Relief systems – valves, flares, scrubbers MCQs With Answer

Relief systems – valves, flares, scrubbers MCQs With Answer

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Relief systems — valves, flares, scrubbers are critical safety components in pharmaceutical process plants to prevent overpressure events, control hazardous releases, and protect personnel, equipment and the environment. This quiz collection focuses on design principles, selection criteria, operating characteristics, standards and practical troubleshooting for pressure-relief valves, rupture discs, flare systems and gas scrubbers. Questions are aimed at M.Pharm students studying Hazards and Safety Management and cover underlying theory (set pressure, blowdown, accumulation), API/ASME guidance, flare types and steam assistance, scrubber mechanisms (wet scrubbing, packed towers, venturi), performance metrics and common safety considerations such as corrosion, backpressure and emissions control. Use these MCQs to test and deepen your understanding for examinations and industrial practice.

Q1. Which standard is primarily used for the sizing and selection of pressure-relieving devices in process industries?

  • API 520
  • ISO 9001
  • IEC 61508
  • NFPA 30

Correct Answer: API 520

Q2. What is the main difference between a relief valve and a safety valve?

  • Relief valves are for liquid service and open proportionally; safety valves are for gas/vapour and lift fully at set pressure
  • Relief valves are manual; safety valves are automatic
  • Relief valves are single-use; safety valves are reusable
  • Relief valves require electricity; safety valves are mechanical

Correct Answer: Relief valves are for liquid service and open proportionally; safety valves are for gas/vapour and lift fully at set pressure

Q3. In pressure relief terminology, what does “accumulation” mean?

  • The allowable pressure rise above set pressure during a relieving scenario
  • The time delay before a relief valve opens
  • The total mass of fluid discharged through the flare
  • The reserve capacity of a surge drum

Correct Answer: The allowable pressure rise above set pressure during a relieving scenario

Q4. Which device is recommended to provide a fast-acting, positive seal and is commonly used in combination with relief valves to prevent leakage?

  • Rupture disc (bursting disc)
  • Globe valve
  • Check valve
  • Pressure transmitter

Correct Answer: Rupture disc (bursting disc)

Q5. What is the principal advantage of a pilot-operated relief valve (pop type) compared with a conventional spring-loaded valve?

  • Higher accuracy in set pressure and capacity with lower leakage and better tightness
  • Lower manufacturing cost
  • No need for periodic testing
  • Operates without any inlet pressure

Correct Answer: Higher accuracy in set pressure and capacity with lower leakage and better tightness

Q6. API 521 provides guidance primarily on which of the following topics?

  • Pressure-relieving and depressuring systems, including flare and venting design
  • Material selection for high-temperature alloys
  • Instrumentation calibration procedures
  • Storage tank painting standards

Correct Answer: Pressure-relieving and depressuring systems, including flare and venting design

Q7. In flare systems, what is the purpose of steam or air assist at the flare tip?

  • Promote smokeless combustion by improving mixing and atomization of hydrocarbons
  • Increase system backpressure for better flare control
  • Cool the flare tip to prevent melting
  • Provide inerting to avoid combustion

Correct Answer: Promote smokeless combustion by improving mixing and atomization of hydrocarbons

Q8. Which scrubber type uses high liquid recirculation and impaction to remove fine particulate and soluble vapours and is effective for gas absorption with low solubility components?

  • Venturi scrubber
  • Packed bed scrubber
  • Knock-out drum
  • Activated carbon adsorber

Correct Answer: Venturi scrubber

Q9. For a pressure relief scenario, what does “blowdown” refer to?

  • The difference between set pressure and reseating pressure expressed as a percentage of set pressure
  • The rate of flow through the relief valve
  • The pressure at which a rupture disc bursts
  • The temperature drop during depressurization

Correct Answer: The difference between set pressure and reseating pressure expressed as a percentage of set pressure

Q10. Which material selection consideration is most important for scrubber internals handling acidic gas streams over long periods?

