After a few days of interval, starting with this post, in this article I'm about to decipher the basics of level and level measurement. Accurately measuring volume of fluid in containers, continuously has always been a challenge in industries specially dealing with hazard environment, where the fluid could be acidic and/or may be maintained in high temperature/pressure. Considering these facts lets proceed to the article.
Some of the basic conventional way of measuring volume or level of fluid are to employ external sight glasses or tubes to view the height of the fluid and hence the volume; while some use floats connected to variable potentiometers or rheostats that will change the resistance according to the amount of motion of the float, this signal is input to transmitter that send a signal to an instrument calibrated to read out the height or volume.
One other way of measuring level is by using pressure. The pressure at the base of any vessel containing liquid is directly proportional to the height of the liquid in that vessel. This is termed as hydrostatic pressure. As the level in the vessel rises, the pressure exerted by the liquid at the base of the vessel also increases. Mathematical presentation for the above theory can be seen, as given below:
P=S*H
Where, P = Pressure (Pa)
S = Weight density of the liquid (N/m3) = ρg
H = Height of liquid column (m)
ρ = Density (kg/m3)
g = acceleration due to gravity (9.81 m/s2)
The level of liquid inside a tank can be determined from the pressure reading if the weight density of the liquid is constant. Differential Pressure (DP) capsules are the most commonly used devices to
measure the pressure at the base of a tank. When a DP transmitter is used for the sole purpose of measuring a level, it will be called as level transmitter. To obtain maximum sensitivity, a pressure capsule has to be used, that has a sensitivity range that closely matches the anticipated pressure of the actual hydrostatic pressure that is to measured. If the process pressure is accidentally applied to only one side of the DP capsule during installation or removal of the DP cell from service, over ranging of the capsule would occur and the capsule could be damaged and indicates error.
The simplest application is the fluid level in an open tank. The below figure shows a typical open tank level measurement installation using a pressure capsule level 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. We have:
Phigh = Patm + S⋅H
Plow = Patm
Differential pressure ΔP = Phigh - Plow = S⋅H
The level transmitter can be calibrated to output 4 mA when the tank is at 0% level and 20 mA when the tank is at 100% level.
Now,
What if the top of the tank is covered. Lets assume the tank to 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 (30 kPa) or so, whereas the steam pressure is typically 5 MPa. Compensation can be achieved by applying the gas pressure to both the 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 also immediately see the need for the three-valve manifold to protect the DP cell against these pressures.
The different arrangement of the sensing lines to the DP cell is indicated in a typical closed tank application, as shown in the below figure.
We have:
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.
Dry Leg System:
A full dry leg installation with three-valve manifold is shown in the below figure.
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 as shown in the above figure. Periodic draining of the condensate in the knockout pot will ensure that the impulse line is free of liquid. 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 helium. In most closed tank applications, a wet leg level measurement system is used.
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.
This can be seen in the figure below,
At the top of 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 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 the 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 as shown in the figure below.
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 high-pressure side of the transmitter will be:
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 so that the output of the transmitter is proportional to the tank level (S⋅H) only. This procedure is called Zero Suppression and it can be done during calibration of the transmitter. A zero suppression kit can be installed in the transmitter for this purpose.
Zero Elevation:
When a wet leg installation is used (refer below figure), the low-pressure side of the level 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:
The differential pressure ΔP sensed by the transmitter is always a negative number (i.e., low pressure side is at a higher pressure than 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 would 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.
Further, when the liquid or the fluid contains suspended solid or is chemically corrosive or radioactive, it is desirable to prevent it from directly contacting the level transmitter. In these cases, a bubbler measurement system, which uses a purge system is used.
Bubbler Level Measurement System:
This measurement system is classified on the following types:
1)Open Tank Application for Bubbler System:
The figure below illustrates the typical bubbler system installation:
As shown in the above figure, a bubbler tube is immersed to the bottom of the vessel in which the liquid level is to be measured. A gas (called purge gas) is allowed to pass through the bubbler tube. Consider that the tank is empty. In this case, the gas will escape freely at the end of the tube and therefore the gas pressure inside the bubbler tube (called back pressure) will be at atmospheric pressure. However, as the liquid level inside the tank increases, pressure exerted by the liquid at the base of the tank (and at the opening of the bubbler tube) increases. The hydrostatic pressure of the liquid in effect acts as a seal, which restricts the escape of, purge gas from the bubbler tube.
