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

10.1.5

Block 10 Steam Distribution

10.1.6

Introduction to Steam Distribution Module 10.1

The Steam and Condensate Loop

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Module 10.2 Pipes and Pipe Sizing

The Steam and Condensate Loop

10.2.1

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Pipes and Pipe Sizing Standards and wall thickness There are a number of piping standards in existence around the world, but arguably the most global are those derived by the American Petroleum Institute (API), where pipes are categorised in schedule numbers. These schedule numbers bear a relation to the pressure rating of the piping. There are eleven Schedules ranging from the lowest at 5 through 10, 20, 30, 40, 60, 80, 100, 120, 140 to schedule No. 160. For nominal size piping 150 mm and smaller, Schedule 40 (sometimes called ‘standard weight’) is the lightest that would be specified for steam applications. Regardless of schedule number, pipes of a particular size all have the same outside diameter (not withstanding manufacturing tolerances). As the schedule number increases, the wall thickness increases, and the actual bore is reduced. For example: o

o

A 100 mm Schedule 40 pipe has an outside diameter of 114.30 mm, a wall thickness of 6.02 mm, giving a bore of 102.26 mm. A 100 mm Schedule 80 pipe has an outside diameter of 114.30 mm, a wall thickness of 8.56 mm, giving a bore of 97.18 mm.

Only Schedules 40 and 80 cover the full range from 15 mm up to 600 mm nominal sizes and are the most commonly used schedule for steam pipe installations. This Module considers Schedule 40 pipework as covered in BS 1600. Tables of schedule numbers can be obtained from BS 1600 which are used as a reference for the nominal pipe size and wall thickness in millimetres. Table 10.2.1 compares the actual bore sizes of different sized pipes, for different schedule numbers. In mainland Europe, pipe is manufactured to DIN standards, and DIN 2448 pipe is included in Table 10.2.1. Table 10.2.1 Comparison of pipe standards and actual bore diameters. Nominal size pipe (mm) 15 20 25 32 40 50 Schedule 40 15.8 21.0 26.6 35.1 40.9 52.5 Schedule 80 13.8 18.9 24.3 32.5 38.1 49.2 Bore (mm) Schedule 160 11.7 15.6 20.7 29.5 34.0 42.8 DIN 2448 17.3 22.3 28.5 37.2 43.1 60.3

65 62.7 59.0 53.9 70.3

80 77.9 73.7 66.6 82.5

100 102.3 97.2 87.3 107.1

150 154.1 146.4 131.8 159.3

In the United Kingdom, piping to BS 1387, (steel tubes and tubulars suitable for screwing to BS 21 threads) is also used in applications where the pipe is screwed rather than flanged. They are commonly referred to as ‘Blue Band’ and ‘Red Band’; this being due to their banded identification marks. The different colours refer to particular grades of pipe: o o

Red Band, being heavy grade, is commonly used for steam pipe applications. Blue Band, being medium grade, is commonly used for air distribution systems, although it is sometimes used for low-pressure steam systems.

The coloured bands are 50 mm wide, and their positions on the pipe denote its length. Pipes less than 4 metres in length only have a coloured band at one end, while pipes of 4 to 7 metres in length have a coloured band at either end.

Fig. 10.2.1 Red band, branded pipe, - heavy grade, up to 4 metres in length

10.2.2

Fig. 10.2.2 Blue band, branded pipe, - heavy grade, between 4-7 metres in length The Steam and Condensate Loop

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Pipe material Pipes for steam systems are commonly manufactured from carbon steel to ANSI B 16.9 A106. The same material may be used for condensate lines, although copper tubing is preferred in some industries. For high temperature superheated steam mains, additional alloying elements, such as chromium and molybdenum, are included to improve tensile strength and creep resistance at high temperatures. Typically, pipes are supplied in 6 metre lengths.

Pipeline sizing The objective of the steam distribution system is to supply steam at the correct pressure to the point of use. It follows, therefore, that pressure drop through the distribution system is an important feature. Bernoulli’s Theorem (Daniel Bernoulli 1700 - 1782) is discussed in Block 4 - Flowmetering. D’Arcy (D’Arcy Thompson 1860 - 1948) added that for fluid flow to occur, there must be more energy at Point 1 than Point 2 (see Figure 10.2.3). The difference in energy is used to overcome frictional resistance between the pipe and the flowing fluid. hf h1

h2

Pipe diameter (D)

Flow velocity (u)

Length (L) Point 1

Point 2 Fig. 10.2.3 Friction in pipes

This is illustrated by the D’Arcy equation (Equation 10.2.1):

K = I/Xò J' I

Equation 10.2.1

Where: hf = Head loss to friction (m) f = Friction factor (dimensionless) L = Length (m) u = Flow velocity (m /s) g = Gravitational constant (9.81 m /s²) D = Pipe diameter (m) It is useful to remember that: o

Head loss to friction (hf) is proportional to the velocity squared (u²).

o

The friction factor (f) is an experimental coefficient which is affected by factors including: - The Reynolds Number (which is affected by velocity). - The reciprocal of velocity².

Because the values for ‘f’ are quite complex, they are usually obtained from charts.

The Steam and Condensate Loop

10.2.3

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Example 10.2.1 - Water pipe Determine the difference in pressure between two points 1 km apart in a 150 mm bore horizontal pipework system. The water flowrate is 45 m³ / h at 15°C and the friction factor for this pipe is taken as 0.005

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P V

K = I/Xò J' I

K

[[P[ò [[

KI

P ≈ EDU

I

Equation 10.2.1

In practice whether for water pipes or steam pipes, a balance is drawn between pipe size and pressure loss.

Oversized pipework means: o

Pipes, valves, fittings, etc. will be more expensive than necessary.

o

Higher installation costs will be incurred, including support work, insulation, etc.

o

For steam pipes a greater volume of condensate will be formed due to the greater heat loss. This, in turn, means that either: - More steam trapping is required, or - Wet steam is delivered to the point of use.

In a particular example: o

o

The cost of installing 80 mm steam pipework was found to be 44% higher than the cost of 50 mm pipework, which would have had adequate capacity. The heat lost by the insulated pipework was some 21% higher from the 80 mm pipeline than it would have been from the 50 mm pipework. Any non-insulated parts of the 80 mm pipe would lose 50% more heat than the 50 mm pipe, due to the extra heat transfer surface area.

Undersized pipework means: o

o o

A lower pressure may only be available at the point of use. This may hinder equipment performance due to only lower pressure steam being available. There is a risk of steam starvation. There is a greater risk of erosion, waterhammer and noise due to the inherent increase in steam velocity.

As previously mentioned, the friction factor (f) can be difficult to determine, and the calculation itself is time consuming especially for turbulent steam flow. As a result, there are numerous graphs, tables and slide rules available for relating steam pipe sizes to flowrates and pressure drops. One pressure drop sizing method, which has stood the test of time, is the ‘pressure factor’ method. A table of pressure factor values is used in Equation 10.2.2 to determine the pressure drop for a particular installation.

) = 3 3 /

Equation 10.2.2

Where: F = Pressure factor P1 = Factor at inlet pressure P2 = Factor at a distance of L metres L = Equivalent length of pipe (m) 10.2.4

The Steam and Condensate Loop

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Example 10.2.2 Consider the system shown in Figure 10.2.4, and determine the pipe size required from the boiler to the unit heater branch line. Unit heater steam load = 270 kg /h. P1 = 7 bar g

P2 = 6.6 bar g

L = 150 m 150 m (original pipe length) + 10 % (allowance for pipe fittings) = 165 m (revised pipe length)

Boiler at 7.0 bar g 286 kg/h

Unit heater at 6.6 bar g 270 kg/h

Revised load to supply the heater battery is 270 kg/h + 5.8% = 286 kg/h

Fig. 10.2.4 System used to illustrate Example 10.2.2

Although the unit heater only requires 270 kg /h, the boiler has to supply more than this due to heat losses from the pipe.

The allowance for pipe fittings

The length of travel from the boiler to the unit heater is known, but an allowance must be included for the additional frictional resistance of the fittings. This is generally expressed in terms of ‘equivalent pipe length’. If the size of the pipe is known, the resistance of the fittings can be calculated. As the pipe size is not yet known in this example, an addition to the equivalent length can be used based on experience. o o

o

If the pipe is less than 50 metres long, add an allowance for fittings of 5%. If the pipe is over 100 metres long and is a fairly straight run with few fittings, an allowance for fittings of 10% would be made. A similar pipe length, but with more fittings, would increase the allowance towards 20%.

In this instance, revised length = 150 m + 10% = 165 m

The allowance for the heat losses from the pipe

The unit heater requires 270 kg /h of steam; therefore the pipe must carry this quantity plus the quantity of steam condensed by heat losses from the main. As the size of the main is yet to be determined, the true calculations cannot be made, but, assuming that the main is insulated, it may be reasonable to add 3.5% of the steam load per 100 m of the revised length as heat losses. In this instance, the additional allowance =

 [ 



Revised boiler load = 270 kg /h + 5.8% = 286 kg /h From Table 10.2.2 (an extract from the complete pressure factor table, Table 10.2.5, which can be found in the Appendix at the end of this Module) ‘F‘ can be determined by finding the pressure factors P1 and P2, and substituting them into Equation 10.2.2. Table 10.2.2 Extract from pressure factor table (Table 10.2.5) Pressure bar g 6.5 6.6 6.7 6.9 7.0 7.1

The Steam and Condensate Loop

Pressure factor (F) 49.76 51.05 52.36 55.02 56.38 57.75

10.2.5

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

From the pressure factor table: P1 (7.0 bar g) = 56.38 P2 (6.6 bar g) = 51.05 Substituting these pressure factors (P1 and P2 ) into Equation 10.2.2 will determine the value for ‘F’:

)

3 3 /

)

 P

)



Equation 10.2.2

Following down the left-hand column of the pipeline capacity and pressure drop factors table (Table 10.2.6 - Extract shown in Table 10.2.3); the nearest two readings around the requirement of 0.032 are 0.030 and 0.040. The next lower factor is always selected; in this case, 0.030. Table 10.2.3 Extract from pipeline capacity and pressure factor table (Table 10.2.6) Pipe size (DN) Factor 15 20 25 32 40 50 65 80 (F) Capacity (kg /h) 0.025 10.99 33.48 70.73 127.3 209.8 459.7 834.6 1 367 0.030 12.00 36.78 77.23 137.9 229.9 501.1 919.4 1 480 0.040 14.46 44.16 93.17 169.2 279.5 600.7 1 093 1 790

100

150

200

2 970 3 264 3 923

8 817 9 792 11 622

19 332 20 917 25 254

Although values can be interpolated, the table does not conform exactly to a straight-line graph, so interpolation cannot be absolutely correct. Also, it is bad practice to size any pipe up to the limit of its capacity, and it is important to have some leeway to allow for the inevitable future changes in design. From factor 0.030, by following the row of figures to the right it will be seen that: o

A 40 mm pipe will carry 229.9 kg /h.

o

A 50 mm pipe will carry 501.1 kg /h.

Since the application requires 286 kg /h, the 50 mm pipe would be selected. Having sized the pipe using the pressure drop method, the velocity can be checked if required.

