API 609

Covers — butterfly valves of gray/ductile iron, bronze, steel, nickel-base, or special alloy in double-flanged, lug, wafer, and butt-welding configurations. Two categories: Category A (manufacturer-rated, typically resilient-seated) and Category B (B16.34-rated, high-performance offset designs).

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

• Ratings basis for Category B → ASME B16.34; testing → API 598; fire test → API 607/6FA.
• Pipeline service → API 6D.
• Damper/HVAC low-pressure butterfly dampers → building-service standards.

Category A vs. B selection drives materials, rating, and allowable service — never mix them up.

API 609 sets minimum requirements for butterfly valves:

• Materials — body, disc, seat, and shaft materials per category; Category B disc/shaft rated to B16.34.
• Design — concentric (Category A) or offset double/triple-offset (Category B); wafer, lug, double-flanged, or butt-weld body styles.
• Examination & Testing — shell and seat testing per API 598; fire test per API 607/6FA where specified.

API 609 covers butterfly valves for on/off and throttling service, split into Category A (manufacturer-rated, resilient-seated, general utility) and Category B (B16.34-rated, high-performance double- or triple-offset for tighter shutoff and higher pressure/temperature).

Butterfly valves give a compact, low-weight, low-cost solution for large bores. Testing follows API 598, and fire-safe qualification (API 607/6FA) is added for hydrocarbon service.

Engineering Note: For tight shutoff and higher-temperature hydrocarbon service, specify a Category B triple-offset (metal-seated) valve; a Category A resilient seat has a lower temperature and tightness ceiling.