As a result, the gas pressure in the bubbler tube will continue to increase until it just balances the hydrostatic pressure (P = S⋅H) of the liquid. At this point the back pressure in the bubbler tube is exactly the same as the hydrostatic pressure of the liquid and it will remain constant until any change in the liquid level occurs. Any excess supply pressure will escape as bubbles through the liquid. As the liquid level rises, the back pressure in the bubbler tube increases proportionally, since the density of the liquid is constant. A level transmitter (DP cell) can be used to monitor this back pressure. In an open tank installation, the bubbler tube is connected to the high-pressure side of the transmitter, while the low pressure side is vented to atmosphere. The output of the transmitter will be proportional to the tank level.
A constant differential pressure relay is often used in the purge gas line to ensure that constant bubbling action occurs at all tank levels. The constant differential pressure relay maintains a constant flow rate of purge gas in the bubbler tube regardless of tank level variations or supply fluctuation. This ensures that bubbling will occur to maximum tank level and the flow rate does not increase at low tank level in such a way as to cause excessive disturbances at the surface of the liquid. Note that bubbling action has to be continuous or the measurement signal will not be accurate. An additional advantage of the bubbler system is that, since it measures only the back pressure of the purge gas, the exact location of the level transmitter is not important. The transmitter can be mounted some distance from the process. Open loop bubblers are used to measure levels in spent fuel bays.
2)Closed Tank Application for Bubbler System:
If the bubbler system is to be applied to measure level in a closed tank, some pressure-regulating scheme must be provided for the gas space in the tank. Otherwise, the gas bubbling through the liquid will pressurize the gas space to a point where bubbler supply pressure cannot overcome the static pressure it acts against. The result would be no bubble flow and, therefore, inaccurate measurement signal. Also, as in the case of a closed tank inferential level measurement system, the low-pressure side of the level transmitter has to be connected to the gas space in order to compensate for the effect of gas pressure. Some typical examples of closed tank application of bubbler systems are the measurement of water level in the irradiated fuel bays and the light water level in the liquid zone control tanks.
2.1)Effect of Temperature on Level Measurement:
Level measurement systems that use differential pressure ΔP as the sensing method, are by their very nature affected by temperature and pressure. Recall that the measured height H of a column of liquid is directly proportional to the pressure P exerted at the base of the column and inversely proportional to the density ρ of the liquid.
Density (mass per unit volume) of a liquid or gas is inversely proportional to its temperature.
Thus, for any given amount of liquid in a container, the pressure P exerted at the base will remain constant, but the height will vary directly with the temperature.
Consider the following scenario. A given amount of liquid in a container [figure a] is exposed to higher process temperatures [figure b].
The above scenario of figure is a common occurrence in plant operations. Consider a level transmitter calibrated to read correctly at 750C.
If the process temperature is increased to 900C as in figure (c), the actual level will be higher than indicated.
The temperature error can also occur in wet-leg systems.
If the reference leg and variable leg are at the same temperature that the level transmitter (LT) is calibrated for, the system will accurately measure liquid level. However, as the process temperature increases, the actual process fluid level increases (as previously discussed), while the indicated measurement remains unchanged. Further errors can occur if the reference leg and the variable (sensing) leg are at different temperatures. The level indication will have increasing positive (high) error as the temperature of the wet reference leg increases above the variable (process) leg. As an example, consider temperature changes around a liquid storage tank with a wet leg. As temperature falls and the wet leg cools off, the density of the liquid inside it increases, while the temperature in the tank remains practically unchanged (because of a much bigger volume and connection to the process). As a result the pressure of the reference leg rises and the indicated level decreases. If it happens to the boiler level measurement for a shutdown system it can even lead to an unnecessary reactor trip on boiler low level. However, high-level trips may be prevented under these circumstances. In an extreme case the wet leg may freeze invalidating the measurement scheme completely, but it could be easily prevented with trace heating as indicated earlier in the wet-leg installation figure.
