Commissioning of Level Instruments – Top 25 interview Q&A

Mastering Your Interview: Top 25 Questions and Answers for Commissioning Level Instruments

The commissioning of level instruments is a critical phase in any process industry, ensuring accurate and reliable measurement for safety, quality, and efficiency. For instrumentation and control professionals, a thorough understanding of this process is paramount. Whether you are a seasoned engineer or a fresh graduate, being well-prepared for interview questions on this topic can significantly boost your career prospects.

This comprehensive guide details the top 25 most frequently asked interview questions concerning the commissioning of level instruments, complete with detailed answers. The questions cover fundamental concepts, specific technologies, procedural steps, troubleshooting, and safety protocols, providing a well-rounded preparation for your next technical interview.

I. Fundamental Concepts and Pre-Commissioning

This section covers the foundational knowledge required before commencing any hands-on commissioning work.

1. What is the primary purpose of commissioning a level instrument?

The primary purpose of commissioning a level instrument is to verify that it is installed correctly, functions according to its design specifications, and provides accurate and reliable level measurements to the control system. This involves a series of checks and tests to ensure the instrument is ready for normal plant operations.

2. What are the essential pre-commissioning checks you would perform for a level transmitter?

Before powering up the instrument, a series of pre-commissioning checks are crucial:

Physical Inspection: Verify that the correct instrument model is installed at the designated tag location as per the P&ID and instrument datasheets. Check for any physical damage during transportation or installation.
Mounting and Orientation: Ensure the instrument is mounted securely and in the correct orientation as specified by the manufacturer. For example, some instruments require vertical mounting.
Process Connections: Confirm that the process connections are correctly made, with no leaks. For differential pressure (DP) transmitters, ensure the high-pressure (HP) and low-pressure (LP) tappings are connected to the correct points on the vessel.
Electrical Wiring: Verify that the power and signal wiring are correctly connected to the appropriate terminals in the junction box and marshalling cabinet. Check for proper grounding and shielding.
Tagging and Labeling: Ensure the instrument has the correct tag number clearly displayed.

3. What is the importance of a Loop Check, and what does it involve for a level instrument?

A loop check is a vital step to verify the integrity of the entire control loop, from the sensor to the control system. For a level instrument, this involves:

Signal Verification: A known input is simulated at the transmitter, and the corresponding output is verified at the Human-Machine Interface (HMI) or Distributed Control System (DCS). For a 4-20 mA transmitter, this typically involves simulating 0%, 25%, 50%, 75%, and 100% of the calibrated range and ensuring the readings on the HMI match.
Alarm and Interlock Checks: Verifying that any associated alarms (e.g., high-level alarm, low-level alarm) and interlocks are triggered at the correct setpoints.
Control Valve Response (if applicable): If the level measurement is part of a control loop, the response of the final control element (e.g., a control valve) is checked.

4. What is the difference between a dry-leg and a wet-leg installation for a DP level transmitter?

Dry Leg: In a dry-leg installation, the low-pressure side of the DP transmitter is connected to the top of the vessel and is filled with a non-condensable gas (e.g., instrument air or nitrogen). This is typically used for measuring the level of non-condensing liquids in an open or closed tank.
Wet Leg: In a wet-leg installation, the low-pressure side impulse line is intentionally filled with a liquid that is compatible with the process fluid. This is used when the process fluid at the top of the vessel is a vapor that could condense, which would otherwise create a fluctuating and erroneous head pressure on the LP side.

5. Why is calibration a crucial step in commissioning, even for factory-calibrated instruments?

While many instruments are calibrated at the factory, on-site calibration is crucial for several reasons:

Verification: To confirm that the calibration has not drifted during transportation and handling.
Process Specifics: To account for the specific gravity (density) of the process fluid, which may differ from the standard fluid used for factory calibration.
Installation Effects: The mounting position and the configuration of the impulse piping can introduce errors that need to be calibrated out.
Customer Requirements: Many plant operating procedures mandate on-site calibration as a quality assurance measure.

II. Commissioning of Specific Level Instrument Technologies

This section delves into the nuances of commissioning different types of level measurement devices.