  • Corrosion resistance to sulfuric or hydrochloric acids (e.g., FRP, Hastelloy, PTFE-lined)
  • High thermal conductivity for heat transfer
  • Magnetic properties for instrumentation
  • Low density for buoyancy

Correct Answer: Corrosion resistance to sulfuric or hydrochloric acids (e.g., FRP, Hastelloy, PTFE-lined)

Q11. What is a typical disadvantage of routing relieved gases to a flare header without any knockout or condensation control?

  • Liquid carryover to the flare tip causing quenching, instability and potential smoke
  • Lower noise levels at the flare tip
  • Improved combustion efficiency
  • Reduced operator intervention

Correct Answer: Liquid carryover to the flare tip causing quenching, instability and potential smoke

Q12. In scrubber design, the term L/G ratio means:

  • Liquid flow rate per unit gas flow rate (important for mass transfer and removal efficiency)
  • Length-to-gauge ratio of the packed bed
  • Load-to-gauge pressure before relief
  • Light-to-gas ratio used for flame stability

Correct Answer: Liquid flow rate per unit gas flow rate (important for mass transfer and removal efficiency)

Q13. Which device is most suitable for providing rapid full-capacity relief in a vapor system and then quickly reseating once pressure drops below reseat value?

  • Conventional spring-operated safety valve (pop-type) with rapid opening characteristic
  • Crack-and-chatter relief valve
  • Manual vent valve
  • Fixed orifice plate

Correct Answer: Conventional spring-operated safety valve (pop-type) with rapid opening characteristic

Q14. Which of the following is an important performance metric for a packed tower scrubber used to remove soluble acid gases?

  • Height of packed bed required to achieve a specified number of transfer units (NTU) or overall mass transfer coefficient
  • The dielectric constant of packing material
  • Vibration frequency of the tower shell
  • Electrical conductivity of the inlet gas

Correct Answer: Height of packed bed required to achieve a specified number of transfer units (NTU) or overall mass transfer coefficient

Q15. When combining a rupture disc upstream of a relief valve, the primary design concern is:

  • Ensuring the disc is non-fragmenting and the combined set-up does not create excessive backpressure or impair valve operation
  • Maximizing disc burst pressure above valve set pressure
  • Minimizing the temperature of the upstream process fluid
  • Using the rupture disc as the primary flow control device in normal operation

Correct Answer: Ensuring the disc is non-fragmenting and the combined set-up does not create excessive backpressure or impair valve operation

Q16. In a flare system layout, what is the purpose of a knockout drum (liquid knockout pot)?

  • Remove condensed liquids and entrained droplets before gas reaches the flare tip to prevent pulsing and smoke
  • Increase the velocity of gas to aid combustion
  • Neutralize acidic components in the gas stream
  • Reduce the chemical reactivity of the gas

Correct Answer: Remove condensed liquids and entrained droplets before gas reaches the flare tip to prevent pulsing and smoke

Q17. Which phenomenon in scrubbers reduces removal efficiency by carrying liquid droplets into the gas outlet?

  • Entrainment
  • Diffusion
  • Osmosis
  • Condensation

Correct Answer: Entrainment

Q18. For sizing relief devices for vapor depressurization during fire exposure, API 521 suggests considering which key effect?

  • Thermal expansion and increased vapor generation from heat input leading to higher required relief capacity
  • Reduced specific gravity of vapour
  • Permanent deformation of piping within 1 minute
  • Electrical grounding requirements

Correct Answer: Thermal expansion and increased vapor generation from heat input leading to higher required relief capacity

Q19. Which scrubber type typically offers the lowest pressure drop for a given gas flow and is commonly used when liquid–gas contact time is important?

  • Packed bed (column) scrubber
  • Venturi scrubber
  • Spray tower with high-velocity nozzle
  • Cyclone separator

Correct Answer: Packed bed (column) scrubber

Q20. When designing a flare system, why is backpressure at the relief device inlet important to control?

  • Excessive backpressure can degrade relief valve capacity, alter set/reseat behavior and may prevent safe depressurization
  • Backpressure increases the temperature of the relief device
  • High backpressure improves combustion at the flare tip
  • Backpressure eliminates the need for a knockout drum

Correct Answer: Excessive backpressure can degrade relief valve capacity, alter set/reseat behavior and may prevent safe depressurization

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