False high level indication can be caused by an increased wet leg temperature, gas or vapour bubbles or a drained wet leg.
A high measured tank level, with the real level being dangerously low, may prevent the actuation of a safety system on a low value of the trip parameter. The real level may even get sufficiently low to cause either the cavitation of the pumps that take suction from the tank or gas ingress into the pumps and result in gas locking and a reduced or no flow condition. If the pumps are associated with a safety system like ECI or a safety related system like PHT shutdown cooling, it can lead to possible safety system impairments and increased probability of resultant fuel damage.
Effect of Pressure on Level Measurement:
Level measurement systems that use differential pressure ΔP as the sensing method, are also affected by pressure, although not to the same degree as temperature mentioned in the previous section. Again the measured height H of a column of liquid is directly proportional to the pressure PL exerted at the base of the column by the liquid and inversely proportional to the density ρ of the liquid:
Density (mass per unit volume) of a liquid or gas is directly proportional to the process or system pressure Ps.
Thus, for any given amount of liquid in a container, the pressure PL (liquid pressure) exerted at the base of the container by the liquid will remain constant, but the height will vary inversely with the process or system pressure.
Most liquids are fairly in compressible and the process pressure will not affect the level unless there is significant vapour content.
Level Measurement System Errors:
The level measurement techniques described in this module use inferred processes and not direct measurements. Namely, the indication of fluid level is based on the pressure exerted on a differential pressure (DP) cell by the height of the liquid in the vessel. This places great importance on the physical and environmental problems that can affect the accuracy of this indirect measurement.
Connections:
As amusing as it may sound, many avoidable errors occur because the DP cell had the sensing line connections reversed. In systems that have high operating pressure but low hydrostatic pressure due to weight of the fluid, this is easy to occur. This is particularly important for closed tank systems. With an incorrectly connected DP cell the indicated level would go down while the true tank level increases.
Over-Pressuring:
Three valve manifolds are provided on DP cells to prevent over-pressuring and aid in the removal of cells for maintenance. Incorrect procedures can inadvertently over-pressure the differential pressure cell. If the cell does not fail immediately the internal diaphragm may become distorted. The
measurements could read either high or low depending on the mode of failure.
Note that if the equalizing valve on the three-valve manifold is inadvertently opened, the level indication will of course drop to a very low level as the pressure across the DP cell equalizes.
Sensing lines:
The sensing lines are the umbilical cord to the DP cell and must be functioning correctly. Some of the errors that can occur are:
Obstructed sensing lines:
The small diameter lines can become clogged with particulate, with resulting inaccurate readings. Sometimes the problem is first noted as an unusually sluggish response to a predicted change in level. Periodic draining and flushing of sensing lines is a must.
Draining sensing lines:
As mentioned previously, the lines must be drained to remove any debris or particulate that may settle to the bottom of the tank and in the line. Also, in closed tank dry leg systems, condensate must be removed regularly to prevent fluid pressure building up on the low-pressure impulse line. Failure to do so will of course give a low tank level reading. Procedural care must be exercised to ensure the DP cell is not over-ranged inadvertently during draining. Such could happen if the block valves are not closed and equalizing valve opened beforehand.
False high level indication can be caused by a leaking or drained wet leg.
A leaking variable (process) leg can cause false low-level indication.
Phigh = Patm + S⋅H
Plow = Patm
Differential pressure ΔP = Phigh - Plow = S⋅H
The level transmitter can be calibrated to output 4 mA when the tank is at 0% level and 20 mA when the tank is at 100% level.
Now,
What if the top of the tank is covered. Lets assume the tank to 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 (30 kPa) or so, whereas the steam pressure is typically 5 MPa. Compensation can be achieved by applying the gas pressure to both the 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 also immediately see the need for the three-valve manifold to protect the DP cell against these pressures.