6. Describe the steps to commission a Differential Pressure (DP) level transmitter for a closed tank.

Isolate the Instrument: Close the block valves on the process impulse lines.
Connect a Calibrator: Connect a pressure calibrator to the high-pressure (HP) and low-pressure (LP) ports of the transmitter.
Zero and Span Calibration:
- Zero (0%): Apply the calculated pressure corresponding to the minimum level (e.g., for a dry leg, this might be zero pressure on the HP side and atmospheric pressure on the LP side). Adjust the transmitter’s zero setting to read 4 mA.
- Span (100%): Apply the calculated pressure corresponding to the maximum level. Adjust the transmitter’s span setting to read 20 mA. The pressure is calculated using the formula: $P = h \times SG \times g$ , where ‘h’ is the height, ‘SG’ is the specific gravity, and ‘g’ is the acceleration due to gravity.
Linearity Check: Check the output at 25%, 50%, and 75% of the range to ensure linearity.
Reinstall and Depressurize: Disconnect the calibrator and reconnect the impulse lines. Slowly open the block valves to bring the instrument into service.

7. How would you commission a non-contacting ultrasonic level transmitter?

Mounting Considerations: Ensure the transmitter is mounted perpendicular to the liquid surface and away from any obstructions in the tank that could cause false echoes (e.g., agitators, inlet streams). The transducer face should be above the maximum expected level.
Power Up and Configuration: Power up the transmitter and access its configuration menu, typically via a local display or a HART communicator.
Enter Parameters: Input key parameters such as:
- Empty Distance (Blocking Distance): The distance from the sensor face to the bottom of the tank or the 0% level mark.
- Span: The measurement range (distance from 0% to 100% level).
- Tank Shape and Dimensions: To enable volume calculation if required.
- Speed of Sound: While many transmitters have built-in temperature compensation, for some applications, the speed of sound in the vapor space may need to be manually entered or compensated for.
False Echo Suppression: Perform a false echo mapping or suppression routine to filter out echoes from known obstructions.
Verification: Verify the level reading against a manual gauge (e.g., a sight glass or dip tape) at different levels.

8. What are the key considerations when commissioning a Guided Wave Radar (GWR) level transmitter?

Probe Selection and Installation: Ensure the correct probe type (e.g., single rod, coaxial) is used for the application (e.g., clean liquids, solids, low dielectric fluids). The probe should be installed vertically and not be in contact with the tank wall unless it’s a coaxial probe.
Dielectric Constant: The dielectric constant of the process medium is a critical parameter for GWR. Ensure the correct value is entered during configuration, as it affects the signal reflection and measurement accuracy.
Stilling Well: For applications with turbulence, a stilling well can be used to provide a calm surface for measurement. Ensure the stilling well is properly vented.
Interface Measurement: If used for interface level detection, both the upper and lower fluid dielectric constants need to be configured correctly.

9. Explain the commissioning process for a Capacitance Level Transmitter.

Probe Installation: The probe must be installed without touching the tank walls (unless it is a concentric shield type). The insulation on the probe must be intact.
Calibration (Two-Point):
- Low Level (0%): With the tank at its minimum level (or empty), the probe is partially wetted. This is calibrated as the 4 mA point.
- High Level (100%): With the tank at its maximum level, the probe is fully wetted. This is calibrated as the 20 mA point.
Dielectric Constant: Similar to GWR, the dielectric constant of the material is crucial. For some transmitters, the calibration process automatically compensates for this. For others, it may need to be entered.

10. How do you commission a Nucleonic (Gamma-Ray) Level Gauge?

Commissioning a nucleonic level gauge requires strict adherence to safety protocols due to the radioactive source.

Source Holder Installation: The radioactive source holder and detector are mounted on opposite sides of the vessel.
Radiation Safety: The area must be cordoned off, and only trained and authorized personnel should be present. Radiation surveys must be conducted to ensure there is no leakage.
Calibration:
- Empty Tank (High Radiation): With the vessel empty, the detector receives the maximum radiation. This is calibrated as the 100% or 0% point depending on the application (e.g., high-level switch or continuous level).
- Full Tank (Low Radiation): With the vessel full, the process medium absorbs the radiation, and the detector receives the minimum amount. This is calibrated as the other end of the range.
Licensing and Documentation: All activities must be documented and comply with local and national radiation safety regulations.

III. Troubleshooting During Commissioning

This section addresses common problems encountered during the commissioning of level instruments.

