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Monday, February 20, 2012

Hydrostatic pressure type : Indirect measurement of level

Static Pressure Measurement of Level

A convenient means of measuring liquid level, where there is a considerable change in level employs conventional industrial instruments which is actuated by changes in hydrostatic pressure head of the liquid as the level changes.

This head is the “weight” of liquid above a reference level or datum line. Head is often expressed in terms of pressure or level height.

Measurement of pressure due to liquid head can be translated to level height above the datum line by the following basic relationship:
where h=height or level
P=pressure due to hydrostatic head
Ρ=density of the liquid
g=acceleration due to gravity

For  the readings to be accurate the density have to be constant. The accuracy will be affected for e.g., temperature variations is sufficient to cause changes in the density of the liquid.

The use of DP transmitter for liquid-level measurement:

 The DP transmitter must be positioned below the minimum liquid level. Corrections must be made for changes in the density ofthe liquid. If there is a pulsating motion in the liquid, the output of the transmitter will be unstable. The tapping tube should be as straight as possible so as not to trap air.

Inferential Level Measurement:

This technique obtains a level indication indirectly by monitoring the pressure exerted by the height of the liquid in the vessel. The pressure at the base of a vessel containing liquid is directly proportional to the height of the liquid in the vessel. This is termed hydrostatic pressure. As the level in the vessel rises, the pressure exerted by the liquid at the base of the vessel will increase linearly. Mathematically,
P=Pressure ( Pa),  S=Weight density of the liquid ( N/m3)= ρ g
H=Height of liquid column ( m),  ρ=Density (kg/m3)
g=acceleration due to gravity ( 9.81 m/s2)

DP capsules are the most commonly used devices to measure the pressure at the base of a tank. The level of liquid inside a tank can be determined from the pressure reading if the weight density of the liquid is constant. Use a pressure capsule that has a sensitivity range that closely matches the anticipated pressure of the measured liquid.

Three valve manifold:

A three-valve manifold is a device that is used to ensure that the capsule will not be over-ranged. It also allows isolation of the transmitter from the process loop. It consists of two block valves-high pressure and low-pressure block valve and an equalizing valve. During normal operation, the equalizing valve is closed and the two block valves are open. When the transmitter is put into or removed from service, the valves must be operated in such a manner that very high pressure is never applied to only one side of the DP capsule.

Open tank measurement:

The simplest application is the fluid level in an open tank. The figure shows a typical open tank level measurement installation using a pressure capsule transmitter. If the tank is open to atmosphere, the high pressure side of the level transmitter will be connected to the base of the tank while the low-pressure side will be vented to atmosphere. In this manner, the level transmitter acts as a simple pressure transmitter.

Phigh = Patm + S.H
Differential pressure,  ΔP = Phigh – Plow = S.H

Closed tank Measurement:

Should the tank be closed and a gas or vapour exists on top of the liquid, the gas pressure must be compensated for. A change in the gas pressure will cause a change in transmitter output. Moreover, the pressure exerted by the gas phase may be so high that the hydrostatic pressure of the liquid column becomes insignificant. For example, the measured hydrostatic head in a boiler may be only three meters (30kPa) or so, whereas the steam pressure is typically 5 MPa.

Compensation can be achieved by applying the gas pressure to both high and low-pressure sides of the level transmitter. This cover gas pressure is thus used as a back pressure (or reference pressure) on the LP side of the DP cell. One can immediately see the need for the threevalve manifold to protect the DP cell against these pressures.

Closed tank measurement- Dry leg system:

A full dry leg installation with three-valve manifold is as shown. If the gas phase is condensable, say steam, condensate will form in the low-pressure impulse line resulting in a column of liquid, which exerts extra pressure on the low-pressure side of the transmitter. A technique to solve this problem is to add a knockout pot below the transmitter in the low-pressure side. Periodic draining of the condensate in the knockout pot will ensure that the impulse line is free of liquid.

Phigh=Pgas + S.H

The effect of the gas pressure is cancelled and only the pressure due to the hydrostatic head of the liquid is sensed. When the low-pressure impulse line is connected directly to the gas phase above the liquid level, it is called a dry leg. In practice, a dry leg is seldom used because frequent maintenance is required. One example of a dry leg application is the measurement of liquid poison level in the poison injection tank, where the gas phase is non-condensable medium. In most closed tank applications, a wet leg level measurement system in used.

Closed tank measurement - Wet Leg System:

In a wet leg system, the low pressure impulse line is completely filled with liquid (usually the same liquid as the process) and hence the name wet leg. A level transmitter, with the associated three-valve manifold, is used in an identical manner to the dry leg system. At the top the low pressure impulse line is a small catch tank. The gas phase or vapour will condense in the wet leg and the catch tank. The catch tank, with the inclined interconnecting line, maintains a constant hydrostatic pressure on the low pressure side of the level transmitter. This pressure, being a constant, can easily be compensated for by calibration. (Note that operating the three-valve manifold in the prescribed manner helps to preserve the wet leg.)

If the tank is located outdoors, trace heating of the wet leg might be necessary to prevent it from freezing. Steam lines or an electric heating element can be wound around the wet leg to keep to keep the temperature of the condensate above its freezing point. Note the two sets of drain valves. The transmitter drain valves would be used to drain (bleed) the transmitter only. The two drain valves located immediately above the three-valve manifold are used for impulse and wet leg draining and filling.