The different arrangement of the sensing lines to the DP cell is indicated in a typical closed tank application, as shown in the below figure.
Phigh = Pgas + S⋅H
Plow = Pgas
ΔP = Phigh - Plow = 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.
Dry Leg System:
A full dry leg installation with three-valve manifold is shown in the below figure.
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.
This can be seen in the figure below,
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 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 the 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 as shown in the figure below.
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 high-pressure side of the transmitter will be:
Phigh = S⋅H + S⋅ X + Patm
Plow = Patm
ΔP = 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 so that the output of the transmitter is proportional to the tank level (S⋅H) only. This procedure is called Zero Suppression and it can be done during calibration of the transmitter. A zero suppression kit can be installed in the transmitter for this purpose.
Zero Elevation:
When a wet leg installation is used (refer below figure), the low-pressure side of the level 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 = Pgas + S⋅ X
ΔP = Phigh - Plow = S⋅H - S⋅ X = - S (X - H)
The differential pressure ΔP sensed by the transmitter is always a negative number (i.e., low pressure side is at a higher pressure than 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 would 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.
Further, when the liquid or the fluid contains suspended solid or is chemically corrosive or radioactive, it is desirable to prevent it from directly contacting the level transmitter. In these cases, a bubbler measurement system, which uses a purge system is used.
Bubbler Level Measurement System:
This measurement system is classified on the following types:
1)Open Tank Application for Bubbler System:
The figure below illustrates the typical bubbler system installation:
As a result, the gas pressure in the bubbler tube will continue to increase until it just balances the hydrostatic pressure (P = S⋅H) of the liquid. At this point the back pressure in the bubbler tube is exactly the same as the hydrostatic pressure of the liquid and it will remain constant until any change in the liquid level occurs. Any excess supply pressure will escape as bubbles through the liquid. As the liquid level rises, the back pressure in the bubbler tube increases proportionally, since the density of the liquid is constant. A level transmitter (DP cell) can be used to monitor this back pressure. In an open tank installation, the bubbler tube is connected to the high-pressure side of the transmitter, while the low pressure side is vented to atmosphere. The output of the transmitter will be proportional to the tank level.
A constant differential pressure relay is often used in the purge gas line to ensure that constant bubbling action occurs at all tank levels. The constant differential pressure relay maintains a constant flow rate of purge gas in the bubbler tube regardless of tank level variations or supply fluctuation. This ensures that bubbling will occur to maximum tank level and the flow rate does not increase at low tank level in such a way as to cause excessive disturbances at the surface of the liquid. Note that bubbling action has to be continuous or the measurement signal will not be accurate. An additional advantage of the bubbler system is that, since it measures only the back pressure of the purge gas, the exact location of the level transmitter is not important. The transmitter can be mounted some distance from the process. Open loop bubblers are used to measure levels in spent fuel bays.
2)Closed Tank Application for Bubbler System:
If the bubbler system is to be applied to measure level in a closed tank, some pressure-regulating scheme must be provided for the gas space in the tank. Otherwise, the gas bubbling through the liquid will pressurize the gas space to a point where bubbler supply pressure cannot overcome the static pressure it acts against. The result would be no bubble flow and, therefore, inaccurate measurement signal. Also, as in the case of a closed tank inferential level measurement system, the low-pressure side of the level transmitter has to be connected to the gas space in order to compensate for the effect of gas pressure. Some typical examples of closed tank application of bubbler systems are the measurement of water level in the irradiated fuel bays and the light water level in the liquid zone control tanks.
2.1)Effect of Temperature on Level Measurement:
Level measurement systems that use differential pressure ΔP as the sensing method, are by their very nature affected by temperature and pressure. Recall that the measured height H of a column of liquid is directly proportional to the pressure P exerted at the base of the column and inversely proportional to the density ρ of the liquid.
H α P/ρ
Density (mass per unit volume) of a liquid or gas is inversely proportional to its temperature.
ρ α 1/T
Thus, for any given amount of liquid in a container, the pressure P exerted at the base will remain constant, but the height will vary directly with the temperature.
