Question 1

According to API RP 520, what is the maximum allowable pressure drop in the inlet piping to a pressure relief valve?

Accepted Answer

The pressure drop in the piping between the protected equipment and its relief valve shall not exceed 3% of the set pressure.

Question 2

What type of pressure relief valve is recommended when the variable back pressure is expected to be between 21% and 50% of the set pressure?

Accepted Answer

A balanced-bellows spring-loaded relief valve is recommended for variable back pressure between 21% and 50% of the set pressure.

Question 3

What does the acronym MAWP stand for in the context of pressure vessels?

Accepted Answer

MAWP stands for Maximum Allowable Working Pressure.

Question 4

In a pressure relief scenario, what is 'accumulation'?

Accepted Answer

Accumulation is the pressure increase over the MAWP of a vessel during a relief event, expressed as a percentage of MAWP.

Question 5

According to API 521, what is the typical maximum accumulation allowed for a single relief device in a non-fire operating contingency?

Accepted Answer

The maximum accumulation for a single relief device in a non-fire operating contingency is typically 10% above the MAWP.

Question 6

Differentiate between superimposed backpressure and built-up backpressure on a PSV.

Accepted Answer

Superimposed backpressure is the pressure in the discharge header before the PSV opens, while built-up backpressure is the additional pressure generated in the discharge header due to flow after the PSV opens.

Question 7

What is the phenomenon called when a pressure relief valve opens and closes rapidly, and what is its primary cause?

Accepted Answer

The phenomenon is called chattering, and it is primarily caused by excessive pressure drop at the inlet of the relief valve.

Question 8

A pilot-operated relief valve can be used when the variable back pressure is up to _____ of the set pressure.

Accepted Answer

70%

Question 9

Which API standard provides guidelines on pressure-relieving and depressurizing systems, including the identification of relief scenarios?

Accepted Answer

API 521 provides guidelines for pressure-relieving and depressurizing systems.

Question 10

Which API standard covers the sizing, selection, and installation of pressure-relieving devices?

Accepted Answer

API RP 520 (Part I for Sizing and Selection, Part II for Installation) covers these aspects.

Question 11

What is a common cause of overpressure due to thermal expansion in a process system?

Accepted Answer

Liquid-filled equipment or piping being blocked in and subsequently heated by an external source, like solar radiation or adjacent hot equipment.

Question 12

Which API standard provides standard orifice designations (e.g., D, E, F, G) and areas for pressure relief valves?

Accepted Answer

API 526 provides the standard orifice areas and designations.

Question 13

For an external fire scenario on a liquid-filled vessel, what determines the heat input according to API 521?

Accepted Answer

The heat input is determined by the wetted surface area of the vessel exposed to the fire.

Question 14

What is a 'blocked outlet' or 'blocked discharge' relief scenario?

Accepted Answer

A blocked outlet scenario occurs when the normal outlet path from a piece of equipment is inadvertently closed while the inlet flow continues, leading to pressure build-up.

Question 15

What is the typical length-to-diameter (L/D) ratio for a vertical gas-liquid separator?

Accepted Answer

The typical L/D ratio for a vertical gas-liquid separator is between 2 and 4.

Question 16

A conventional spring-loaded relief valve should be used when the total back pressure does not exceed _____ of the set pressure.

Accepted Answer

10%

Question 17

What safety device is often installed in combination with a PSV to provide protection against corrosive fluids and prevent leakage?

Accepted Answer

A rupture disc is often installed upstream of the PSV for this purpose.

Question 18

The coefficient of discharge for a pressure relief device, often used in sizing calculations, is represented by the symbol _____.

Accepted Answer

$K_d$

Question 19

What is the purpose of the 'weep hole' in the space between a rupture disc and a PSV?

Accepted Answer

The weep hole provides a visual indication of a ruptured or leaking disc, preventing pressure from building up between the devices which would alter the PSV set pressure.

Question 20

In a relief scenario, if the relieving fluid is a flashing liquid released above its bubble point pressure, what model is recommended by API RP 520 for sizing calculations?

Accepted Answer

The Homogeneous Equilibrium Model (HEM) is recommended for sizing in two-phase flashing flow scenarios.

Question 21

What is 'set pressure' for a pressure relief valve?

Accepted Answer

Set pressure is the inlet static pressure at which the pressure relief valve is set to start opening under service conditions.

Question 22

What is the consequence of selecting a PSV orifice that is significantly oversized for the required relief load?

Accepted Answer

An oversized PSV may be unstable and prone to chattering, leading to damage to the valve's seating surfaces and potential failure.

Question 23

A relief scenario involving the failure of a heat exchanger tube, where a high-pressure fluid leaks into a low-pressure side, is known as _____.

Accepted Answer

tube rupture

Question 24

If the relieving temperature of a gas or vapor is unknown for a fire case, what value should be assumed for PSV sizing calculations as per API guidelines?

Accepted Answer

The relieving temperature should be assumed as the temperature corresponding to the saturated vapor at the relieving pressure.

Question 25

What is the purpose of a flare K.O. (knock-out) drum in a pressure relief system?

Accepted Answer

A flare K.O. drum is used to separate and remove any liquid droplets from the vapor stream before it enters the flare stack to prevent burning liquid rain.

Question 26

If the back pressure on a conventional PSV exceeds 10% of its set pressure, what is the likely operational consequence?

Accepted Answer

The lift of the valve will be significantly reduced, leading to a decrease in its relieving capacity.

Question 27

What is a 'choked flow' condition in a pressure relief valve?

Accepted Answer

Choked flow, or critical flow, is a condition where the fluid velocity at the valve orifice reaches the local sonic velocity, and the mass flow rate becomes independent of further decreases in downstream pressure.

Question 28

A relief scenario caused by the failure of an automatic controller, such as a pressure or level controller, is known as a(n) _____.

Accepted Answer

instrument failure

Question 29

What is the 'credit for insulation' in a fire case relief calculation according to API 521?

Accepted Answer

It is a reduction factor applied to the heat input calculation, accounting for the thermal resistance provided by the vessel's insulation, which slows down heat transfer from the fire.

Question 30

What is the purpose of the 'depressurizing' or 'blowdown' system in a process plant, as described in API 521?

Accepted Answer

A depressurizing system is designed to rapidly reduce the pressure and inventory of a unit during an emergency, such as a fire, to prevent vessel rupture.

Pressure Relieving Systems (PSV)

Fundamentals of Pressure Relieving Systems

PSV Types and Selection

Backpressure Considerations

Relief Scenarios and Sizing

System Components

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