API 609 Butterfly Categories — Category A vs B Selection
API 609 classifies butterfly valves into categories. Category A typically covers concentric resilient-seated butterflies; Category B covers double- and triple-offset designs. Name the category or construction clearly.
Quick selection
Water / HVAC: often Category A concentric. Higher P/T / tighter shutoff: Category B offset. See wafer vs lug.
Comparison table
| Factor | Category A (typical) | Category B (typical) |
|---|---|---|
| Construction | Concentric resilient seat | Double / triple offset |
| Service | Water, air, mild process | Higher P/T process |
| Seats | EPDM, NBR, PTFE, etc. | PTFE / metal / laminated |
| Body styles | Wafer, lug, flanged | Wafer, lug, flanged / double flange |
| Buyer action | Seat elastomer + PN/Class | Offset type + Class + seat |
Pair with end style
Category does not replace wafer vs lug selection. Specify both.
- Fire-safe soft-seat Q-turn may reference API 607 — separate from 609.
- Torque and gear sizing grow with size and differential pressure.
RFQ wording that works
- “API 609 Category A, concentric wafer butterfly, PN16, EPDM, gear, DN__.”
- “API 609 Category B, triple offset, Class 300, RF, metal seat.”
FAQ
Is every concentric butterfly Category A?
Category A is the usual bucket for concentric resilient designs under API 609.
Does Category B mean metal seat only?
No. Offset valves may use soft or metal seats.
API 609 vs EN butterfly standards?
Different documents. Follow the project specification.
Do I need both 609 and 598?
Design/category and testing are often both cited.
Related RFQs and hubs
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