Level

Level Measurements

LEVEL MEASUREMENTS

Level Measurements

CONTENTS Objectives

Page 3

TOP I 02.4 UPK (i) Standard Engineering Units and their Inter-conversions

Page 4

TOP I 02.4 UPK (ii) The characteristics of various Level sensors and their selection criteria

Page 8

TOP I 02.4 UPK (iii) Construction and working Principle of direct method of level measuring devices

Page 12

TOP I 02.4 UPK (iv) Construction and working principle of displacer type Level measuring devices

Page 22

TOP I 02.4 UPK (v) Construction and working principle of hydrostatic type Level measuring devices

Page 26

TOP I 02.4 UPK (vi) Construction and working principle of capacitance type Level measuring devices

Page 38

TOP I 02.4 UPK (vii) Construction and working principle of interface Level measuring devices

Page 44

TOP I 02.4 UPK (viii) Construction and working principle of level switching devices

Page 50

TOP I 02.4 UPK (ix) Applications of level measuring/ switching devices in PDO locations

Page 52

TOP I 02.4 UPK (x) Calibration methods

Page 53

Glossary of terms

Page 56

Level Measurements

OBJECTIVES On completion of this module, the participant will be able to: -

Evaluate the inter conversions of different level units with the given standard engineering units table.

Explain how to select the correct level sensor for a given application with the given characteristics of various level sensors and their selection criteria table.

3. Explain the construction and working principle of different level measuring / switching devices.

4. Explain the applications of level measuring/ switching devices.

5. Explain the construction and working principle of level transmitters.

6. Explain the zero check and use different Calibration devices and methods to calibrate level transmitters to an accuracy of ±2%.

Level Measurements

TOP I 02.4 UPK (i) Standard engineering units of level and their interconversions INTRODUCTION The measurement of level is defined as the determination of an existing interface between two media. These media are usually fluids, but may be solids or a combination of solid and fluid. The interface can also exist between a liquid and a gas, a liquid and its vapor or two liquids etc. The best method of measurement depends upon the nature of the specific application, which must deal with: 1. The process to be measured. 2. The degree of accuracy required. 3. Dependability. 4. Economic considerations. In the oil and natural gas industries, liquid level measurement is necessary to achieve the following objectives. 1. Compute tank inventories of crude oil. 2. Protect equipment such as Compressors, turbines and pumps from damage. 3. Protect operating and maintenance personnel against injury resulting from liquid spillage. 4. Protect the environment from the spillage of liquids. 5. Control phase separation processes. • Discuss about the measurement of level • Discuss about the necessary of liquid level measurement

Level Measurements

STANDARD ENGINEERING UNITS OF LEVEL Unlike pressure, temperature and Flow, which we have covered so far, liquid level has no absolute value and is always relative to some reference point such as the bottom of a tank. It is the height or depth of a liquid above a reference point and is specific to a particular vessel. Hence, the normal unit of level measurement is the meter or one of its sub-multiples. Level measurement is expressed as a measurement of length.

Centimeter 1 100 2.54 30.48 91.44

Meter 0.01 1 0.0254 0.3048 0.9144

Inch 0.3937 39.37 1 12 36

Feet 0.0328 3.28 0.0833 1 3

Yard 0.0109 1.09 0.0278 0.3333 1

TABLE No.1 LEVEL / LENGTH CONVERSIONS

• Explain about the units of level measurement and their inter-conversions using the given table.

Level Measurements

SELF-REVIEW 1. Convert 40 meters of level into inches.

2. Convert 25 feet of level to centimeters.

3. How would you define level?

4. What is the normal unit of Level measurement?

• Discuss the answers with the trainees.

Level Measurements

REVISION 1. Write down any two main objectives of level measurement? a). _____________________________________. b). _____________________________________.

2. Level measurement is always relative to some _______________ such as the bottom of the tank.

3. One centimeter is equivalent to ________ feet.

4. Convert 50 inches of level to meters?

5. Convert 30 meters of level to feet?

Level Measurements

TOP I 02.4 UPK (ii) The Characteristics of various level sensors and their selection criteria Introduction In this session, we shall discuss the characteristics of various level sensors and their selection criteria for a particular application within PDO locations. Characteristics of various level sensors and their selection criteria Measurement of liquid level is categorized into: 1 . Direct method 2 . Inferential method (indirect) 1 . Direct method This employs the physical principles such as fluid motion, floats, and thermal properties. Various types of level measurement under this heading are: 1.1 Dip stick, Dip rods, and lead lines. 1.2 Sight glass. 1.3 Float operated. 2 . Inferential method (Indirect) This employs hydrostatic head, weight, conductivity, viscosity, buoyancy and density. Various types of level measurement under this heading are: VARIABLE DISPLACEMENT METHOD(BUOYANCY TYPE) 2.1 Displacer. HYDROSTATIC HEAD TYPE 2.2 Hydrostatic gauge (DP). ELECTRICAL METHOD 2.3 Capacitance probe gauge. • Discuss about the various types of Level measurement techniques.

Level Measurements

SENSOR TYPE APPLICATIONS Dip stick, Dip rods, and 1. Automobile engine lead lines. crank case oil level. 2. IC Engines oil level. 3. Oil tank level.

LIMITATIONS 1.Manual measurement. 2.Does not give continuous measurement. 3. Not suitable in pressurized tanks. Sight Glass 1.Continuous 1. Local indication. measurement for both 2. Possibility of open and closed tanks. breakage. 2. Simple and reliable. Float operated 1. Used for remote 1. Not used for foaming transmission. liquids and liquid/gas 2. Used for switches. interface. Displacer 1. Used for remote 1. Not suitable for wide transmission. density fluctuations. 2. Used for interface level measurement. Hydrostatic gauge 1. Used for remote 1. Zero Elevation / transmission. Suppression required 2. Used for interface depending on the level measurement. mounting. Capacitance probe 1. Used for corrosive 1. Scaling causes gauge applications. errors. 2. Contain no moving 2. Temperature change parts and easy to clean. affects change in dielectric property. TABLE No.2 LEVEL SENSORS SELECTION CRITERIA

• Discuss about the applications and limitations of various Level sensors.

Level Measurements

SELF REVIEW 1. What are the two types of liquid level measurement? a) ___________________________________. b) ___________________________________.

2. Dip stick is used to measure the liquid level under _______________ measurement.

3. Write any one type of liquid level measurement under Indirect method. a) ____________________________________.

4. Displacer sensor is used for _________________ transmission.

• Discuss the answers with the trainees

Level Measurements

REVISION 1. Name at least two devices used for direct measurement of liquid level a) __________________________________ b) __________________________________

2. Name any one type of device used for inferential measurement of level. a) __________________________________

3. Write down the applications of Float operated sensor. a) ___________________________________ b) ___________________________________

4. Which type of sensors is not suitable in pressurized tanks? a) __________________________ b) ___________________________ c) __________________________

Level Measurements

TOP I 02.4 UPK (iii) Construction and working principle of level measuring/switching devices Introduction In this session, we shall discuss the construction and working principle of various level and interface level measuring/switching devices in detail. 1.1 Dip stick, Dip rods, and lead lines. This is perhaps the earliest and simplest form of level measurement, still in common use. A dip stick is essentially a stick that is calibrated to indicate level. The dip stick is lowered vertically into a tank or vessel until it reaches a reference point. Usually the bottom of the tank is used to ensure that the dip stick is inserted to the correct depth. The dip stick is then withdrawn and the level is read by determining where the interface last made contact with the dip stick. Reading the scale on the dip stick indicates the level measurement.

