Calculators β€Ί Equipment Design β€Ί Relief Valve Sizing
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Relief Valve Sizing Calculator - API 520

Per API 520 9th Ed. / API 521 6th Ed. with Fire Case Analysis

Pressure Relief Valve Sizing Calculator
Sizes relief valves for gas, liquid, and two-phase service per API 520 9th Edition / API 521 6th Edition. Handles multiple scenarios including fire exposure, blocked outlet, and thermal expansion with standard orifice selection per API 526.
⚠️ Important Notice:
This calculator is for preliminary engineering sizing only. Final valve selection must be verified with manufacturer sizing software, considering specific valve designs, materials, and installation requirements. Always consult API 520/521 and applicable codes for detailed design.

Relief Scenario & Flow

Required relief capacity

Pressure & Temperature

psig
PRV set pressure
%
Standard: 10% (Fire: 21%, Emergency: 16%)
psig
Built-up or superimposed
Β°F
Temperature at relief conditions
Affects backpressure correction (Kb/Kw)
Reduces Kd by 10% per API 520 (Kc = 0.9)

Fluid Properties

Auto-calculates MW, Z-factor using Hall-Yarborough
-
Typical: 0.55-0.65 for pipeline gas
-
k = Cp/Cv, auto-estimated if empty
-
Auto-calculated for natural gas via Hall-Yarborough

Discharge Coeff (Kd)

Vapor: 0.975
Liquid: 0.65
Two-phase: 0.85

Overpressure Limits

Non-fire: 10% (single) / 16% (multiple)
Fire case: 21% (any config)

Max Backpressure

Conventional: ≀10% Pset
Balanced: ≀50% Pset
Pilot: ≀90% Pset

Fire Heat Input

Adequate drain: 21,000
Inadequate: 34,500
Γ— F Γ— A0.82 BTU/hr

πŸ“˜ Engineering Reference Notes

πŸ”§ Valve Type Selection

  • Conventional: BP <10%. Simple, lowest cost.
  • Balanced Bellows: Variable BP up to 50%. Corrosion protection.
  • Pilot Operated: High BP up to 90%. Tight shutoff, large orifices.

🌊 Critical Pressure Ratios

Flow chokes when Pβ‚‚/P₁ < critical:

  • Natural Gas (k=1.28): 0.549
  • Methane (k=1.31): 0.544
  • Air/Nβ‚‚ (k=1.40): 0.528
  • Propane (k=1.13): 0.574

πŸ“Š Z-Factor Ranges (Nat. Gas)

  • Low P (<500 psia): Z β‰ˆ 0.95–1.0
  • Medium (500–2000): Z β‰ˆ 0.80–0.95
  • High P (>2000 psia): Z β‰ˆ 0.70–0.90
  • Hβ‚‚S/COβ‚‚ >5%: Apply Wichert-Aziz

πŸ”₯ Fire Case Overview

Fire relief loads depend on wetted area and drainage.

  • Overpressure: 21% allowed
  • Heat Input: Q = C Γ— F Γ— A0.82

πŸ› οΈ Installation Rules

  • Inlet pipe β‰₯ PRV inlet size, short as possible
  • Inlet Ξ”P <3% of set pressure
  • No pockets in discharge (drain liquids)
  • Support outlet for reaction forces
  • No isolation valves without CSO

⚑ Accumulation Limits

  • Single valve, non-fire: 10% MAWP
  • Multiple valves: 16% MAWP
  • Fire case (any config): 21% MAWP
  • Supplemental PRV: Set ≀ 105% MAWP

API 526 Standard Orifices

Orifice Area (inΒ²) Orifice Area (inΒ²)
D0.110L2.853
E0.196M3.60
F0.307N4.34
G0.503P6.38
H0.785Q11.05
J1.287R16.0
K1.838T26.0

Correction Factors

Factor Vapor Liquid Notes
Kd0.9750.65Discharge coeff.
Kb0.5–1.0β€”Backpressure (vapor)
Kwβ€”0.6–1.0Backpressure (liquid)
Kvβ€”0.65–1.0Viscosity (iterative)
Kc0.90.9With rupture disk

