Interface Level in Oil & Water Separator – Top 25 commonly asked interview Q&A

Top 25 Interview Questions on Oil & Water Separator Interface Level

The accurate measurement and control of the interface level between oil and water in a separator are critical for efficient and safe operations in the oil and gas industry. A failure to maintain this crucial parameter can lead to off-spec products, equipment damage, and environmental hazards. Consequently, interviewers for roles in instrumentation, process operations, and maintenance frequently probe a candidate’s understanding of this subject.

Here are the top 25 commonly asked interview questions and their answers regarding the interface level in oil and water separators, designed to prepare you for your next technical interview.

Fundamental Concepts

1. What is the “interface level” in an oil and water separator?

In an oil and water separator, the interface level is the distinct boundary layer between the lighter crude oil on top and the denser water at the bottom. Due to their immiscibility and different densities, the two liquids naturally separate, forming this interface.

2. Why is it so important to accurately measure and control the interface level?

Accurate interface level control is paramount for several reasons:

Product Quality: It prevents “water carryover” into the oil outlet, which would contaminate the crude and require further, costly separation.
Environmental Compliance: It avoids “oil carry-under” into the water outlet, which can lead to environmental pollution and hefty fines.
Process Efficiency: It maximizes the separation efficiency of the vessel, ensuring optimal recovery of oil.
Equipment Protection: It prevents downstream equipment like pumps and pipelines from being damaged by processing fluids for which they were not designed.

3. What is an “emulsion layer” or “rag layer,” and how does it affect interface level measurement?

An emulsion layer is a mixture of oil and water that has not fully separated. It forms a third, often thick and indistinct, layer between the clean oil and water. This “rag layer” is a major challenge for many interface level measurement technologies because it can dampen, scatter, or falsely reflect the measurement signal, leading to inaccurate readings. The properties of the emulsion can also vary, further complicating measurement.

4. What are the primary factors that influence the separation of oil and water in a separator?

The primary factors influencing separation are:

Density Difference: The greater the difference in density between the oil and water, the faster and more distinct the separation.
Temperature: Higher temperatures generally reduce the viscosity of the oil, aiding in faster separation.
Retention Time: The amount of time the fluid mixture remains in the separator allows for gravity to act and separate the phases.
Droplet Size: Larger water droplets in the oil phase will settle more quickly.
Chemicals: The presence of demulsifiers can help to break down emulsion layers and improve separation.
Turbulence: A calm, laminar flow within the separator is ideal for effective separation.

5. Can you describe the basic principle of a three-phase separator?

A three-phase separator is designed to separate the well stream into its three constituent components: oil, water, and gas. The fluid enters the separator, where the velocity is reduced. The gas, being the lightest, separates out and exits from the top. The liquids (oil and water) settle to the bottom. Due to density differences, the oil forms a layer on top of the water. An interface level controller manages the oil-water interface, while a separate level controller manages the overall liquid level (oil).

Measurement Technologies

6. What are the most common technologies used for measuring the interface level in oil and water separators?

The most common technologies are:

Guided Wave Radar (GWR): Highly popular due to its accuracy and ability to often measure both the interface and the total level.
Capacitance Probes: Effective when there is a significant difference in the dielectric constants of the two liquids.
Differential Pressure (DP) Transmitters: A traditional method that infers level based on the pressure difference.
Displacers and Floats: A mechanical method that relies on the buoyancy of a float in the different liquids.
Nucleonic (Gamma) Profilers: Used in very challenging applications with thick emulsions or when non-intrusive measurement is required.

7. How does a Guided Wave Radar (GWR) transmitter measure the interface level?

A GWR transmitter sends low-energy microwave pulses down a probe (waveguide). A portion of the pulse is reflected from the surface of the upper liquid (oil) due to the change in the dielectric constant between the vapor space and the oil. The remaining energy continues down the probe and is reflected from the oil-water interface, again due to the significant change in dielectric constant between oil ( $ϵ_{r}$ ≈ 2-5) and water ( $ϵ_{r}$ ≈ 80). The transmitter’s electronics analyze the time difference between these two reflected signals to calculate both the upper level and the interface level.

8. What are the main advantages and disadvantages of using GWR for interface measurement?

Advantages:

High accuracy and reliability.
Can often measure both the total level and the interface level with a single instrument.
Relatively unaffected by changes in density, pressure, and temperature.
No moving parts, leading to lower maintenance.

