DP Range Calculation for Closed Tank – Top 20 interview Q&A

Mastering DP Range Calculation for Closed Tanks: Top 20 Interview Q&A

The accurate measurement of liquid level in closed or pressurized tanks is a critical task in numerous industrial processes. Differential Pressure (DP) transmitters remain a robust and widely used solution for this application. A thorough understanding of the principles behind DP range calculation is, therefore, a key skill for instrumentation and control engineers. Excelling in an interview for a related role often hinges on the ability to articulate these concepts clearly and apply them to various scenarios.

Here are the top 20 interview questions and answers to help you prepare for your next technical interview, covering the essential aspects of DP range calculation for closed tanks.

1. What is the fundamental principle of level measurement using a Differential Pressure (DP) transmitter in a closed tank?

In a closed tank, a DP transmitter measures the difference between the pressure at the bottom of the tank and the pressure in the vapor space at the top. The pressure at the bottom is a combination of the hydrostatic pressure exerted by the liquid column and the static pressure of the gas/vapor above the liquid. The top connection, known as the low-pressure (LP) side, measures only this static gas/vapor pressure. The DP transmitter effectively cancels out the common static pressure, leaving only the hydrostatic pressure, which is directly proportional to the liquid level.

2. Why is a DP transmitter necessary for a closed tank instead of a simple pressure transmitter?

A simple pressure transmitter installed at the bottom of a closed tank would measure the sum of the hydrostatic pressure and the tank’s internal (vapor) pressure. This vapor pressure can fluctuate due to temperature changes or process conditions, leading to inaccurate level readings. A DP transmitter compensates for these fluctuations by measuring the vapor pressure at the top of the tank and subtracting it from the total pressure at the bottom, thus providing a true reading of the liquid level.

3. What are the key parameters required for calculating the DP range?

To accurately calculate the DP range for a closed tank level measurement, you need the following parameters:

H1: The minimum level to be measured (0% level).
H2: The maximum level to be measured (100% level or the span).
SGp: The specific gravity of the process liquid.
SGf: The specific gravity of the fill fluid in the wet leg (if applicable).
h: The vertical distance between the HP tapping point and the transmitter.
L: The vertical distance of the wet leg from the top tapping point.

4. What is the formula for calculating the Lower Range Value (LRV) and Upper Range Value (URV) in a closed tank with a dry leg installation?

In a dry leg installation, the low-pressure impulse line is filled with the tank’s vapor.

The pressure on the High-Pressure (HP) side at 0% level is: $P_{H P (0%)} = (h_{1} \times S G_{p}) + P_{g a s}$

The pressure on the Low-Pressure (LP) side is: $P_{L P} = P_{g a s}$

The differential pressure at 0% level (LRV) is: $L R V = P_{H P (0%)} - P_{L P} = (h_{1} \times S G_{p})$

Similarly, the differential pressure at 100% level (URV) is: $U R V = P_{H P (100%)} - P_{L P} = ((h_{1} + h_{2}) \times S G_{p})$

Where:

$h_{1}$ is the distance from the transmitter’s HP tapping point to the 0% level.
$h_{2}$ is the span (distance between 0% and 100% level).
$S G_{p}$ is the specific gravity of the process liquid.
$P_{g a s}$ is the pressure of the gas in the tank.

Note: If the transmitter is installed at the same level as the 0% tapping point, $h_{1} = 0$ , so the LRV will be 0.

5. What is a “wet leg” installation, and when is it used?

A wet leg installation is used in closed tank level measurement where the vapor in the tank is likely to condense in the low-pressure impulse line. To ensure a constant and stable pressure reference on the LP side, this line is intentionally filled with a non-volatile, process-compatible liquid. This prevents measurement errors that would occur due to a fluctuating liquid head in the LP leg.

6. How do you calculate the LRV and URV for a closed tank with a wet leg?

In a wet leg scenario, the LP leg exerts a constant hydrostatic pressure.

The pressure on the HP side at 0% level is: $P_{H P (0%)} = (h_{1} \times S G_{p}) + P_{g a s}$

The pressure on the LP side (with a wet leg of height L and fill fluid specific gravity $S G_{f}$ ) is: $P_{L P} = (L \times S G_{f}) + P_{g a s}$

The differential pressure at 0% level (LRV) is: $L R V = P_{H P (0%)} - P_{L P} = (h_{1} \times S G_{p}) - (L \times S G_{f})$

The differential pressure at 100% level (URV) is: $U R V = P_{H P (100%)} - P_{L P} = ((h_{1} + h_{2}) \times S G_{p}) - (L \times S G_{f})$

7. What is “zero suppression”? In which scenario does it occur?

Zero suppression occurs when the DP transmitter is mounted below the bottom (HP) tapping point of the tank. In this case, even when the tank level is at 0%, the transmitter senses a positive pressure due to the liquid head in the impulse line connecting to the bottom tap. This offset from true zero must be “suppressed” in the transmitter’s calibration. The LRV will be a positive value.

8. What is “zero elevation”? In which scenario does it occur?

Zero elevation occurs in a wet leg installation where the hydrostatic pressure from the wet leg on the LP side is greater than the pressure on the HP side when the tank is at its minimum level (0%). This results in a negative differential pressure. The transmitter’s zero point must be “elevated” from a negative value to correspond to the 4 mA output. In this case, the LRV will be a negative value.

9. A closed tank contains a liquid with a specific gravity of 0.9. The level measurement range is from a tapping point 1 meter above the tank bottom to 5 meters above the tank bottom. The transmitter is installed at the 1-meter tapping point. Calculate the LRV and URV for a dry leg installation.

