Here are 25 interview questions and answers for open and closed tank level measurement, compiled from various instrumentation and automation resources.
Open Tank Level Measurement
1. What is the fundamental principle of level measurement using a Differential Pressure (DP) transmitter in an open tank?
In an open tank, the level is measured by determining the hydrostatic pressure exerted by the liquid column. The high-pressure (HP) side of the DP transmitter is connected to the bottom of the tank, while the low-pressure (LP) side is vented to the atmosphere. The transmitter measures the pressure difference, which is directly proportional to the liquid level.
2. How do you calculate the calibration range for a DP transmitter in an open tank?
The calibration range is determined by the Lower Range Value (LRV) and Upper Range Value (URV).
- LRV (4mA): Corresponds to the 0% level. In a typical installation, this is 0.
- URV (20mA): Corresponds to the 100% level. This is calculated as:
URV = H x SG, where H is the height of the liquid at 100% and SG is the specific gravity of the liquid.
3. What is “zero suppression” in open tank level measurement?
Zero suppression is when the DP transmitter is installed below the bottom tapping point of the tank. In this case, the transmitter always senses a minimum pressure, even when the tank is empty, due to the liquid in the impulse line. The LRV will be a positive value.
4. What is “zero elevation” in open tank level measurement?
Zero elevation is less common in open tanks but involves mounting the transmitter above the bottom tap. This is generally avoided due to the potential for air traps in the impulse line.
5. What is the effect of specific gravity changes on open tank level measurement?
If the specific gravity of the process fluid changes, the level reading will be inaccurate. If the SG increases, the transmitter will indicate a higher level than the actual level, and if the SG decreases, it will indicate a lower level.
Closed Tank Level Measurement
6. How does level measurement in a closed tank differ from an open tank?
In a closed tank, the space above the liquid is not open to the atmosphere and can have a variable pressure. To measure the level accurately, this vapor pressure must be compensated for. This is done by connecting the LP side of the DP transmitter to the top of the tank to sense the vapor pressure.
7. What is a “dry leg” installation in a closed tank?
A dry leg installation is used when the vapor in the tank is non-condensing and non-corrosive. The LP impulse line is filled with a non-condensing gas (like air or nitrogen) and connects the top of the tank to the LP side of the transmitter.
8. When is a “wet leg” installation necessary?
A wet leg installation is required when the vapor in the tank is condensable. The LP impulse line is intentionally filled with a liquid (either the process fluid itself or a compatible fill fluid) to prevent condensation and ensure a constant reference pressure on the LP side.
9. How do you calculate the calibration range for a dry leg installation?
- LRV (4mA) at 0% level:
(H1 x SG_fluid) - (H2 x SG_dry_gas). Since the specific gravity of the gas is negligible, the formula is approximatelyH1 x SG_fluid. - URV (20mA) at 100% level:
((H1 + H) x SG_fluid) - (H2 x SG_dry_gas). Approximately(H1 + H) x SG_fluid. (Where H1 is the distance from the HP tap to the transmitter, H is the measuring span, and H2 is the height of the dry leg).
10. How do you calculate the calibration range for a wet leg installation?
- LRV (4mA) at 0% level:
(H1 x SG_fluid) - (H2 x SG_fill_fluid) - URV (20mA) at 100% level:
((H1 + H) x SG_fluid) - (H2 x SG_fill_fluid)(Where H1 is the distance from the HP tap to the transmitter, H is the measuring span, and H2 is the height of the wet leg).
11. Why is the fill fluid in a wet leg important?
The fill fluid must be compatible with the process fluid (in case of leakage), have a stable specific gravity over the operating temperature range, and not freeze or boil at process conditions.
12. What are some common problems with dry leg installations?
The primary issue is the potential for condensation in the LP leg, which will cause an error in the level reading.
13. What are some common problems with wet leg installations?
Issues include changes in the specific gravity of the fill fluid due to temperature fluctuations, and the potential for the fill fluid to leak or evaporate.
14. What is a “bubbler” system for level measurement?
A bubbler system is an alternative for corrosive or high-temperature liquids. A dip tube is installed in the tank, and a constant flow of air or inert gas is “bubbled” through it. The back pressure required to bubble the gas out of the tube is proportional to the liquid level. This pressure is then measured by a pressure transmitter.
15. How do you choose between a DP transmitter and other level measurement technologies like radar or ultrasonic?
DP transmitters are robust, well-understood, and suitable for a wide range of applications. However, radar and ultrasonic transmitters are non-contact and are unaffected by changes in liquid density, making them a better choice for certain applications. The choice depends on factors like the process fluid, temperature, pressure, and required accuracy.
General DP Level Measurement Questions
16. What is the purpose of a 3-valve manifold?
A 3-valve manifold is used to isolate, equalize, and vent the DP transmitter. It allows for safe removal and calibration of the transmitter without shutting down the process.
17. How do you perform a zero check on a DP transmitter?
By closing the block valves and opening the equalizing valve on the 3-valve manifold, the same pressure is applied to both the HP and LP sides of the transmitter. The output should then be at the zero point (4mA for a raised zero, or the calculated LRV).
18. What are impulse lines?
Impulse lines are the pipes or tubes that connect the process tapping points to the DP transmitter.
19. What precautions should be taken when installing impulse lines?
- They should be as short as possible.
- They should be sloped to allow for drainage (for liquids) or venting (for gases) to prevent trapped fluids or bubbles.
- They should be insulated if temperature variations can affect the fluid density.
20. What is the effect of temperature on DP level measurement?
Temperature changes can affect the specific gravity of both the process fluid and the fill fluid in a wet leg, leading to inaccuracies. Temperature compensation may be required for high-accuracy applications.
21. What is “interfacing level measurement”?
This is the measurement of the level between two immiscible liquids with different specific gravities in the same tank. A DP transmitter can be used for this, but the calibration calculations are more complex.
22. Can you use a DP transmitter for solid level measurement?
DP transmitters are generally not used for measuring the level of solids. Technologies like guided-wave radar, ultrasonic, or weight-based systems are more suitable.
23. What is a capillary seal system?
A capillary seal system is used when the process fluid is highly corrosive, viscous, or at a very high temperature. The DP transmitter is connected remotely to diaphragm seals via filled capillary tubes. This isolates the transmitter from the harsh process conditions.
24. What is the “turndown ratio” of a DP transmitter?
The turndown ratio is the ratio of the maximum calibrated span to the minimum calibrated span. A higher turndown ratio means the transmitter can be used for a wider range of applications.
25. How do you troubleshoot a DP level transmitter that is giving an incorrect reading?
Troubleshooting steps include:
- Checking for leaks in the impulse lines.
- Verifying that the block and equalizing valves are in the correct position.
- Checking for blockages in the impulse lines.
- Verifying the specific gravity of the process fluid.
- Recalibrating the transmitter.