11. You are commissioning a DP level transmitter, and the reading is fluctuating wildly. What are the possible causes?

Turbulence: The liquid surface in the vessel might be turbulent due to agitators or high flow rates.
Boiling or Flashing: The process liquid might be boiling or flashing, causing rapid pressure changes.
Air/Gas in Impulse Lines: Trapped air or gas in the impulse lines can cause erratic readings.
Hunting of Control Loop: If it’s part of a control loop, the controller tuning might be causing oscillations.
Instrument Malfunction: The transmitter itself could be faulty.

12. An ultrasonic level transmitter is showing a “loss of echo” alarm. What steps would you take to troubleshoot this?

Check for Obstructions: Ensure there are no new obstructions in the beam path.
Verify Mounting: Confirm the sensor is still mounted correctly and is perpendicular to the surface.
Check for Foam or Vapors: Heavy foam or dense vapors can absorb or scatter the ultrasonic signal.
Inspect the Transducer Face: The face of the transducer might be coated with condensation or product buildup.
Review Configuration: Double-check the configured parameters, especially the empty distance and span.

13. A guided wave radar is giving an incorrect reading. What are the potential issues?

Incorrect Dielectric Constant: An incorrect dielectric value is a common cause of inaccuracy.
Probe Coating: Buildup of material on the probe can affect the signal propagation.
Foam: While more tolerant than ultrasonic, very thick or dense foam can still dampen the signal.
Multiple Reflections: In complex tank geometries, multiple reflections can sometimes be misinterpreted.
Probe Damage: The probe might be bent or damaged.

14. During the commissioning of a capacitance level probe, the output is non-linear. What could be the reason?

Inconsistent Material Composition: If the dielectric constant of the material being measured is not uniform, the output will be non-linear.
Improper Grounding: Poor grounding of the vessel or the instrument can lead to stray capacitance and non-linear readings.
Probe Coating: A non-uniform coating on the probe can also cause non-linearity.

15. What is “zero suppression” and “zero elevation” in the context of DP level measurement, and why is it important during commissioning?

Zero Elevation: This is used in wet-leg installations. The pressure exerted by the liquid in the wet leg is always present on the LP side. To compensate for this and make the transmitter read zero at the minimum level, the zero of the transmitter is “elevated.”
Zero Suppression: This is used when the level tapping point is above the bottom of the tank. The pressure from the liquid below the tapping point is “suppressed” so that the transmitter reads zero at the desired minimum level.

Properly setting the zero elevation or suppression during commissioning is critical for accurate level measurement across the entire range.

IV. Safety and Documentation

This section highlights the importance of safety procedures and proper documentation.

16. What are the key safety precautions to take when commissioning a level instrument in a hazardous area?

Work Permit: Always obtain a valid work permit (e.g., hot work, cold work permit) before starting any job.
Area Classification: Be aware of the hazardous area classification (e.g., Zone 1, Zone 2) and use appropriately certified equipment (e.g., intrinsically safe, explosion-proof).
Personal Protective Equipment (PPE): Use the required PPE, which may include safety glasses, a hard hat, safety shoes, and gloves. For certain processes, a gas detector might be necessary.
Isolation: Ensure the instrument is electrically and process-wise isolated before working on it.
Emergency Procedures: Be aware of the emergency shutdown procedures and the location of safety equipment like emergency showers and eyewash stations.

17. What is Intrinsic Safety (IS), and how does it apply to commissioning level instruments?

Intrinsic Safety is a protection technique that limits the electrical and thermal energy in a circuit to a level below that which can cause ignition of a specific hazardous atmospheric mixture. When commissioning an intrinsically safe instrument, you must ensure that:

The entire loop (transmitter, barrier, and wiring) is designed and installed as an intrinsically safe system.
The correct type of Zener barrier or galvanic isolator is used.
The cable parameters (capacitance and inductance) are within the limits specified in the control drawing.

18. What documents are essential for the commissioning of a level instrument?

Piping and Instrumentation Diagram (P&ID): To understand the instrument’s location and its role in the process.
Instrument Datasheet: Contains all the technical specifications of the instrument.
Vendor Manuals: Provide detailed information on installation, configuration, and calibration.
Loop Diagrams: Show the complete wiring from the instrument to the control system.
Calibration Certificates: To record the as-found and as-left calibration data.
Commissioning Checklists: To ensure all steps are completed and documented.