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Where:

6WHDPIORZUDWH

 NJ K UHYLVHGORDG

6SHFLILFYROXPHY J

 P ó NJ $WEDUJ

3LSHGLDPHWHU'

PFDOFXODWHGDERYH

6WHDPYHORFLW\

 NJ K [ P ó NJ[ P V  V K[π [ò  ( P ò )

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P V

Viewed in isolation, this velocity may seem low in comparison with maximum permitted velocities. However, this steam main has been sized to limit pressure drop, and the next smaller pipe size would have given a velocity of over 47 m/s, and a final pressure less than the requirement of 6.6 bar g. 10.2.6

The Steam and Condensate Loop

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

As can be seen, this procedure is fairly complex and can be simplified by using the nomogram shown in Table 10.2.7 (in the Appendix at the end of this Module). The method of use is explained in Example 10.2.3. Example 10.2.3 Using the data from Example 10.2.2, determine the pressure drop using the nomogram shown in Figure 10.2.5 (same as Table 10.2.7). Inlet pressure = 7 bar g Steam flowrate = 286 kg /h

0D[LPXPSUHVVXUHGURSSHUP 0D[LPXPSUHVVXUHGURSSHUP

( 3 3 ) [ / (  ) [ 

0D[LPXPSUHVVXUHGURSSHUP EDU Method: o

Select the point on the saturated steam line at 7 bar g, and mark Point A.

o

From point A, draw a horizontal line to the steam flowrate of 286 kg /h, and mark Point B.

o

From point B, draw a vertical line towards the top of the nomogram (Point C).

o

Draw a horizontal line from 0.24 bar /100 m on the pressure loss scale (Line DE). The point at which lines DE and BC cross will indicate the pipe size required. In this case, a 40 mm pipe is too small, and a 50 mm pipe would be used. 20

C

0.3 0.2

D

E

0.1 0.05

Ins

0.03 0.02

mm

0.5

400 500 ide pip 600 ed iam ete r

1

200 250 300

15

3 2

20 25 30

Pressure loss bar / 100 m

5

10

10

40 50 60 70 80 100 125 150

o

0.01

100

0.5 1 2 3 5 7 10 15 20 30

100 200 300 500 100 0 20 3 0 00 00 50 00 10 000 20 30 000 000 50 000 100 000 Ste am 200 0 flow 00 rat ek g/h

A Saturation temperature curve

10

um 50% vacu 0 bar g

B

50 70 100

200 300 400 Steam temperature °C

20 30 50

Steam pressure bar g

500

Fig. 10.2.5 Steam pipeline sizing chart - Pressure drop The Steam and Condensate Loop

10.2.7

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Sizing pipes on velocity

From the knowledge gained at the beginning of this Module, and particularly the notes regarding the D’Arcy equation (Equation 10.2.1), it is acknowledged that velocity is an important factor in sizing pipes. It follows then, that if a reasonable velocity could be used for a particular fluid flowing through pipes, then velocity could be used as a practical sizing factor. As a general rule, a velocity of 25 to 40 m /s is used when saturated steam is the medium. 40 m /s should be considered an extreme limit, as above this, noise and erosion will take place particularly if the steam is wet. Even these velocities can be high in terms of their effect on pressure drop. In longer supply lines, it is often necessary to restrict velocities to 15 m /s to avoid high pressure drops. It is recommended that pipelines over 50 m long are always checked for pressure drop, no matter what the velocity. By using Table 10.2.4 as a guide, it is possible to select pipe sizes from known data; steam pressure, velocity and flowrate. Table 10.2.4 Saturated steam pipeline capacities in kg /h for different velocities (Schedule 40 pipe) Pipe size (nominal) 15 20 25 32 40 50 65 80 100 125 150 Pressure Velocity Actual inside pipe diameter Schedule 40 bar g m/s 15.80 20.93 26.64 35.04 40.90 52.50 62.70 77.92 102.26 128.20 154.05 Pipeline capacity kg /h 15 9 15 25 43 58 95 136 210 362 569 822 0.4 25 14 25 41 71 97 159 227 350 603 948 1 369 40 23 40 66 113 154 254 363 561 965 1 517 2 191 15 10 18 29 51 69 114 163 251 433 681 983 0.7 25 17 30 49 85 115 190 271 419 722 1 135 1 638 40 28 48 78 136 185 304 434 671 1 155 1 815 2 621 15 12 21 34 59 81 133 189 292 503 791 1 142 1 25 20 35 57 99 134 221 315 487 839 1 319 1 904 40 32 56 91 158 215 354 505 779 1342 2 110 3 046 15 18 31 50 86 118 194 277 427 735 1 156 1 669 2 25 29 51 83 144 196 323 461 712 1 226 1 927 2 782 40 47 82 133 230 314 517 737 1 139 1 961 3 083 4 451 15 23 40 65 113 154 254 362 559 962 1 512 2 183 3 25 38 67 109 188 256 423 603 931 1 603 2 520 3 639 40 61 107 174 301 410 676 964 1 490 2 565 4 032 5 822 15 28 50 80 139 190 313 446 689 1 186 1 864 2 691 4 25 47 83 134 232 316 521 743 1 148 1 976 3 106 4 485 40 75 132 215 371 506 833 1 189 1 836 3 162 4 970 7 176 15 34 59 96 165 225 371 529 817 1 408 2 213 3 195 5 25 56 98 159 276 375 619 882 1 362 2 347 3 688 5 325 40 90 157 255 441 601 990 1 411 2 180 3 755 5 901 8 521 15 39 68 111 191 261 430 613 947 1 631 2 563 3 700 6 25 65 114 184 319 435 716 1 022 1 578 2 718 4 271 6 167 40 104 182 295 511 696 1 146 1 635 2 525 4 348 6 834 9 867 15 44 77 125 217 296 487 695 1 073 1 848 2 904 4 194 7 25 74 129 209 362 493 812 1 158 1 788 3 080 4 841 6 989 40 118 206 334 579 788 1 299 1 853 2 861 4 928 7 745 11 183 15 49 86 140 242 330 544 775 1 198 2 063 3 242 4 681 8 25 82 144 233 404 550 906 1 292 1 996 3 438 5 403 7 802 40 131 230 373 646 880 1 450 2 068 3 194 5 501 8 645 12 484 15 60 105 170 294 401 660 942 1 455 2 506 3 938 5 686 10 25 100 175 283 490 668 1 101 1 570 2 425 4 176 6 563 9 477 40 160 280 453 785 1 069 1 761 2 512 3 880 6 682 10 502 15 164 15 80 141 228 394 537 886 1 263 1 951 3 360 5 281 7 625 14 25 134 235 380 657 896 1 476 2 105 3 251 5 600 8 801 12 708 40 214 375 608 1 052 1 433 2 362 3 368 5 202 8 960 14 082 20 333

10.2.8

The Steam and Condensate Loop

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Alternatively the pipe size can be calculated arithmetically. The following information is required, and the procedure used for the calculation is outlined below. Information required to calculate the required pipe size: u = Flow velocity (m /s) vg = Specific volume (m³ /kg) ms = Mass flowrate (kg /s) V = Volumetric flowrate (m³ /s) = ms x vg From this information, the cross sectional area (A) of the pipe can be calculated:

&URVVVHFWLRQDODUHD$ π ['ò

LH



9ROXPHIORZUDWH (  ) )ORZYHORFLW\ ( X )  X

Rearranging the formula to give the diameter of the pipe (D) in metres:

' = '=

[ π [X [ π [X

Example 10.2.4 A process requires 5 000 kg /h of dry saturated steam at 7 bar g. For the flow velocity not to exceed 25 m /s, determine the pipe size. Where:

)ORZYHORFLW\X 6SHFLILFYROXPHDWEDUJY J



0DVVIORZUDWH

 P V  Pó NJ  NJ KRU NJ V

9ROXPHWULFIORZUDWH

[Y

9ROXPHWULFIORZUDWH

 NJ V [Pó NJ

9ROXPHWULFIORZUDWH

Therefore, using:

&URVVVHFWLRQDODUHD$ π['ò



'ò ' 3LSHGLDPHWHU' 3LSHGLDPHWHU'

J

Pó V

9ROXPHWULFIORZUDWH (  ) )ORZYHORFLW\ ( X )  X [ π[X [

π[X

[ π[

PRUPP

Since the steam velocity must not exceed 25 m /s, the pipe size must be at least 130 mm; the nearest commercially available size, 150 mm, would be selected. Again, a nomogram has been created to simplify this process, see Figure 10.2.6. The Steam and Condensate Loop

10.2.9

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Example 10.2.5 Using the information from Example 10.2.4, use Figure 10.2.6 to determine the minimum acceptable pipe size Inlet pressure = 7 bar g Steam flowrate = 5 000 kg /h Maximum velocity = 25 m /s Method: o Draw a horizontal line from the saturation temperature line at 7 bar g (Point A) on the pressure scale to the steam mass flowrate of 5 000 kg /h (Point B). o

o

From point B, draw a vertical line to the steam velocity of 25 m /s (Point C). From point C, draw a horizontal line across the pipe diameter scale (Point D). A pipe with a bore of 130 mm is required; the nearest commercially available size, 150 mm, would be selected. 600 500 400 300 200 150 C

30

100

50 0 10 50 1

50

D

Pipe diameter mm

/s m ity c lo ve 5 m ea St 10 20

40 30 20

Steam pressure bar g

10

/h kg te 10 a r w 20 0 flo 3 m a 50 e St 0 10

50% va

0 20 00 3 0 50

00 10 00 2 0 000 B 3 00 50 0 00 10 0 00 0 20 0 00 0 3 00 50 00 00 10 00 00 20

A Saturation temperature curve

cuum

0 bar g 0.5 1 2 3 5 7 10 15 20 30

50 70 100

100

200 300 400 Steam temperature °C

500

Fig. 10.2.6 Steam pipeline sizing chart - Velocity

10.2.10

The Steam and Condensate Loop

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Sizing pipes for superheated steam duty

Superheated steam can be considered as a dry gas and therefore carries no moisture. Consequently there is no chance of pipe erosion due to suspended water droplets, and steam velocities can be as high as 50 to 70 m/s if the pressure drop permits this. The nomograms in Figures 10.2.5 and 10.2.6 can also be used for superheated steam applications. Example 10.2.6 Utilising the waste heat from a process, a boiler /superheater generates 30 t /h of superheated steam at 50 bar g and 450°C for export to a neighbouring power station. If the velocity is not to exceed 50 m /s, determine: 1. The pipe size based on velocity (use Figure 10.2.8). 2. The pressure drop if the pipe length, including allowances, is 200 m (use Figure 10.2.7). Part 1 o Using Figure 10.2.8, draw a vertical line from 450°C on the temperature axis until it intersects the 50 bar line (Point A). o

o

o

From point A, project a horizontal line to the left until it intersects the steam ‘mass flowrate’ scale of 30 000 kg /h (30 t /h) (Point B). From point B, project a line vertically upwards until it intersects 50 m /s on the ‘steam velocity’ scale (Point C). From Point C, project a horizontal line to the right until it intersects the ‘inside pipe diameter’ scale.

The ‘inside pipe diameter’ scale recommends a pipe with an inside diameter of about 120 mm. From Table 10.2.1 and assuming that the pipe will be Schedule 80 pipe, the nearest size would be 150 mm, which has a bore of 146.4 mm. Part 2 o

o

o

o

Using Figure 10.2.7, draw a vertical line from 450°C on the temperature axis until it intersects the 50 bar line (Point A). From point A, project a horizontal line to the right until it intersects the ‘steam mass flowrate’ scale of 30 000 kg /h (30 t /h) (Point B). From point B, project a line vertically upwards until it intersects the ‘inside pipe diameter’ scale of (approximately) 146 mm (Point C). From Point C, project a horizontal line to the left until it intersects the ‘pressure loss bar/100 m’ scale (Point D).