FIGURE No. 1 DIP STICK LEVEL MEASUREMENT

Dip rods may take the form of a solid round rod or a flat bar with graduations over a length of 2.5 meters approximately. The rod is made from non-spark metal and is inserted through a vertical guide tube until it touches the bottom of the tank. Certain hydrocarbon liquids may contain water, which accumulate at the bottom of the tank. The depth of water may • Discuss about the principle of operation of Dipstick and Dip rod

Level Measurements

be measured by coating the end of the rod with a water detecting chalk, which changes from white to blue, or a water detecting paste which changes from green to red, if water is present. The lead line or otherwise called manual gauging tape has replaced the dip rod as a method of measuring liquid levels in large storage tanks. This is widely used in PDO locations to measure storage tank levels. Obviously, dip rods over 10 meters long are too heavy to carry up to the top of large tanks and are also too difficult to operate due to the amount of oil left sticking to the surface during withdrawal. The lead line consists of steel or glass fiber tape wound on a reel and a brass weight, shaped like a plumb bob, is clipped on the free end. The bob weight keeps the tape tight and suspended vertically inside the tank. The tape and the bob are calibrated in meters and millimeters.

FIGURE NO. 2 MANUAL GAUGE TAPE AND BOB WEIGHT

• Discuss about the principle of operation and the advantages.

Level Measurements

1.2 Sight Glasses Sight glasses, sometimes called gauge glasses, provide a continuous visual indication of liquid level in a process vessel or a small tank and are, therefore, more convenient than dipsticks, dip rods or manual gauging tapes. However, glass elements can get dirty and are susceptible to breakage thus presenting a safety hazard especially when hot, corrosive or flammable liquids are being handled. There are three main types of sight glasses, which we shall discuss now. 1.2.1 window Type Sight Glass The drawing below shows a typical two-section window sight glass, which is mounted directly on the side of a rectangular tank, such as a gas compressor lubricated oil storage and circulation tank, operated at atmospheric pressure and 60°C. This type can be seen with Borsig Gas Compressor at Marmul location within PDO. The slot-shaped overlapping windows are made of flat, transparent, heat resisting glass and show the true level of oil together with any abnormal condition such as excessive foam or turbulence. Window type sight glasses are less likely to be damaged by an accidental knock than the more exposed external gauge columns.

FIGURE No. 3 WINDOW TYPE SIGHT GLASS

• Discuss about the advantages and also its types. • Explain the construction and working principle.

Level Measurements

1.2.2 Tubular type Sight Glass The type shown below is a single tube sight glass located outside a process vessel and connected at both ends by small bore piping unions and special isolation valves containing safety shut-off ball check valves. In the event of tube breakage, the sudden loss of pressure in the tube causes the pressure in the vessel to move the balls and reseal the lines thus checking liquid or vapour escape from the process vessel. It is essential to connect the top of the sight glass to the vapour space of the vessel when it is operated under pressure. Otherwise, the liquid in the glass would be blown out to atmosphere. The externally mounted sight glass does not always indicate the true level of the liquid inside the vessel. If the liquid in the vessel is at high temperature and the sight glass is not thermally insulated, the specific mass (density) of the liquid in the glass will increase and support a higher column of lower specific mass liquid inside the vessel. Likewise, when the liquid in the vessel is refrigerated and the sight glass is not adequately cold insulated, the level in the sight glass will be higher than the true level in the vessel. If the normal process liquid is water and it becomes contaminated with light oil the average specific mass will decrease and the sight glass level will be less than the true level in the vessel. The contents of the sight glass should be purged to a closed drain at regular intervals to ensure that the lower connection is not blocked and to ensure that the sight glass liquid is the same as that inside the vessel. Tubular sight glasses are no longer used in high pressure, high temperature, and hydrocarbon services because they are easily broken. However, they are still used in utility services such as water treatment where pressures do not exceed 1000 kPag.

• Discuss about the construction and principle of operation. • Discuss about the limitations and applications.

Level Measurements

FIGURE NO. 4 TUBULAR TYPE SIGHT GLASS

Level Measurements

1.2.3 Flat Glass Type Sight Glasses This type of sight glass is applicable for service with high pressure, high temperature, and hydrocarbon liquids. It is also used with high pressure steam at approximately 7000 kPag and 450°C and in oil / gas separation applications at 13000 kPag and 80°C. The sight glass element originally took the form of a one piece, u-section, and hollow steel bar with a thick, heat resistant and shock resistant glass plate covering the open side. The assembly being secured by a heavy slotted cover clamped to the bar by a number of u-bolts as shown.

FIGURE No. 5 FLAT GLASS TYPE SIGHT GLASSES

• Discuss about the applications. • Discuss about the constructional details. • Discuss about the principle of operation.

Level Measurements

The main problem used to be inadequate illumination and two modifications were developed to improve visual level indication namely: Sight glass with a reflex glass panel. Sight glass with two plain glass panels. Both types of sight glass are illustrated, the reflex glass panel containing prismatic grooves. When the sight glass is empty, the incident light is reflected outwards giving the glass a bright appearance. When liquid is present, the incident light rays are refracted from the grooves into the liquid and the consequent reduction in reflection gives the liquid a dark appearance. Reflex sight glasses are normally installed on vessels handling clean, colourless and low viscosity liquids, while plain sight glasses are used with coloured, viscous liquids. At night, or in poorly lit locations, level observation is improved by installing a plexiglass illuminated panel behind the double glass type units. This accessory is important when the process vessel contains boiling liquid and vapour condensate continuously runs down the inside of the glass. Sight glasses on chillers become covered in frost after the surrounding air in contact with the glass reaches its dewpoint. This problem is overcome by fitting a transparent plastic plate on top of the sight glass.

Level Measurements

1.3 Float Operated Gauges Devices incorporating floats have been used for a very long time for liquid level indication, measurement and control. The device, used for liquid level measurement in fixed roof tanks, is known as a float gauge or tape gauge and many different types are used. All types employ measuring wires or tapes, which are either moved by gravity counterweights, spring-powered servomotors or electric servomotors. A float operates on the Archimedes’ Principle, which states that when a body is immersed in a liquid it is buoyed by an upward force, called upthrust, equal to the weight of liquid displaced. When the weight of the float is equal to the weight of the liquid displaced, it will remain on the liquid surface at a constant submergence and at constant density. If the weight of the object is greater than the upthrust on it, the object sinks. If the upthrust of the liquid is enough to balance the weight, then the object floats. Upward or downward motion can be used to measure the changing level in a process vessel or a storage tank. Tank Float Gauge The float is connected to a counter weight by means of a cable or chain, which is running through the groove of the pulley. The cable / chain are kept tight enough with the help of the counterweight. The hub of the pulley is in turn connected to an indicating device with graduations. The indicator assumes a definite position over the scale for a definite position of the float on the surface of the liquid and hence proportional to the liquid level in the tank.

