What is a safety valve?

A safety valve (or pressure relief valve) is a device that automatically releases fluid when pressure exceeds a predetermined set point. It protects equipment from overpressure per ASME Section VIII, ISO 4126, or PED requirements.

How is orifice area calculated for gas service?

For gas/vapor service per API 520: A = W / (C × Kd × P₁ × Kb × √(M/TZ)), where W is required capacity, C is gas constant factor, Kd is discharge coefficient, P₁ is relieving pressure, Kb is back pressure correction, M is molecular weight, T is temperature, and Z is compressibility factor.

What are standard API orifice designations?

API 526 defines standard orifice sizes designated by letters: D (0.110 in²), E (0.196 in²), F (0.307 in²), G (0.503 in²), H (0.785 in²), J (1.287 in²), K (1.838 in²), L (2.853 in²), M (3.60 in²), N (4.34 in²), P (6.38 in²), Q (11.05 in²), R (16.0 in²), T (26.0 in²).

What is the difference between set pressure and relieving pressure?

Set pressure is the pressure at which the valve begins to open. Relieving pressure is the set pressure plus allowable overpressure (typically 10% for single valves per ASME VIII). The sizing calculation uses relieving pressure (absolute) as P₁.

Do I need to account for back pressure?

Yes. Constant back pressure reduces the effective differential across the valve. For conventional valves, the back pressure correction factor Kb adjusts the capacity. Balanced bellows or pilot-operated valves are less affected by back pressure.

Free Engineering Tool

Safety Valve Sizing Calculator

Size pressure safety valves per ISO 4126 / API 520. Calculate required orifice area and select standard orifice letter for gas, steam, or liquid service.

ISO 4126API 520/526Gas / Steam / Liquid

Fluid Type

Required Capacity (kg/h)

Set Pressure (bar g)

Back Pressure (bar g, 0 if atm)

Relieving Temperature (°C)

Molecular Weight (for gas, e.g. 29 for air)

Ratio of Specific Heats k (Cp/Cv)

Compressibility Z

Quick presets

Results

Required Orifice Area

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Standard Orifice Letter

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Standard Orifice Area

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Relieving Pressure (abs)

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Kd (Discharge Coeff)

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Valve Cross-Section

Gas / Vapor Sizing (API 520)

Standard API Orifice Designations (API 526)

Letter	Area (mm²)	Area (in²)	Typical Inlet × Outlet
D	71.0	0.110	1″ × 2″
E	126.5	0.196	1″ × 2″
F	198.1	0.307	1½” × 2½”
G	324.5	0.503	1½” × 3″
H	506.5	0.785	2″ × 3″
J	830.3	1.287	3″ × 4″
K	1186	1.838	3″ × 6″
L	1841	2.853	4″ × 6″
M	2323	3.600	4″ × 6″
N	2800	4.340	4″ × 6″
P	4116	6.380	6″ × 8″
Q	7129	11.05	6″ × 10″
R	10323	16.00	8″ × 10″
T	16774	26.00	8″ × 12″

⚠️ Note: This calculator provides preliminary sizing. Final valve selection must be verified by the valve manufacturer and comply with applicable codes (ASME VIII, PED, etc.).

Safety Valve Sizing Calculator | ISO 4126 | Relief Valve

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Gas / Vapor Sizing (API 520)

Standard API Orifice Designations (API 526)