Remote Seal Transmitters – top 25 Interview Q&A for Field Engineers

Top 25 Interview Questions and Answers for Field Engineers on Remote Seal Transmitters

For field engineers working in the process control and instrumentation domain, a thorough understanding of remote seal transmitters is crucial. These instruments are vital for reliable pressure and level measurement in challenging process conditions. This curated list of 25 interview questions and answers will help you prepare for your next technical interview, covering fundamental concepts, installation, calibration, troubleshooting, and maintenance of remote seal transmitters.

Category 1: Fundamentals and Operating Principles

1. What is a remote seal transmitter and why is it used?

A remote seal transmitter is a pressure or differential pressure transmitter equipped with one or two diaphragm seals connected via a capillary tube filled with a specific fluid. They are used to isolate the transmitter’s sensing element from the process medium. This is necessary when the process fluid is corrosive, highly viscous, contains suspended solids, is at an extreme temperature, or has a tendency to solidify or polymerize in the impulse lines.

2. Explain the working principle of a remote seal transmitter.

A remote seal system operates on the principle of hydraulic pressure transmission. The process pressure exerted on the flexible diaphragm of the remote seal is transferred through the incompressible fill fluid within the capillary tube to the transmitter’s sensing diaphragm. This pressure change is then converted into an electrical signal (typically 4-20 mA with HART) by the transmitter’s electronics. For differential pressure measurement with two seals, the pressure difference between the high and low process connections is transmitted.

3. What are the main components of a remote seal system?

The primary components are:

Pressure/DP Transmitter: The main body that measures the pressure and converts it to an output signal.
Diaphragm Seal: A flexible membrane that interfaces with the process fluid. It’s available in various materials to ensure chemical compatibility.
Capillary Tube: A small-diameter tube that connects the diaphragm seal to the transmitter. Its length can vary based on the application’s needs.
Fill Fluid: An incompressible fluid that fills the capillary and seal system, transmitting the pressure.
Process Connection: Flanges or threaded connections that mount the seal to the process vessel or pipe.

4. What is a “balanced system” in the context of two remote seals?

A balanced system refers to a differential pressure transmitter with two remote seals where the capillaries are of equal length. This is the ideal configuration to minimize temperature-induced errors. When the ambient temperature changes, the fill fluid in both capillaries expands or contracts equally, and the resulting pressure changes on the high and low sides of the transmitter cancel each other out, leading to a stable and accurate measurement.

5. What are the common fill fluids used in remote seals and how are they selected?

Common fill fluids include silicone oils (like DC200, DC704), glycerin, and various inert fluids like Halocarbon or Fluorinert. The selection depends on several factors:

Process Temperature: The fluid must remain stable and have low vapor pressure across the entire operating temperature range.
Process Pressure (especially vacuum): The fluid should not boil under vacuum conditions.
Chemical Compatibility: To prevent reaction with the process fluid in case of a diaphragm rupture.
Application: For food and pharmaceutical applications, FDA-approved fluids like food-grade glycerin are required.

Category 2: Installation and Commissioning

6. What are the crucial considerations when installing a remote seal transmitter for level measurement in a closed tank?

Transmitter Mounting Location: For vacuum service, the transmitter must be mounted below the bottom process tap to maintain a positive head on the transmitter. In pressurized service, it can be mounted at, above, or below the taps, but the effect of the fill fluid head must be compensated for during calibration.
Capillary Length: Keep capillaries as short as possible to minimize temperature effects and response time.
Capillary Routing: Avoid sharp bends and protect the capillaries from physical damage and extreme temperature gradients.
Zero Elevation/Suppression: The hydrostatic head created by the fill fluid in the capillaries must be calculated and compensated for in the transmitter’s configuration (LRV/URV settings).

7. How does the mounting position of the transmitter relative to the seals affect calibration?

Transmitter below the seal(s): The fill fluid creates a positive head (pressure) on the transmitter’s sensor. This requires zero suppression, where the Lower Range Value (LRV) is set to a positive value to counteract this head pressure at the minimum process level.
Transmitter above the seal(s): The fill fluid creates a negative head (vacuum effect) on the sensor. This necessitates zero elevation, where the LRV is set to a negative value.

8. What is a “wet leg” versus a “dry leg,” and how does a remote seal on the low-pressure side eliminate the issues of a conventional wet leg?

In a conventional DP level measurement on a closed tank, the low-pressure side impulse line can be either a “dry leg” (filled with non-condensing gas) or a “wet leg” (intentionally filled with a liquid). A wet leg is often problematic due to temperature-induced density changes and the potential for evaporation or contamination, leading to measurement errors. A remote seal on the low-pressure side replaces the wet leg with a stable, sealed hydraulic system, eliminating these issues and providing a reliable and maintenance-free reference pressure.

