อัปเดต 2026-09-07

Relief Valve / PSV Sizing

API RP 520 Part I (10th ed.)

Sizes the required PSV orifice area for the three most common relieving fluid states — gas/vapor (critical or subsonic flow), liquid (incompressible), and steam (Napier form) — then selects the next-larger standard API 526 letter (D through T2).

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Each fluid state has its own closed-form API 520 sizing equation with correction factors (Kd discharge coefficient, Kb back-pressure, Kc rupture-disk, Kv liquid-viscosity via an iterated Reynolds number, Kn steam Napier high-pressure correction). No iteration is needed except the liquid Kv viscosity loop (≤5 passes).

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Gas/vapor — critical (choked) flow, API 520 §5.6 Eq 5

A = \frac{W}{C K _{d} P _{1} K _{b} K _{c}} \frac{T Z}{M}

A in mm², W in kg/h, P1 = relieving pressure in kPa abs. C = 0.03948·√(k·(2/(k+1))^((k+1)/(k-1))) is a function of the isentropic exponent k. Applies when P_back/P1 ≤ the critical ratio (2/(k+1))^(k/(k-1)).

Liquid — incompressible flow, API 520 §5.8

A = \frac{11.78 Q}{K _{d} K _{w} K _{c} K _{v}} \frac{G}{Δ P}

Q in L/min, ΔP in kPa, G = specific gravity. Kv (viscosity correction) is solved iteratively: Kv = (0.9935 + 2.878/√Re + 342.75/Re^1.5)^-1, Re depending on the bore diameter implied by A.

Steam — Napier form, API 520 §5.7 Eq 17 (SI)

A = \frac{190.4 W}{K _{d} P _{1} K _{b} K _{c} K _{n} K _{s h}}

Kn (Napier high-pressure correction) = 1 below ≈10.34 MPa abs, else (0.02764·P−1000)/(0.03324·P−1061); Ksh is the superheat correction (=1 for saturated steam). Not valid above the steam critical pressure 22.064 MPa.

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