Relief Valve (PSV) Sizing

Quick pick (TL;DR)

Identify the governing relief case (fire, blocked outlet, control-valve failure, tube rupture, thermal expansion) and its required rate W.
Use the API 520 area equation for the phase, with overpressure 10% (process) or 21% (fire).
Round the calculated area up to the next API 526 standard orifice (D…T).
Choose conventional for stable back-pressure, balanced-bellows for variable back-pressure, pilot-operated for tight/high-capacity duty.
Route to flare or a safe location.

Step 1 — Governing scenario

Scenario	Overpressure	Note
Blocked outlet	10%	Common governing
Fire	21%	API 521 heat input
Thermal expansion	10%	Small thermal PSV
Tube rupture	10–16%	Exchanger case

Step 2 — Worked vapour sizing

Fire case, gas 5000 kg/h

API 520 vapour area A = (W / (C·Kd·P1)) · √(T·Z / M) Given W = 5000 kg/h , T = 523 K , M = 44 , Z = 1.0 k = 1.3 → C = 0.0263 , Kd = 0.975 Step 1 — Relieving pressure P1 = (10 × 1.21) + 1.01 = 13.1 bara = 1311 kPa Step 2 — √(TZ/M) √(523 × 1.0 / 44) = √11.89 = 3.448 Step 3 — Required area A = (5000 / (0.0263×0.975×1311)) × 3.448 A = 148.7 × 3.448 = 512 mm² Step 4 — Pick orifice H = 0.785 in² (506 mm²) < 512 → too small J = 1.287 in² (830 mm²) ≥ 512 → OK Answer → orifice 'J', balanced bellows, to flare

Round the area UP to the next API 526 orifice — 512 mm² → 'J' (830 mm²). C depends on k; P1 includes overpressure.

PSV sizing calculator (vapour / gas)

API 520 vapour area & API 526 orifice

Critical-flow vapour relief (API 520). Liquid & two-phase use different equations.

Relief rate W (kg/h)

Set pressure (barg)

Overpressure (%)

Relieving temp (K)

Molecular weight

k (Cp/Cv)

Compressibility Z

Discharge coeff Kd

Screening tool — confirm with full API 520 (Kb, Kc, Kv).

Reference — API 526 orifice areas

Letter	in²	mm²	Letter	in²	mm²
D	0.110	71	L	2.853	1841
E	0.196	126	M	3.60	2323
F	0.307	198	N	4.34	2800
G	0.503	325	P	6.38	4116
H	0.785	506	Q	11.05	7129
J	1.287	830	R	16.0	10323
K	1.838	1186	T	26.0	16774

Decision flow

1. Governing case → rate W

2. Compute area A (API 520)

OrificeRound up to next API 526 letter

Valve typeBellows / pilot per back-pressure

Inlet lineLoss ≤ 3% of set

Liquid & two-phase relief use their own equations.

Step 3 — Valve type & disposal

Conventional: stable back-pressure only.
Balanced bellows: variable back-pressure.
Pilot-operated: tight/high-capacity.
Disposal: flare for hazardous fluids.

Common mistakes

Wrong governing scenario.
Conventional on variable back-pressure.
Inlet loss > 3% → chatter.

Requisition & datasheet

Case, fluid, rate, set, T, M, k.
Area, orifice, valve type, disposal.

Frequently asked questions

What should I know about Relief Valve (PSV) Sizing?

Identify the governing relief case (fire, blocked outlet, control-valve failure, tube rupture, thermal expansion) and its required rate W. Use the API 520 area equation for the phase, with overpressure 10% (process) or 21% (fire). Round the calculated area up to the next API 526 standard orifice (D…T). Choose conventional for stable back-pressure, balanced-bellows for variable back-pressure, pilot-operated for tight/high-capacity duty. Route to flare or a safe location.

Which standards or codes apply?

API 520 Part I (sizing) & II (installation), API 521 (relief loads & disposal), API 526 (orifice areas), ASME VIII (overpressure protection).

Standards — API 520/521/526; ASME Section VIII.