The ‘pressure loss bar /100 m’ scale reads about 0.9 bar /100 m. The pipe length in the example is 200 m, so the pressure drop is:

3UHVVXUHGURS

P [EDU EDU P

This pressure drop must be acceptable at the process plant.

The Steam and Condensate Loop

10.2.11

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Using formulae to establish steam flowrate on pressure drop Empirical formulae exist for those who prefer to use them. Two formulae are shown below that have been tried and tested over many years, and which appear to give results close to the pressure factor method. The advantage of using these formulae is that they can be programmed into a scientific calculator, or a spreadsheet, and consequently used without the need to look up tables and charts. The second formula requires the specific volume of steam to be known, which means it is necessary to look up this value from a steam table. Pressure drop formula 1

( 3 )   ( 3 ) 

Where: P1 = Upstream pressure (bar a) P2 = Downstream pressure (bar a) L = Length of pipe (m) m = Mass flowrate (kg /h) D = Pipe diameter (mm) Pressure drop formula 2

∆3

/



'



/YJ  ò '

Where: DP = Pressure drop (bar) L = Length of pipe (m) vg = Specific volume of steam (m³ /kg) m = Mass flowrate (kg /h) D = Pipe diameter (mm)

Summary o

o

10.2.12

The selection of piping material and the wall thickness required for a particular installation is stipulated in standards such as BS 806 (1993) and ASME 31.1. Selecting the appropriate pipe size (nominal bore) for a particular application is based on accurately identifying pressure and flowrate. The pipe size may be selected on the basis of: - Velocity (usually pipes less than 50 m in length). - Pressure drop (as a general rule, the pressure drop should not normally exceed 0.1 bar /50 m.

The Steam and Condensate Loop

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Appendix Table 10.2.5 Pressure drop factor (F) table Pressure Pressure Pressure bar a factor (F) bar g

Pressure factor (F)

Pressure bar g

Pressure factor (F)

8.748 9.026 9.309 9.597 9.888

7.60 7.70 7.80 7.90 8.00

64.84 66.31 67.79 69.29 70.80

0.05 0.10 0.15 0.20 0.25

0.0301 0.0115 0.0253 0.0442 0.0681

2.05 2.10 2.15 2.20 2.25

0.30 0.35 0.40 0.45 0.50

0.0970 0.1308 0.1694 0.2128 0.2610

2.30 2.35 2.40 2.45 2.50

10.18 10.48 10.79 11.40 11.41

8.10 8.20 8.30 8.40 8.50

72.33 73.88 75.44 77.02 78.61

0.55 0.60 0.65 0.70 0.75

0.3140 0.3716 0.4340 0.5010 0.5727

2.55 2.60 2.65 2.70 2.75

11.72 12.05 12.37 12.70 13.03

8.60 8.70 8.80 8.90 9.00

80.22 81.84 83.49 85.14 86.81

0.80 0.85 0.90 0.95 1.013

0.6489 0.7298 0.8153 0.9053 1.0250

2.80 2.85 2.90 2.95 3.00

13.37 13.71 14.06 14.41 14.76

9.10 9.20 9.30 9.40 9.50

88.50 90.20 91.92 93.66 95.41

Pressure bar g

Pressure factor (F)

0 0.05 0.10 0.15 0.20 0.25

1.025 1.126 1.230 1.339 1.453 1.572

3.10 3.20 3.30 3.40 3.50

15.48 16.22 16.98 17.75 18.54

9.60 9.70 9.80 9.90 10.00

97.18 98.96 100.75 102.57 104.40

3.60 3.70 3.80 3.90 4.00

19.34 20.16 21.00 21.85 22.72

10.20 10.40 10.60 10.80 11.00

108.10 111.87 115.70 119.59 123.54

0.30 0.35 0.40 0.45 0.50

1.694 1.822 1.953 2.090 2.230

4.10 4.20 4.30 4.40 4.50

23.61 24.51 25.43 26.36 27.32

11.20 11.40 11.60 11.80 12.00

127.56 131.64 135.78 139.98 144.25

0.55 0.60 0.65 0.70 0.75

2.375 2.525 2.679 2.837 2.999

4.60 4.70 4.80 4.90 5.00

28.28 29.27 30.27 31.29 32.32

12.20 12.40 12.60 12.80 13.00

148.57 152.96 157.41 161.92 166.50

0.80 0.85 0.90 0.95 1.00

3.166 3.338 3.514 3.694 3.878

5.10 5.20 5.30 5.40 5.50

33.37 34.44 35.52 36.62 37.73

13.20 13.40 13.60 13.80 14.00

171.13 175.83 180.58 185.40 190.29

1.05 1.10 1.15 1.20 1.25

4.067 4.260 4.458 4.660 4.866

5.60 5.70 5.80 5.90 6.00

38.86 40.01 41.17 42.35 43.54

14.20 14.40 14.60 14.80 15.00

195.23 200.23 205.30 210.42 215.61

1.30 1.35 1.40 1.45 1.50

5.076 5.291 5.510 5.734 5.961

6.10 6.20 6.30 6.40 6.50

44.76 45.98 47.23 48.48 49.76

15.20 15.40 15.60 15.80 16.00

220.86 226.17 231.50 236.97 242.46

1.55 1.60 1.65 1.70 1.75

6.193 6.429 6.670 6.915 7.164

6.60 6.70 6.80 6.90 7.00

51.05 52.36 53.68 55.02 56.38

16.20 16.40 16.60 16.80 17.00

248.01 253.62 259.30 265.03 270.83

1.80 1.85 1.90 1.95 2.00

7.417 7.675 7.937 8.203 8.473

7.10 7.20 7.30 7.40 7.50

57.75 59.13 60.54 61.96 63.39

17.20 17.40 17.60 17.80 18.00

276.69 282.60 288.58 294.52 300.72

The Steam and Condensate Loop

10.2.13

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Table 10.2.6 Pipeline capacity and pressure factor table Pipe size (mm) Factor 15 20 25 32 40 50 65 80 F Capacity (kg /h) 0.00016 0.00020 0.00025

10.2.14

100

150

200

250

300

10.84

16.18 17.92

30.40 34.32 38.19

55.41 62.77 69.31

90.72 103.0 113.2

199.1 225.6 249.9

598.2 662.0 735.5

1 275 1 437 1 678

2 329 2 623 2 904

3 800 4 276 4 715

11.95 12.44 14.56

19.31 20.59 23.39

41.83 43.76 50.75

75.85 80.24 92.68

124.1 130.0 150.9

271.2 285.3 333.2

804.5 845.3 979.7

1 733 1 823 2 118

4 172 3 346 3 884

5 149 5 406 6 267

0.00030 0.00035 0.00045

3.62

6.86 7.94

0.00055 0.00065 0.00075

4.04 4.46 4.87

8.99 9.56 10.57

16.18 17.76 19.31

26.52 29.14 31.72

57.09 62.38 68.04

103.8 113.8 124.1

170.8 186.7 203.2

373.1 409.8 445.9

1 101 1 207 1 315

2 382 2 595 2 836

4 338 4 781 5 172

7 057 7 741 8 367

0.00085 0.00100 0.00125

1.96 2.10

5.52 5.84 6.26

11.98 12.75 13.57

21.88 23.50 24.96

35.95 38.25 40.72

77.11 81.89 87.57

140.7 148.6 159.8

230.2 245.2 261.8

505.4 539.4 577.9

1 490 1 579 1 699

3 215 3 383 3 634

5 861 6 228 6 655

9 482 10 052 10 639

0.00150 0.00175 0.0020

2.39 2.48 2.84

7.35 7.51 8.58

15.17 16.30 18.63

28.04 29.61 33.83

45.97 49.34 56.39

98.84 103.4 118.2

179.3 188.8 215.8

295.1 311.1 355.5

652.8 686.5 784.6

1 908 2 017 2 305

4 091 4 291 4 904

7 493 7 852 8 974

11 999 13 087 14 956

0.0025 0.0030 0.0040

3.16 3.44 4.17

9.48 10.34 12.50

20.75 22.5 26.97

37.25 40.45 48.55

61.30 66.66 80.91

132.0 143.4 173.1

240.5 262.0 313.8

391.3 429.8 514.9

881.7 924.4 1 128

2 456 2 767 3 330

5 422 6 068 7 208

10 090 11 033 13 240

16 503 18 021 21 625

0.0050 0.0060 0.0080

4.71 5.25 6.08

14.12 15.69 18.34

30.40 35.80 39.23

54.92 60.31 70.12

90.23 99.05 116.2

196.1 215.8 251.5

354.0 392.3 456.0

578.6 647.3 750.3

1 275 1 412 1 648

3 727 4 148 4 879

8 189 9 072 10 543

14 858 16 476 19 173

24 469 26 970 31 384

0.0100 0.0125 0.0150

6.86 7.35 8.27

20.64 22.20 25.00

44.13 47.28 53.33

79.44 81.00 95.62

130.4 140.1 157.2

283.9 302.1 342.0

514.9 547.3 620.6

845.9 901.9 1 020

1 863 1 983 2 230

5 492 5 867 6 620

11 867 12 697 14 251

21 576 23 074 25 974

35 307 37 785 42 616

0.0175 0.0200 0.0250

8.58 9.80 10.99

26.39 30.16 33.48

55.78 63.75 70.73

100.4 114.7 127.3

165.6 189.3 209.8

360.4 411.9 459.7

665.1 760.1 834.6

1 073 1 226 1 367

2 360 2 697 2 970

6 994 7 993 8 817

15 017 17 163 19 332

27 461 31 384 34 750

44 194 50 508 56 581

0.0300 0.0400 0.0500

12.00 14.46 16.43

36.78 44.16 49.53

77.23 93.17 104.4

137.9 169.2 191.2

229.9 279.5 313.8

501.1 600.7 676.7

919.4 1 093 1 231

1 480 1 790 2 020

3 264 3 923 4 413

9 792 11 622 13 044

20 917 25 254 28 441

37 697 45 604 51 489

62 522 75 026 85 324

0.060 0.080 0.100

18.14 21.08 24.03

52.96 62.28 70.12

115.7 134.8 152.0

210.8 245.2 277.0

343.2 402.1 456.0

750.3 872.8 980.7

1 373 1 594 1 804

2 231 2 599 2 942

4 855 5 688 6 424

14 368 16 672 18 879

31 384 36 532

57 373

0.120 0.150 0.200

25.99 28.50 34.32

77.48 84.13 102.0

167.7 183.9 220.7

306.5 334.2 402.1

500.2 551.7 622.0

1 079 1 195 1 427

1 986 2 161 2 599

3 236 3 494 4 217

7 110 7 769 9 317

20 841

0.250 0.300 0.350

37.72 41.37 43.34

112.7 122.7 128.7

245.2 266.6 283.2

447.9 487.3 514.9

735.5 804.5 841.0

1 565 1 710 1 802

2 876 3 126 3 2.61

4 668 5 057

0.400 0.450 0.500

49.93 50.31 55.90

147.1 150.0 166.7

323.6 326.6 362.9

588.4 600.2 666.9

961.1 979.9 1 089

2 059 2 083 23 214

3 727

0.600 0.700 0.800

62.28 63.07 72.08

185.3 188.8 215.8

402.1 407.6 465.8

735.5 750.9 858.1

1 201

0.900

73.28

218.4

476.6

The Steam and Condensate Loop

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Table 10.2.7 Steam pipeline sizing chart - Pressure drop 20 10