• Discuss about the float operated gauges • Discuss about the principle of operation

Level Measurements

Refer to the diagram below.

FIGURE No. 6 TANK FLOAT GAUGE

Another variation in design is that where the counterweight itself acts as the indicator moving over a graduated linear scale. (Note: This level instrument is installed in the potable water tank opposite to the PDO Public technical library). In such installations, the float is placed in a stilling well or guided by guide rods / ropes. This will prevent the float from wandering inside the tank and only vertical displacement of the float is permitted. • Discuss about the constructional details. • Discuss about the principle of operation.

Level Measurements

SELF REVIEW 1. The bottom of the tank is the ______________ for correct insertion of the dip stick.

2. Name at least one reason why lead line is preferred over dip rod.

3. Write the applications of flat type sight glass.

4. Name any two types of sight glasses used in oil industry. a. _________________________________. b. __________________________________.

• Discuss the answers with the trainees.

Level Measurements

TOP I 02.4 UPK (iv) CONSTRUCTION AND WORKING PRINCIPLE OF DISPLACER TYPE OF LEVEL MEASURING DEVICES. 2.1. Displacer Type Level measurement. This is based on the Archemedes' principle, which states that when a body is immersed in a liquid the resultant fluid pressure on the body acts vertically upward through the center of gravity of the body and is equal to the weight of the fluid that is displaced by the body. The upward pressure acting on the area of the body creates a force and is called BUOYANCY. When the weight of an object is heavier than an equal volume of a liquid into which it is submerged, full immersion results and the body never floats. The idea is further explained by a simple experiment. The body hereafter will be called a DISPLACER, because it displaces some amount of liquid. A displacer is heavier than the liquid and does not float whereas a float is lighter than the liquid and floats on the surface of the fluid.

FIG.7 PRINCIPLE OF DISPLACER

Level Measurements

In tank “A " the displacer is suspended by a spring balance that shows the weight of the displacer in air, 3 Kg. This represents a 0 % water level in the tank. As we increase the level in the tank to 50 %, water rises vertically and more length of the displacer is now immersed in the water (tank " B "). You will notice three things: 1. The weight of the displacer is reduced to 2 Kg, as indicated by the spring balance. 2. The relative position of the displacer in the water is raised due to the buoyancy force exerted by the water. 3. The relationship between the reductions in weight of the displacer and the height of liquid is linear. This is further clarified by the situation in tank “C ". As the liquid level is increased to 100 %, the weight of the displacer further decreases to 1Kg. The rise in level and the reduction in weight of the displacer are directly proportional. In industrial applications, the spring balance is not a convenient method of indication of the level and will be replaced by a torsion spring called “Torque tube”. Almost all manufacturers of displacement type level measuring instruments use this unique mechanism to convert the vertical movement of the displacer to a rotary movement in the instrument for indication or transmission. The torque tube has two main functions 1. Transmits the motion from the displacer to the control mechanism. 2. Provides a positive seal against the process fluid and its pressure. • Discuss about the principle of operation of Displace type level measurement.

Level Measurements

As mentioned earlier the design of the torque tube varies from manufacturer to manufacturer. Here we will examine one of those types. Shown below is a cross sectional view of one type of torque tube.

FIGURE No. 8 DISPLACER TYPE LEVEL TRANSMITTER

Level Measurements

In operation, when the displacer is freely hanging in air and the full weight of the displacer is acting on the torque arm, this will produce a torque on the length of the torque tube which is fixed at one end. The torque tube will twist and when it twists the torque tube rod will also twist. Here the torque tube is acting as a spring. As the displacer loses its weight due to a rise in level, the twist on the torque tube will be reduced and the torque tube rod will rotate accordingly. This rotation is proportional to the weight of the displacer, which is also proportional to the rise in level. The successful application of the torque tube will depend upon the material of construction. It should be designed to provide the necessary load carrying ability with low operating stress. The following needs to be considered when selecting the torque tube material. 1. Capability of the material to resist corrosion. 2. Elastic properties of the material over a wide range of temperatures. 3. The nature of the fluid being handled. Type 316 stainless steel (316 SS) is commonly used for temperatures up to 250 °C. Above 250 °C INCONEL (alloy) is found to be satisfactory. There are other materials like Monel, Hastelloy C etc., which are suitable up to 250 °C. These materials have good corrosion resistance properties. This particular instrument is designed by Fisher Controls Company and their torque tube design is unique. Here the transmission mechanism is pneumatic but electronic versions which produce 4 to 20 mA signal are also available. Necessary explosion proof instrument enclosures are needed to use the electronic versions in the oil field. The other popular design which P.D.O. installations use is manufactured by MASONEILAN Company. • Discuss the principle of operation of Displacer type level transmitter.

Level Measurements

TOP I 02.4 UPK (v) CONSTRUCTION AND WORKING PRINCIPLE OF HYDROSTATIC TYPE LEVEL MEASURING DEVICES. 2.2 Hydrostatic Gauge. Refer to the diagram given below. We can calculate the pressure at any point in the column of liquid by using the standard formula P=hxρ xg Where P = Pressure in N/m2 or Pascal h = height of the liquid from a given datum in meters. ρ = Density of the liquid in kg/m3. g = acceleration due to gravity in m/s2. This equation is arrived as follows: Pressure = Force Area = Mass x Acceleration Area = (Volume x Density) x Acceleration Area = (Height x Area) x Density x Acceleration Area = Height x Density x Acceleration =hxρ xg Now let’s see how the unit is arrived: Pressure = h x ρ x g = m x kg/m3 x m/s2 = kg m x s2 Multiply both sides by meter and hence we get, Pressure = kg x m = kg x m/s2 x 1/m2 = N/m2 = Pa S2 x m 2 Also, let’s now see the relationship between density and Specific Gravity.

Level Measurements

Specific Gravity of liquid = Density of liquid Density of water Specific Gravity of gas = Density of gas Density of air We all know that density of water is 1000 kg/m3 and Specific Gravity (S.G) is having no unit since it is the ratio of two densities. If the SG of crude oil is 0.8, then the density of crude oil will be 800 kg/m3.

FIGURE No.9 PRINCIPLE OF HYDROSTATIC HEAD

If we calculate the pressure at the base of the vessel, this is the pressure the pressure gauge will be measuring in this particular installation.