Disadvantages:

Performance can be affected by thick and stable emulsion layers, which can dampen the signal.
The probe is in contact with the process fluid and can be susceptible to coating or corrosion.
Requires a sufficient difference in the dielectric constants of the two liquids.

9. Explain the working principle of a capacitance probe for interface level measurement.

A capacitance probe works on the principle that the capacitance of a capacitor changes with the dielectric constant of the material between its plates. The probe and the vessel wall act as the two plates of a capacitor. Since water has a much higher dielectric constant than oil, as the water level rises along the probe, the overall capacitance of the system increases. This change in capacitance is measured and correlated to the interface level.

10. In what situations is a capacitance probe a good choice for interface measurement, and when is it not?

Good Choice:

In applications with a clean and distinct interface.
When one liquid is conductive (like water) and the other is non-conductive (like oil).
It can be less affected by thin emulsion layers compared to some other technologies.

Not a Good Choice:

If the dielectric constants of the two liquids are very similar.
If the conductivity of the liquids changes significantly, it can affect accuracy.
The probe is prone to errors from coating, especially if the coating material has a different dielectric constant.

11. How is a Differential Pressure (DP) transmitter used to measure the interface level?

A DP transmitter measures the pressure difference between two points at different elevations on the separator. Assuming the total liquid level is constant and known, any change in the measured differential pressure is directly proportional to the change in the interface level. This is because the overall density of the liquid column between the two taps changes as the proportion of oil to water changes. A higher interface level (more oil) results in a lower DP, and a lower interface level (more water) results in a higher DP.

12. What are the main limitations of using a DP transmitter for interface measurement?

The primary limitations are:

It assumes a constant total level and constant densities of both liquids. Any variation in these parameters will lead to measurement errors.
It provides an inferred measurement, not a direct one.
It is not suitable for applications with significant emulsion layers as the density of the emulsion is often unknown and variable.
The impulse lines can get clogged, leading to inaccurate readings.

13. Describe how a displacer-type level instrument works for interface applications.

A displacer is a buoyant element with a specific weight that is suspended in the process fluid. The buoyant force acting on the displacer is proportional to the density of the fluid it is submerged in. In an interface application, the displacer is submerged across the oil-water interface. As the interface level changes, the proportion of the displacer submerged in the denser water versus the lighter oil changes. This alters the apparent weight of the displacer, which is then measured by a torque tube and translated into a level measurement.

14. What are the key considerations when calibrating a displacer for interface service?

The key considerations for calibration are the specific gravities (densities) of both the upper and lower liquids at their operating temperatures. The instrument must be calibrated to produce a 0% output when fully submerged in the lighter liquid (oil) and a 100% output when fully submerged in the heavier liquid (water) over the desired measurement span.

Operational and Troubleshooting Scenarios

15. You receive a report of an incorrect interface level reading from the control room. What are your initial troubleshooting steps?

My initial steps would be:

Gather Information: Understand the current process conditions (pressure, temperature, flow rates), any recent changes, and the history of the instrument.
Check Local Indication: If available, compare the control room reading with a local indicator on the instrument or a sight glass on the vessel.
Manual Verification: If safe and permissible, take a manual sample from the separator at different heights to determine the actual interface level. This is often done using try cocks.
Inspect the Instrument: Visually inspect the level transmitter and its installation for any obvious issues like loose wiring, damage, or leaks.
Review Instrument Diagnostics: Check the device diagnostics for any error codes or abnormal readings (e.g., signal strength for a GWR).

16. How would you troubleshoot a Guided Wave Radar that is giving an erratic or “lost signal” reading?

For an erratic GWR reading, I would investigate:

Emulsion Layer: The presence of a thick or changing emulsion layer is a common cause.
Coating on the Probe: A conductive or thick coating can attenuate the signal.
Turbulence: Excessive turbulence around the probe can cause fluctuating readings. A stilling well can mitigate this.
Incorrect Dielectric Settings: Ensure the dielectric constants for both fluids are correctly configured in the transmitter.
Probe Integrity: Check for any bends or damage to the probe.
Signal Strength: Analyze the echo curve to see if the signal from the interface is weak or being lost.

17. What is a “stilling well,” and why is it sometimes used with level instruments in separators?

A stilling well is a vertical pipe installed inside the separator, open at the top and bottom. The level instrument is installed inside this well. Its purpose is to create a calm, stable liquid surface for the measurement device, protecting it from the effects of turbulence, agitation, and foaming within the main body of the separator. This leads to a more stable and reliable level reading.