0.3 0.2 0.1 0.05

400 500 Insi de p ipe 600 diam eter mm

0.5

200 250 300

100 125 15 0

1

40 50 60 70 80

20 25 30

Pressure loss bar/100 m

2

15

3

10

5

0.03 0.02 0.01 Steam pressure bar g

100 0 2 00 3 00 0 0 5 00 0 10 0 00

5 7 10 15 20 30

20 30 0000 00 50 0 00 100 000 Ste 2 am 00 00 flow 0 rate kg / h

Saturation temperature curve

g

10

0 bar 0.5 1 2 3

100 200 300 5 00

cuum

20 30 50

50% va

50 70

100

100

200 300 400 Steam temperature °C

The Steam and Condensate Loop

500

10.2.15

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Table 10.2.8 Steam pipeline sizing chart - Velocity 600 500 400 300 200

/s

10

20

100 30

50 0 10 50 1

50

Pipe diameter mm

m ea St

m ity oc l ve 5

40 30 20

Steam pressure bar g

10 /h kg e t ra w 0 flo 1 m 20 0 ea 3 St 50

50% va

0 10

0 bar g 0.5 1 0 20 00 3 0 50

2 3

00 10 00 2 0 000 3 00 50

0 00 10 0 00 0 20 00 0 30 00 50 00 00 0 1 00 00 0 2

Saturation temperature curve

5 7 10 15 20 30 50 70 100

100

10.2.16

cuum

200 300 400 Steam temperature °C

500

The Steam and Condensate Loop

Block 10 Steam Distribution

Pipes and Pipe Sizing Module 10.2

Questions 1. A boiler is operated at 10 bar g and is required to supply 500 kg /h of saturated steam at 9.8 bar g to equipment 110 m away. The pipe run is torturous and contains many fittings adding 20% to the equivalent length. What size pipe should be selected?

¨ ¨ ¨ ¨

a | 100 mm nominal bore b | 80 mm nominal bore c | 50 mm nominal bore d | 65 mm nominal bore

2. A 100 mm steam pipe has been selected for a particular steam flowrate with 8.3 bar g at the inlet and 7.7 bar g at the end of the run. Calculations show that, for this flowrate and size of pipe, the pressure at the end of the run will actually be 7.9 bar g. Which of the following is true? a | The steam velocity is higher than expected, and could cause noise b | The pipe has some additional spare capacity for future additional loads c | The resistance to flow is higher than expected d | A larger pipe is required

¨ ¨ ¨ ¨

3. A 40 m long 5 bar g saturated steam pipe is to be sized to carry 850 kg /h of steam. Should the pipe be sized on velocity or pressure drop? a | Pressure drop to limit the steam velocity b | On a velocity over 40 m/s c | On a velocity of about 25 m/s d | Either, provided the steam velocity does not exceed, approximately 5 m /s

¨ ¨ ¨ ¨

4. A 40 m pipe incorporating a number of bends and fittings is to be sized by the velocity method to handle 1 200 kg /h of saturated steam at 4 bar g. What size pipe is required? a | 100 mm b | 80 mm c | 125 mm d | The pipe should be sized on pressure drop, and not by velocity

¨ ¨ ¨ ¨

5. A straight run of pipe 30 m long and carrying saturated steam at 10 bar g is to be sized by the velocity method to pass 20 000 kg /h. What size pipe is required?

¨ ¨ ¨ ¨

a | 175 mm b | 150 mm c | 200 mm d | 250 mm

6. From the following, what is the effect of sizing a 100 m long, 8 bar g steam pipe by the velocity method? a | Sizing by velocity takes no account of pressure drop along the pipe

¨

b | If the velocity is more than 40 m /s, the pressure drop along the pipe may be very small and in practice a small pipe may be used

¨

c | If a low velocity is selected, the chosen pipe will probably be undersized resulting in steam starvation at the plant d | Over a length of 100 m, the noise of steam flow can be unacceptable

¨ ¨

Answers

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

10.2.17

Block 10 Steam Distribution

10.2.18

Pipes and Pipe Sizing Module 10.2

The Steam and Condensate Loop

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

Module 10.3 Steam Mains and Drainage

The Steam and Condensate Loop

10.3.1

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

Steam Mains and Drainage Throughout the length of a hot steam main, an amount of heat will be transferred to the environment, and this will depend on the parameters identified in Block 2 - ‘Steam Engineering and Heat Transfer’, and brought together in Equation 2.5.1.

 = N$

∆7 ì

Equation 2.5.1

Where: Q = Heat transferred per unit time (W) k = Thermal conductivity of the material (W /m K or W /m °C) A = Heat transfer area (m²) DT = Temperature difference across the material (K or °C) ƒ = Material thickness (m) With steam systems, this loss of energy represents inefficiency, and thus pipes are insulated to limit these losses. Whatever the quality or thickness of insulation, there will always be a level of heat loss, and this will cause steam to condense along the length of the main. The effect of insulation is discussed in Module 10.5. This Module will concentrate on disposal of the inevitable condensate, which, unless removed, will accumulate and lead to problems such as corrosion, erosion, and waterhammer. In addition, the steam will become wet as it picks up water droplets, which reduces its heat transfer potential. If water is allowed to accumulate, the overall effective cross sectional area of the pipe is reduced, and steam velocity can increase above the recommended limits.

Piping layout The subject of drainage from steam lines is covered in the UK British Standard BS 806:1993, Section 4.12. BS 806 states that, whenever possible, the main should be installed with a fall of not less than 1:100 (1 m fall for every 100 m run), in the direction of the steam flow. This slope will ensure that gravity, as well as the flow of steam, will assist in moving the condensate towards drain points where the condensate may be safely and effectively removed (See Figure 10.3.1). 30 - 50 metre intervals Gradient 1:100

Steam

Gradient 1:100

Trap set

Trap set

Steam Trap set

Condensate Condensate

Condensate

Fig. 10.3.1 Typical steam main installation

Drain points

The drain point must ensure that the condensate can reach the steam trap. Careful consideration must therefore be given to the design and location of drain points. Consideration must also be given to condensate remaining in a steam main at shutdown, when steam flow ceases. Gravity will ensure that the water (condensate) will run along sloping pipework and collect at low points in the system. Steam traps should therefore be fitted to these low points. 10.3.2

The Steam and Condensate Loop

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

The amount of condensate formed in a large steam main under start-up conditions is sufficient to require the provision of drain points at intervals of 30 m to 50 m, as well as natural low points such as at the bottom of rising pipework. In normal operation, steam may flow along the main at speeds of up to 145 km/h, dragging condensate along with it. Figure 10.3.2 shows a 15 mm drain pipe connected directly to the bottom of a main. Steam

Flow

Condensate Steam trap set Fig. 10.3.2 Trap pocket too small

Although the 15 mm pipe has sufficient capacity, it is unlikely to capture much of the condensate moving along the main at high speed. This arrangement will be ineffective. A more reliable solution for the removal of condensate is shown in Figure 10.3.3. The trap line should be at least 25 to 30 mm from the bottom of the pocket for steam mains up to 100 mm, and at least 50 mm for larger mains. This allows a space below for any dirt and scale to settle. Steam

Flow

Condensate

Pocket Steam trap set Fig. 10.3.3 Trap pocket properly sized

The bottom of the pocket may be fitted with a removable flange or blowdown valve for cleaning purposes. Recommended drain pocket dimensions are shown in Table 10.3.1 and in Figure 10.3.4. Table 10.3.1 Recomended drain pocket dimensions Mains diameter - D Pocket diameter - d1 Up to 100 mm nb d1 = D 125 - 200 mm nb d1 = 100 mm 250 mm and above d1 ³ D / 2

Steam

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

Steam main

D d2

d1

Float trap with in-built sensor Fig. 10.3.4

The Steam and Condensate Loop

Condensate return

10.3.3

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

Waterhammer and its effects Waterhammer is the noise caused by slugs of condensate colliding at high velocity into pipework fittings, plant, and equipment. This has a number of implications: o

o

o

Because the condensate velocity is higher than normal, the dissipation of kinetic energy is higher than would normally be expected. Water is dense and incompressible, so the ‘cushioning’ effect experienced when gases encounter obstructions is absent. The energy in the water is dissipated against the obstructions in the piping system such as valves and fittings. Steam Condensate Steam Slug Steam Fig. 10.3.5 Formation of a ‘solid’ slug of water

Indications of waterhammer include a banging noise, and perhaps movement of the pipe. In severe cases, waterhammer may fracture pipeline equipment with almost explosive effect, with consequent loss of live steam at the fracture, leading to an extremely hazardous situation. Good engineering design, installation and maintenance will avoid waterhammer; this is far better practice than attempting to contain it by choice of materials and pressure ratings of equipment. Commonly, sources of waterhammer occur at the low points in the pipework (See Figure 10.3.6). Such areas are due to: o o

Sagging in the line, perhaps due to failure of supports. Incorrect use of concentric reducers (see Figure 10.3.7) - Always use eccentric reducers with the flat at the bottom.

o

Incorrect strainer installation - They should be fitted with the basket on the side.

o

Inadequate drainage of steam lines.

o

Incorrect operation - Opening valves too quickly at start-up when pipes are cold. Steam Concentric reducer

Riser

Condensate Steam

Condensate

Steam Condensate

Strainer with hanging basket Fig. 10.3.6 Potential sources of waterhammer

10.3.4

The Steam and Condensate Loop

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

Eccentric reducer Correct Steam

Condensate Incorrect Steam

Concentric reducer

Condensate

Fig. 10.3.7 Eccentric and concentric pipe reducers

To summarise, the possibility of waterhammer is minimised by: o

o

o

Installing steam lines with a gradual fall in the direction of flow, and with drain points installed at regular intervals and at low points. Installing check valves after all steam traps which would otherwise allow condensate to run back into the steam line or plant during shutdown. Opening isolation valves slowly to allow any condensate which may be lying in the system to flow gently through the drain traps, before it is picked up by high velocity steam. This is especially important at start-up.

Branch lines

Steam

Steam main

Steam

Branch line

Steam Fig. 10.3.8 Branch line

Branch lines are normally much shorter than steam mains. As a general rule, therefore, provided the branch line is not more than 10 metres in length, and the pressure in the main is adequate, it is possible to size the pipe on a velocity of 25 to 40 m/s, and not to worry about the pressure drop. Table 10.2.4 ‘Saturated steam pipeline capacities for different velocities’ in Module 10.2 will prove useful in this exercise.

The Steam and Condensate Loop

10.3.5

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

Branch line connections

Branch line connections taken from the top of the main carry the driest steam (Figure 10.3.8). If connections are taken from the side, or even worse from the bottom (as in Figure 10.3.9 (a)), they can accept the condensate and debris from the steam main. The result is very wet and dirty steam reaching the equipment, which will affect performance in both the short and long term. The valve in Figure 10.3.9 (b) should be positioned as near to the off-take as possible to minimise condensate lying in the branch line, if the plant is likely to be shutdown for any extended periods.

