How to Size a Relief Valve: API 520

Pressure relief valve sizing per API 520 Part I determines the minimum effective discharge area (orifice area) required to relieve a specified mass or volume flow rate at a given set pressure. The calculated area is matched to a standard API orifice letter designation, and the valve inlet and outlet sizes are selected accordingly.

When This Calculation Is Used

Process safety engineers size relief valves for every pressure vessel and piping system that can be exposed to overpressure. Common relieving scenarios include fire case, blocked outlet, thermal expansion, control valve failure, and tube rupture. API 520 Part I covers sizing; API 521 defines the relieving scenarios; API 526 standardizes the orifice sizes.

API 520 Orifice Area Formula (Gas/Vapor)

A = W / (C x K x P1 x Kb x Kc) x sqrt(T x Z / M)

Symbol	Parameter	Unit
A	Required effective discharge area	in2
W	Required relieving rate	lb/hr
C	Gas constant coefficient (function of k = Cp/Cv)	Dimensionless
K	Effective coefficient of discharge (0.975 for API-certified valves)	Dimensionless
P1	Relieving pressure (set pressure x 1.10 + atmospheric)	psia
Kb	Back-pressure correction factor	Dimensionless (1.0 for conventional; < 1.0 for balanced bellows)
Kc	Combination correction factor (1.0 without rupture disc; 0.9 with)	Dimensionless
T	Relieving temperature	Rankine (F + 460)
Z	Gas compressibility factor at relieving conditions	Dimensionless
M	Molecular weight of gas	lb/lb-mol

API 520 Orifice Area Formula (Liquid)

A = Q / (38 x K x Kw x Kv) x sqrt(G / (P1 - P2))

Where Q is in US GPM, G is specific gravity, P1 is set pressure x 1.10 + atmospheric (psia), and P2 is total back pressure (psia).

Standard API 526 Orifice Designations

Letter	Effective Area (in2)	Effective Area (mm2)	Typical Inlet x Outlet
D	0.110	71.0	1” x 2”
E	0.196	126.5	1” x 2”
F	0.307	198.1	1.5” x 2.5”
G	0.503	324.5	1.5” x 3”
H	0.785	506.5	2” x 3”
J	1.287	830.3	3” x 4”
K	1.838	1,186	3” x 4”
L	2.853	1,841	4” x 6”
M	3.600	2,323	4” x 6”
N	4.340	2,800	4” x 6”
P	6.380	4,116	4” x 6” or 6” x 8”
Q	11.05	7,129	6” x 8” or 6” x 10”
R	16.00	10,323	6” x 10” or 8” x 10”
T	26.00	16,774	8” x 10”

Sizing Steps

1. Identify all overpressure scenarios per API 521 (fire, blocked outlet, thermal expansion, etc.)
2. Calculate the required relieving rate (W or Q) for each scenario
3. Apply the appropriate API 520 formula (gas, liquid, steam, or two-phase)
4. Select the smallest standard API 526 orifice letter that provides an effective area equal to or greater than the calculated area
5. Verify inlet pressure drop does not exceed 3% of set pressure (API 520 Part II)
6. Verify that the back pressure is within the valve’s allowable range

Common Mistake

Using design pressure instead of relieving pressure in the formula. Relieving pressure for a single relief valve is 110% of the set pressure (which equals the MAWP) plus atmospheric pressure. For fire case or multiple-valve installations, the accumulation allowance may be 116% or 121% per ASME Section VIII. Using the wrong pressure results in undersized orifice selection.

For information on safety and pressure relief valve types and their applications in piping systems, see the detailed reference guide.

Read the full pipe sizes guide