PSV Liquid Relief Sizing Calculator

API RP 520 Part I (2008) — Section 5.8 (Certified) & Section 5.9 (Non-certified) — SI units — Standard orifices per API Std 526

Input Data
Sizing Method
Section 5.8 (Certified): ASME capacity-certified valve. Kd = 0.65. Overpressure entered by user (typically 10%). Uses P1−P2 as differential pressure.
Flow Rate

Q in the API 520 Eq. (29)/(39) is in L/min; mass input is converted via ρ

kg/h
kg/m³

Water = 1000 kg/m³ @ 15°C  |  Specific gravity Gl = ρ / 1000

cP

Use value at the relieving temperature  |  Water @ 20°C ≈ 1.0 cP

Pressures
kPag
%

10 % standard  |  P1 = Ps × (1 + OP/100)

kPag

Calculated: Ps × (1 + OP/100) — Sec. 5.8

kPag
kPa

Sec 5.8: P1 − P2  |  Sec 5.9: 1.25×Ps − P2

Valve Parameters
–

Sec 5.8: 0.65 (certified)  |  Sec 5.9: 0.62 (non-certified) — use manufacturer value if known

Conventional & pilot-operated valves require no back-pressure correction (API 520 §5.3)

Calculation Results
Method appliedAPI RP 520 Part I section
–
Volumetric flow, QL/min — used in sizing equation
–
Specific gravity, Glρ / 1000
–
Preliminary area, ARwith Kv = 1.0 (first pass)
–
Reynolds number, ReAPI 520 Eq. (33) — at next API 526 orifice ≥ AR
–
Viscosity correction, KvAPI 520 Eq. (30) — applied to final area
–
Required discharge area, AA = AR / Kv
–
Selected standard orificeper API Std 526 — smallest ≥ required A
– –
Calculation Trace
Sizing method
–
P1 relieving pressure
–
Differential pressure (ΔP)
–
Gl = ρ / 1000
–
Kd × Kw × Kc (× Kp)
–
AR (Kv=1) = 11.78×Q/(Kd·Kw·Kc·[Kp])×√(G/ΔP)
–
Next API 526 orifice ≥ AR used for Re
–
Re = Q×18800×Gl / (μ×√A)
–
Kv = (0.9935 + 2.878/Re0.5 + 342.75/Re1.5)−1
–
Final A = AR / Kv
–
Formulas & reference data (API RP 520 Part I, 2008)

Section 5.8 — Certified PRVs (Equations 28/29)

For ASME-capacity-certified valves in liquid (incompressible, non-flashing) service — SI units (Q in L/min, P in kPag, A in mm²):

A = (11.78 × Q) / (Kd × Kw × Kc × Kv) × √(Gl / (P1 − P2))

P1 = relieving pressure = Pset × (1 + OP/100). For preliminary sizing Kd = 0.65.

Section 5.9 — Non-certified PRVs (Equations 38/39)

For valves where capacity certification is not required or was never established — SI units:

A = (11.78 × Q) / (Kd × Kw × Kc × Kv × Kp) × √(G / (1.25×Ps − P2))

Design basis is 25% overpressure; Kd = 0.62; Kp from API 520 Fig. 38 (at 10% OP → Kp = 0.6; at 25% OP → Kp = 1.0). Note: typically gives a larger (more conservative) area than Sec. 5.8.

Reynolds number (Eq. 33 — SI)

Re = Q × (18 800 × Gl) / (μ × √A)

Q in L/min, Gl = ρ/1000, μ in cP, A in mm². Use the next standard API 526 orifice larger than AR (first-pass area with Kv=1) to calculate Re. Then compute Kv and final A.

Viscosity correction Kv (Eq. 30)

Kv = ( 0.9935 + 2.878 / Re0.5 + 342.75 / Re1.5 )−1

Kv is capped at 1.0. If Kv × AR/Kv > the orifice used for Re, repeat with the next larger orifice (per API 520 §5.8.1.5).

Kp — overpressure correction for non-certified PRVs (Fig. 38)

Overpressure (%)10152025304050
Kp (approx.)0.600.740.861.001.041.071.09

Standard effective orifice areas — API Std 526 5th ed., Table 1

LetterDEFGHJKLMNPQRT
mm²71.0126.5198.1324.5506.5830.31186184123232800411671291032316774

Notes & limitations

  • This calculator covers liquid, non-flashing, incompressible relief only (API 520 §5.8.1.2).
  • Kd = 0.65 (certified) / 0.62 (non-certified) are preliminary values. Always confirm with the manufacturer's certified Kd before final procurement.
  • Kw for balanced-bellows valves must be read from API 520 Fig. 31 — this tool accepts a manually entered value.
  • Per API 520 §5.9.2, non-certified valves operating at <10% overpressure may chatter — avoid this operating regime.
  • Always select the next larger standard orifice; never interpolate between sizes.