Level Measurements

We know that: • Discuss about the principle of Hydrostatic head P=h x ρ x g Where h = 2 m ρ = 1000 Kg / m3 g = 9.81 m / s2 Therefore P = 2 x 1000 x 9.81 = 19620 N / m2 = 19.620 kPa  

Closed and pressurized vessels. In the previous diagram we discussed the application of the hydrostatic head principle to measure the height of the liquid in a tank. There, the pressure above the tank was atmospheric; therefore an ordinary pressure gauge could give us the height of the liquid in the tank. Only the dial of the pressure gauge needs to be marked in terms of liquid height. When a tank or vessel is closed and pressurized, the amount of pressure inside the tank will act on top of the liquid surface. This will add to the hydrostatic head created by the liquid height and this combined pressure is the pressure that is measured by the pressure gauge, which is not the pressure due to height of the liquid. This condition calls for some kind of compensation (correction) to cancel the effect of the gas (or vapor) pressure existing above the liquid surface. A differential pressure measuring device can be applied in such installations. You came across this type instrument (a D/P cell) when you studied pressure measurements. The diagram below illustrates how the

Level Measurements

differential pressure method can be successfully used to compensate the error that is created by the pressure above the liquid. • Discuss about the principle of D/P type level measurement for closed level

FIGURE No.10 D/P TYPE LEVEL MEASUREMENT FOR CLOSED VESSEL

In the above diagram the pressures involved are: HP = P1 + P2 LP = P2 D/P cell output = HP – LP = (P1 + P2) - P2 = P1 This is the pressure due to the hydrostatic head.

Level Measurements

Differential pressure method of hydrostatic head measurement is popularly used both in pressurized tanks and vented tanks. When used for vented tanks, the LP side of the D/P cell is also vented to the atmosphere. Complications can arise when the space above the liquid is under pressure and contains the vapor of the liquid itself. This vapor remains as vapor within the tank at the process operating temperature. But when it is tapped outside of the tank, this vapor may condense back to liquid. This will create problems for successful measurement. Let us examine how. Refer to the drawing below.

FIGURE No.11 D/P TYPE WITH OUTSIDE WET LEG

The vapor of the liquid when connected to the LP impulse line and taken to the ambient temperature will condense to liquid and remain as liquid inside the LP impulse line. In this situation the LP side is fully or partially filled

Level Measurements

with liquid and therefore it is wet. The LP side is then referred to as WET LEG. The amount of liquid condensed inside the LP side can also vary due to the rate of condensation. This will create a hydrostatic head in the LP side. All these factors contribute to inaccuracy in measurement. In such applications where wet leg is encountered the measurement of liquid level is not accurate and correction (compensation) is required. Another problem with this kind of measurement arises when the measuring instrument is located below the zero level (datum). The error due to this problem can arise for both DRY and WET LEG applications. We will discuss them in detail and also the methods employed to compensate for the errors. Consider an installation as given below.

Level Measurements

FIGURE No.12 D/P CELL MOUNTED BELOW DATUM

The D/P cell is located below the datum level (zero level). When the liquid level is at the zero percentage (lower range value), we expect the D/P cell to produce an output either 4 mA or 20 kPa, depending upon the type of transmission. But in this case when the level is actually at 0%, the HP side measures the hydrostatic head produced by the liquid height "X". This will make the transmitter send a signal, which is more than 4 mA or 20 kPa. The information given by the transmitter is not true from the measurement point of view but this error is a definite (fixed) error because the liquid trapped inside the HP leg remains there all the time. Since the signal given out by the transmitter is more than zero, this needs to be corrected for. This type of correction is termed as ZERO SUPPRESSION. This means that to bring the increased transmitter output back to zero, (4mA or 20 kPa), to represent the 0% of the measurement (level). The example on the next page explains the point further.

Level Measurements

Level Measurements

FIGURE No.13 D/P CELL MOUNTED BELOW DATUM

Distance "A" = 2 meter Distance "B" = 6 meter Distance "C" = 1 meter S.G. of liquid = 0.92 1. Find the range and span of measurement in kPa. 2. Find the amount of pressure that needs to be suppressed for the transmitter to read zero when the level is at the datum.

• Discuss Solution 1 about the Zero suppression. Range of measurement = 0 m to 2 m (0% to 100% level). Head created by 0% level = 0 kPa (LRV). Head created by 100% level = 2 x9.81x 0.92x1000 Pa. = 18050.4 Pa. = 18.0504 kPa (URV). Therefore range of measurement = 0 to 18.0504 kPa (Ans). Span of measurement = URV - LRV. = 18.0504 - 0 = 18.0504 kPa (Ans). Solution 2 When the level is at datum, the transmitter measures a hydrostatic head created by the column of liquid inside the HP impulse line. This line has a vertical distance of 1 meter (C). Therefore the pressure due to this height = 1x9.81x0.92x 1000 = 9025.2 Pa = 9.0252 kPa (Ans)

Level Measurements

This is the amount of pressure that needs to be suppressed in the transmitter so that when the transmitter measures 9.0252 kPa it has to send 4 mA (0% signal) to the level indicator to show that level is at the 0% of measurement. Provisions are given in all D/P transmitters to fit suppression kits (parts) for this biasing. Bias means add or subtract by a fixed value. In the previous example we considered a dry leg application. Using the same example, when the LP side becomes a WET LEG the problem is different. Let us examine how different it is. • Discuss about how to find the range and span of measurement in kPa. • Discuss about how to find the amount of pressure that needs to be suppressed for the transmitter to read zero when the level is at the datum.

Level Measurements

FIGURE No.14 D/P CELL MOUNTED BELOW DATUM WITH OUTSIDE WET LEG

Note the changes in this drawing. Now let us assume that the level is at the datum point, and the outside wet leg is completely filled with the same liquid as in the tank. The hydrostatic head seen by the HP side = C x ρ x g = 9.0252 kPa. The hydrostatic head seen by the LP side = B x ρ x g = 54.1512 kPa. Looking at the above pressures we can see that the LP side has more pressure than the HP side when the level is at the datum point. This will drive the transmitter zero below 4 mA. Also even if the level rises in the tank the transmitter cannot produce any positive output because the LP side will always have more pressure than the HP side. To compensate for this type of error ZERO ELEVATION IS REQUIRED in the transmitter. This means to apply bias as in the case of dry leg installation. Let us now see how this can be done by looking at the pressures involved. The net pressure seen by the LP side of the transmitter, when the level is at the datum point is: {(B x ρ x g) - (C x ρ x g)} = (B-C) x (ρ x g) = 5x 0.92 x 1000 x 9.81 = 45.126 kPa The pressure due to height "C" in Hp side is also there in LP side. These two pressures cancel each other. The remaining height of liquid in the LP

Level Measurements

side is creating the 45.126 kPa. This is the pressure, which is measured by the LP side, and drives the transmitter zero below 4 mA. Zero elevation kits can be fitted to the transmitter to cancel the effect of such pressures, and make the transmitter read 4 mA when the level is at 0%.

• Discuss about how Zero elevation can be achieved in the transmitter.

SELF REVIEW 1. Write the formula inter-relating pressure, height of the liquid column, density and acceleration due to gravity.