H1 (0% level): 1 m
H2 (100% level): 5 m
Span = 5 m – 1 m = 4 m
SGp = 0.9
Transmitter is at the 0% tapping point, so the effective head at 0% is 0.

LRV (at 0% level): The pressure is due to the level above the tapping point. At 0% level (1m from bottom), the head on the transmitter is 0. $L R V = 0 mmH_{2} O$

URV (at 100% level): At 100% level, the height of the liquid above the transmitter is 4 meters. $U R V = Span \times S G_{p} = 4 m \times 0.9 = 3.6 mH_{2} O = 3600 mmH_{2} O$

Calibration Range: 0 to 3600 mmH₂O

10. Consider the same tank as in Q9, but now with a wet leg. The LP tapping is at 6 meters from the bottom, and the wet leg is filled with a fluid of SG = 1.1. Calculate the new LRV and URV.

H1 (0% level): 1 m
H2 (100% level): 5 m
Span = 4 m
SGp = 0.9
LP tapping (L): 6 m from the bottom. The wet leg height is from the top tapping to the transmitter, which is 6m – 1m = 5m.
SGf = 1.1

LRV (at 0% level): $P_{H P (0%)} = 0$ (as the level is at the tapping point) $P_{L P} = 5 m \times 1.1 = 5.5 mH_{2} O$ $L R V = 0 - 5.5 = - 5.5 mH_{2} O = - 5500 mmH_{2} O$

URV (at 100% level): $P_{H P (100%)} = 4 m \times 0.9 = 3.6 mH_{2} O$ $P_{L P} = 5.5 mH_{2} O$ $U R V = 3.6 - 5.5 = - 1.9 mH_{2} O = - 1900 mmH_{2} O$

Calibration Range: -5500 to -1900 mmH₂O

11. How does the density of the process fluid affect the DP range calculation?

The hydrostatic pressure exerted by a liquid column is directly proportional to its density (or specific gravity). Therefore, if the density of the process fluid changes, the DP transmitter’s output will change even if the level remains constant. It is crucial to use the correct specific gravity for the process liquid at its operating temperature for accurate range calculation and calibration.

12. What happens if the process liquid’s specific gravity is higher than what the transmitter was calibrated for?

If the actual SG is higher than the calibrated SG, the transmitter will indicate a level that is higher than the actual level. This is because the higher density liquid will exert more pressure for the same height.

13. What happens if the process liquid’s specific gravity is lower than what the transmitter was calibrated for?

Conversely, if the actual SG is lower than the calibrated SG, the transmitter will indicate a level that is lower than the actual level because the lower density liquid exerts less pressure for the same height.

14. Can you use a DP transmitter to measure the interface level between two immiscible liquids?

Yes, a DP transmitter can be used to measure the interface level between two immiscible liquids in a tank. The calculation is more complex as it involves the specific gravities of both the lighter and the heavier liquids. The DP range will be determined by the difference in the hydrostatic pressures exerted by the two liquids as the interface level changes.

15. What is the purpose of a 3-valve or 5-valve manifold in a DP transmitter installation?

A valve manifold is used to isolate, equalize, and vent the DP transmitter from the process.

Isolate: The block valves allow the transmitter to be safely removed for maintenance or calibration without shutting down the process.
Equalize: The equalizing valve connects the HP and LP sides, allowing for a zero check of the transmitter at line pressure.
Vent/Drain: The vent/drain valves (in a 5-valve manifold) are used to bleed off any trapped process fluid or gas from the transmitter and impulse lines.

16. How does the location of the transmitter (above or below the bottom tap) influence the range calculation?

Transmitter below the tap (Zero Suppression): The liquid head in the impulse line creates a positive offset. This offset must be added to both the LRV and URV calculations.
Transmitter above the tap: This is generally avoided as it can create a vacuum in the impulse line or lead to gas entrapment, causing measurement errors.

17. What are the advantages and disadvantages of using capillary seals with DP transmitters for level measurement?

Advantages:

Eliminate wet or dry legs and their associated maintenance issues.
Isolate the transmitter from extreme process temperatures or corrosive fluids.
Faster response time compared to long impulse lines.

Disadvantages:

Higher initial cost.
Temperature fluctuations along the capillary can affect the fill fluid density, leading to errors.
The system is sealed, so leaks in the capillary require replacement of the entire assembly.

18. If a DP transmitter is calibrated for a specific range, can it be re-ranged for a different application?

Yes, modern smart DP transmitters are highly flexible and can be re-ranged. Using a HART communicator or a fieldbus interface, you can adjust the LRV and URV to match the requirements of a new application, provided the new range falls within the transmitter’s overall sensor limits.

19. What is “turndown ratio” in the context of a DP transmitter?

The turndown ratio indicates the rangeability of a transmitter. It is the ratio of the maximum calibrated span to the minimum calibrated span. For example, a transmitter with a turndown ratio of 100:1 can be accurately calibrated for a span as low as 1/100th of its maximum possible span. A higher turndown ratio offers greater application flexibility.

20. How would you troubleshoot an incorrect level reading from a DP transmitter on a closed tank?

A systematic approach is key:

Check the Manifold: Ensure the block valves are open and the equalizing valve is closed.
Inspect Impulse Lines: Look for leaks, blockages, or (in a dry leg) condensation. For a wet leg, ensure it is completely filled.
Verify Pressures: Use a pressure calibrator to check the pressures on the HP and LP sides independently to see if they match expected values.
Check Transmitter Calibration: Use a HART communicator to verify the configured LRV, URV, and specific gravity values. Perform a zero trim if necessary.
Review Calculations: Double-check the original range calculations for any errors in the specific gravity values or dimensions used.
Consider Process Conditions: Investigate if there have been any changes in the process fluid’s density, temperature, or the tank’s operating pressure that could affect the reading.