API 594

Covers — check valves in wafer, dual-plate, lug, and butt-welding configurations for preventing reverse flow, addressing design, materials, face-to-face, ratings, and testing.

Does NOT cover (exclusions) — the following are governed elsewhere:

• Swing/lift check valves in gate-valve bodies → API 600 / 602 (as check variants).
• Pipeline check valves → API 6D.
• Ratings basis → ASME B16.34; testing → API 598.

API 594 targets compact wafer/dual-plate check valves; use API 6D for pipeline check duty.

API 594 sets minimum requirements for compact check valves:

• Materials — body, plate/disc, spring, and hinge-pin materials suited to the service.
• Design — dual-plate or wafer construction with spring assist for fast closure and minimal reverse-flow slam.
• Examination & Testing — shell and seat testing per API 598 to defined leakage.

API 594 covers compact wafer and dual-plate check valves that stop reverse flow with a light, short body clamped between flanges. Spring-assisted plates close quickly on flow reversal, limiting slam and protecting pumps and compressors from back-flow.

Correct sizing matters: a check valve oversized for the actual flow will not hold its plates fully open, causing flutter and premature wear. Ratings trace to B16.34; testing follows API 598.

Engineering Note: Size the check valve to the real flow, not the line size — full-open operation is what keeps a dual-plate check valve from fluttering.