(a) Incorrect

(b) Correct

Fig. 10.3.9 Steam off-take

Drop leg

Low points will also occur in branch lines. The most common is a drop leg close to an isolating valve or a control valve (Figure 10.3.10). Condensate can accumulate on the upstream side of the closed valve, and then be propelled forward with the steam when the valve opens again consequently a drain point with a steam trap set is good practice just prior to the strainer and control valve. Steam Drop leg

Isolation valve

Control valve

Strainer

Unit heater Isolation valve Isolation valve

Trap set

Trap set Condensate

Condensate Fig. 10.3.10 Diagram of a drop leg supplying a unit heater

10.3.6

The Steam and Condensate Loop

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

Rising ground and drainage There are many occasions when a steam main must run across rising ground, or applications where the contours of the site make it impractical to lay the pipe with the 1:100 fall proposed earlier. In these situations, the condensate must be encouraged to run downhill and against the steam flow. Good practice is to size the pipe on a low steam velocity of not more than 15 m /s, to run the line at a slope of no less than 1:40, and install the drain points at not more than 15 metre intervals (see Figure 10.3.11). The objective is to prevent the condensate film on the bottom of the pipe increasing in thickness to the point where droplets can be picked up by the steam flow.

Steam velocity 30 m/s

Steam velocity 15 m/s

1:100 Fall

30 - 50 m

Increase in pipe diameter Fall 1:40 Fall 30 m/s 15 m

15 m

Fig. 10.3.11 Reverse gradient on steam main

Steam separators Modern packaged steam boilers have a large evaporating capacity for their size and have limited capacity to cope with rapidly changing loads. In addition, as discussed in Block 3 ‘The Boiler House’, other circumstances, such as . . . o

Incorrect chemical feedwater treatment and /or TDS control

o

Transient peak loads in other parts of the plant

. . . can cause priming and carryover of boiler water into the steam mains. Separators, as shown by the cut section in Figure 10.3.12, may be installed to remove this water. Air and incondensable gases vented

Dry steam out

Wet steam in

Moisture to trap set Fig. 10.3.12 Cut section through a separator The Steam and Condensate Loop

10.3.7

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

As a general rule, providing the velocities in the pipework are within reasonable limits, separators will be line sized. (Separators are discussed in detail in Module 12.5) A separator will remove both droplets of water from pipe walls and suspended mist entrained in the steam itself. The presence and effect of waterhammer can be eradicated by fitting a separator in a steam main, and can often be less expensive than increasing the pipe size and fabricating drain pockets. A separator is recommended before control valves and flowmeters. It is also wise to fit a separator where a steam main enters a building from outside. This will ensure that any condensate produced in the external distribution system is removed and the building always receives dry steam. This is equally important where steam usage in the building is monitored and charged for.

Strainers When new pipework is installed, it is not uncommon for fragments of casting sand, packing, jointing, swarf, welding rods and even nuts and bolts to be accidentally deposited inside the pipe. In the case of older pipework, there will be rust, and in hard water districts, a carbonate deposit. Occasionally, pieces will break loose and pass along the pipework with the steam to rest inside a piece of steam using equipment. This may, for example, prevent a valve from opening / closing correctly. Steam using equipment may also suffer permanent damage through wiredrawing - the cutting action of high velocity steam and water passing through a partly open valve. Once wiredrawing has occurred, the valve will never give a tight shut-off, even if the dirt is removed. It is therefore wise to fit a line-size strainer in front of every steam trap, flowmeter, reducing valve and regulating valve. The illustration shown in Figure 10.3.13 shows a cut section through a typical strainer.

A

C B

D

Fig. 10.3.13 Cut section through a Y-type strainer.

Steam flows from the inlet ‘A’ through the perforated screen ‘B’ to the outlet ‘C’. While steam and water will pass readily through the screen, dirt cannot. The cap ‘D’, can be removed, allowing the screen to be withdrawn and cleaned at regular intervals. A blowdown valve can also be fitted to cap ‘D’ to facilitate regular cleaning. Strainers can however, be a source of wet steam as previously mentioned. To avoid this situation, strainers should always be installed in steam lines with their baskets to the side. Strainers and screen details are discussed in Module 12.4. 10.3.8

The Steam and Condensate Loop

Block 10 Steam Distribution

Steam Mains and Drainage Module 10.3

How to drain steam mains Steam traps are the most effective and efficient method of draining condensate from a steam distribution system. The steam traps selected must suit the system in terms of: o

Pressure rating

o

Capacity

o

Suitability

Pressure rating Pressure rating is easily dealt with; the maximum possible working pressure at the steam trap will either be known or should be established. Capacity Capacity, that is, the quantity of condensate to be discharged, which needs to be divided into two categories; warm-up load and running load. Warm-up load - In the first instance, the pipework needs to be brought up to operating temperature. This can be determined by calculation, knowing the mass and specific heat of the pipework and fittings. Alternatively, Table 10.3.2 may be used. o

o

o

The table shows the amount of condensate generated when bringing 50 m of steam main up to working temperature; 50 m being the maximum recommended distance between trapping points. The values shown are in kilograms. To determine the average condensing rate, the time taken for the process must be considered. For example, if the warm-up process required 50 kg of steam, and was to take 20 minutes, then the average condensing rate would be: PLQXWHV [NJ $YHUDJHFRQGHQVLQJUDWH PLQXWHV $YHUDJHFRQGHQVLQJUDWH NJ K When using these capacities to size a steam trap, it is worth remembering that the initial pressure in the main will be little more than atmospheric when the warm-up process begins. However, the condensate loads will still generally be well within the capacity of a DN15 ‘low capacity’ steam trap. Only in rare applications at very high pressures (above 70 bar g), combined with large pipe sizes, will greater trap capacity be needed.

Running load - Once the steam main is up to operating temperature, the rate of condensation is mainly a function of the pipe size and the quality and thickness of the insulation. For accurate means of calculating running losses from steam mains, refer to Module 2.12 ‘Steam consumption of pipes and air heaters’. Alternatively, for quick approximations of running load, Table 10.3.3 can be used which shows typical amounts of steam condensed each hour per 50 m of insulated steam main at various pressures.

The Steam and Condensate Loop

10.3.9

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

Table 10.3.2 Amount of steam condensed to warm-up 50 m of schedule 40 pipe (kg) Note: Figures are based on an ambient temperature of 20°C, and an insulation efficiency of 80% Steam -18°C Steam main size (mm) pressure correction bar g 50 65 80 100 125 150 200 250 300 350 400 450 500 600 factor 1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 25 30 40 50 60 70 80 90 100 120

5 6 7 8 8 9 9 9 10 10 10 11 12 17 17 19 21 22 24 27 29 32 34 35 42

9 10 11 12 13 13 14 14 15 16 17 17 19 23 26 29 32 34 37 41 44 49 51 54 64

11 13 14 16 17 18 18 19 20 20 22 23 24 31 35 39 41 46 50 54 59 65 69 72 86

16 19 20 22 24 25 26 27 28 29 31 32 35 45 51 56 62 67 73 79 86 95 100 106 126

22 25 25 30 33 34 35 37 38 40 42 44 47 62 71 78 86 93 101 135 156 172 181 190 227

28 33 36 39 42 43 45 47 50 51 54 57 61 84 97 108 117 127 139 181 208 232 245 257 305

44 49 54 59 63 66 68 71 74 77 84 85 91 127 148 164 179 194 212 305 346 386 409 427 508

60 69 79 83 70 93 97 101 105 109 115 120 128 187 220 243 265 287 214 445 510 568 598 628 748

79 92 101 110 119 124 128 134 139 144 152 160 172 355 302 333 364 395 432 626 717 800 842 884 1 052

94 108 120 131 142 147 151 158 164 171 180 189 203 305 362 400 437 473 518 752 861 960 1011 1062 1265

123 142 156 170 185 198 197 207 216 224 236 247 265 393 465 533 571 608 665 960 1 100 1 220 1 288 1 355 1 610

155 179 197 215 233 242 250 261 272 282 298 311 334 492 582 642 702 762 834 1 218 1 396 1 550 1 635 1 720 2 050

182 210 232 254 275 285 294 307 320 332 350 366 393 596 712 786 859 834 1 020 1 480 1 694 1 890 1 990 2 690 2 490

254 296 324 353 382 396 410 428 436 463 488 510 548 708 806 978 1 150 1 322 1 450 2 140 2 455 2 730 2 880 3 030 3 600

1.39 1.35 1.32 1.29 1.28 1.27 1.26 1.25 1.24 1.24 1.23 1.22 1.21 1.21 1.20 1.19 1.18 1.16 1.15 1.15 1.15 1.14 1.14 1.14 1.13

Table 10.3.3 Condensing rate of steam in 50 m of schedule 40 pipe - at working temperature (kg / h) Note: Figures are based on an ambient temperature of 20°C, and an insulation efficiency of 80% Steam -18°C Steam main size (mm) pressure correction bar g 50 65 80 100 125 150 200 250 300 350 400 450 500 600 factor 1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 25 30 40 50 60 70 80 90 100 120

10.3.10

5 5 6 7 7 8 8 9 9 10 11 12 12 14 15 15 17 20 24 27 29 34 38 41 52

5 6 7 9 9 10 10 11 11 12 13 14 15 16 17 19 21 25 29 32 35 42 46 50 63

7 8 9 10 11 11 12 14 14 15 16 17 18 19 21 23 25 30 34 39 43 51 56 61 77

9 10 11 12 13 14 15 16 17 17 18 20 23 24 25 28 31 38 44 50 56 66 72 78 99

10 12 14 16 17 18 19 20 21 21 23 26 29 30 31 35 39 46 54 62 70 81 89 96 122

13 14 16 18 20 21 23 24 25 25 26 30 34 36 37 42 47 56 65 74 82 97 106 114 145

16 18 20 23 24 26 28 30 32 33 36 39 42 44 46 52 51 70 82 95 106 126 134 149 189

19 22 25 28 30 33 35 37 39 41 45 49 52 55 58 66 73 87 102 119 133 156 171 186 236

23 26 30 33 36 39 42 44 47 49 53 58 62 66 69 78 87 104 121 140 157 187 204 220 280

25 28 32 37 40 43 46 49 52 54 59 64 68 72 76 86 96 114 133 155 173 205 224 242 308

28 32 37 42 46 49 52 57 60 62 67 73 78 82 86 97 108 130 151 177 198 234 265 277 352

31 35 40 46 49 53 56 61 64 67 73 79 85 90 94 106 118 142 165 199 222 263 287 311 395

35 39 45 51 55 59 63 68 72 75 81 93 95 100 105 119 132 158 184 222 248 293 320 347 440

41 46 54 61 66 71 76 82 88 90 97 106 114 120 125 141 157 189 220 265 296 350 284 416 527

1.54 1.50 1.48 1.45 1.43 1.42 1.41 1.40 1.39 1.38 1.38 1.37 1.36 1.36 1.35 1.34 1.33 1.31 1.29 1.28 1.27 1.26 1.26 1.25 1.22

The Steam and Condensate Loop

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

Suitability A mains drain trap should consider the following constraints: o

o

o

Discharge temperature - The steam trap should discharge at, or very close to saturation temperature, unless cooling legs are used between the drain point and the trap. This means that the choice is a mechanical type trap (such as a float, inverted bucket type, or thermodynamic traps). Frost damage - Where the steam main is located outside a building and there is a possibility of sub-zero ambient temperature, the thermodynamic steam trap is ideal, as it not damaged by frost. Even if the installation causes water to be left in the trap at shutdown and freezing occurs, the thermodynamic trap may be thawed out without suffering damage when brought back into use. Waterhammer - In the past, on poorly laid out installations where waterhammer was a common occurrence, float traps were not always ideal due to their susceptibility to float damage. Contemporary design and manufacturing techniques now produce extremely robust units for mains drainage purposes. Float traps are certainly the first choice for proprietary separators as high capacities are readily achieved, and they are able to respond quickly to rapid load increases.

