Pressure Relieving Systems – Procedures for Sizing

Modifications to API RP 520

API Effective Area and Effective Coefficient of Discharge
1) The minimum size of PR valves shall be as follows:
a) 1 in. NPS (25 mm) inlet size with flanged inlet and outlet connections.
b) 3/4 in. NPS (20 mm) when used only for protection against liquid thermal expansion. This size may be used with threaded inlet and outlet connections.
2) The use of smaller sizes shall be approved by Owner’s Engineer. Vessels fabricated from pipe less than 24 in. NPS (600 mm), which are not ASME Code stamped, may not require PR devices for protection against fire.

API Effective Area and Effective Coefficient of Discharge
1) Unless otherwise specified, pressure relief valves with Q, R, and T size orifices shall not be used where the temperature exceeds 350?F (177?C) and the molecular weight is less than 10 without Owner Engineer’s approval. Pressure relief valves with orifice sizes larger than T (16,775 mm2 or 26.0 in2) or with inlet flanges larger than NPS 8 in. (200 mm) shall not be used without Owner Engineer’s approval.

: Balanced Pressure Relief Valves
1) In new designs, the total back pressure (built-up plus superimposed) shall be limited to 50 percent of the set pressure for balanced bellows type PR valves for either operating or fire contingencies. In retrofits, the maximum backpressure published by the valve manufacturer for correction factors, Kb may be allowed with approval by the Owner’s Engineer. The built up backpressure shall be calculated based on the frictional plus kinetic pressure changes.
2) When total backpressure for balanced bellows relief valves exceeds 30 percent of the set pressure, the valve manufacturer shall be consulted. The capacity of the valve and the maximum allowable backpressure on the bellows shall be verified with the manufacturer.

 

Effect of Back Pressure and Header Design on PRV Sizing and Selection

  • The maximum superimposed backpressure for non-discharging PR valves during a maximum system release (from either single or multiple valve releases under a design contingency) shall be as follows:

For spring-loaded, conventional PR valves:

For balanced bellows and pilot operated valves:

Where:
Psi (max) = Maximum superimposed back pressure
Pset = Pressure relief valve set pressure
Pd = Differential spring pressure
C = Multiplier applied to design pressure to obtain hydrostatic test pressure per GP 05-03-01, dimensionless

Operating Contingencies
1) Where not prohibited by local legislation and in accordance with the requirements of the ASME Code, Section VIII, Division 1 (ASME SEC VIII D1), accumulated pressure can be increased when multiple PRVs are installed as follows:
a) 116% of the maximum allowable working pressure for operating contingencies,
b) 121% of the maximum allowable working pressure when fire is the only contingency requiring a PRV,
c) 106% of the maximum allowable working pressure for boilers.


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