Specific Heat Ratios (k = Cp/Cv)

Gas k Gas k
Hydrogen1.41Propane1.13
Methane1.31n-Butane1.09
Nat. Gas1.28Steam1.33
Ethane1.19Air / Nβ‚‚1.40
COβ‚‚1.29Ammonia1.31

Environment Factor (F) - Fire Case

Condition F
Bare vessel1.0
Water spray / deluge0.5
Insulation 1"0.3
Insulation 2"0.15
Insulation 3"0.075
Insulation 4"0.05
Fireproofing0.03
⚠️ Key Sizing Rules:
  • Always select the next larger standard orificeβ€”never interpolate
  • Inlet piping Ξ”P must be ≀3% of set pressure at rated flow
  • Conventional valves require balanced bellows or pilot-operated if backpressure >10%
  • Fire case uses 21% overpressure regardless of valve configuration
  • Use 34,500 constant (not 21,000) if drainage is inadequate per API 521
  • Two-phase: Use Kd = 0.85 (not 0.975 or 0.65)

Standards: API 520 Part I (9th Ed.) Β· API 520 Part II (6th Ed.) Β· API 521 (6th Ed.) Β· API 526 (7th Ed.) Β· ASME BPVC VIII Div. 1

πŸ“š Learn the Theory

Learn API 520/521 sizing methods, relief scenarios, discharge coefficients, and installation

Read Engineering Guide β†’

Sizing Formulas (API 520 9th Edition)

Vapor/Gas (Equation 3a):

A = (W / (C Γ— Kd Γ— P1 Γ— Kb Γ— Kc)) Γ— √(T Γ— Z / M)

Where C = 520 Γ— √[k Γ— (2/(k+1))(k+1)/(k-1)]

Liquid (Equation 3.1):

A = Q / (38 Γ— Kd Γ— Kw Γ— Kv Γ— Kc) / √(Ξ”P / SG)

Two-Phase (Annex C - Omega Method):

Ω = x√ρv + (1-x)√ρl
A = W / (C Γ— Kd Γ— P1 Γ— Kb) Γ— √(T Γ— Z) / (Ξ© Γ— √M)

Fire Case Heat Input (API 521 Eq. 5):

Q = 21,000 Γ— F Γ— A0.82 (BTU/hr)
W = Q / Ξ» (lb/hr)

Critical Pressure Ratio:

(P2/P1)critical = (2/(k+1))k/(k-1)

Notes:
β€’ Standard orifice areas per API 526
β€’ Kd = 0.975 (vapor), 0.65 (liquid), 0.85 (two-phase)
β€’ Kb/Kw from API 520 Figures 30-32 (backpressure correction)
β€’ Kv from API 520 Figure 31 (viscosity correction, iterative)
β€’ Kc = 0.9 if rupture disk upstream, else 1.0
β€’ Z-factor via Hall-Yarborough correlation for natural gas
β€’ Installation factor 0.9 applied to required area
β€’ Fire case supports adequate/inadequate drainage per API 521

Frequently Asked Questions

How do you size a relief valve per API 520?

Relief valve sizing per API 520 calculates the required orifice area based on the relief flow rate, set pressure, allowable overpressure, fluid properties, and correction factors. The calculator determines the minimum orifice area and selects the next standard API 526 orifice designation.

What is the standard allowable overpressure for PSV sizing?

Standard allowable overpressure is 10% of set pressure for single relief device installations. Fire case scenarios allow 21% overpressure, and multiple valve installations allow 16% overpressure per API 520/521 guidelines.

What relief scenarios does API 521 require evaluation for?

API 521 requires evaluation of blocked outlet, fire case exposure, thermal expansion, gas blowby, and runaway reaction scenarios. Each scenario may produce different relief flow rates, and the governing case determines the required valve size.

How does backpressure affect relief valve selection?

Backpressure reduces relief valve capacity and affects valve type selection. Conventional valves are limited to 10% backpressure of set pressure, while balanced bellows and pilot-operated valves can tolerate higher backpressure with appropriate correction factors.

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