9. What precautions should be taken when handling and installing capillary tubes?

Do not kink or crush the capillary: This can create a restriction and dampen the pressure response.
Maintain a minimum bending radius: Typically specified by the manufacturer (e.g., 3 inches).
Protect from high temperatures and welding splatter: Excessive heat can damage the fill fluid and the capillary.
Support the capillary: Avoid letting it hang freely over long distances to prevent strain and vibration-induced damage.

10. During commissioning, how do you perform a zero check on a remote seal level transmitter?

With the process at a known minimum level (or the vessel empty), the transmitter’s output should correspond to the calculated Lower Range Value (LRV). If not, a zero adjustment is required. It’s critical to ensure the calculated LRV correctly accounts for the hydrostatic head of the fill fluid (zero suppression or elevation) and the specific gravity of the process fluid. A common mistake is to zero the transmitter at atmospheric pressure without considering the fill fluid head.

Category 3: Calibration and Maintenance

11. Can you explain the formula for calculating the LRV and URV for a remote seal level transmitter on a closed tank?

Let:

$H$ = Measuring Span (in inches or mm)
$h_{1}$ = Distance from the HP seal to the transmitter (for head calculation)
$h_{2}$ = Distance from the LP seal to the transmitter (for head calculation)
$S G_{p}$ = Specific Gravity of the process fluid
$S G_{f}$ = Specific Gravity of the fill fluid
$P_{g a s}$ = Pressure of the gas in the tank’s vapor space

The pressure on the High side ( $P_{H P}$ ) and Low side ( $P_{L P}$ ) is calculated at both the 0% and 100% levels.

At 0% Level (tank empty to the HP tap):

$P_{H P}$ = Pressure from fill fluid in HP capillary + $P_{g a s}$
$P_{L P}$ = Pressure from fill fluid in LP capillary + $P_{g a s}$
$L R V = Δ P = P_{H P} - P_{L P}$ (This will be a function of the difference in capillary head pressures)

At 100% Level:

$P_{H P}$ = Pressure from process fluid + Pressure from fill fluid in HP capillary + $P_{g a s}$
$P_{L P}$ = Pressure from fill fluid in LP capillary + $P_{g a s}$
$U R V = Δ P = P_{H P} - P_{L P}$

The final LRV and URV will be in units of pressure (e.g., inH2O, mbar).

12. Why should you never fill a remote seal system in the field?

Remote seal systems are filled under a high vacuum in a controlled factory environment. This process removes all air and moisture from the system. Attempting to fill a system in the field would inevitably introduce air bubbles. Since air is compressible and highly affected by temperature changes, its presence would lead to significant measurement errors and an unreliable transmitter.

13. How does ambient temperature affect the accuracy of a remote seal transmitter, and how can this effect be minimized?

Ambient temperature changes cause the fill fluid to expand or contract, which alters the pressure within the sealed system, leading to a zero shift in the measurement. This effect is more pronounced with longer capillaries and larger temperature fluctuations. Minimization techniques include:

Using a balanced system with equal capillary lengths.
Keeping capillaries as short as possible.
Routing both capillaries together so they are exposed to the same ambient conditions.
Using a fill fluid with a low coefficient of thermal expansion.

14. What is hydrogen permeation, and in which applications is it a concern for remote seals?

Hydrogen permeation is the phenomenon where small hydrogen ions from the process fluid migrate through the molecular structure of the diaphragm seal material. Over time, these ions accumulate in the fill fluid, forming hydrogen gas bubbles. This increases the internal pressure, causing a significant upward drift in the transmitter’s reading. It is a major concern in applications like refineries (hydrocrackers), chemical plants, and processes involving high-pressure hydrogen. Using gold-plated diaphragms or special non-permeable materials can mitigate this issue.

15. What are the key steps in a bench calibration of a remote seal transmitter?

Isolate and remove: Safely disconnect the transmitter from the process.
Setup: Mount the transmitter in a stable position, ideally at the same orientation as its final installation.
Connect: Connect a pressure calibrator to the high-pressure seal and vent the low-pressure seal to the atmosphere (for a single seal system). For a dual seal system, apply pressure to the HP seal while the LP seal is at atmospheric pressure.
Power up and connect HART: Power the transmitter and connect a HART communicator.
Check LRV and URV: Verify the configured range.
Perform a 5-point check: Apply pressure corresponding to 0%, 25%, 50%, 75%, and 100% of the calibrated range (both upscale and downscale) and check the mA output and the digital reading on the communicator.
Adjust if necessary: Perform a “Digital Trim” (Sensor Trim) if the readings are outside the acceptable tolerance.
Document: Record the “as found” and “as left” data.

Category 4: Troubleshooting

16. A remote seal level transmitter is showing a constant high reading, even when the tank level is known to be low. What are the possible causes?

Blockage on the low-pressure side: A clog in the process connection of the LP seal can trap pressure.
Loss of fill fluid on the low-pressure side: A leak in the LP capillary or seal will cause the HP side to always read higher.
Hydrogen permeation: Gas buildup inside the high-pressure seal.
Incorrect calibration: The LRV might be set incorrectly.
Transmitter malfunction: An internal fault in the transmitter.

