API 594 vs API 6D: Check Valve Standards

Quick Answer

API 594 covers wafer-type and lug-type check valves for refinery and process plant piping. API 6D covers pipeline valves (including check valves) for hydrocarbon transmission and distribution systems. API 594 focuses on compact, lightweight check valves installed between flanges. API 6D covers full-bore pipeline check valves with flanged, butt-weld, or compact (wafer) ends designed for pipeline duty.

Key Differences

Application scope: API 594 is a process plant standard for check valves that fit between two flanges (wafer-style or lug-style). API 6D is a pipeline standard that covers gate, ball, plug, and check valves for pipeline systems governed by ASME B31.4 and B31.8.

Valve types: API 594 covers only wafer and lug-type check valves (dual-plate, single-plate, tilting disc). API 6D covers flanged and butt-weld swing check valves, piston-lift check valves, and wafer/dual-plate check valves designed for pipeline service.

Testing requirements: API 6D mandates more stringent testing, including shell test, backseat test (where applicable), low-pressure closure test, and high-pressure closure test. API 594 requires shell test and low-pressure closure test per API 598.

Specifications Table

Feature	API 594	API 6D
Scope	Process piping check valves	Pipeline valves (gate, ball, plug, check)
Check valve types	Wafer dual-plate, wafer single-plate, lug-type	Swing, piston-lift, dual-plate, tilting disc
End connections	Wafer (between flanges), lug (bolted to one flange)	Flanged, butt-weld, wafer, compact
Size range	NPS 2 to NPS 48	NPS 2 to NPS 60+
Pressure classes	150, 300, 600, 900, 1500, 2500	150, 300, 600, 900, 1500, 2500
Face-to-face	Per API 594 tables (short pattern)	Per ASME B16.10 or API 6D tables
Body material	Per ASME B16.34	Per ASME B16.34 or API 6D material requirements
Shell test	Per API 598	Per API 6D (1.5x rated pressure)
Closure test	Low-pressure air/gas per API 598	Low-pressure + high-pressure closure per API 6D
Fire safe	Optional (per API 607)	Optional (per API 607/6FA)
Piping codes	ASME B31.3, B31.1	ASME B31.4, B31.8
Typical applications	Refinery, chemical plant, offshore topsides	Cross-country pipelines, terminals, pump stations

Selection Guide

Application	Standard	Typical Valve Type
Refinery process piping	API 594	Wafer dual-plate check
Chemical plant	API 594	Wafer or lug dual-plate
Offshore topsides process piping	API 594	Wafer dual-plate (weight savings)
Oil/gas transmission pipeline	API 6D	Flanged swing check or piston check
Pipeline pump station	API 6D	Flanged swing check (full bore)
Pipeline terminal	API 6D	Flanged or BW check valve
Compressor station	API 6D	Nozzle-type or piston-lift check

Pro Tip

For offshore process piping, API 594 dual-plate check valves offer significant weight and space savings compared to API 6D swing check valves. However, for pipeline risers and subsea tie-ins governed by ASME B31.4/B31.8, API 6D valves with full NDT and enhanced closure testing are required. Verify which standard applies based on the piping code governing that specific section.

For a overview of check valve types and valve specifications, refer to the full guides.

Read the full guide to piping engineering