Overpressure Protection: Top 20 Interview Q&A Explained
Navigating the critical field of process safety, a thorough understanding of overpressure protection is paramount for any engineer. Whether you are a seasoned professional or a recent graduate, being well-versed in the principles, equipment, and standards governing this domain is essential for a successful technical interview. Here are the top 20 interview questions and their detailed explanations to help you confidently demonstrate your expertise.
1. What is the primary purpose of overpressure protection?
Answer: The primary purpose of overpressure protection is to prevent the catastrophic failure of equipment, such as pressure vessels and piping, and the associated release of hazardous materials, by ensuring that the internal pressure does not exceed the Maximum Allowable Working Pressure (MAWP). This is a critical layer of protection for ensuring the safety of personnel, the environment, and the asset itself.
2. What is the difference between a relief valve and a safety valve?
Answer: While often used interchangeably, there is a key distinction:
- Relief Valve: Typically used for liquid service. It opens proportionally to the increase in pressure over the set pressure.
- Safety Valve: Primarily used for compressible fluids like steam and gases. It exhibits a rapid, full-opening “pop” action once the set pressure is reached.
A safety relief valve is a versatile device designed to function as either a safety valve or a relief valve.
3. What are the most common causes of overpressure in a process plant?
Answer: The most common causes of overpressure can be broadly categorized as:
- Blocked Discharge: A downstream valve is inadvertently closed, preventing the normal outflow of fluid.
- External Fire: Exposure to an external fire (pool fire or jet fire) vaporizes the liquid within a vessel, rapidly increasing the internal pressure. This often represents the governing case for relief sizing.
- Thermal Expansion: A liquid-full line or vessel is blocked in and subsequently heated, causing the liquid to expand and generate high pressures.
- Runaway Reaction: An uncontrolled exothermic chemical reaction generates heat and/or gases at a rate faster than they can be removed.
- Equipment Failure: Failures of cooling systems, reflux pumps, or control valves can lead to a buildup of pressure.
- Utility Failure: Loss of cooling water, instrument air, or power can disrupt normal operations and cause overpressure scenarios.
4. Explain the key terms: MAWP, Set Pressure, Overpressure, and Blowdown.
Answer:
- MAWP (Maximum Allowable Working Pressure): The maximum gauge pressure permissible at the top of a piece of equipment in its normal operating position at a designated temperature.
- Set Pressure: The inlet gauge pressure at which the pressure relief device is set to begin opening. For safety valves, this is the pressure at which the “pop” action occurs.
- Overpressure: The pressure increase over the set pressure, usually expressed as a percentage of the set pressure. This is the additional pressure required for the relief device to achieve its fully rated flow.
- Blowdown: The difference between the set pressure and the reseating pressure of a relief valve, expressed as a percentage of the set pressure. It represents how much the pressure needs to drop below the set pressure before the valve closes.
5. What is a Rupture Disk (or Bursting Disc) and how does it work?
Answer: A rupture disk is a non-reclosing pressure relief device designed to burst at a pre-determined pressure. It consists of a thin, calibrated membrane held between flanges. When the system pressure reaches the disk’s burst pressure, the membrane ruptures, providing a full-bore opening for pressure relief.
6. What are the advantages and disadvantages of a Rupture Disk compared to a Pressure Relief Valve (PRV)?
Answer:
Advantages of a Rupture Disk:
- Leak-tightness: Provides a bubble-tight seal, making it ideal for highly toxic or valuable process fluids.
- Fast-acting: Offers a very rapid response to pressure surges.
- Full-bore opening: Provides a large, unobstructed flow path.
- Lower initial cost: Generally less expensive than a PRV.
- Corrosion resistance: Can be made from a wide variety of materials to handle corrosive fluids.
Disadvantages of a Rupture Disk:
- Non-reclosing: Once it ruptures, the entire system content is released until the system is depressurized. This results in process downtime and potential environmental release.
- Fatigue: Susceptible to fatigue and premature failure due to pressure cycling.