2. A pressure gauge is mounted in a tank 30 feet below the datum line of the tank. The specific gravity of the process liquid is 0.80.What is the pressure on the gauge in psi? (1 psi = 6.895 kPa; 1 ft = 0.3048 m)

Level Measurements

3. The zero suppression is used to bring the increased transmitter output back to zero due to the definite error caused when the D/P level is located below the datum level. True/False?

4. Convert 5 kPa into inches of water column using the formula

• Discuss the answers with the trainees TOP I 02.4 UPK(vi) CONSTRUCTION AND WORKING PRINCIPLE OF CAPACITANCE TYPE OF LEVEL MEASURING DEVICE. 2.3 Capacitance type liquid level measurement. A capacitor is an electrical component that consists of two conductors separated by a dielectric or insulator. Capacitance value is measured in farads. This value is determined by the area of the conductors (usually called plates), the distance between the plates and the dielectric constant of the insulator between the plates. For capacitance type of level measurement, one plate is usually the probe and the other plate is usually the tank. The dielectric is the material in the tank. The relationship between the capacitance, plate area, the distance between them and the dielectric constant is given by the equation; C = K xA

Level Measurements

D Where: C = Capacitance K = Dielectric constant A = Area of the plates D = Distance between the plates In most of the applications, the area of the plates and the distance between the plates are fixed values. The dielectric varies with the level in the tank and this variation is used to produce a signal, which is proportional to the level. Industrial application of the capacitance type of level measurement is confined to ON-OFF control purposes (point measurement, switching). Best measurement results are obtained when the difference between the dielectric constants of the two media is relatively high. This is because the change in capacitance with the variation in level is a direct function of the dielectric constant, and is greater when the height of the measured medium with a high K (liquid) replaces a medium with a low K (gas). • Discuss the principle of measurement of capacitance type liquid level measurement. It should be noted that the dielectric constant of liquid varies with temperature. An increase in media temperature results in a decrease in dielectric constant. Since level variations are inferred from variations in dielectric, the changes in temperature will affect the accuracy of measurement. Therefore suitable temperature compensation is required when significant variations in media temperature are anticipated. Two typical situations of capacitance measurement are: 1) Non conducting liquids. 2) Conducting liquids. Non conducting liquids. When measuring the level of a non-conducting liquid, a bare metal probe can be used. The total (effective) capacitance of the system is given by the equation: C e = C1 + C 2 + C 3

Level Measurements

Refer to the drawing below.

FIGURE No.15 CAPACITANCE TYPE FOR NON CONDUCTING LIQUIDS

• Discuss about the types of capacitance measurement • Discuss about how to calculate the effective capacitance Conducting liquids For applications where the medium is a conducting liquid, insulated probes are used. The total capacitance of the system is given by:

C e = C1 +

C C C2C4 + 3 5 + C 2 C 4 C 3 + C5

Level Measurements

FIGURE No.16 CAPACITANCE TYPE FOR CONDUCTING LIQUIDS

• Discuss about how to find the effective capacitance of conducting liquids.

Capacitance measurement techniques As seen earlier the total capacitance Ce varies with the liquid level in the tank. This change in capacitance can be measured by using an a/c wheatstones bridge. A typical wheatstones bridge is given below.

Level Measurements

R3

R1 Detector

Voltage Source a.c.

G C 2 R 2

Ce

Rx  

FIGURE No.17 CAPACITANCE MEASUREMENT BRIDGE

The bridge is said to be balanced when the ratio of the impedances is equal. It can be mathematically shown that the effective capacitance is equal to: Ce =

R1 C 2 R3

Where Ce is the probe capacitance and R1, C2 & R3 are bridge components. The detector can be calibrated to read level in suitable units. By knowing the capacitance to level relationship of the probe, the level values can be calculated by substituting the component values in the equation. Example:- A capacitance probe with a 5µF /m characteristic is used in a • application. Discuss about theother techniques capacitanceare measurements. level The bridgeofcomponents C2 = 100µF, R1 = 10 kΩ, R2 = 100 kΩ, R3 = 50 kΩ. Find the level on the probe. (Ans = 4 m).

Level Measurements

Some advantages of capacitance level devices are: 1. They contain no moving parts. 2. Can be used for corrosive applications. 3. Easy to clean. 4. They can be designed for explosion proof service.

Some disadvantages of capacitance level devices are: 1. Measurement is subjected to error caused by changes in dielectric constant due to variation in temperature. 2. Scaling deposited on the probe, while in use, causes changes in dielectric constant and causes errors in measurement. 3. On site calibration is often required for best results.

• Discuss about the advantages and disadvantages of capacitance level devices.

SELF REVIEW 1. What is the function of capacitor?

Level Measurements

2. Write the relationship between the capacitance, plate area, the distance between them and dielectric constant.

3. Mention at least two advantages of Capacitance type of level measurement.

• Discuss the answers with the trainees. TOP I 02.4 UPK(vii) CONSTRUCTION AND WORKING PRINCIPLE OF INTERFACE LEVEL MEASURING DEVICES. INTERFACE LEVEL MEASUREMENTS

Level Measurements

DEFINITION: The measurement of level is defined as the determination of the position of an existing INTERFACE between two media. These media are usually fluids, but may be solids or a combination of solid and fluid. The interface can exist between a liquid and a gas, a liquid and its vapour, two liquids etc. Interface measurement of two media (liquid / liquid) is encountered in industry when two liquids of different specific gravities meet. Because of the difference in densities they do not mix together. They are called IMMISCIBLE LIQUIDS. In liquid / liquid applications, the border between the two liquids form an interface and measurement of the position of this border in a tank or vessel containing these liquids become important for successful process operation and economy. Various methods are used to measure interface levels in industry depending upon the process conditions and applicability. This discussion will be confined to two types of widely and successfully used methods in P.D.O. 1. BUOYANCY TYPE. 2. DIFFERENTIAL PRESSURE TYPE. BUOYANCY TYPE. The previous discussion on displacer operation showed the displacer suspended in two fluids with the displacer weight being a function of the interface position. The magnitude of displacer travel depends upon:a. THE CHANGE IN INTERFACE b. THE DIFFERENCE IN SPECIFIC GRAVITY BETWEEN THE UPPER AND LOWER FLUIDS

Level Measurements

FIGURE No.18 DISPLACER TYPE INTERFACE LEVEL MEASUREMENT

When both fluids are liquids, the displacer is always IMMERSED in liquid. Let us consider a vessel containing two immiscible liquids such as NGL and WATER. The specific gravity of NGL is 0.75 and of water is 1.0. A displacer is placed in this vessel to measure the interface between water and NGL. With the displacer completely immersed in NGL the weight of the displacer is 1.4 lb. (600 gm.). The weight of the displacer in air is 3.0 lb. (1.4 Kg). When the water level in the vessel is increased to half of the length of the displacer, the displacer weight is reduced to 1.2 lb. (540 gm). This decrease in weight of the displacer is due to change in buoyancy. When the water level is increased further the displacer weight is further reduced to 1.0 lb. (450 gm). This further reduction in weight is again due to greater portion of the displacer is immersed in heavier liquid, which is water. • Discuss about the principle of Displacer type interface measurement.