Steam traps used to drain condensate from steam mains, are shown in Figure 10.3.14. The thermostatic trap is included because it is ideal where there is no choice but to discharge condensate into a flooded return pipe. The subject of steam trapping is dealt with in detail in the Block 11, ‘Steam Trapping’.

Ball float type

Thermodynamic type Thermostatic type Fig. 10.3.14 Steam traps suitable for steam mains drainage

Inverted bucket type

Steam leaks Steam leaking from pipework is often ignored. Leaks can be costly in both the economic and environmental sense and therefore need prompt attention to ensure the steam system is working at its optimum efficiency with a minimum impact on the environment. Figure 10.3.15 illustrates the steam loss for various sizes of hole at various pressures. This loss can be readily translated into a fuel saving based on the annual hours of operation. Hole size 12.5 mm

Steam leak rate kg/h

500 400

10 mm

300 200

7.5 mm

100

5 mm 3 mm

0 1

The Steam and Condensate Loop

2

3 4 5 Steam pressure bar g Fig. 10.3.15 Steam leakage rate through holes

10

10.3.11

Block 10 Steam Distribution

Steam Mains and Drainage Module 10.3

Summary Proper pipe alignment and drainage means observing a few simple rules: o

o

o

o o

o

o

10.3.12

Steam lines should be arranged to fall in the direction of flow, at not less than 100 mm per 10 metres of pipe (1:100). Steam lines rising in the direction of flow should slope at not less than 25 mm per 10 metres of pipe (1:40). Steam lines should be drained at regular intervals of 30 - 50 m and at any low points in the system. Where drainage has to be provided in straight lengths of pipe, then a large bore pocket should be used to collect condensate. If strainers are to be fitted, then they should be fitted on their sides. Branch connections should always be taken from the top of the main from where the driest steam is taken. Separators should be considered before any piece of steam using equipment ensuring that dry steam is used. Traps selected should be robust enough to avoid waterhammer damage and frost damage.

The Steam and Condensate Loop

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

Questions 1. Which of the following is true of wet steam? a| It can cause waterhammer if allowed to build up

¨

b| It can corrode pipes if allowed to continue

¨

c| It causes erosion of bends

¨

d| All of the above

¨

2. What is the effect of installing a steam main horizontally level? a| None, provided the pipe is drained at 30 - 50 m intervals

¨

b| Complete drainage will be less effective, and waterhammer could result

¨

c| Larger diameter drain points should be fitted

¨

d| Condensate will not reach the drain points

¨

3. Steam pipeline strainers should be fitted with their baskets on the side to: a| Prevent condensate filling the body and being carried over to the equipment being protected

¨

b| Provide a greater screening area

¨

c| Extend the periods between cleaning the strainer

¨

d| Provide more effective removal of the debris

¨

4. Using the velocity method, what size pipe is required to carry 500 kg /h of steam at 6 bar g over a 40 m run with a rising slope? (The specific volume of steam at 6 bar g is 0.272 m³ /kg a| 40 mm

¨

b| 80 mm

¨

c| 50 mm

¨

d| 65 mm

¨

The Steam and Condensate Loop

10.3.13

Steam Mains and Drainage Module 10.3

Block 10 Steam Distribution

5. A correctly sized pilot operated reducing valve has been installed in a pressure reducing station supplying an autoclave, as shown in Figure 10.3.16. What is wrong with the installation? DN20 pressure reducing valve

DN25 stop valve Steam at 7 bar g DN25 separator

DN25 strainer Steam trap set

Safety valve

280 kg /h of steam at 5 bar g

DN32 stop valve

Condensate

Fig. 10.3.16

a| The pipe after the PRV is at a lower pressure, and steam has a higher volume, so the pipe should be larger than 32 mm

¨

b| The upstream strainer and isolation valve should be the same size as the reducing valve

¨

c| The separator should be one size larger than the pipework to avoid excessive pressure drop

¨

d| There is no downstream pressure gauge before the DN32 stop valve

¨

6. As a minimum, horizontal runs of 150 mm steam main should be drained at intervals of: a| Every 15 metres via 100 mm bore drain pockets, 100 mm deep

¨

b| Every 30 - 50 metres via 150 mm bore drain pockets, 100 mm deep

¨

c| Every 15 metres via 100 mm bore drain pockets, 150 mm deep

¨

d| Every 30 - 50 metres via 100 mm bore drain pockets, 150 mm deep

¨

Answers

1: d, 2: b, 3: a, 4: d, 5: d, 6: d

10.3.14

The Steam and Condensate Loop

Pipe Expansion and Support Module 10.4

Block 10 Steam Distribution

Module 10.4 Pipe Expansion and Support

The Steam and Condensate Loop

10.4.1

Pipe Expansion and Support Module 10.4

Block 10 Steam Distribution

Pipe Expansion and Support Allowance for expansion All pipes will be installed at ambient temperature. Pipes carrying hot fluids such as water or steam operate at higher temperatures. It follows that they expand, especially in length, with an increase from ambient to working temperatures. This will create stress upon certain areas within the distribution system, such as pipe joints, which, in the extreme, could fracture. The amount of the expansion is readily calculated using Equation 10.4.1, or read from an appropriate chart such as Figure 10.4.1. Expansion ( mm ) = L ∆T α

Equation 10.4.1

Where: L = Length of pipe between anchors (m) ∆T = Temperature difference between ambient temperature and operating temperatures (°C) α = Expansion coefficient (mm /m °C) x 10-3 α) (mm /m °C x 10-3) Table 10.4.1 Expansion coefficients (α Material Carbon steel 0.1% - 0.2% C Alloy steel 1% Cr 0.5% Mo Stainless steel 18% Cr 8% Ni

<0 12.8 13.7 9.4

0 - 100 13.9 14.5 20.0

Temperature range (°C) 0 - 200 0 - 300 0 - 400 0 - 500 14.9 15.8 16.6 17.3 15.2 15.8 16.4 17.0 20.9 21.2 21.8 22.3

0 - 600 17.9 17.6 22.7

0 - 700 23.0

Example 10.4.1 A 30 m length of carbon steel pipe is to be used to transport steam at 4 bar g (152°C). If the pipe is installed at 10°C, determine the expansion using Equation 10.4.1. Expansion ( mm ) = L ∆T α Where:

L = 30 m ∆T = 152°C - 10 °C ∆T = 142°C α in the range 0 - 200 = 14.9 x 10-3 mm m °C for carbon steel pipe Expansion = 30 m x 142°C x 14.9 x 10 -3 mm m °C Expansion = 63.5 mm

Alternatively, the chart in Figure 10.4.1 can be used for finding the approximate expansion of a variety of steel pipe lengths - see Example 10.4.2 for explanation of use. Example 10.4.2 Using Figure 10.4.1. Find the approximate expansion from 15°C, of 100 metres of carbon steel pipework used to distribute steam at 265°C. Temperature difference is 265 - 15°C = 250°C. Where the diagonal temperature difference line of 250°C cuts the horizontal pipe length line at 100 m, drop a vertical line down. For this example an approximate expansion of 330 mm is indicated.

10.4.2

The Steam and Condensate Loop

Pipe Expansion and Support Module 10.4

Block 10 Steam Distribution

50

220 200

Length of pipe (m)

100

Temperature difference °C 100 200 300 400 500

Example 10.4.2

50 40 30 20 10 0 10

20

30 40 50

100 200 300 500 1 000 2 000 Expansion of pipe (mm) Fig. 10.4.1 A chart showing the expansion in various steel pipe lengths at various temperature differences Table 10.4.2 Temperature of saturated steam bar g 1 2 3 4 °C 120 134 144 152

5 159

7.5 173

10 184

15 201

20 215

25 226

30 236

Pipework flexibility The pipework system must be sufficiently flexible to accommodate the movements of the components as they expand. In many cases the flexibility of the pipework system, due to the length of the pipe and number of bends and supports, means that no undue stresses are imposed. In other installations, however, it will be necessary to incorporate some means of achieving this required flexibility. An example on a typical steam system is the discharge of condensate from a steam mains drain trap into the condensate return line that runs along the steam line (Figure 10.4.2). Here, the difference between the expansions of the two pipework systems must be taken into account. The steam main will be operating at a higher temperature than that of the condensate main, and the two connection points will move relative to each other during system warm-up. Steam

Steam main

Steam

Trap set

Condensate Condensate Fig. 10.4.2 Flexibility in connection to condensate return line

The amount of movement to be taken up by the piping and any device incorporated in it can be reduced by ‘cold draw’. The total amount of expansion is first calculated for each section between fixed anchor points. The pipes are left short by half of this amount, and stretched cold by pulling up bolts at a flanged joint, so that at ambient temperature, the system is stressed in one direction. When warmed through half of the total temperature rise, the piping is unstressed. At working temperature and having fully expanded, the piping is stressed in the opposite direction. The effect is that instead of being stressed from 0 F to +1 F units of force, the piping is stressed from -½ F to + ½ F units of force. The Steam and Condensate Loop

10.4.3

Pipe Expansion and Support Module 10.4

Block 10 Steam Distribution

In practical terms, the pipework is assembled cold with a spacer piece, of length equal to half the expansion, between two flanges. When the pipework is fully installed and anchored at both ends, the spacer is removed and the joint pulled up tight (see Figure 10.4.3). L

Position after cold draw Neutral position

Spacer piece

Half calculated expansion over length L

Hot position Fig. 10.4.3 Use of spacer for expansion when pipework is installed

The remaining part of the expansion, if not accepted by the natural flexibility of the pipework will call for the use of an expansion fitting. In practice, pipework expansion and support can be classified into three areas as shown in Figure 10.4.5.

Anchor point A

Sliding support point B

Expansion fitting point C

Sliding support point B

Anchor point A

Fig. 10.4.4 Diagram of pipeline with fixed point, variable anchor point and expansion fitting

The fixed or ‘anchor’ points ‘A’ provide a datum position from which expansion takes place. The sliding support points ‘B’ allow free movement for expansion of the pipework, while keeping the pipeline in alignment. The expansion device at point ‘C’ is to accommodate the expansion and contraction of the pipe.

Fig. 10.4.5 Chair and roller

Fig. 10.4.6 Chair roller and saddle

Roller supports (Figure 10.4.5 and 10.4.6) are ideal methods for supporting pipes, at the same time allowing them to move in two directions. For steel pipework, the rollers should be manufactured from ferrous material. For copper pipework, they should be manufactured from non-ferrous material. It is good practice for pipework supported on rollers to be fitted with a pipe saddle bolted to a support bracket at not more than distances of 6 metres to keep the pipework in alignment during any expansion and contraction. 10.4.4

The Steam and Condensate Loop

Pipe Expansion and Support Module 10.4

Block 10 Steam Distribution

Where two pipes are to be supported one below the other, it is poor practice to carry the bottom pipe from the top pipe using a pipe clip. This will cause extra stress to be added to the top pipe whose thickness has been sized to take only the stress of its working pressure. All pipe supports should be specifically designed to suit the outside diameter of the pipe concerned.