- Sensitivity to installation: Performance is highly dependent on proper installation and bolt torqueing.
7. Why would you install a rupture disk upstream of a pressure relief valve?
Answer: Installing a rupture disk upstream of a PRV offers several benefits:
- Isolation from corrosive/fouling service: The rupture disk protects the expensive and complex internals of the PRV from corrosive or polymerizing fluids, reducing maintenance costs.
- Absolute sealing: It prevents fugitive emissions of toxic or valuable materials through the PRV seat, which may have a slight leakage.
- Testing in-situ: Allows for in-line testing of the PRV’s set pressure without removing it from the process.
It is crucial to have a pressure gauge or tell-tale indicator in the space between the rupture disk and the PRV to detect any leakage through the disk, which could prevent the PRV from opening at its set pressure.
8. What are the main types of pressure relief valves?
Answer: The main types of PRVs are:
- Conventional Spring-loaded PRV: The most common type. The set pressure is determined by a spring that holds a disc against the valve seat. Its performance is affected by backpressure.
- Balanced-Bellows PRV: Uses a bellows to balance the effect of superimposed backpressure on the valve’s opening pressure. This allows it to be used in applications with higher or variable backpressure.
- Pilot-operated PRV: Consists of a main valve and a pilot valve. The pilot valve senses the system pressure and controls the opening and closing of the main valve. They can operate closer to the set pressure with better leak-tightness and can handle very high pressures and flow rates.
9. How does backpressure affect the performance of a conventional PRV?
Answer: Backpressure is the pressure at the outlet of the PRV. It can be categorized as:
- Superimposed Backpressure: The pressure present at the outlet of the PRV before it opens.
- Built-up Backpressure: The pressure that develops at the outlet as a result of flow through the valve and the discharge piping.
For a conventional PRV, superimposed backpressure acts on the valve disc, adding to the spring force. This increases the set pressure. For example, 10 psig of superimposed backpressure will increase the set pressure by approximately 10 psig. Built-up backpressure can reduce the lift of the valve disc, thereby decreasing its relieving capacity and potentially causing chattering.
10. What are the API standards that govern overpressure protection?
Answer: The primary American Petroleum Institute (API) standards are:
- API Standard 520: “Sizing, Selection, and Installation of Pressure-Relieving Devices.” Part I covers sizing and selection, while Part II provides guidance on installation.
- API Standard 521: “Guide for Pressure-Relieving and Depressuring Systems.” This standard provides comprehensive guidance on identifying potential overpressure scenarios, determining relief loads, and designing the disposal system (e.g., flare system).
- API Standard 526: “Flanged Steel Pressure Relief Valves.” This standard specifies standard dimensions for flanged steel PRVs to ensure interchangeability.
- API Recommended Practice 527: “Seat Tightness of Pressure Relief Valves.”
11. What is the “fire case” for relief valve sizing, and why is it often the governing scenario?
Answer: The “fire case” refers to the overpressure scenario caused by an external fire heating the wetted surface of a vessel containing a volatile liquid. The heat input vaporizes the liquid, leading to a rapid pressure increase. The required relief rate is calculated based on the heat input from the fire, as specified in API 521. This scenario often results in the largest required relief area because of the high rate of vapor generation, making it the “governing” or “controlling” scenario for sizing the relief device.
12. What is a “Thermal Relief Valve” (TRV) and where is it used?
Answer: A Thermal Relief Valve (TRV) is a small relief valve used to protect equipment or piping from overpressure caused by the thermal expansion of a trapped liquid. They are typically found on heat exchanger shells where the liquid-filled side can be blocked in, or on long pipelines that can be isolated and exposed to solar heating.
13. What is “chattering” in a pressure relief valve, and how can it be prevented?
Answer: Chattering is the rapid, abnormal opening and closing of a pressure relief valve. It can cause severe damage to the valve’s seating surfaces and internals. The primary cause of chattering is an excessive inlet pressure drop to the PRV. When the valve opens, the flow creates a pressure drop in the inlet piping. If this pressure drop is large enough, the pressure at the valve inlet will fall below the reseating pressure, causing the valve to close. Once closed, the pressure builds up again, and the cycle repeats.
