API 520 Relief Contingency Reference

Common overpressure contingencies (fire, blocked outlet, control failure, tube rupture, etc.) and the API 521 / 520 sizing basis for each.

Contingency Matrix

API 521 Relief Contingency Matrix · Source: API STD 521 (7th Ed) §4 'Causes of Overpressure'

Contingency	API 521 §	Double-jeopardy allowed?	Typical sizing basis
Blocked outlet (closed valve)	§4.2	No — always single	Total inlet flow @ relief T,P
Tube rupture (HX HP shell, LP tube)	§4.4.8	Yes vs. coincident	API 521 §4.4.8 choked/2-phase
Fire exposure — gas-filled	§4.4.13	Yes — unlikely combo	API 521 eq 5: Q=21000·F·A⁰·⁸²
Fire exposure — liquid wetted	§4.4.13	Yes	Q=C₁·F·A^0.82; latent-heat flash
Control valve failure (FO/FC)	§4.3	No	Full open Cv at max inlet P
Thermal expansion (liquid blocked in)	§4.4.2	Yes	API 521 §4.4.2 — small RV
Power failure (pumps trip, no cooling)	§4.4.3	No — common-mode	All cooling lost, reflux off
Instrument air failure	§4.4.4	No — common-mode	All pneumatic valves fail-safe
Cooling water failure	§4.4.5	No — common-mode	All HX non-condensing
Loss of fan / air-cooler	§4.4.5	Yes	Single cell or all, per design
Reflux failure	§4.4.6	Yes	Reflux pump trip, no recovery
Overfilling (liquid)	§4.4.9	Yes	Max inlet flow, 2-phase capable
Chemical reaction (runaway)	§4.4.11	No if catalyst load	DIERS method; 2-phase
Vapor displacement from filling	§4.4.12	Yes	Max fill rate x liquid vapor
Utility cooling medium loss (refrig)	§4.4.7	No	All refrigerant vapor from HX

Backpressure Kb

Back-Pressure Correction Factor K_b (Balanced Bellows) · Source: API 520 Pt 1 §5.4.1.5 Fig 30 (vapor service, balanced bellows) · Conventional: K_b=1.0 for Pb/Pset <= 0.30; derate for higher built-up.

Pb/Pset (gauge)	10% overpressure	16% overpressure	21% overpressure	25% overpressure
0.3	1	1	1	1
0.35	0.99	1	1	1
0.4	0.97	0.99	1	1
0.45	0.94	0.98	0.99	1
0.5	0.89	0.95	0.98	0.99
0.55	0.83	0.92	0.96	0.98
0.6	0.75	0.88	0.93	0.96
0.65	0.65	0.82	0.89	0.93
0.7	0.54	0.75	0.84	0.9
0.75	0.42	0.66	0.77	0.85
0.8	0.29	0.54	0.68	0.78

Correction Factors

API 520 Sizing Correction Factors

Factor	Typical value	Source / Notes
K_d (discharge coeff., vapor)	0.975 (certified) / 0.62 (preliminary)	API 520 §5.4.1.1 — use MFG cert
K_d (liquid)	0.65 (certified) / 0.62 (prelim)	API 520 §5.7 — use MFG cert
K_c (rupture disc upstream)	0.9 with disc / 1.0 w/o	API 520 §5.4.1.3
K_w (liquid back-press.)	Fig 35 API 520 Pt1	1.0 if Pb/Pset < 15%
K_v (viscosity)	1.0 if Re > 100,000; else Fig 36	API 520 §5.8.3
K_n (Napier for steam >1500 psia)	Eq (14) API 520 §5.6.3	For steam only, above 1515 psia
K_sh (superheat, steam)	Fig 37 API 520	<1.0 above sat T
K_b (vapor back-pressure)	See Backpressure_Kb sheet	Balanced bellows only
Z (compressibility)	Calculate from Tr, Pr via Lee-Kesler	API 520 §5.5.3
C (gas constant)	Table 8 API 520 — function of k=Cp/Cv	e.g. k=1.0→315, k=1.4→356, k=1.67→378
F (fire environment factor)	0.3-1.0 per API 521 Tbl 5	Insulated & drained = 0.3; bare = 1.0

API526 Orifices

API 526 Orifice Areas & Flow Capacity · Source: API STD 526 (8th Ed, 2023) Table 1

Letter	Effective area (in²)	Inlet x Outlet	Service range
D	0.11	1 x 2	Small PSVs, thermal relief
E	0.196	1 x 2	Small process
F	0.307	1.5 x 2	Process
G	0.503	1.5 x 3	Process
H	0.785	1.5 x 3	Process
J	1.287	2 x 3	Process
K	1.838	3 x 4	Process
L	2.853	3 x 4	Process
M	3.6	4 x 6	Large units
N	4.34	4 x 6	Large units
P	6.38	4 x 6	Major relief
Q	11.05	6 x 8	Major relief
R	16	6 x 8	Major relief
T	26	8 x 10	Fire / depressuring

Installation Rules

Key Installation Rules — API 520 Part 2 (7th Ed)

Item	Rule	Section
Inlet pressure drop (non-modulating)	≤ 3% of set pressure at rated flow	§4.2.2
Inlet pressure drop (modulating pilot)	≤ as-installed pilot tolerance (typically 7%)	§4.2.2
Discharge to flare - built-up BP	≤ 10% of set for conv; ≤ 50% for bellows	§4.3.2
Discharge to flare - superimposed BP	Check each mode; may require bellows	§4.3.1
Block valves upstream	Full-port, car-sealed open, locked per OSHA	§4.5.1
Block valves downstream	Full-port, car-sealed open	§4.5.2
Inlet piping orientation	No horizontal pockets; self-draining to vessel	§4.2.4
Rupture disc upstream of PSV	Allowed with K_c=0.9; excess-flow monitor req.	§4.4
Multiple PSVs	Stagger set pressures ≤ 105% MAWP max	§4.6
Test / calibration interval	5 yr max for clean, 2-3 yr for fouling	API RP 576

Source: API_520_Relief_Contingency_Reference_v1.xlsx

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