In addition to the three-valve manifold most transmitter installations have valves where the impulse lines connect to the process. These isolating valves, sometimes referred to as the root valves, are used to isolate the transmitter for maintenance.

Level Compensation:

It would be idealistic to say that the DP cell can always be located at the exact bottom of the vessel we are measuring fluid level in. Hence, the `measuring system’ has to consider the hydrostatic pressure of the fluid in the sensing lines themselves. This leads to two compensations required.

Zero Suppression:

In some cases, it is not possible to mount the level transmitter right at the base level of the tank. Say, for maintenance purposes, the level transmitter has to be mounted X meters below the base of an open tank.

The liquid in the tank exerts a varying pressure that is proportional to its level H on the high-pressure side of the transmitter. The liquid in the high pressure impulse line also exerts a pressure on the high-pressure side. However, this pressure is a constant (P=S.X) and is present at all times.

 When the liquid level is at H meters, pressure on the highpressure side of the transmitter will be:
Phigh=S.H + S.X + Patm
P=Phigh -Plow=S.H + S.X

That is, the pressure on the high-pressure side is always higher than the actual pressure exerted by the liquid column in the tank ( by a value of S.X). This constant pressure would cause an output signal that is higher than 4 mA when the tank is empty and above 20 mA when it is full. The transmitter has to be negatively biased by a value of -S.X, only. This procedure is called Zero Suppression and it can be done during calibration of the transmitter.

Zero Elevation:

When a wet leg installation is used, the low-pressure side of the leg transmitter will always experience a higher pressure than the high pressure side. This is due to the fact that the height of the wet leg (X) is always equal to or greater than the maximum height of the liquid column (H) inside the tank. When the liquid level is at H meters, we have:
Phigh = Pgas +S.H
Plow = P gas +S.X
P=Phigh –Plow = S.H – S.X - = -S(X-H)

The differential pressure,  Δ P a negative number ( ie., low pressure side is at a higher pressure than the high pressure side).   ΔP increases from P=-S.X to P=-S(X-H) as the tank level rises from 0% to 100%. If the transmitter were not calibrated for this constant negative error (-S.X), the transmitter output read low at all times. To properly calibrate the transmitter, a positive bias (+S.X) is needed to elevate the transmitter output. This positive biasing technique is called zero elevation.

Example: Zero Suppression in Level Measurements of open vessels ( tanks):

A d/p transmitter is connected to the tank by a pressure tapping tube.
The liquid is tapped for the high pressure side, and the open air is tapped for the low pressure side.
The following relationship exists:
P=ρ1g ( H+h1)
P = the pressure
ρ1=density of the liquid
H=distance between the surface and the minimum liquid level
h1=distance between the minimum liquid level and the pressure detector.

Liquid-level measurement: Open tank:

A pressure transmitter connected at a position 10 cm below the bottom of a tank sends 13.57 mA to a computer. The transmitter was calibrated for a range of 0-200kPa to produce 4-20 mA. If the liquid has a specific gravity of 1.26, calculate the level of the liquid in the tank.
ρw=1000 kg/m2                                        g=9.81 m/s2
Reading 13.57 mA :(200 kPa *13.57mA/16mA)-50 kPa=119.62 kPa
h=P/  ρwgRD=119.62kPa/(1000 kg/m2 *9.81 m/s2*1.26)=967.75cm
hactual =h-10 cm =957.75 cm

Example: Zero Elevation in Level Measurements of closed vessels (tanks)

Dry leg method:

A DP transmitter is connected to the closed tank: the low pressure tap is the pressure of the gas above the liquid in the upper  art of the tank.

The pressure of the gas is also applied to the high pressure tap at the same time. Hence, when taking the pressure differential, it cancels out and so does not affect the transmitter output.

If condensation from the gas in the upper part of the tank collects inside the tapping tube, the low pressure tapping pressure in the tube will change and the output of the d/p transmitter will be affected. To avoid this, the condensation is collected in a drain pot.

Liquid-level measurement: Closed tank –- Dry Leg


The following relationship exists:
High pressure tap pressure, PH = ρ1g ( H+h1)+PG
Low pressure tap pressure, PL=PG
Pressure differential, PH-PL= ρ1g (H+h1)
PG = the pressure of the gas in the upper part of the tank
ρ1=density of the liquid
H=distance between the surface and the minimum liquid level
h1=distance between the minimum liquid level and the pressure detector.

Example: Zero Elevation in Level Measurements of closed vessels (tanks) -  Wet Leg

Wet leg method:

Similarly, a d/p transmitter is connected to the closed tank: the low pressure tap is the pressure of the gas above the liquid in the upper part of the tank.

A relatively heavy liquid (high density) that does not easily evaporate to fill the tube. The pressure of the gas in the tank is then applied to the pressure detector through this liquid.

Liquid-level measurement: Closed tank –- Wet Leg 

The following relationship exists:
High pressure tap pressure, PH =ρ1g (H+h1)+PG
Low pressure tap pressure, PL= ρ2gh2 +PG
Pressure differential, PH-PL= ρ1g (H+h1)- ρ2gh2
ρ2=density of the liquid in the wet leg, (kg/m3)
h2=the height of the liquid in the wet leg, (m) .

Article Source: Dr. Rosdiazli Ibrahim,Universiti Teknologi Petronas (EEB5223/EAB4223 Industrial Automation & Control Systems)


Basics of Instrumentation & Control

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