Relief Scenario & Flow

Relief Scenario

Service Phase

Relief Flow Rate

Required relief capacity

Wetted Surface Area

ft²

For fire case heat input (API 521 Eq. 5)

Insulation Type

Fire Scenario Type Affects heat flux constant and exponent

Vapor Fraction

0-1 for two-phase flow (API 520 Annex C)

Pressure & Temperature

Set Pressure

psig

PRV set pressure

Allowable Overpressure

Standard: 10% (Fire: 21%, Emergency: 16%)

Back Pressure

psig

Built-up or superimposed

Relieving Temperature

°F

Temperature at relief conditions

Valve Type Affects backpressure correction (Kb/Kw)

Rupture Disk Upstream Reduces Kd by 10% per API 520 (Kc = 0.9)

Fluid Properties

Use Natural Gas Properties Auto-calculates MW, Z-factor using Hall-Yarborough

Gas Gravity (Air=1)

Typical: 0.55-0.65 for pipeline gas

Molecular Weight

For custom gas composition

Specific Heat Ratio (k)

k = Cp/Cv, auto-estimated if empty

Compressibility (Z)

Auto-calculated for natural gas via Hall-Yarborough

Liquid Specific Gravity

Relative to water (water = 1.0)

Viscosity

For Kv correction (API 520 Fig. 31)

Latent Heat (Fire Case)

BTU/lb

Heat of vaporization at relief conditions

Discharge Coeff (K_d)

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% P_set
Balanced: ≤50% P_set
Pilot: ≤90% P_set

Fire Heat Input

Adequate drain: 21,000
Inadequate: 34,500
× F × A^0.82 BTU/hr

📘 Engineering Reference Notes

🔧 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 P₂/P₁ < critical:

Natural Gas (k=1.28): 0.549
Methane (k=1.31): 0.544
Air/N₂ (k=1.40): 0.528
Propane (k=1.13): 0.574

📊 Z-Factor Ranges (Nat. Gas)

Low P (<500 psia): Z ≈ 0.95–1.0
Medium (500–2000): Z ≈ 0.80–0.95
High P (>2000 psia): Z ≈ 0.70–0.90
H₂S/CO₂ >5%: Apply Wichert-Aziz

🔥 Fire Case Overview

Fire relief loads depend on wetted area and drainage.

Overpressure: 21% allowed
Heat Input: Q = C × F × A^0.82

Use Fire Relief Calculator →

🛠️ Installation Rules

Inlet pipe ≥ PRV inlet size, short as possible
Inlet ΔP <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 (in²)	Orifice	Area (in²)
D	0.110	L	2.853
E	0.196	M	3.60
F	0.307	N	4.34
G	0.503	P	6.38
H	0.785	Q	11.05
J	1.287	R	16.0
K	1.838	T	26.0

Correction Factors

Factor	Vapor	Liquid	Notes
K_d	0.975	0.65	Discharge coeff.
K_b	0.5–1.0	—	Backpressure (vapor)
K_w	—	0.6–1.0	Backpressure (liquid)
K_v	—	0.65–1.0	Viscosity (iterative)
K_c	0.9	0.9	With rupture disk

Specific Heat Ratios (k = Cp/Cv)

Gas	k	Gas	k
Hydrogen	1.41	Propane	1.13
Methane	1.31	n-Butane	1.09
Nat. Gas	1.28	Steam	1.33
Ethane	1.19	Air / N₂	1.40
CO₂	1.29	Ammonia	1.31

Environment Factor (F) - Fire Case

Condition	F
Bare vessel	1.0
Water spray / deluge	0.5
Insulation 1"	0.3
Insulation 2"	0.15
Insulation 3"	0.075
Insulation 4"	0.05
Fireproofing	0.03

⚠️ Key Sizing Rules:

Always select the next larger standard orifice—never interpolate
Inlet piping ΔP 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 K_d = 0.85 (not 0.975 or 0.65)

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

📚 Learn the Theory

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

Read Engineering Guide →

Sizing Formulas (API 520 9th Edition)

Vapor/Gas (Equation 3a):

A = (W / (C × K_d × P₁ × K_b × K_c)) × √(T × Z / M)

Where C = 520 × √[k × (2/(k+1))^(k+1)/(k-1)]

Liquid (Equation 3.1):

A = Q / (38 × K_d × K_w × K_v × K_c) / √(ΔP / SG)

Two-Phase (Annex C - Omega Method):

Ω = x√ρ_v + (1-x)√ρ_l
A = W / (C × K_d × P₁ × K_b) × √(T × Z) / (Ω × √M)

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

Q = 21,000 × F × A^0.82 (BTU/hr)
W = Q / λ (lb/hr)

Critical Pressure Ratio:

(P₂/P₁)_critical = (2/(k+1))^k/(k-1)

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

Relief Valve Sizing Calculator

Relief Scenario & Flow

Pressure & Temperature

Fluid Properties

🔧 Valve Type Selection

🌊 Critical Pressure Ratios

📊 Z-Factor Ranges (Nat. Gas)

🔥 Fire Case Overview

🛠️ Installation Rules

⚡ Accumulation Limits

API 526 Standard Orifices

Correction Factors

Specific Heat Ratios (k = Cp/Cv)

Environment Factor (F) - Fire Case

📚 Learn the Theory

Recommendations

Sizing Formulas (API 520 9th Edition)

Sizing Results

Performance Data

Gas Dynamics

Relief Valve Sizing Calculator

Relief Scenario & Flow

Pressure & Temperature

Fluid Properties

🔧 Valve Type Selection

🌊 Critical Pressure Ratios

📊 Z-Factor Ranges (Nat. Gas)

🔥 Fire Case Overview

🛠️ Installation Rules

⚡ Accumulation Limits

API 526 Standard Orifices

Correction Factors

Specific Heat Ratios (k = Cp/Cv)

Environment Factor (F) - Fire Case

📚 Learn the Theory

Recommendations

Sizing Formulas (API 520 9th Edition)

Sizing Results

Performance Data

Gas Dynamics

Related Calculators

Gas Density

Pipeline Hydraulics

Line Sizing

Pipe Volume

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