PSV Liquid Relief Sizing Calculator

API RP 520 Part I (2020) — 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. K_d = 0.65. Overpressure entered by user (typically 10%). Uses P₁−P₂ as differential pressure.

Flow Rate

Relief rate basis

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

Mass flow rate Wkg/h

kg/h

Liquid density ρkg/m³

kg/m³

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

Viscosity μcP

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

Pressures

Set pressure P_skPag

kPag

Allowable overpressure OP%

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

Relieving pressure P₁kPag

kPag

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

Total back pressure P₂kPag

kPag

Differential pressure ΔPkPa

kPa

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

Valve Parameters

Discharge coefficient K_d–

–

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

Valve type & back-pressure correction K_w

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

Rupture disk combination K_c–

Calculation Results

Method appliedAPI RP 520 Part I section

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Volumetric flow, QL/min — used in sizing equation

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Specific gravity, G_lρ / 1000

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Preliminary area, A_Rwith K_v = 1.0 (first pass)

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Reynolds number, ReAPI 520 Eq. (37) — at next API 526 orifice ≥ A_R

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Viscosity correction, K_vAPI 520 Eq. (34) — applied to final area

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Required discharge area, AA = A_R / K_v

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Selected standard orificeper API Std 526 — smallest ≥ required A

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Calculation Trace

Sizing method

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P₁ relieving pressure

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Differential pressure (ΔP)

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G_l = ρ / 1000

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K_d × K_w × K_c (× K_p)

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A_R (Kv=1) = 11.78×Q/(K_d·K_w·K_c·[K_p])×√(G/ΔP)

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Next API 526 orifice ≥ A_R used for Re

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Re = Q × (18,800 × G_l) / (μ × √A_selected)Eq. (37) — μ in cP

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K_v = (170/Re_L + 1)^−0.5Eq. (34), valid for Re_L ≥ 80

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Final A = A_R / K_v

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Formulas & reference data (API RP 520 Part I, 2020)

Section 5.8 — Certified PRVs (Equations 32/33)

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) / (K_d × K_w × K_c × K_v) × √(G_l / (P₁ − P₂))

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

Section 5.9 — Non-certified PRVs (Equations 42/43)

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

A = (11.78 × Q) / (K_d × K_w × K_c × K_v × K_p) × √(G / (1.25×P_s − P₂))

Design basis is 25% overpressure; K_d = 0.62; K_p from API 520 Fig. 39 (at 10% OP → K_p = 0.6; at 25% OP → K_p = 1.0). Note: typically gives a larger (more conservative) area than Sec. 5.8.

Reynolds number (Eq. 37 — SI)

Re = Q × (18,800 × G_l) / (μ × √A_selected)

Q in L/min, G_l = ρ/1000, μ in cP, A_selected in mm². Use the next standard API 526 orifice larger than A_R (first-pass area with K_v=1) to calculate Re. Then compute K_v and final A. Alternative: Eq. (38) uses Saybolt universal seconds (U) instead of cP — not recommended below 100 SUS.

Viscosity correction K_v (Eq. 34)

K_v = (170 / Re_L + 1)^−0.5

K_v is capped at 1.0. Applicable for Re_L ≥ 80. If the corrected area A_R/K_v exceeds the orifice used for Re, repeat with the next larger orifice (per API 520 §5.8.1.5).

K_p — overpressure correction for non-certified PRVs (Fig. 39)

Overpressure (%)	10	15	20	25	30	40	50
K_p (approx.)	0.60	0.74	0.86	1.00	1.04	1.07	1.09

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

Letter	D	E	F	G	H	J	K	L	M	N	P	Q	R	T
mm²	71.0	126.5	198.1	324.5	506.5	830.3	1186	1841	2323	2800	4116	7129	10323	16774

Notes & limitations

This calculator covers liquid, non-flashing, incompressible relief only (API 520 §5.8.1.2).
K_d = 0.65 (certified) / 0.62 (non-certified) are preliminary values. Always confirm with the manufacturer's certified K_d before final procurement.
K_w 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.

PSV Liquid Relief Sizing Calculator

Section 5.8 — Certified PRVs (Equations 32/33)

Section 5.9 — Non-certified PRVs (Equations 42/43)

Reynolds number (Eq. 37 — SI)

Viscosity correction Kv (Eq. 34)

Kp — overpressure correction for non-certified PRVs (Fig. 39)

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

Notes & limitations

Viscosity correction K_v (Eq. 34)

K_p — overpressure correction for non-certified PRVs (Fig. 39)