What is the set pressure of a Pressure Relief Valve (PSV)?
The pressure (inlet gage) the PSV is set to open, generally at Design Pressure
Set pressure is critical for determining when the valve will activate.
Define overpressure in the context of a PSV.
The pressure increase over the set pressure, expressed as % of set pressure
Overpressure is the same as accumulation when set at MAWP.
What is the opening pressure of a PSV?
Inlet static pressure at which there is a measurable lift (start-to-discharge pressure)
This is the point where the valve begins to relieve pressure.
What does closing pressure or reseating pressure refer to?
Inlet static pressure at which disc contacts with the seat again
This indicates when the valve stops discharging.
What is blowdown pressure?
Decreasing inlet static pressure at which no further discharge is detected at the PSV outlet after the valve has been subjected to pressure equal to or above the set pressure
This is important for understanding valve performance after activation.
Define back pressure in relation to a PSV.
Pressure that exists at the outlet of a PSV due to the discharge system
Back pressure can affect the performance of the PSV.
What is the difference between superimposed back pressure and built-up back pressure?
- Superimposed Back Pressure: Pressure in the discharge system from coincident discharges, may be constant or variable
- Built-up Back Pressure: Increase in pressure at the outlet of the PSV due to the valve opening, variable
Understanding these types of back pressure is crucial for PSV design.
What does accumulation refer to?
Pressure increase over the maximum allowable working pressure or design pressure of the vessel, expressed as % of MAWP or in pressure units
Accumulation is critical for ensuring safety margins.
What are relieving conditions?
Inlet pressure and temperature on a pressure relief device during an overpressure condition
Relieving pressure = Set pressure + Overpressure.
What is simmer in the context of a PSV?
Audible or visible escape of fluid between the seat and disc at an inlet static pressure below the popping pressure
This applies to safety or safety relief valves on compressible fluid.
What is the rated capacity of a pressure relief device?
Rated flow determined in accordance with the pressure design code or regulation and supplied by the manufacturer
This is essential for sizing and safety assessments.
Define relief gas.
All gases and vapors sent to the flare tip, excluding entrained air, including organic material, nitrogen, and other gases
Other terms include flared gas, vent gas, waste gas, and waste vapor.
What is the required relief rate?
Estimated flow rate that needs to be relieved to prevent the equipment pressure from exceeding the specified design value
This is critical for ensuring equipment safety.
True or false: Overpressure is the same as accumulation when set at MAWP.
TRUE
Understanding this relationship is important for pressure relief calculations.
What is the maximum allowable design pressure plus accumulation for operational failure and fire?
- Operational failure: 10%
- Fire: 21%
These percentages are critical for safety margins in pressure vessel design.
What should be considered when sizing PRV piping?
- Rated capacity for inlet and outlet piping
- Discharge line sizing per API 520
- Required relief rate for common relief header
Key exceptions apply for modulating type PRVs.
What are some PSV overpressure scenarios?
- External fire
- Closed/Blocked Outlet
- Open inlet (Blocked Outlet)
- Reverse flow
- Reflux or Pump-around failure
- Liquid Overfill
- Control valve failure
- Abnormal process heat input
- Internal explosion
- Runaway chemical reaction
Identifying these scenarios is essential for safety assessments.