To prevent chattering, API 520 Part II recommends that the total non-recoverable pressure drop at the inlet of a PRV should not exceed 3% of its set pressure for valves at 10% overpressure.
14. How do you determine the required orifice area for a relief valve?
Answer: The required orifice area is calculated using sizing equations provided in API 520 Part I. These equations differ for gas/vapor service, liquid service, and steam service. The key parameters required for the calculation are:
- Required relief rate (mass or volumetric flow)
- Relieving pressure and temperature
- Fluid properties (molecular weight, compressibility factor, specific gravity, viscosity)
- Set pressure and overpressure
- Backpressure
Once the required area is calculated, a standard orifice size (as designated by letters like D, E, F, etc., in API 526) with an area equal to or greater than the calculated area is selected.
15. What are the considerations for the design of inlet and outlet piping for a PRV?
Answer:
- Inlet Piping: Must be as short and direct as possible with a diameter at least equal to the PRV inlet size. This is to minimize the inlet pressure drop and prevent chattering. The “3% rule” from API 520 is a key design criterion.
- Outlet Piping: Must be sized to handle the relief load without generating excessive built-up backpressure, which could affect the performance of a conventional PRV or exceed the pressure rating of the discharge system. The outlet piping must be sloped away from the PRV to allow for drainage and prevent the accumulation of liquids.
16. What is the purpose of a flare system?
Answer: A flare system is a crucial part of the disposal system for relieved fluids in many industrial facilities. Its primary purpose is to safely and efficiently combust flammable, toxic, or corrosive vapors released during overpressure events or planned depressurizing. This converts the hazardous materials into less harmful combustion products like carbon dioxide and water.
17. How often should pressure relief valves be tested and inspected?
Answer: The frequency of testing and inspection depends on the service conditions (corrosiveness, fouling potential), regulatory requirements, and the facility’s risk-based inspection program. A common interval for PRVs in clean, non-corrosive service is 5 years. However, for critical or fouling services, this interval could be as short as every year. These intervals should be documented and justified.
18. What is a depressurizing system, and when is it used?
Answer: A depressurizing system is designed to rapidly reduce the pressure within a process system in a controlled manner during an emergency, such as a fire. This is often achieved by opening a dedicated depressurizing valve that vents to a flare or other safe location. The purpose is to reduce the stress on the equipment at elevated temperatures, thereby preventing rupture, and to reduce the inventory of hazardous material that could be released if the equipment were to fail.
19. Can a single pressure relief device protect multiple pieces of equipment?
Answer: Yes, under certain circumstances. A single pressure relief device can be used to protect multiple pieces of equipment if the piping connecting them is of sufficient size to prevent the pressure from exceeding the MAWP of any of the protected equipment. The relief device must be set at or below the lowest MAWP of all the equipment it is protecting, and its capacity must be sufficient to handle the combined relief load from all credible overpressure scenarios.
20. Describe the steps you would take to perform a relief system design verification.
Answer: A relief system design verification involves a systematic review to ensure the adequacy of the overpressure protection. The key steps include:
- Identify all protected equipment and their MAWP.
- For each piece of equipment, identify all credible overpressure scenarios in accordance with API 521.
- For each scenario, determine the required relief load (flow rate) and the relieving conditions (pressure, temperature, fluid properties).
- Identify the governing (worst-case) scenario that requires the largest relief area.
- Verify the sizing of the existing or proposed pressure relief device using API 520 Part I to ensure it has adequate capacity for the governing scenario.
- Check the set pressure to ensure it is at or below the MAWP.
- Evaluate the inlet and outlet piping for excessive pressure drop (inlet) and backpressure (outlet) according to API 520 Part II.
- Verify that the discharge location and disposal system (e.g., flare) are adequate and safe.
- Ensure all documentation is complete and accurate.