Safety Valve Gas Orifice Calculator (API 520)

Size pressure safety relief valve (PSV) effective orifice areas and select standard API 526 letter designations (D through T) for gas and vapor service.

Relief Sizing Parameters

Relieving Rate W (lb/hr):

Gas Molecular Weight M:

Set Pressure Pset (psig):

Allowable Overpressure:

Relieving Temperature (°F):

Specific Heat Ratio k (Cp/Cv):

Backpressure & Coefficients

Backpressure (psig):

Compressibility Factor (Z):

Valve Type (API 520 Derating):

PSV Sizing & API 526 Orifice

Required Orifice Effective Area

-- sq in

-- mm²

Selected API 526 Orifice

-- sq in actual

Certified Rated Capacity

-- lb/hr

--% overcapacity

Relieving Pressure P1: -- psia

Critical Flow Constant C: --

Flow Condition: Critical Sonic Flow

API 526 standard orifice sizes ensure certified nameplate relieving capacity matches or exceeds relief demand.

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Frequently Asked Questions

What are API 526 standard orifice letter designations?

API Standard 526 standardizes relief valve nozzle sizes and body dimensions into 14 distinct orifice letters ranging from D (0.110 in²) up to T (26.0 in²). Relief valve manufacturers design their cast bodies to match these exact standard throat areas.

When does API 520 require a Balanced Bellows instead of a Conventional PSV?

Conventional valves vent the back of the disc holder to the discharge header. If superimposed or built-up backpressure exceeds 10% of set pressure, the backpressure acts on top of the disc, artificially raising the opening pressure and choking capacity. Balanced bellows isolate the disc, permitting backpressures up to 30 - 50%.

What allowable overpressures are permitted by ASME Section VIII?

For non-fire single valve protection: 10% (or 3 psi, whichever is greater). For multiple valve installations: 16%. For external fire exposure scenarios: 21% accumulation above vessel Maximum Allowable Working Pressure (MAWP).