Differences between API 609 Category A and Category B Butterfly Valves

API 609 is the design standard for the American standard butterfly valve, which specifies the structure length is divided into Category A and B, in Chinese the structure length of API 609 A butterfly valve and B butterfly valve, also called the body length. Also different in design/Inspection. Here we focus on Face to face of API 609 Valve.

Butterfly valves are categorized according to their structure. Nowadays, there are more midline/center line butterfly valves and eccentric butterfly valves on the market. The standard length of the American standard butterfly valve used in the center line butterfly valve is the API 609 Class A butterfly valve. The eccentric butterfly valve is mainly a three-offset butterfly valve. The structure length standard adopted is the Class B butterfly valve regulated by API 609.

Two butterfly valve categories:

Category A Butterfly Valves:

Manufacturer’s rated cold working pressure (CWP) butterfly valves, usually with a concentric disc and seat configuration.  Sizes covered are NPS 2 to NPS 48 for valves having ASME Class 125 or Class 150 flange bolting patterns.

Category B Butterfly Valves:

Pressure-temperature rated butterfly valves that have an offset seat and either an eccentric or a concentric disc configuration.  These valves may have a seat rating less than the body rating.  Sizes covered are NPS 3 to NPS 24 for Classes 150, 300, and 600.

The Class B butterfly valve of the API 609 standard specifies the structure length of the American standard butterfly valve, flange butterfly valve, and lug butterfly valve. In comparison with the national standard GB butterfly valve, the flange butterfly valve, and the national standard butterfly valve structure length is the same, but there is a gap between the clip butterfly valve, especially the 6 inch, 8 inch difference between the two specifications/standard is largest. This is also a way to judge a manufacturer if they are professional in producing American standard butterfly valves.

In the API 609 Category B, the 6-inch to 150LB butterfly valve is specified, the structure length is only 57mm, and many manufacturers, or 70mm, is the same with the national standard valve body, it can be seen that many American standard butterfly valve manufacturers directly to use the national standard valve mold. The valve body was modified to the American standard connection size.

Differences between API 609 Category A and Category B Butterfly Valves

Category A Butterfly Valves

“Concentric” type. Also known as rubber-lined butterfly valves. These are popular in low-pressure systems (Class 150 or less), relatively lower cycle life, rubber liners are also good for abrasive slurry services.

Category B Butterfly valves

“Offset” type, meaning the shaft is offset from the disc, so it can handle higher differential pressures. Most of the offset types cause the disc to rotate freely from the seat after a few degrees of rotation, which minimizes seat wear and increases valve cycle life. The offset type can have rubber, polymer (PTFE or other), metal &graphite, or even metal seats. Lots of variety which makes these suitable for a variety of temperatures and pressures.



API Std 598 – Valve Inspection and Test

API Std 609 – Butterfly Valves: Double-Flanged, Lug- and Wafer-Type


ASME B1.1 – Unified Inch Screw Threads, (UN and UNR Thread Form)

ASME B16.1 – Gray Iron Pipe Flanges and Flanged Fittings: Classes 25, 125, and 250

ASME B16.5 – Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard

ASME B16.20 – Metallic Gaskets for Pipe Flanges: Ring-Joint, Spiral-Wound, and Jacketed

ASME B16.24 – Cast Copper Alloy Pipe Flanges and Flanged Fittings: Classes 150, 300, 600, 900, 1500 and 2500

ASME B16.34 – Valves Flanged, Threaded and Welding End

ASME B16.42 – Malleable Iron Threaded Pipe Unions: Classes 150, 250, and 300

ASME B16.47 – Large Diameter Steel Flanges: NPS 26 Through NPS 60 Metric/Inch Standard

ASME B31.3 – Process Piping

ASME B46.1 – Surface Texture (Surface Roundness, Waviness, and Lay)


AWWA C504 – Rubber Seated Butterfly Valves

AWWA M49 – Butterfly Valves: Torque, Head Loss & Cavitation Analysis


BS 5155 – Specification for butterfly valves


ISO 5752 – Metal valves for use in flanged pipe systems — Face-to-face and center-to-face dimensions

ISO 10631 – Metallic butterfly valves for general purposes


MSS SP-6 – Standard Finishes for Contact Faces of Pipe Flanges and Connecting-End Flanges of Valves and Fittings

MSS SP-25 – Standard Marking Systems for Valves, Fittings, Flanges and Unions

MSS SP-44 – Steel Pipeline Flanges

MSS SP-45 – Bypass and Drain Connections

MSS SP-67 – Butterfly Valves

MSS SP-91 – Guidelines for Manual Operation of Valves

Our API 609 standard butterfly valve has a unique set of molds, which are designed and manufactured according to API 609 and other related regulations. Therefore, regardless of the valve body thickness, flange thickness, and structural length can reach the relevant standards.

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ZECO Valve is established in 1991, as a professional industrial valve manufacturer and water valve manufacturer in Shanghai, China.

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