PSV Inlet/Outlet Piping — API 520 Pt 2

Inlet pressure-loss and outlet back-pressure rules of thumb per API Std 520 Part 2 with worked examples.

API 520 Pt 2 PSV Piping

PSV INLET/OUTLET PIPING — API 520 PART 2 RULES · Source: API Std 520 Part 2, 7th Ed (2020) — Installation of Pressure-Relieving Devices

Rule	Value / Limit	API 520 Pt 2 Section	Rationale	Typical Mitigation
Max non-recoverable inlet pressure loss (spring-loaded PSV)	3% of CDP (set pressure)	5.2.2, 7.3.3.2	Prevents chatter; ensures reseat & stable operation	Shorten inlet; enlarge line; eliminate fittings
Max non-recoverable inlet loss (modulating pilot PSV)	Vendor-specified (often 7-50%)	5.2.2	Pilot sense-line isolates downstream effects	Verify w/ vendor; sense line per pilot spec
Max superimposed back pressure (conventional PSV)	10% of set pressure (gauge)	5.3.2.1	Conventional spring: constant back pressure shifts set point ≈1:1	Use balanced bellows or pilot if exceeded
Max built-up back pressure (conventional PSV)	10% of set pressure	5.3.2.2	Lift force reduced by back pressure; 10% keeps capacity ≥ 97%	Increase outlet size; use bellows/pilot
Max back pressure (balanced bellows PSV)	50% of set pressure (with Kb correction)	5.3.3.2	Bellows isolates disc from back pressure	Use Kb chart from API 520 Pt 1 Fig 30
Max back pressure (pilot-operated PSV)	50% of set pressure (typ); up to 90% w/ backflow preventer	5.3.4.2	Pilot vents to atmosphere for reset; some lift off BP	Confirm w/ vendor; add backflow preventer for >50%
Minimum horizontal inlet piping slope (liquid service)	Self-draining to PSV	5.2.3	Prevents liquid holdup, chatter, and freeze risk	Elevate PSV; install drip pocket
Minimum outlet piping slope (open discharge to flare header)	Self-draining to header	5.6.3	Avoid liquid trap; prevent slug loads on flare KO drum	Avoid pockets; slope ≥1:50 toward header
Max rated velocity in PSV outlet (discharge tail pipe)	0.5 Mach (subsonic); sonic only w/ design check	5.6.2	Avoid shock cell damage, noise, tail pipe loads	Enlarge tail pipe; limit rated mass flow
Block valve upstream of PSV (when allowed)	Full port, car-sealed/locked open	4.6.2, Table 2	Continued overpressure protection during service/test	Full-port gate or ball valve; car seal + tag
Block valve downstream of PSV	Full port, car-sealed/locked open	4.6.3	Isolation for header maintenance / valve change-out	Full-port gate/ball; interlocked w/ inlet
Inlet line size	≥ PSV inlet flange size; never smaller	5.2.4	Back-propagation of losses into inlet nozzle hurts operation	Match or enlarge; reducer only if P-drop OK
Outlet line size	≥ PSV outlet flange size	5.6.4	Lower pressure drop = less back pressure = more capacity	Enlarge if needed to meet back-pressure limit
Relief device reaction force (liquid & gas)	Compute per Part 2 App C (mass flow × velocity)	App C	Protects piping/supports during relief event	Brace support per C / SwRI / vendor tables
Long horizontal drip legs on inlet	Avoid; drain to safe location	5.2.3	Collect condensate/water — chatter risk	Remove drip leg; install continuous slope

3% Inlet Loss Check

QUICK-CHECK: DOES PSV PASS 3% INLET LOSS RULE?

Input	Value	Unit	Notes
PSV Set Pressure (gauge)	150	psig	User input
Rated PSV mass flow	50000	lb/hr	From PSV sizing (nameplate × Kv)
Inlet pipe ID	3.826	in	NPS 4 Sch 40
Inlet piping length (eq)	15	ft	Incl equiv length of fittings (L + ΣLe)
Fluid density at rated flow	4.5	lb/ft3	From process data
Friction factor (Moody)	0.018	-	CS, turbulent
Calc	Formula	Result
Velocity, ft/s
ΔP (psi)
3% limit (psi)
Pass?

References

Source: PSV_Inlet_Outlet_Piping_API520Pt2_Reference_v1.xlsx

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