Expansion fittings The expansion fitting (‘C’ Figure 10.4.4) is one method of accommodating expansion. These fittings are placed within a line, and are designed to accommodate the expansion, without the total length of the line changing. They are commonly called expansion bellows, due to the bellows construction of the expansion sleeve. Other expansion fittings can be made from the pipework itself. This can be a cheaper way to solve the problem, but more space is needed to accommodate the pipe. Full loop This is simply one complete turn of the pipe and, on steam pipework, should preferably be fitted in a horizontal rather than a vertical position to prevent condensate accumulating on the upstream side. The downstream side passes below the upstream side and great care must be taken that it is not fitted the wrong way round, as condensate can accumulate in the bottom. When full loops are to be fitted in a confined space, care must be taken to specify that wrong-handed loops are not supplied. The full loop does not produce a force in opposition to the expanding pipework as in some other types, but with steam pressure inside the loop, there is a slight tendency to unwind, which puts an additional stress on the flanges.

Flow

Flow

Fig. 10.4.7 Full loop

This design is used rarely today due to the space taken up by the pipework, and proprietary expansion bellows are now more readily available. However large steam users such as power stations or establishments with large outside distribution systems still tend to use full loop type expansion devices, as space is usually available and the cost is relatively low. Horseshoe or lyre loop When space is available this type is sometimes used. It is best fitted horizontally so that the loop and the main are on the same plane. Pressure does not tend to blow the ends of the loop apart, but there is a very slight straightening out effect. This is due to the design but causes no misalignment of the flanges. If any of these arrangements are fitted with the loop vertically above the pipe then a drain point must be provided on the upstream side as depicted in Figure 10.4.8.

Side elevation

Flow

Flow

Trap set Fig. 10.4.8 Horseshoe or lyre loop The Steam and Condensate Loop

10.4.5

Pipe Expansion and Support Module 10.4

Block 10 Steam Distribution

Expansion loops

Welded bend ∅ radius = 1.5∅

2W

W

Welded joint Fig. 10.4.9 Expansion loop

The expansion loop can be fabricated from lengths of straight pipes and elbows welded at the joints (Figure 10.4.9). An indication of the expansion of pipe that can be accommodated by these assemblies is shown in Figure 10.4.10. It can be seen from Figure 10.4.9 that the depth of the loop should be twice the width, and the width is determined from Figure 10.4.10, knowing the total amount of expansion expected from the pipes either side of the loop. Expansion from neutral position (mm) 50 75 100 125

25

400

150

175

200

300

Nominal pipe size (mm)

200

100 90 80 70 60 50 40

30 25

10.4.6

0.5

1.0

1.5

2.0

2.5 3.0 3.5 4.0 W = width (metres) Fig. 10.4.10 Expansion loop capacity for carbon steel pipes

4.5

5.0

The Steam and Condensate Loop

Pipe Expansion and Support Module 10.4

Block 10 Steam Distribution

Sliding joint These are sometimes used because they take up little room, but it is essential that the pipeline is rigidly anchored and guided in strict accordance with the manufacturers’ instructions; otherwise steam pressure acting on the cross sectional area of the sleeve part of the joint tends to blow the joint apart in opposition to the forces produced by the expanding pipework (see Figure 10.4.11). Misalignment will cause the sliding sleeve to bend, while regular maintenance of the gland packing may also be needed. Stay bolts

Pressure acts on this area

Gland packing

Movement due to pipework expansion Fig. 10.4.11 Sliding joint

Expansion bellows An expansion bellows, Figures 10.4.12, has the advantage that it requires no packing (as does the sliding joint type). But it does have the same disadvantages as the sliding joint in that pressure inside tends to extend the fitting, consequently, anchors and guides must be able to withstand this force.

Fig. 10.4.12 Simple expansion bellows

Bellows may incorporate limit rods, which limit over-compression and over-extension of the element. These may have little function under normal operating conditions, as most simple bellows assemblies are able to withstand small lateral and angular movement. However, in the event of anchor failure, they behave as tie rods and contain the pressure thrust forces, preventing damage to the unit whilst reducing the possibility of further damage to piping, equipment and personnel (Figure 10.4.13 (b)). Where larger forces are expected, some form of additional mechanical reinforcement should be built into the device, such as hinged stay bars (Figure 10.4.13 (c)). There is invariably more than one way to accommodate the relative movement between two laterally displaced pipes depending upon the relative positions of bellows anchors and guides. In terms of preference, axial displacement is better than angular, which in turn, is better than lateral. Angular and lateral movement should be avoided wherever possible. The Steam and Condensate Loop

10.4.7

Pipe Expansion and Support Module 10.4

Block 10 Steam Distribution

Figure 10.4.13 (a), (b), and (c) give a rough indication of the effects of these movements, but, under all circumstances, it is highly recommended that expert advice is sought from the bellows’ manufacturer regarding any installation of expansion bellows. Guides

Axial movement Short distance

Fixing point Axial movement Guides Fig. 10.4.13 (a) Axial movement of bellows

Guides

Limit rods

Medium distance

Small lateral movement

Large lateral movement

Fixing point Large lateral movement

Small lateral movement

Limit rods Guides

Fig. 10.4.13 (b) Lateral and angular movement of bellows

Hinged stay bars

Small angular movement

Axial movement

Long distance

Small angular movement Fixing point

Fig. 10.4.13 (c) Angular and axial movement of bellows

10.4.8

The Steam and Condensate Loop

Block 10 Steam Distribution

Pipe Expansion and Support Module 10.4

Pipe support spacing The frequency of pipe supports will vary according to the bore of the pipe; the actual pipe material (i.e. steel or copper); and whether the pipe is horizontal or vertical. Some practical points worthy of consideration are as follows: o

o

o

o

o

Pipe supports should be provided at intervals not greater than shown in Table 10.4.3, and run along those parts of buildings and structures where appropriate supports may be mounted. Where two or more pipes are supported on a common bracket, the spacing between the supports should be that for the smallest pipe. When an appreciable movement will occur, i.e. where straight pipes are greater than 15 metres in length, the supports should be of the roller type as outlined previously. Vertical pipes should be adequately supported at the base, to withstand the total weight of the vertical pipe and the fluid within it. Branches from vertical pipes must not be used as a means of support for the pipe, because this will place undue strain upon the tee joint. All pipe supports should be specifically designed to suit the outside diameter of the pipe concerned. The use of oversized pipe brackets is not good practice.

Table 10.4.3 can be used as a guide when calculating the distance between pipe supports for steel and copper pipework. Table 10.4.3 Recommended support for pipework Nominal pipe size (mm) Interval of horizontal run Steel Copper (metre) bore outside diameter Mild steel Copper 15 1.2 15 1.8 20 22 2.4 1.2 25 28 2.4 0.5 32 35 2.4 1.8 40 42 2.4 1.8 50 54 2.4 1.8 65 67 3.0 2.4 80 76 3.0 2.4 100 108 3.0 2.4 125 133 3.7 3.0 150 159 4.5 3.7 200 6.0 250 6.5 300 7.0

Interval of vertical run (metre) Mild steel Copper 2.4 1.8 3.0 3.0 1.8 3.0 2.4 3.7 3.0 3.7 3.0 4.6 3.0 4.6 3.7 4.6 3.7 5.5 3.7 5.5 3.7 5.5 8.5 9.0 10.0

The subject of pipe supports is covered comprehensively in the European standard EN 13480, Part3.

The Steam and Condensate Loop

10.4.9

Pipe Expansion and Support Module 10.4

Block 10 Steam Distribution

Questions 1.

A DN100 Schedule 40 pipe carries steam at 10 bar g over a length of 80 m. If the pipe is installed at 10°C, using Equation 10.4.1 and Table 10.4.1, by how much will it expand?

¨ ¨ ¨ ¨

a| 291 mm b| 196 mm c| 352 mm d| 207 mm 2.

If the expansion of a pipe from installation to working temperature was 352 mm, what length of spacer would be used in ‘cold drawing’ the pipe being installed?

¨ ¨ ¨ ¨

a| 352 mm b| 704 mm c| 176 mm d| 88 mm 3.

A 100 m run of 80 mm pipe at 15 bar g is supported at its ends and three intermediate points. It is trapped at intervals of 40 m. Noise and vibration often occurs at start-up. What do you think is required to put things right?

¨ ¨ ¨ ¨

a| Fit more supports at 3 m intervals b| Check that the steam traps are removing condensate properly c| Check that the steam main isolating valve is opened slowly d| All of the above 4.

A 150 mm steam pipe is to incorporate a fabricated expansion loop to take up 125 mm of expansion. Using Figures 10.4.9 and 10.4.10, what should be the width and length of the loop?

¨ ¨ ¨ ¨

a| Width : 2.6 m; Depth : 5.2 m b| Width : 5.2 m; Depth : 2.6 m c| Width : 5.2 m; Depth : 10.4 m d| Width : 1.3 m; Depth : 2.6 m 5.

What is one advantage of a bellows expansion fitting over a horseshoe loop?

a| It is less expensive b| Its operating movement can be observed c| Fewer pipe supports are required d| It takes up less space 6.

¨ ¨ ¨ ¨

Condensate from a heater battery operating at 3.8 bar g returns to a vented pump set from where it is pumped through a carbon steel pipe to an atmospheric boiler feedtank which is 85 m away. Using the chart in Figure 10.4.1, what will be the approximate pipe expansion from an ambient temperature of 0°C?

¨ ¨ ¨ ¨

a| 130 mm b| 200 mm c| 160 mm d| 100 mm

Answers

1: d, 2: c, 3: d, 4: a, 5: d, 6: d

10.4.10

The Steam and Condensate Loop

Air Venting, Heat Losses and a Summary of Various Pipe Related Standards Module 10.5

Block 10 Steam Distribution

Module 10.5 Air Venting, Heat Losses and a Summary of Various Pipe Related Standards

The Steam and Condensate Loop

10.5.1

Air Venting, Heat Losses and a Summary of Various Pipe Related Standards Module 10.5

Block 10 Steam Distribution

Air Venting, Heat Losses and a Summary of Various Pipe Related Standards Air venting

When steam is first admitted to a pipe after a period of shutdown, the pipe is full of air. Further amounts of air and other non-condensable gases will enter with the steam, although the proportions of these gases are normally very small compared with the steam. When the steam condenses, these gases will accumulate in pipes and heat exchangers. Precautions should be taken to discharge them. The consequence of not removing air is a lengthy warming up period, and a reduction in plant efficiency and process performance. Air in a steam system will also affect the system temperature. Air will exert its own pressure within the system, and will be added to the pressure of the steam to give a total pressure. Therefore, the actual steam pressure and temperature of the steam /air mixture will be lower than that suggested by a pressure gauge. Of more importance is the effect air has upon heat transfer. A layer of air only 1 mm thick can offer the same resistance to heat as a layer of water 25 µm thick, a layer of iron 2 mm thick or a layer of copper 15 mm thick. It is very important therefore to remove air from any steam system. Automatic air vents for steam systems (which operate on the same principle as thermostatic steam traps) should be fitted above the condensate level so that only air or steam /air mixtures can reach them. The best location for them is at the end of the steam mains as shown in Figure 10.5.1. Balanced pressure air vent

Discharge air to a safe place

Steam main

Drain to a safe place Condensate Fig. 10.5.1 Draining and venting at the end of a steam main

10.5.2

The Steam and Condensate Loop

Block 10 Steam Distribution

Air Venting, Heat Losses and a Summary of Various Pipe Related Standards Module 10.5

The discharge from an air vent must be piped to a safe place. In practice, a condensate line falling towards a vented receiver can accept the discharge from an air vent. In addition to air venting at the end of a main, air vents should also be fitted: o

o o

In parallel with an inverted bucket trap or, in some instances, a thermodynamic trap. These traps are sometimes slow to vent air on start-up. In awkward steam spaces (such as at the opposite side to where steam enters a jacketed pan). Where there is a large steam space (such as an autoclave), and a steam /air mixture could affect the process quality.