H α T
Consider the following scenario. A given amount of liquid in a container [figure a] is exposed to higher process temperatures [figure b].
Low Pressure
High Pressure
As the amount (mass) of liquid does not change from figure (a) to (b), the pressure exerted on the base of the container has not changed and the indicated height of the liquid does not change. However, the volume occupied by the liquid has increased and thus the actual height has increased.The above scenario of figure is a common occurrence in plant operations. Consider a level transmitter calibrated to read correctly at 750C.
If the process temperature is increased to 900C as in figure (c), the actual level will be higher than indicated.
The temperature error can also occur in wet-leg systems.
False high level indication can be caused by an increased wet leg temperature, gas or vapour bubbles or a drained wet leg.
A high measured tank level, with the real level being dangerously low, may prevent the actuation of a safety system on a low value of the trip parameter. The real level may even get sufficiently low to cause either the cavitation of the pumps that take suction from the tank or gas ingress into the pumps and result in gas locking and a reduced or no flow condition. If the pumps are associated with a safety system like ECI or a safety related system like PHT shutdown cooling, it can lead to possible safety system impairments and increased probability of resultant fuel damage.
Effect of Pressure on Level Measurement:
Level measurement systems that use differential pressure ΔP as the sensing method, are also affected by pressure, although not to the same degree as temperature mentioned in the previous section. Again the measured height H of a column of liquid is directly proportional to the pressure PL exerted at the base of the column by the liquid and inversely proportional to the density ρ of the liquid:
H α PL/ρ
Density (mass per unit volume) of a liquid or gas is directly proportional to the process or system pressure Ps.
ρ α Ps
Thus, for any given amount of liquid in a container, the pressure PL (liquid pressure) exerted at the base of the container by the liquid will remain constant, but the height will vary inversely with the process or system pressure.
H α 1/Ps
Most liquids are fairly in compressible and the process pressure will not affect the level unless there is significant vapour content.
Level Measurement System Errors:
The level measurement techniques described in this module use inferred processes and not direct measurements. Namely, the indication of fluid level is based on the pressure exerted on a differential pressure (DP) cell by the height of the liquid in the vessel. This places great importance on the physical and environmental problems that can affect the accuracy of this indirect measurement.
Connections:
As amusing as it may sound, many avoidable errors occur because the DP cell had the sensing line connections reversed. In systems that have high operating pressure but low hydrostatic pressure due to weight of the fluid, this is easy to occur. This is particularly important for closed tank systems. With an incorrectly connected DP cell the indicated level would go down while the true tank level increases.
Over-Pressuring:
Three valve manifolds are provided on DP cells to prevent over-pressuring and aid in the removal of cells for maintenance. Incorrect procedures can inadvertently over-pressure the differential pressure cell. If the cell does not fail immediately the internal diaphragm may become distorted. The
measurements could read either high or low depending on the mode of failure.
Note that if the equalizing valve on the three-valve manifold is inadvertently opened, the level indication will of course drop to a very low level as the pressure across the DP cell equalizes.
Sensing lines:
The sensing lines are the umbilical cord to the DP cell and must be functioning correctly. Some of the errors that can occur are:
Obstructed sensing lines:
The small diameter lines can become clogged with particulate, with resulting inaccurate readings. Sometimes the problem is first noted as an unusually sluggish response to a predicted change in level. Periodic draining and flushing of sensing lines is a must.
Draining sensing lines:
As mentioned previously, the lines must be drained to remove any debris or particulate that may settle to the bottom of the tank and in the line. Also, in closed tank dry leg systems, condensate must be removed regularly to prevent fluid pressure building up on the low-pressure impulse line. Failure to do so will of course give a low tank level reading. Procedural care must be exercised to ensure the DP cell is not over-ranged inadvertently during draining. Such could happen if the block valves are not closed and equalizing valve opened beforehand.
False high level indication can be caused by a leaking or drained wet leg.
A leaking variable (process) leg can cause false low-level indication.
No comments:
Post a Comment