Valve Standard Reference
API 598
AmericanValve Inspection and Testing
API 598 is the universal valve testing and inspection standard, defining acceptable leakage rates (leak classes) for hydrostatic shell tests, seat leakage tests, and backseat tests.
Overview — API 598
API 598 is the universal valve testing and inspection standard, defining acceptable leakage rates (leak classes) for hydrostatic shell tests, seat leakage tests, and backseat tests. It is referenced by virtually every valve purchasing specification globally and defines the distinction between Class A (zero leakage — metal seat) and Class D (maximum soft seat leakage) tests.
Scope
All valve types: gate, globe, plug, ball, check, and butterfly valves. Covers shell test, seat leakage test, backseat test, and drain/vent plug test requirements. Defines leak classes (A through D) and test durations by valve size.
Applicable Valve Types
Gate Valves
Globe Valves
Ball Valves
Check Valves
Butterfly Valves
Plug Valves
Applicable Pressure Classes
Key Requirements — API 598
Shell test: 1.5× rated pressure, water or air, duration per table
Low-pressure seat test (air): 80–100 kPa (12–15 psig), duration per table
High-pressure seat test (water): 1.1× rated pressure, duration per table
Backseat test (for OS&Y gate/globe valves with backseat): 1.1× rated pressure
Leak Class A: zero leakage (bubble-tight) — metal seats, safety valves
Leak Class D: soft-seat maximum — up to 0.18 mL/min/NPS for NPS ≤2" in liquid
Testing Requirements — API 598
Shell test: 1.5× rated CWP, minimum duration per table
High-pressure seat: 1.1× rated CWP, liquid, duration per table
Low-pressure seat: 80–100 kPa, gas (air/N₂), duration per table
Backseat test where specified
Documentation Required for API 598 Compliance
Hydrostatic test certificate signed by inspector
Leak class specification on purchase order or data sheet
Test pressure record for all pressure levels
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