18. How can changes in the process fluid properties (e.g., API gravity of the oil) affect interface level measurement?

Changes in fluid properties can significantly impact certain measurement technologies:

DP Transmitters: A change in the density of either the oil or water will directly cause an error in the inferred level reading.
Displacers: Changes in the specific gravity of the fluids will alter the buoyant force on the displacer, leading to inaccurate readings unless the instrument is recalibrated.
GWR and Capacitance Probes: These are less affected by density changes but can be impacted by significant changes in the dielectric constant of the fluids.

19. Describe the procedure for a “zero and span” calibration of a DP transmitter for interface service.

A “zero and span” calibration for interface service involves simulating the 0% and 100% interface level conditions:

Zero Calibration (0%): The lower pressure (LP) and higher pressure (HP) sides of the transmitter are subjected to a pressure equivalent to the vessel being filled with the lighter fluid (oil) up to the top tap. This simulates the highest possible interface level.
Span Calibration (100%): The pressures are adjusted to simulate the condition where the heavier fluid (water) has risen to the top tap, representing the lowest possible interface level. This is typically done on a bench using a pressure calibrator and requires calculating the pressures based on the fluid densities and the distance between the taps.

20. What is a “wet leg” versus a “dry leg” installation for a DP transmitter, and which is more common for interface level?

Dry Leg: The low-pressure side impulse line is filled with a non-process gas (like the vapor in the separator).
Wet Leg: The low-pressure side impulse line is intentionally filled with a process liquid of known and stable density.

For interface measurement in a pressurized separator, a wet leg installation is often preferred. This is because it provides a more stable reference pressure on the low-pressure side, as it is not susceptible to condensation of vapors in the impulse line, which would change the head pressure and introduce errors.

Advanced and Application-Specific Questions

21. How can you measure the thickness of an emulsion layer?

Measuring the emulsion layer thickness is challenging. Some advanced instrumentation can provide an indication:

Multi-parameter Guided Wave Radar: Some GWR transmitters can analyze the signal attenuation through the emulsion layer to provide an estimate of its thickness.
Nucleonic (Gamma) Profilers: These instruments use a radioactive source and multiple detectors to create a density profile of the vessel’s contents, which can clearly identify the top of the oil, the emulsion layer, and the water level.
Sampling: Taking manual samples at different elevations through sample cocks is a common, though less precise, method.

22. What safety precautions must be taken when working with and maintaining interface level instruments on a separator?

Safety is paramount. Key precautions include:

Work Permits: Always obtain the necessary work permits (e.g., hot work, confined space entry if applicable).
Lock-Out/Tag-Out (LOTO): Isolate the instrument from the process and any energy sources before starting work.
Personal Protective Equipment (PPE): Wear appropriate PPE, including safety glasses, gloves, and flame-retardant clothing, as the process fluids can be hazardous.
Pressure Release: Safely depressurize the instrument and any associated piping before disconnecting it.
Awareness of H2S: Be aware of the potential for hydrogen sulfide (H2S) gas, which is highly toxic and often present in crude oil.

23. For a new separator installation, what information would you need to select the most appropriate interface level measurement technology?

To select the right technology, I would need:

Fluid Properties: Densities (or API gravity) and dielectric constants of the oil and water at operating conditions.
Process Conditions: Operating pressure and temperature range.
Vessel Design: Dimensions of the separator, nozzle locations, and the presence of internal structures.
Expected Emulsion: An estimation of the likelihood and thickness of an emulsion layer.
Accuracy and Performance Requirements: The required level of accuracy and the desired control performance.
Safety Requirements: Whether the instrument needs to be part of a Safety Instrumented System (SIS).

24. What is the role of interface level control in desalting operations?

In a desalter, which is a type of oil and water separator used to remove salt from crude oil, the interface level control is critical. An electrostatic grid is used to coalesce the water droplets. Maintaining a stable interface level below the electrostatic grid is essential to prevent the water from short-circuiting the grid, which would trip the desalter and disrupt the entire refining process.

25. Can you explain the concept of a “boot” or “water leg” in a horizontal separator and its relevance to interface level control?

In some horizontal separators, a “boot” or “water leg” is a section at the bottom of the vessel, often with a smaller diameter, where the water is collected. The interface level is controlled within this boot. This design provides a more quiescent zone for the water to settle and allows for more precise control of the interface level due to the smaller volume in the boot compared to the main body of the separator. Accurate interface level measurement in the boot is crucial for effective water drainage.