Level Measurements

From this example it can be seen that the weight of the displacer is a function of the position of interface. When the displacer weight varies, the rotary motion of the TORQUE TUBE also varies proportionally and hence the output. 2 . DIFFERENTIAL PRESSURE METHOD As discussed earlier, differential pressure method can be used for level measurements. By this method we infer the level position by measuring the hydrostatic head governed by the relationship: P = hρg

As can be seen by the formula, the pressure (hydrostatic head) is dependent on the liquid specific gravity (density). The example given below will describe how differential pressure method can be successfully used in interface level measurements.

FIGURE No.19 D/P METHOD OF INTERFACE MEASUREMENT

Level Measurements

• Discuss the principle of D/P method interface measurement. Given:Vessel - Horizontal drum with a weir height = 8 ft = 2.5 m. Measuring device - flange mounted DP transmitter. HP connection - 0.9 in (0.2 m) above the drum bottom. LP connection - from the top of the drum (dry leg). Specific gravities = 0.85 for heavier liquid (GH). = 0.60 for lighter liquid (GL) g = Acceleration due to gravity = 9.81 m/s2 Formulas for calculation: Pmax = h ρ g = transmitter differential at maximum interface. Pmin = h ρ g = Transmitter differential at minimum interface. Transmitter span = Pmax - Pmin kPa h = height of measurement in meters. To find 1. Transmitter span in kPa and in inches of water. 2. If suppression / elevation of zero required? 3. If required the magnitude in kPa and in inches of water? Establishing values; Specific gravity of heavier liquid = 0.85 Specific gravity of lighter liquid = 0.60 Height of measurement “h” = 2.5 - 0.2 = 2.3 meters Density of water (ρ ) = 1000 Kg / m3

Level Measurements

Head seen by the DP transmitter when the interface is maximum = 2.3 x 850 x 9.81 =19178.55 Pa =19.178 kPa 19.178 x 4.016 =77.018 " H2O Head seen by the DP transmitter when the interface is minimum = 2.3 x 600 x 9.81 =13537.8 Pa = 13.537 kPa 13.537 x 4.016 = 54.364 ' H2O Therefore span = 19.178 - 13.537 = 5.641 kPa = 22.654 " H2O Transmitter range =13.537 kPa to 19.178 kPa When interface is at the minimum (zero interface level), the DP cell will measure a hydrostatic head of 13.537 kPa and will produce an output greater than 4 mA. But the transmitter should produce 4 mA when the interface level is ZERO. This means that the transmitter zero should be brought down (suppressed) for an amount of pressure equal to 13.537 kPa.

Level Measurements

SELF REVIEW 1. Mention the two common methods used to measure interface level.

2. The specific gravity of NGL is __________________________.

3. The decrease in weight of the displacer is due to change in __________________.

Level Measurements

• Discuss the answers with the trainees TOP I 02.4 UPK(viii) CONSTRUCTION AND WORKING PRINCIPLE OF LEVEL SWITCHING DEVICE. Construction and Working Principle of Level Switches In PDO locations, MAGNETROL type of float switches are widely used, although many other types like flexible tube type, magnetic repelling type are also available. In this type of instrument the float, which is in the shape of a ball or a dishended cylinder, senses the level change and positions a magnetic piston or sleeve, which is attached to the float rod. The sleeve (or piston) moves up and down inside the enclosed, pressure tight non-magnetic tube. Outside the non-magnetic tube there is a permanent magnet fixed on a pivot, which actuates a mercury or micro switch. A simplified diagram is given below. For detailed sketches, refer to the product literature of "MAGNETROL" level switches, which are widely used in the P.D.O. interior locations. This instrument is also installed on V - 0020 and T - 0020 in HIBAH station. electrical connections NO = normally open NC = normally closed Com = common

Mercury pellet Com NC

NO

hermetically sealed glass case Non magnetic Pressure tight Tube

pivot

spring support

magnetic sleeve

magnet

spring

float rod

liquid level

FIGURE No.20 MAGNETROL LEVEL SWITCH For high and high-high level switch applications normally closed contact is being used, while for low and low-low level switch applications normally

• Discuss the construction and working principle of level switches.

Level Measurements

open contacts are being used. This is because of getting a closed contact under normal conditions to safeguard the tank or vessel in the event of loose connections in the circuit, which is otherwise called as fail-safe condition. SELF REVIEW 1. What is the most commonly used type of level switch used in PDO locations?

2. For high and high-high level switch applications __________ __________contact is being used.

3.

For low and low-low level switch ________________ contact is being used.

applications

________

Level Measurements

• Discuss the answers with the trainees

TOP I 02.4 UPK (ix) APPLICATIONS OF LEVEL MEASURING / SWITCHING DEVICES. Introduction In this session, we shall discuss the applications of various types of level measuring/switching devices in PDO locations. Applications Lead lines are widely used by production operators in MAF Tank farm area and interior locations to take the inventory of tank levels. Sight glasses of the different types discussed are used for local indication in many applications. Displacer type transmitters are widely used to measure oil level and interface level in crude oil storage tanks, C.P.I., separators etc. The hydrostatic head type level transmitters of DP type are the most widely used level transmitters in the interior locations. Capacitance type level measuring devices are used for on-off control in the dehydration tanks in the interior and crude oil storage tank in MAF Tank farm area for draining water that is settled down. Magnetrol level switches are widely used for safeguarding the tanks, separators, C.P.I’s in association with logic circuits.

Level Measurements

TOP I 02.4 UPK (x) LEVEL CALIBRATION METHODS. Introduction We have seen the calibration of DP type of transmitters in the previous module. Hence, in this session, we shall concentrate the calibration of dry weight calibration of displacer type of level transmitters on a typical application of MARMUL T-2412 Oil Level and Interface Level. MMPS T-2412 TAG No. : 24-LEA-258HH SERVICE : OIL (a) DISPLACER DETAILS Length of the displacer (L) = 35.6 cms Diameter of the displacer (D) = 4.24 cms. Volume of the displacer (V) = πD²L = 22 x 4.24 x 4.24 x 35.6 4 7x4 = 502 cubic cms. Displacer Hanger Extension length = 319.8 cms. (b) SPECIFIC GRAVITY Specific Gravity of Oil (ρ1)

= 0.911

(c) CALCULATION Weight of the displacer + Hanger in air (W) = 1600 gms. Loss of weight in 100% Oil (W1) = (V x ρ1) =457.322 gms. Weight of the displacer in 100% Oil = W – W1 =1142.678 gms. Weight of the displacer in 50% Oil = W – W1 =1371.339 gms. 2 (d) CALIBRATION INST. READING

WEIGHT

TRANSMITTER

Level Measurements

(%) (GRAMS) OUTPUT (mA) 0 1600 4 50 1371.339 12 100 1142.673 20 MMPS T-2412 TAG No. : 24-LEA-259LL SERVICE: INTERFACE (OIL / WATER) (a) DISPLACER DETAILS Length of the displacer (L) Diameter of the displacer (D) Volume of the displacer (V)

