Relief Valve Sizing Calculator — API 520 521 526

CAPCO

FEL-3 Relief Valve Design Engine — Pressure-Relieving Device Sizing

API 520 PARTS I & II API 521 / API 526 ASME SEC. VIII DIV. 1

Relief Scenario — Select Governing Case

Blocked Outlet API 521 §5.15

Fire Case API 521 §5.16

Control Valve Fail Open API 521 §5.11

Thermal Expansion API 521 §5.14

Tube Rupture API 521 §5.18

Utility Failure API 521 §5.12

Protected Equipment — MAWP Basis

MAWP (Maximum Allowable Working Pressure)

psig

Operating pressure

psig

Set pressure (≤ MAWP)

psig

Number of PRVs on this vessel

Back pressure — superimposed

psig

Back pressure — built-up (est.)

psig

Pressure accumulation diagram

Fluid Phase at Relieving Conditions

Fluid / service description

Relieving temperature

°F

Molecular weight (vapor)

g/mol

Compressibility factor (Z)

—

Specific heat ratio (k = Cp/Cv)

—

Required Relieving Rate

Relief load (mass flow)

Combination correction factor (Kc)

Capacity correction — viscosity (Kv)

— (1.0 typical)

Inlet Line — 3% ΔP Check (API 520)

Inlet line diameter

Inlet line length

API 520 Pt.II: inlet ΔP must not exceed 3% of set pressure to prevent valve chatter and instability.

Discharge Disposal Route