What is the 3% inlet pressure loss rule for PSVs?

API 520 Part II Section 6.2 requires that the non-recoverable pressure loss in the inlet piping to a pressure safety valve (PSV) shall not exceed 3% of the valve set pressure. Exceeding this limit can cause valve chatter (rapid opening and closing), which damages the valve seat, reduces relieving capacity, and can lead to catastrophic overpressure.

Why does excessive inlet pressure loss cause PSV chatter?

When the PSV opens, flow accelerates through the inlet pipe causing a pressure drop at the valve inlet. If this drop exceeds 3% of set pressure, the pressure at the valve falls below the reseat pressure, causing the valve to close. Once closed, pressure rebuilds and the valve reopens. This rapid cycling is called chatter and can destroy the valve in seconds.

How do I reduce PSV inlet pressure loss?

The most effective methods are: (1) increase inlet pipe size to at least one size larger than the PSV inlet flange, (2) minimize inlet pipe length and number of fittings, (3) eliminate unnecessary elbows and tees, (4) mount the PSV directly on the vessel nozzle when possible, (5) use a full-bore block valve instead of a reduced-port valve.

What pipe fittings are included in PSV inlet pressure loss?

All fittings between the pressure source (vessel nozzle) and the PSV inlet flange must be included: elbows (90 and 45 degree), tees (branch and run-through), block valves (gate or ball), reducers, and straight pipe. Each fitting is converted to equivalent length using Crane TP-410 L/D ratios multiplied by the pipe internal diameter.

PSV Inlet Pressure Loss Calculator | API 520 Part II 3% Rule

PSV Inlet Pressure Loss Calculator
Verify that inlet piping pressure loss to a pressure safety valve does not exceed the 3% non-recoverable pressure loss limit per API 520 Part II. Excessive inlet loss causes valve chatter, seat damage, and reduced relieving capacity. This calculator determines friction loss, equivalent length of fittings, inlet velocity, Mach number, and recommends minimum pipe size if the current configuration fails.

PSV Parameters

Set Pressure

psig

PSV Orifice Size (API 526)

Orifice area: 1.287 in²

Overpressure

10% non-fire, 21% fire case per ASME VIII

Relieving Fluid

Fluid Type

Relieving Rate

lb/hr

Molecular Weight

lb/lbmol

Cp/Cv Ratio (k)

—

Natural gas ~1.27, Air ~1.40, Steam ~1.33

Fluid Temperature at Relief

°F

Liquid Density

lb/ft³

Liquid Viscosity

Inlet Piping

Inlet Pipe Size (NPS)

Pipe Schedule

Straight Pipe Length

Pipe Roughness

Commercial steel: 0.0018 in, Stainless: 0.0006 in

Inlet Fittings

90° Elbows

45° Elbows

Tees (Branch Flow)

Tees (Run-Through)

Inlet Block Valve

Block valves must be Car-Sealed-Open (CSO) per API 520

Understanding PSV Inlet Pressure Loss

The 3% Rule
API 520 Part II requires that the non-recoverable pressure loss in PSV inlet piping shall not exceed 3% of set pressure. This prevents valve chatter — rapid open/close cycling that destroys the valve seat and reduces relief capacity.

Chatter Mechanism:
PSV opens → flow accelerates → inlet pressure drops → pressure at valve falls below reseat → valve closes → pressure rebuilds → valve reopens. Cycle repeats at 10-100 Hz.

Prevention Methods:
Use larger inlet pipe, minimize fittings, mount PSV directly on vessel nozzle, use full-bore block valves, keep pipe runs as short as possible.

📚 Learn the Theory

Understand the API 520 Part II 3% rule, chatter mechanism, inlet piping best practices, and Crane TP-410 fitting L/D values

Read Engineering Guide →

Results

Inlet Pressure Loss

psi

Loss as % of Set Pressure -

3% Limit -

Compliance -

Pressure at PSV Inlet -

Inlet Velocity -

Mach Number -

Reynolds Number -

Friction Factor (Darcy) -

Total Equivalent Length -

Recommended Pipe Size -

Formula

ΔP = f × (L_eq/D) × ρv²/2

ΔP = Pressure drop (psi)

f = Darcy friction factor (Colebrook)

L_eq = Total equivalent length (ft)

D = Pipe internal diameter (ft)

ρ = Fluid density (lb/ft³)

v = Fluid velocity (ft/s)

3% Rule: ΔP ≤ 0.03 × P_set

Standards & References

API 520 Part II (6th Ed., 2015)
Installation — Section 6.2: Inlet Piping
API 521 (6th Ed., 2014)
Pressure-Relieving and Depressuring Systems
ASME VIII UG-127
Pressure Relief Device Requirements
NFPA 30
Flammable and Combustible Liquids Code
Crane TP-410
Flow of Fluids Through Valves, Fittings, and Pipe

Engineering Notes

3% Rule: Non-recoverable inlet loss must not exceed 3% of set pressure at rated relieving capacity
Chatter: Exceeding 3% causes rapid open/close cycling that destroys valve seat in seconds
Mach limit: Inlet velocity should be < Mach 0.5 to prevent sonic effects and flow instability
Pipe size: Inlet pipe should be at least equal to PSV inlet flange size; one size larger is preferred
Block valves: Must be Car-Sealed-Open (CSO) or locked open per API 520
Direct mount: Best practice is to mount PSV directly on vessel nozzle to minimize inlet loss

Quick Reference — API 526 Orifice Sizes

D: 0.110 in² | E: 0.196 in² | F: 0.307 in²
G: 0.503 in² | H: 0.785 in² | J: 1.287 in²
K: 1.838 in² | L: 2.853 in² | M: 3.600 in²
N: 4.340 in² | P: 6.380 in² | Q: 11.050 in²
R: 16.000 in² | T: 26.000 in²

PSV Inlet Pressure Loss Calculator

PSV Parameters

Relieving Fluid

Inlet Piping

Inlet Fittings

Understanding PSV Inlet Pressure Loss

📚 Learn the Theory

Results

Formula

Standards & References

Engineering Notes

Quick Reference — API 526 Orifice Sizes

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