Which Kd values are used?

Preliminary API 520 values: Kd = 0.975 (gas/vapor) and Kd = 0.650 (liquid). Certified ASME UG-131 nameplate Kd overrides these for purchase.

What are API 526 orifice letters?

Standard effective areas D through T (e.g. F = 0.307 in², H = 0.785 in², J = 1.287 in²). The calculator picks the smallest letter with A_API ≥ A_req.

Is this certified PSV selection?

No. It is preliminary MTO / line screening. Backpressure, two-phase HEM, viscosity (Kv), and rupture disks (Kc) are out of scope.

PSV / PRV Orifice Area Screening Calculator (API 520 / API 526)

Engineering Reference & ASME Code Basis

1. Core Formula & Variable Definitions

API Standard 520 Part I (10th Edition, 2020) — §5.2 gas/vapor, §5.6 liquid sizing · API Standard 526 (7th Edition, 2017) — Table 1 standard effective orifice areas D–T · ASME BPVC Section VIII Division 1 UG-131 — certified relieving capacity (nameplate Kd)

Gas / vapor (API 520 §5.2, critical): A_req = W / (C · K_d · P₁) · √(T · Z / M)

Liquid (API 520 §5.6): A_req = Q / (38 · K_d) · √(G / ΔP)

P₁ = P_set · (1 + OP%/100) + P_atm · select smallest API 526 letter with A_API ≥ A_req

API 520 Part I (10th Ed.) · API 526 Table 1 · ASME VIII UG-131 screening Kd

Hero API 526 letterGas Kd 0.975Liquid Kd 0.650Letters D–T

US customary API 520 equations with metric/imperial I/O conversion. Screening Kd = 0.975 (gas) / 0.650 (liquid); Kb = Kw = Kv = Kc = Z = 1 (atmospheric discharge, non-viscous, no rupture disk). Orifice letters D–T from API 526 effective areas.

A_req (Required effective orifice area) — Minimum API 520 discharge area (mm² / in²).
P1 (Relieving absolute pressure) — P_set × (1 + OP/100) + Patm (bar abs / psia).
W / Q (Required relieving capacity) — Gas mass flow (kg/h or lb/h) or liquid volume flow (L/min or GPM).
Kd (Effective discharge coefficient) — Preliminary API 520 Kd (0.975 gas / 0.650 liquid).
C (Gas constant from k) — C = 520 √[k (2/(k+1))^((k+1)/(k−1))] (US customary).
A_API (API 526 effective orifice area) — Standard letter area D–T selected ≥ A_req.

2. Screening Rules Matching This Calculator

Preliminary API 520 required effective area with screening Kd, then maps to the smallest API 526 letter D–T with A_API ≥ A_req. Certified nameplate Kd and backpressure corrections are purchase steps.

Relieving pressureP1 = P_set × (1 + OP/100) + P_atm

Absolute relieving pressure. Typical non-fire overpressure OP = 10%; fire cases often 21%.

Screening Kd0.975 gas · 0.650 liquid

Preliminary API 520 defaults. ASME VIII UG-131 certified Kd overrides for final selection.

Correction factorsKb = Kw = Kv = Kc = Z = 1

Atmospheric discharge, non-viscous, no rupture disk in series — screening simplification.

Out of scope hereBackpressure · two-phase HEM · viscosity Kv

Balanced bellows Kb, superimposed backpressure, two-phase relief, and rupture-disk Kc need full API 520 Part I analysis.

Quick Reference Lookup Table

Default gas duty (air · P_set = 10 bar g · W = 5000 kg/h · OP = 10% · T = 25 °C · k = 1.40)
Quantity	Value	Unit	Notes
P1	12.01	bar abs	10×1.10 + 1.013
C	356.1	—	k = 1.40
A_req	506	mm²	≈ 0.785 in²
Orifice	H	—	API 526 A = 506 mm²
Margin	~0%	—	Selected ≈ required

Hero reports the selected API 526 letter and effective area. Default air duty (10 bar g · 5000 kg/h · 10% OP) selects orifice H (~506 mm²).

3. Fluid & Service Limits

PSV orifice screening is fluid- and service-driven. Body metallurgy is a separate corrosion / temperature selection — not computed here.

Group / Regime	Range / Condition	Limit / Criterion	Engineering Notes
Gas / vapor (API 520 §5.2 critical)	T at relieving conditions	k sets C constant	Mass flow W with M, Z = 1 screening. Choked critical flow assumed.
Liquid (API 520 §5.6)	N/A	G = specific gravity	Volumetric Q with Kd = 0.650. Viscous correction Kv not applied in this tool.
API 526 orifice letters D–T	N/A	Smallest A_API ≥ A_req	Hero reports selected letter and area margin — confirm inlet loss and chatter on the data sheet.
Two-phase / reactive / viscous relief	N/A	Out of scope	HEM, runaway reaction, and high-viscosity duties need specialist API 520 methods — not this screening path.

Code Applicability & Safety Boundaries

API 520 Part I preliminary sizing — not ASME UG-125 / UG-131 certified relieving capacity
Backpressure, inlet loss, dual-phase flow, and rupture-disk combinations require full API 520 analysis before purchase

4. Step-by-Step Worked Example

Field VerificationShow

Worked example — air PSV, 10 bar g · 5000 kg/h

Preliminary gas PSV sizing for air at 10 bar g set, 10% overpressure, 25 °C, W = 5000 kg/h.

Fluid:Air (M = 28.97, k = 1.40)P_set / OP:10 bar g / 10%W:5000 kg/h

Relieving pressure

P1 = 10 × 1.10 + 1.013

Result:12.01 bar abs

Required area

API 520 critical gas (Kd = 0.975, Z = 1)

Result:A_req ≈ 506 mm²

API 526 letter

Smallest A_API ≥ A_req

Result:H (506 mm²)

Conclusion: Select API 526 orifice H for this screening duty. Confirm manufacturer Kd, backpressure, and installation before purchase.

5. How to screen PSV / PRV orifice area

1
Choose gas or liquid
Gas uses W, M, T, and k; liquid uses Q and specific gravity G.
2
Enter set pressure and overpressure
Typical non-fire OP = 10%; fire case often 21%.
3
Enter relieving capacity
Mass flow for gas or volumetric flow for liquid in the active unit system.
4
Read letter and margin
Hero shows the API 526 letter; badges show A_req, P1, and capacity margin.

6. Frequently Asked Questions & Technical References

Show

A screening tool that sizes the API 520 required effective discharge area and maps it to the nearest API 526 orifice letter (D–T).