Relief Sizing Overview

Chapter from the Pressure Relief Design Guide.

RELIEF VALVE SIZING METHODOLOGY — API 520

Relief Device Type Selection

Device Type	Mechanism	Advantages	Limitations	Backpressure Limit	Typical Application	API 520 Reference	Cost
Conventional spring	Spring-loaded, direct acting	Simple, reliable, lowest cost	Affected by backpressure	≤10% of set pressure	Most common, clean service	Section 3	Low
Balanced bellows	Bellows isolates backpressure	Tolerates variable backpressure	Bellows can fail, higher cost	30-50% of set pressure	Variable backpressure systems	Section 3	Medium
Pilot operated	Pilot controls main valve	Full lift at set, tight shutoff	Pilot susceptible to fouling	Up to 70% of set	High backpressure, tight shutoff needed	Section 3	High
Rupture disk	Bursting membrane	No leak path, fast opening	Single use, must be replaced	N/A (one-time device)	Corrosive, high P, fast-acting needed	Section 3	Low per unit
Rupture disk + PRV	Disk in series with PRV	Corrosion protection for PRV	Disk fragments, monitoring needed	Per PRV type selected	Corrosive service, lethal service	Section 3	Medium

Set Pressure & Accumulation Rules

Scenario	Set Pressure	Allowable Overpressure	Max Accumulation	Code Basis
Single device (non-fire)	≤ MAWP	10%	110% MAWP	API 520 / ASME VIII UG-134
Multiple devices (non-fire)	First at MAWP, add'l at 105%	16%	116% MAWP	API 520 / ASME VIII UG-134
Fire case (supplemental)	≤ 105% MAWP	21%	121% MAWP	API 521 / ASME VIII UG-134
Rupture disk only	≤ MAWP	10%	110% MAWP	ASME VIII UG-127
Power-actuated PRV	≤ MAWP	10%	110% MAWP	ASME VIII UG-134

Sizing Equation Summary (API 520 Part I)

VAPOR/GAS SERVICE:
A = W × √(T×Z) / (C × K_d × P_1 × K_b × K_c × √M)
Where: A=orifice area (in²), W=mass flow (lb/hr), T=relieving temp (°R), Z=compressibility, C=gas constant function, K_d=discharge coeff (0.975), P_1=relieving pressure (psia), K_b=backpressure correction, K_c=combination correction (0.9 w/ rupture disk), M=molecular weight

LIQUID SERVICE:
A = Q × √(G) / (38 × K_d × K_w × K_c × K_v × √(P_1 - P_2))
Where: Q=flow rate (GPM), G=specific gravity, K_d=discharge coeff (0.65), K_w=backpressure correction, K_v=viscosity correction, P_1=relieving pressure (psig), P_2=total backpressure (psig)

STEAM SERVICE:
A = W / (51.5 × K_d × P_1 × K_b × K_c × K_sh × K_N)
Where: K_sh=superheat correction, K_N=Napier correction for high pressure steam

Source: Pressure_Relief_Design_Guide_v1.xlsx · sheet “Relief Sizing Overview”

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