Pressure Relief Valve Sizing Basis: API 520, Fire Case and Two-Phase Flow

Quick Answer

Relief valve sizing starts from the required relieving rate, which is set by the governing overpressure scenario (blocked outlet, external fire, thermal expansion, control failure). API 520 Part I then converts that rate into a required effective orifice area, and you select the next larger certified API 526 orifice (D through T). Backpressure and two-phase flow determine whether you use a conventional, balanced-bellows or pilot-operated valve.

Start With the Relieving Rate

The valve is only as good as the rate it must pass. API 521 defines the credible overpressure scenarios and how to calculate the relieving load for each:

• Blocked outlet / closed valve: full pump or compressor capacity into a blocked-in segment.
• External fire: API 521 wetted-area heat-input formula, generating vapor at the relief set pressure.
• Thermal expansion: trapped liquid heated with no outlet; a small thermal-relief valve usually suffices.
• Control failure: a downstream control valve failing open can overload upstream equipment.

Apply API 520 Part I

API 520 Part I gives the sizing equations. For vapor you use the critical- or sub-critical-flow gas equation; for liquid a Darcy-based equation with the rated Cv of the orifice. The result is a *required effective area*, and you round up to the next standard API 526 orifice.

Backpressure Rules

Two-Phase Flow

When the relieving stream is vapor-liquid (common in fire cases on liquefied-gas or two-phase systems), use the omega or homogeneous-equilibrium method. Sizing as all-vapor understates the required area and leaves the equipment under-protected.

See our safety relief valve sizing overview and the relief vs safety valve distinction for related detail, or the standards hub for the code map.