19. What is a “punch list,” and what is its relevance to commissioning?

A punch list is a document that lists all the outstanding items or discrepancies found during the pre-commissioning and commissioning phases. These could be minor issues like a missing tag or more significant problems like a faulty instrument. The punch list items must be cleared or “punched off” before the system can be handed over for startup.

20. What is the role of HART protocol in the commissioning of a level transmitter?

The HART (Highway Addressable Remote Transducer) protocol is a hybrid analog+digital communication protocol. During commissioning, a HART communicator is invaluable for:

Remote Configuration: Setting up the transmitter’s parameters without having to be physically at the instrument’s local display.
Diagnostics: Accessing detailed diagnostic information from the instrument to troubleshoot problems.
Calibration: Performing zero and span adjustments electronically.
Loop Testing: Forcing the analog output to a specific value to test the loop.

V. Advanced and Scenario-Based Questions

This section includes more challenging questions that test your problem-solving skills and in-depth knowledge.

21. You need to select a level instrument for a vessel containing a highly corrosive liquid with heavy fuming. Which technology would you recommend and why?

A non-contacting radar level transmitter would be the most suitable choice.

Corrosion Resistance: Being non-contact, the sensor is not exposed to the corrosive liquid. The antenna material can also be selected for chemical compatibility (e.g., PTFE).
Unaffected by Fumes: Radar signals are largely unaffected by vapors and fumes, unlike ultrasonic transmitters.
Accuracy: Radar provides high accuracy and reliability in such challenging environments.

22. How would you handle level measurement in a tank where the specific gravity of the liquid changes with temperature?

There are a few approaches:

DP Transmitter with Temperature Compensation: Use a DP transmitter along with a temperature transmitter. The control system can then be configured to use the live temperature reading to calculate the real-time density and correct the level measurement.
Technologies Unaffected by Density: Use a level measurement technology that is not affected by changes in specific gravity, such as:
- Guided Wave Radar (GWR)
- Non-contacting Radar
- Ultrasonic (though vapors might still be an issue)
- Magnetostrictive level transmitters

23. Describe a situation where a stilling well would be necessary for level measurement and how it is commissioned.

A stilling well would be necessary in a tank with:

High Turbulence: Caused by agitators or high inlet/outlet flow rates.
Foam: To provide a relatively foam-free surface for measurement.

Commissioning a Stilling Well:

Venting: Ensure the stilling well has adequate vents at the top to allow the liquid level inside the well to equalize with the level in the tank. The bottom of the well should also be open to the tank.
Instrument Installation: The level instrument (e.g., GWR or ultrasonic) is then installed inside the stilling well.
Calibration: The calibration and commissioning of the instrument proceed as they would in a normal installation, but the measurement is now taken within the calmer environment of the stilling well.

24. What are the advantages of using a laser level transmitter, and in what applications are they typically used?

Advantages:

High Accuracy: Lasers have a very narrow beam, allowing for precise targeting, even in vessels with many internal obstructions.
Long Range: They can measure over very long distances.
Unaffected by Vapors: The laser beam is not significantly affected by most vapors.

Applications:

Solids Level Measurement: They are excellent for measuring the level of bulk solids in silos and hoppers, as they can map the surface profile.
Molten Metals: Used in the steel and foundry industries for measuring the level of molten metals.
Narrow Vessels: Ideal for tall, narrow vessels where other non-contact methods might struggle with beam divergence.

25. What do you understand by the term “interface level,” and how would you commission an instrument to measure it?

Interface level is the boundary between two immiscible liquids in the same vessel (e.g., oil and water).

Commissioning an Interface Level Instrument (e.g., a GWR or a DP transmitter):

Technology Selection: A Guided Wave Radar is often the preferred choice for clean interfaces. A DP transmitter can also be used, but it is more complex to set up.
Configuration for GWR:
- Both the dielectric constant of the upper liquid and the lower liquid must be entered into the transmitter’s configuration.
- The total level and the interface level can often be measured simultaneously.
Calibration for DP Transmitter:
- This requires careful calculation of the pressures exerted by both liquids. The calibration will be based on the specific gravities of both the upper and lower fluids. The zero and span will be set based on the minimum and maximum interface levels.

By mastering these questions and their underlying principles, you will be well-equipped to demonstrate your expertise and confidence in any interview focused on the commissioning of level instruments.