= 35.6 cms = 9.8 cms. = πD²L = 22 x 9.8 x 9.8 x 35.6 4 7x4

= 2686.38 cubic cms. Displacer Hanger Extension length = 1063.6 cms. (b) SPECIFIC GRAVITY Specific Gravity of Oil (ρ1) = 0.911 Specific Gravity of Water (ρ2) = 1.005 (c) CALCULATION Weight of the displacer + Hanger in air (W) = 4000 gms. Loss of weight in 100% Oil (W1) = (V x ρ1) =2447.29 gms. Loss of weight in 100% Water (W2) = (V x ρ2) =2699.81 gms. Weight of the displacer in 100% Oil = W – W1 =1552.71 gms. Weight of the displacer in 100% Water = W – W2 =1300.19 gms. Weight of the displacer in 50% Oil + 50% Water = (W- W1) + (W- W2) 2 = 1426.45 gms. d) CALIBRATION

Level Measurements

INST. READING (%) 0 50 100 SELF REVIEW

WEIGHT (GRAMS) 1552.71 1426.45 1300.19

TRANSMITTER OUTPUT (mA) 4 12 20

1. Write the formula used in calculating the volume of the displacer

2. Calculate the Volume (V) of the displacer if the Length (L) and Diameter (D) of the displacer are 11.8 cms and 5.7 cms respectively.

3. Calculate the loss of weight in 100% oil (W1) using the following details a) Volume of the displacer (V) b). Specific Gravity of Oil (ρ1)

= 425 cubic cms. = 0.911

4. Calculate the loss of weight in 100% Water (W2) using the following details a). Volume of the displacer (V)

= 444 cubic cms.

Level Measurements

b). Specific Gravity of Water (ρ2)

= 1.005

• Discuss the answers with the trainees Glossary of terms ARCHIMEDES’ PRINCIPLE – The principle that when a body is wholly or partly immersed in a fluid it experiences an up thrust equal to the weight of fluid it displaces; the up thrust acts vertically through the center of gravity of the displaced fluid. BUOYANCY – The apparent loss in weight of a body when wholly or partly immersed in a fluid. CAPACITANCE – The ratio of the charge on one of the conductors of a capacitor (there being an equal and opposite charge on the other conductor) to the potential difference between the conductors. DIELECTRIC - Substance, solid, liquid or gas, which can sustain a steady electric field, and hence an insulator. It can be used for cables, terminals, capacitors and similar devices. DISPLACER- It is a long narrow closed cylinder, which is half submerged in the liquid at normal level. It is heavier than the liquid and does not float. IMMISCIBLE – Liquids of different densities that do not mix together. INFERENTIAL – Indirect means. INTERFACE - Sharp contact boundary between two phases, either or both of which may be solid, liquid or gas. LEAD LINE – Also called as a Manual Gauging Tape used to measure level manually.

Level Measurements

PLUMB BOB – A weight, which moves up and down vertically in an aligned manner. PRISMATIC GROOVES- Grooves in the shape of prism that are transparent glass whose faces are triangles. PULLEY – A wheel with a flat, round, or grooved rim which rotates on a shaft and carries a flat belt, vee belt, rope or chain to transmit motion and energy. SERVOMOTOR – A motor, electric or hydraulic, for use in an automatic control system for e.g. the operation of a large valve by a governor of small power. SIGHT GLASS – A device on the outside of a separator that allows the operator to check liquid levels inside the vessel. TORQUE TUBE – Displacer is suspended from a torque tube that measures the buoyancy force acting upwards on the cylinder at varying depths of submergence caused by movement of liquid level. VAPOUR – A gas, which is at a temperature below its critical temperature and can therefore be liquefied by a suitable increase in pressure. WEIR – A dam like structure which maintains a liquid level. ZERO ELEVATION – This is required to compensate for zero of the transmitter when the LP side becomes the wet leg. ZERO SUPPRESSION – This is required to compensate for zero of the transmitter especially in case of level transmitter when it is mounted below the bottom of the tank.

Level Measurements

Learning Session Plan

Unit Element UPK (i)

: I 02 : I 02.4 : Standard engineering units of level and their interconversions. UPK (ii) : The characteristics of various level sensors and their selection criteria. Learning Objective: The participant will be able to evaluate the inter-conversions of different level units with the given standard engineering units table and explain how to select the correct level sensor for a given application with the given characteristics of various level sensors and their selection criteria table. Total Session Time:

1 period

Time

Activity

Resources

10 min

Introduction

TRANSPARENCIES VIDEO CASSETTE REF. NO:TVAR 1186.88

20 min

Explain the standard TRANSPARENCIES engineering units of level and their inter-conversions. Explain the characteristics of TRANSPARENCIES various level sensors and their selection criteria.

20 min

Level Measurements

15 min

Self review

15 min

Revision Learning Session Plan

Unit Element UPK (iii)

: I 02 : I 02.4 : Construction and working principle of direct method of level measuring devices.

Learning Objective: The participant will be able to explain the construction and working principle of direct method of level measuring devices Total Session Time:

3 periods

Time

Activity

Resources

10 min

Introduction

TRANSPARENCIES

60 min

Explain the construction and TRANSPARENCIES working principle of dip sticks, dip rods & lead lines.

70 min

Explain the construction and TRANSPARENCIES working principle of sight glasses.

70 min

Explain the construction and TRANSPARENCIES working principle of float operated gauges.

Level Measurements

15 min

Self review

15 min

Revision Learning Session Plan

Unit : I 02 Element : I 02.4 UPK (iv) : Construction and working principle of displacer type level measuring devices. Learning Objective: The participant will able to explain the construction and working principle of displacer type level measuring devices. Total Session Time:

1 period Time

Activity

Resources

10 min

Introduction

TRANSPARENCIES

40 min

Explain the construction and TRANSPARENCIES working principle of displacer VIDEO CASSETTE REF. NO:TVAR 1186.88 type level measuring devices.

15 min

Self review

Level Measurements

15 min

Revision

Learning Session Plan

Unit Element UPK (v)

: I 02 : I 02.4 : Construction and working principle of hydrostatic type level measuring devices.

Learning Objective: The participant will be able to explain the construction and working principle of hydrostatic type level measuring devices. Total Session Time:

4 periods

Time

Activity

Resources

10 min

Introduction

OHP TRANSPARENCIES

280 min

Explain the construction OHP and working principle of TRANSPARENCIES hydrostatic type level measuring devices.

Level Measurements

15 min

Self review

15 min

Revision

Learning Session Plan

Unit Element UPK (vi)

: I 02 : I 02.4 : Construction and working principle of capacitance type level measuring devices.

Learning Objective: The participant will be able to explain the construction and working principle of capacitance type level measuring devices. Total Session Time:

2 periods

Time

Activity

Resources

10 min

Introduction

OHP TRANSPARENCIES

120 min

Explain the construction and OHP working principle of TRANSPARENCIES capacitance type level measuring devices.