Reduction of heat losses Even when a steam main has warmed up, steam will continue condensing as heat is lost by radiation. The condensing rate will depend upon the steam temperature, the ambient temperature, and the efficiency of the pipe insulation. For a steam distribution system to be efficient, appropriate steps should be taken to ensure that heat losses are reduced to the economic minimum. The most economical thickness of insulation will depend upon several factors: o

Installation cost.

o

The heat carried by the steam.

o

Size of the pipework.

o

Pipework temperature.

When insulating external pipework, dampness and wind speed must be taken into account. The effectiveness of most insulation materials depends on minute air cells which are held in a matrix of inert material such as mineral wool, fibreglass or calcium silicate. Typical installations use aluminium clad fibreglass, aluminium clad mineral wool and calcium silicate. It is important that insulating material is not crushed or allowed to waterlog. Adequate mechanical protection and waterproofing are essential, especially in outdoor locations. The heat loss from a steam pipe to water, or to wet insulation, can be as much as 50 times greater than from the same pipe to air. Particular care should be taken to protect steam lines, running through waterlogged ground, or in ducts, which may be subjected to flooding. The same applies to protecting the lagging from damage by ladders etc., to avoid the ingress of rainwater. It is important to insulate all hot parts of the system with the exception of safety valves. This includes all flanged joints on the mains, and also the valves and other fittings. It was, at one time, common to cut back the insulation at each side of a flanged joint, to leave access to the bolts for maintenance purposes. This is equivalent to leaving about 0.5 m of bare pipe. Fortunately, prefabricated insulating covers for flanged joints and valves are now more widely available. These are usually provided with fasteners so that they can readily be detached to provide access for maintenance purposes.

Calculation of heat transfer The calculation of heat losses from pipes can be very complex and time consuming, and assume that obscure data concerning pipe wall thickness, heat transfer coefficients and various derived constants are easily available, which, usually, they are not. The derivations of these formulae are outside the scope of this Module, but further information can be readily found in any good thermodynamics textbook. To add to this, an abundance of contemporary computer software is available for the discerning engineer.

The Steam and Condensate Loop

10.5.3

Block 10 Steam Distribution

Air Venting, Heat Losses and a Summary of Various Pipe Related Standards Module 10.5

This being so, pipe heat losses can easily be found by reference to Table 10.5.1 and a simple equation (Equation 10.5.1). The table assumes ambient conditions of between 10 - 21°C, and considers heat losses from bare horizontal pipes of different sizes with steam contained at various pressures. Table 10.5.1 Heat emission from pipes Note: Heat emission from bare horizontal pipes with ambient temperatures between 10°C and 20°C and still air conditions Pipe size (DN) Temperature 15 20 25 32 40 50 65 80 100 150 difference steam to air °C W/m 60 60 72 88 111 125 145 172 210 250 351 70 72 87 106 132 147 177 209 253 311 432 80 86 104 125 155 171 212 248 298 376 519 90 100 121 146 180 196 248 291 347 443 610 100 116 140 169 207 223 287 336 400 514 706 110 132 160 193 237 251 328 385 457 587 807 120 149 181 219 268 282 371 436 517 664 914 130 168 203 247 301 313 417 490 581 743 1 025 140 187 226 276 337 347 464 547 649 825 1 142 150 208 250 306 374 382 514 607 720 911 1 263 160 229 276 338 413 418 566 670 794 999 1 390 170 251 302 372 455 457 620 736 873 1 090 1 521 180 275 330 407 499 497 676 805 955 1 184 1 658 190 299 359 444 544 538 735 877 1 041 1 281 1 800 200 325 389 483 592 582 795 951 1 130 1 381 1 947

Other factors can be included in the equation, for instance, if a pipe is lagged with insulation providing a reduction in heat losses to 10% of the uninsulated pipe, then it is multiplied by a factor of 0.1. V =

 /I  KIJ

Equation 2.12.2

Where: ms = Rate of condensation (kg /h) Q = Heat emission (W/m) (from Table 10.5.1) L = Effective length of pipe, allowing for flanges and fittings (m) f = Insulation factor. e.g.: 1 for bare pipes, 0.1 for good insulation hfg = Specific enthalpy of evaporation (kJ /kg) Equivalent lengths: Pair of mating flanges 0.5 m Line size valve 1.0 m Example 10.5.1 50 m of 100 mm pipe has 8 pairs of flanges and two valves, and carries saturated steam at 7 bar g. Ambient temperature is 10°C, and the insulation efficiency is given as 0.1 With reference to Table 10.5.1 and the application of Equation 10.5.1: determine the quantity of steam that will be condensed per hour: Part 1 - Without insulation. Part 2 - With the pipe insulated, but the valves and flanges are left without insulation. Part 3 - Completely insulated.

10.5.4

The Steam and Condensate Loop

Air Venting, Heat Losses and a Summary of Various Pipe Related Standards Module 10.5

Block 10 Steam Distribution

Equivalent length of fittings: (8 pairs of flanges @ 0.5 m) + (2 valves @ 1.0 m) = 6.0 m of pipe Saturated steam at 7 bar g:

Steam temperature Temperature difference (pipe to ambient temperature) Enthalpy of evaporation (hfg) Heat loss per metre of 100 mm pipe (from Table 10.5.1)

= 170°C = 170°C - 10°C = 160°C = 2 048 kJ /kg = 999 W/m

Part 1 - Without insulation: V =

 /I  KIJ

V =

 /I  KIJ

V =

[[ [   

&RQGHQVLQJUDWH

Equation 2.12.2

NJ K

Part 2 - Pipe insulated, but without insulation on the valves and flanges: Consider the two elements separately: ,QVXODWHGSLSH 

V =

 /I  KIJ

V =

[[[   

+HDWORVVIURPSLSHV V 8QLQVXODWHGILWWLQJV 

NJ K

V =

 /I KIJ

V =

[[[  

+HDWORVVIURPILWWLQJV V

NJ K

Total condensing rate = heat loss from pipes + heat loss from fittings Total condensing rate = 8.78 kg /h + 10.54 kg /h = 19.32 kg /h

Part 3 - Pipe and fittings insulated: V =

/I  K IJ

V = &RQGHQVLQJUDWH

The Steam and Condensate Loop

[[ [    NJ K

10.5.5

Air Venting, Heat Losses and a Summary of Various Pipe Related Standards Module 10.5

Block 10 Steam Distribution

Relevant UK and International Standards Symbols have been used to indicate, technically equivalent standards (=), and related standards (¹) respectively. Table 10.5.2 BS 10 BS 21 = ISO 7/1 ¹ ISO 7/2 EN 13480 BS 1306 BS 1387

BS 1560 BS 1600 EN 10253-1 BS 1710 BS 2779= IS0 228/1, ISO 228/2

Specification for flanges and bolting for pipes, valves and fittings. Specification for pipe threads for tubes and fittings where pressure tight joints are made on the threads. Specification for metallic industrial piping. Specification for copper and copper alloy piping systems. Specification for screwed and socketed tubes and tubulars and for plain end steel tubes suitable for welding and screwing to BS 21 pipe threads. Circular flanges for pipes, valves and fittings (Class designated): - Part 3, Section 3.1 - Specification for steel flanges (¹ ISO 7005). - Part 3, Section 3.2 - Specification for cast iron flanges (¹ ISO 7005-2). - Part 3, Section 3.3 - Specification for copper alloy and composite flanges (¹ ISO 7005-3). Dimensions of steel pipe for the petroleum industry. Specification for butt welding pipe fittings for pressure purposes. Specification for identification of pipelines. Specification for pipe threads for tubes and fittings where pressure tight joints are not made onthe threads.

Specification for dimensions and masses per unit length of welded and seamless steel pipes and tubes for pressure purposes. Specification for steel pipes and tubes with specified room temperature properties for pressure BS 3601 purposes. EN 10216-2 Specification for steel pipes and tubes for pressure purposes: EN 10217-2/3/5 carbon and carbon manganese steel with specified elevated temperature properties. EN 10216-4 Specification for carbon and alloy steel pipes and tubes with EN 10217-4 specified low temperature properties for pressure purposes. EN 10216-2 Steel pipes and tubes for pressure purposes: EN 10217-2 ferritic alloy steel with specified elevated temperature properties. BS 3604-2 BS 3605-1/2 Austenitic stainless steel pipes and tubes for pressure purposes. BS 3799 Specification for steel pipe fittings, screwed and socket welded for the petroleum industry. BS 3974 Specification for pipe supports. EN 1092-1 3.1 - Specification for steel flanges; EN 1092-2 3.2 - Specification for cast iron flanges (¹ ISO 7005-2); BS 4504 3.3 - Specification for copper alloy and composite flanges (¹ ISO 7005-3). EN 10220

Summary

To summarise the ‘Steam Distribution’ Block of The Steam and Condensate Loop, the following checklist may be used to ensure that a steam distribution system will operate efficiently and effectively:

10.5.6

o

Are steam mains properly sized?

o

Are steam mains properly laid out?

o

Are steam mains adequately drained?

o

Are steam mains adequately air vented?

o

Is adequate provision made for expansion?

o

Can separators be used to improve steam quality?

o

Are there leaking joints, glands or safety valves and why?

o

Can redundant piping be blanked off or removed?

o

Is the system effectively insulated? The Steam and Condensate Loop

Air Venting, Heat Losses and a Summary of Various Pipe Related Standards Module 10.5

Block 10 Steam Distribution

Questions 1. As a general rule, where should air vents be fitted in a steam system? a | At the highest points b | On a bypass around a steam trap c | At points where air is driven by the incoming steam d | Around all steam traps or points adjacent to them

o o o o

2. On what principle do automatic air vents operate? a | They sense the difference in pressure between steam pressure and water pressure in a steam /air mixture b | They are temperature sensitive and remain open until steam at any pressure reaches them

o

c | They remain open until the air passing through them reaches steam temperature

o o

d | They remain open until steam at saturation temperature reaches them. They will then close and will remain closed until, the temperature drops by approximately 12°C.

o

3. From the following, what is the effect of air in a steam and condensate system? a | Erosion of pipes b | Reduced heat output from the plant c | The steam traps will close as they would on sensing steam d | The air will prevent steam and condensate reaching the traps

o o o o

4. The surface cladding of insulation on a steam main is damaged and allows rain to enter the lagging. What is the effect?

o

a | No significant effect b | Less condensation will occur in the pipe because the heat transfer rate through water is less than the heat transfer rate through air c | The water will be evaporated and the steam formed will destroy the insulation

o o

d | Heat losses will increase because the heat transfer rate to water is much greater than to air

o

5. A 75 m long, 150 mm steam main operates at 10 bar g. The main runs outside and the insulation is claimed to be 80% efficient. Approximately how much steam will be condensed in meeting heat losses from the pipe?

o o o o

a | 200 kg /h b | 40 kg /h c | 97 kg /h d | 28 kg /h

6. If, in Question 5, the insulation was 90% efficient, what would the heat losses now be?

o o o o

a | 180 kg /h b | 20 kg /h c | 194 kg /h d | 14 kg /h

Answers

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

10.5.7

Block 10 Steam Distribution

10.5.8

Air Venting, Heat Losses and a Summary of Various Pipe Related Standards Module 10.5

The Steam and Condensate Loop