⚠️ Safety & Relief Valve Sizing

Size pressure relief valves (PRV) for gas, vapor, steam, or liquid service per API 520, and select the standard API 526 orifice designation.

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📝 Configuration

📊 Results

Configure inputs and click Size.

📘 Methodology

API 520 Gas/Vapor

A = W·√(TZ/(Mk)) / (C·Kd·P1·Kb). The coefficient C depends on the isentropic exponent k. Kd is the discharge coefficient (0.975 for gas). Kb corrects for backpressure.

API 520 Steam & Liquid

Steam: A = W/(51.5·Kd·P1·Kn·Ksh). Liquid: A = Q·√(ρ/ΔP)/(38·Kd·Kw·Kv). Each service type uses specific correction factors per the standard.

API 526 Orifice Designations

Standard orifice letters D through T define discrete valve sizes from 71 mm² to 16,774 mm². The selected orifice must equal or exceed the calculated required area.

📘 Calculation Methodology: API 520 Pressure Relief Valve (PRV) Sizing

Mathematical Model & Theory

Sizes safety relief valves for vapor/gas and liquid emergency overpressure relief per API 520 Part I standards, calculating required effective discharge area $A$:

$$A = \frac{W}{C K_d P_1 K_b K_c} \sqrt{\frac{T Z}{M}} \quad (\text{Vapor/Gas}), \quad A = \frac{Q}{38 K_d K_w K_v} \sqrt{\frac{G}{\Delta P}} \quad (\text{Liquid})$$

$$P_1 = \text{Set Pressure} \times (1 + \text{Overpressure Fraction}) + P_{atm}$$

Assumptions

Critical acoustic choked nozzle flow across orifice.
Discharge coefficient $K_d = 0.975$ for certified ASME Section VIII relief valves.

Academic References

API Standard 520 Part I: Sizing, Selection, and Installation of Pressure-relieving Devices, 10th Edition.
ASME Boiler and Pressure Vessel Code: Section VIII Division 1.

Worked Engineering Example

Problem Statement:
Size a PRV for natural gas ($M = 18\text{ g/mol}$, $Z = 0.95$, $T = 310\text{ K}$, $k = 1.30 \implies C = 345$) relieving mass flow $W = 15,000\text{ kg/h}$ ($33,069\text{ lb/h}$) at relieving pressure $P_1 = 12.0\text{ bar, abs}$ ($174\text{ psia}$), $K_d = 0.975, K_b = 1.0$. Find required orifice area.

Step-by-step Solution:
1. Calculate required area $A$ in standard API units (or metric):
$$A \approx 942\text{ mm}^2 = 1.46\text{ in}^2$$
2. Select standard API orifice letter: **\"J\" Orifice** (Standard area $A = 1.287\text{ in}^2$) or **\"K\" Orifice** ($A = 1.838\text{ in}^2 = 1186\text{ mm}^2$).
Final Result:
Required effective discharge area is $\mathbf{942\text{ mm}^2}$ (Select **API 520 \"K\" Letter Orifice**).