Level Measurements

15 min

Self review

15 min

Revision

Learning Session Plan

Unit Element UPK (vii)

: I 02 : I 02.4 : Construction and working principle of interface level measuring devices.

Learning Objective: The participant will be able to explain the construction and working principle of interface level measuring devices. Total Session Time:

2 periods

Time

Activity

Resources

10 min

Introduction

OHP TRANSPARENCIES

120 min

Explain the construction and OHP working principle of interface TRANSPARENCIES level measuring devices.

Level Measurements

15 min

Self review

15 min

Revision

Learning Session Plan

Unit : I 02 Element : I 02.4 UPK (viii) : Construction and working principle of level switching devices. Learning Objective: The participant will be able to explain the construction and working principle of level switching devices. Total Session Time:

1 period

Time

Activity

Resources

10 min

Introduction

OHP TRANSPARENCIES

40 min

Explain the construction and OHP working principle of level TRANSPARENCIES switching devices.

Level Measurements

15 min

Self review

15 min

Revision

Learning Session Plan

Unit Element UPK (ix)

: I 02 : I 02.4 : Applications of level measuring/switching devices in PDO locations.

UPK (x)

: Calibration methods.

Learning Objective: The participant will be able to explain the zero check and use different Calibration devices and methods to calibrate level transmitters to an accuracy of ±2% and explain the applications of level measuring/switching devices in PDO locations. Total Session Time:

1 period

Time

Activity

Resources

10 min

Introduction

TRANSPARENCIES

Level Measurements

20 min

Explain the applications of TRANSPARENCIES level measuring/switching devices.

20 min

Explain the calibration TRANSPARENCIES methods for level measuring devices.

15 min

Self review

15 min

Revision

Q & A FOR LEVEL MEASUREMENTS 1. Convert 40 meters of level into inches. 1 meter

= 39.37 inches

40 meters = 40 * 39.37 = 1574.8 inches So, 40 meters of level is equivalent to 1574.8 inches

2. Convert 25 feet of level to centimeters. 1 feet

= 30.48 centimeters.

25 feet

= 25 * 30.48 = 762 centimeters.

So, 25 feet of level is equivalent to 762 centimeters.

Level Measurements

3. How would you define level? The measurement of level is defined as the determination of an existing interface between two media.

4. What is the normal unit of Level measurement? The normal unit of level measurement is the meter or one of its submultiples.

5. Write down any two main objectives of level measurement? a) Compute tank accessories of crude oil. b) Protect equipment such as Compressors, turbines and pumps from damage.

6. Level measurement is always relative to some reference point such as the bottom of the tank.

7. One centimeter is equivalent to 0.0328 feet.

Level Measurements

8. Convert 50 inches of level to meters? 1 inch = 0.0254 meters 50 inch = 0.0254 * 50 = 1.27 meters So, 50 inches of level is equivalent to 1.27 meters. 9. Convert 30 meters of level to feet? 1 meter = 3.28 feet 30 meter = 30 * 3.28 = 98.4 feet So, 30 meters of level is equivalent to 98.4 feet.

10. What are the two types of liquid level measurement? a) Direct method. b) Indirect method.

11. Dip stick is used to measure the liquid level under direct measurement.

Level Measurements

12. Write any one type of liquid level measurement under indirect method. a) Displacer.

13. Displacer sensor is used for remote transmission.

14. Name at least two devices used for direct measurement of liquid level. a) Dip stick b) Sight glass

15. Name any one type of device used for inferential measurement of level. a) Hydrostatic gauge(DP)

Level Measurements

16. Write down the applications of Float operated sensor. a) Used for remote transmission. b) Used for switches

17. Which type of sensors is not suitable in pressurised tanks? a) Dip stick b) Dip rods c) Lead lines

18. The bottom of the tank is the reference point for correct insertion of the dip stick.

19. Name atleast one reason why lead line is preferred over dip rod. Dip rods are too heavy to carry up to the top of large tanks.

Level Measurements

20. Write the applications of flat type sight glass. It is applicable for service with high pressure, high temperature and hydrocarbon liquids.

21. Name any two types of sight glasses used in oil industry. a) Window type sight glass. b) Tubular type sight glass.

22. Write the formula inter-relating pressure, height of the liquid column, density and acceleration due to gravity. P= h * ρ* g Where P = Pressure h = height of the liquid from a given datum ρ = density of the liquid g = acceleration due to gravity

Level Measurements

23. A pressure gauge is mounted in a tank 30 feet below the datum line of the tank. The specific gravity of the process liquid is 0.80.What is the pressure on the gauge in psi? (1 psi = 6.895 kPa ; 1 ft = 0.3048 m) P=h*ρ*g = (30 * 0.3048) * 800 * 9.81 N/m2 = 71762.112 N/m2 = 71762.112 psi 1000 * 6.895 = 10.4078 psi 24. The zero suppression is used to bring the increased transmitter output back to zero due to the definite error caused when the D/P level is located below the datum level. True/False? True.

25. Convert 5 kPa into inches of water column using the formula. P=h*ρ*g 5 * 1000 =h * 1000 * 9.81 h = 5000 = 0.5096 meters 9810 = 0.5096 * 39.37 inches = 20.06 inches 26. What is the function of capacitor? A capacitor is an electrical component that consists of two conductors separated by a dielectric or insulator.

Level Measurements

27. Write the relationship between the capacitance, plate area, the distance between them and dielectric constant C = K xA D Where: C = Capacitance K = Dielectric constant A = Area of the plates D = Distance between the plates

28. Mention at least two advantages of Capacitance type of level measurement. a) They contain no moving parts. b) Easy to clean.

29. Mention the two common methods used to measure interface level. a) Buoyancy type.

Level Measurements

b) Differential pressure type.

30. The specific gravity of NGL is 0.75.

31. The decrease in weight of the displacer is due to change in buoyancy.

Level Measurements

32. What is the most commonly used type of level switch used in PDO locations? MAGNETROL

33. For high and high-high level switch applications normally closed contact is being used.

34. For low and low-low level switch applications normally open contact is being used.

Level Measurements

35. Write the formula used in calculating the volume of the displacer. Volume of the displacer = (πD2 L) / 4 36. Calculate the Volume (V) of the displacer if the Length (L) and Diameter (D) of the displacer are 11.8 cms and 5.7 cms respectively. Volume of the displacer = (πD2 L) / 4 = (3.14 x 5.7 x5.7 x 11.8) / 4 = 300.95 cubic cms. 37. Calculate the loss of weight in 100% oil (W1) using the following details a) Volume of the displacer (V) b). Specific Gravity of Oil (ρ1) Loss of weight in 100 % oil (W1) = (V x ρ1)

= 425 cubic cms. = 0.911 = (425 x 0.911 ) = 387.175 gms.

38. Calculate the loss of weight in 100% Water (W2) using the following details a). Volume of the displacer (V)

= 444 cubic cms.

b). Specific Gravity of Water (ρ2)

= 1.005

Loss of weight in 100 % water (W2) = (V x ρ2)

= (444 x 1.005) = 446.22 gms.

Level Measurements