17. The output of a remote seal transmitter is erratic or noisy. What would you investigate?

Process-induced pulsation: Check for pumps or agitators causing rapid pressure fluctuations. The transmitter’s damping function might need to be adjusted.
Air/gas bubbles in the fill fluid: This is a factory defect but can occur. It often manifests as instability with temperature changes.
Electrical interference: Check for proper grounding and shielding of the signal wires.
Vibration: Ensure the transmitter and capillaries are securely mounted and not subject to excessive vibration.
Loose electrical connections.

18. What could cause a slow or sluggish response from a remote seal transmitter?

Long capillary tubes: The longer the capillary, the slower the response time due to fluid friction.
High viscosity fill fluid: At low temperatures, the viscosity of the fill fluid increases, slowing down the pressure transmission.
Kinked or restricted capillary: A partial blockage will dampen the pressure signal.
High damping value: The electronic damping in the transmitter may be set too high.

19. If you suspect a leak in the remote seal system, how would you confirm it?

A visual inspection for fill fluid at the process connections, welds, or along the capillary is the first step. For a non-obvious leak, the transmitter will likely show a drift in its reading over time, often towards a lower value as the fill fluid escapes. A definitive check involves taking the transmitter out of service and applying a stable pressure. If the reading does not hold steady and drifts, a leak is highly probable. A vacuum test during a bench calibration can also reveal leaks.

20. A transmitter in vacuum service is reading incorrectly. What are some specific issues to look for?

Incorrect mounting: The transmitter must be mounted below the lowest process connection to ensure the fill fluid head prevents the internal pressure from dropping to the fluid’s vapor pressure, which would cause the fluid to boil.
Inappropriate fill fluid: The fill fluid must have a very low vapor pressure to be suitable for vacuum applications.
System leaks: Even a minuscule leak will allow air to be drawn into the system, causing significant errors.

Category 5: Advanced Concepts and Application-Specific Questions

21. What is “turndown,” and why is it an important specification for a remote seal transmitter?

Turndown ratio refers to the ratio of the maximum calibrated span to the minimum calibrated span of a transmitter. For example, a transmitter with a 100:1 turndown can be accurately calibrated for a range of 1000 inH2O as well as a range of 10 inH2O. This is important for remote seal transmitters as they often have to measure relatively small differential pressures, and a high turndown allows a standard transmitter model to be used across a wide variety of applications, reducing the need for multiple models.

22. When would you choose a pancake-style seal versus a flanged seal with an extension?

Pancake-style (or flush-mount) seal: This type is mounted flush with the vessel wall. It is ideal for viscous or slurry-type fluids as it prevents any dead zones where the process medium can accumulate and clog the connection.
Flanged seal with an extension: This design places the diaphragm seal further into the process vessel. It is used to bypass nozzle or flange-related dead spaces and get the measurement diaphragm directly into the representative process fluid, which is useful in tanks with agitators or where wall effects could influence the measurement.

23. Explain how a DP transmitter with two remote seals can be used to measure interface level between two immiscible liquids.

This application works by measuring the hydrostatic pressure of the column of liquid. The transmitter is calibrated based on the specific gravities of the two liquids.

The LRV is set to correspond to the pressure when the vessel is filled to the measurement span with the lighter liquid.
The URV is set to correspond to the pressure when the vessel is filled with the heavier liquid. The transmitter’s output will then proportionally represent the position of the interface between the two liquids. It is crucial that the total level remains constant for this measurement to be accurate.

24. What are some safety considerations when working with remote seal transmitters?

Process Isolation: Always ensure the transmitter is properly isolated from the process before starting any work.
De-pressurization and Draining: Safely vent any trapped pressure and drain any hazardous process fluid.
Personal Protective Equipment (PPE): Use appropriate PPE based on the nature of the process fluid (e.g., chemical gloves, safety glasses, face shield).
Temperature: Be aware of high process temperatures and allow the equipment to cool down before handling.
Material Safety Data Sheets (MSDS): Be familiar with the hazards of both the process fluid and the fill fluid.

25. Describe a challenging remote seal transmitter application you have worked on and how you resolved the issues.

This is a behavioral question designed to assess your practical experience and problem-solving skills. A good answer would involve:

S.T.A.R. Method: Describe the Situation (e.g., a high-temperature, corrosive application with level measurement issues), the Task you were assigned, the Action you took (e.g., investigated the existing installation, found an incorrect fill fluid and material selection, calculated the correct calibration, specified a new transmitter with Hastelloy seals and a high-temperature silicone fill, and oversaw the installation and commissioning), and the Result (e.g., stable and accurate level measurement, reduced maintenance, and improved process control).

By mastering these questions, a field engineer can confidently demonstrate their expertise in handling the complexities of remote seal transmitters in a variety of industrial settings.