API 609 vs EN 593 Butterfly Valves: Key Differences and How to Choose

September 10, 2026
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API 609 and EN 593 are both commonly used product standards for industrial butterfly valves, but they belong to different standard systems.

API 609 is more commonly used together with API / ASME piping standards, while EN 593 is mainly used within the European EN standards system.

For actual projects, choosing between API 609 and EN 593 is not simply a matter of deciding which standard is “better.” The selection should be based on the project’s pressure rating, flange standard, face-to-face dimensions, testing requirements, and the existing piping system.

What Is API 609?

API 609 is a butterfly valve standard published by the American Petroleum Institute (API), mainly covering metallic butterfly valves used in industrial piping systems.

The standard specifies requirements for butterfly valve design, materials, pressure-temperature ratings, dimensions, inspection, and testing.

Depending on the valve category and project requirements, API 609 can apply to different butterfly valve configurations, including wafer, lug, and double-flanged types.

API 609 divides butterfly valves into Category A and Category B, which differ in valve design and the way pressure ratings are established.

What Is EN 593?

EN 593 is a European standard for metallic butterfly valves used in industrial applications. It mainly defines basic requirements for valve design, materials, pressure ratings, operating performance, inspection, and testing.

Unlike API 609, EN 593 does not use the Category A / Category B classification. Its scope can include concentric butterfly valves as well as different offset designs.

It is important to note that API 609 and EN 593 do not operate as standalone standards.

In actual engineering projects, they are normally used together with the relevant flange, face-to-face, and testing standards.

API 609 projects are generally used within the API / ASME standards system, while EN 593 projects are more commonly used within the EN standards system.

API 609 vs EN 593: Quick Comparison

Comparison	API 609	EN 593
Standard System	API / U.S. industrial standards system	EN / European standards system
Connection Types	Wafer, Lug, Flanged, Butt-welding, etc.	Wafer, Lug, Flanged, Butt-welding, etc.
Pressure Rating	Category A generally uses manufacturer-rated CWP; Category B uses ASME Class pressure-temperature ratings	Can use PN ratings as well as Class 150 / 300 / 600 / 900
Size Designation	Usually expressed in NPS	Usually expressed in DN
Valve Classification	Category A / Category B	Does not use Category A / B; can cover concentric and different offset designs
Face-to-Face Dimensions	According to the applicable dimensional requirements of API 609	Commonly used together with EN 558
Flange Compatibility	Usually matched with ASME / MSS flange standards	PN ratings are commonly matched with EN 1092; Class ratings can be matched with the EN 1759 series
Testing Requirements	API 609 includes inspection and testing requirements; API 598 is also commonly specified in projects	Commonly used together with EN 12266-1

The main differences between API 609 and EN 593 are not in the appearance of the valves, but in their classification methods and the related pressure rating, flange, dimensional, and testing standard systems.

API 609 vs EN 593 Butterfly Valve Design

API 609 and EN 593 use different approaches to classifying butterfly valve designs. API 609 divides butterfly valves into Category A and Category B.

API 609 Category A: Commonly associated with concentric butterfly valves, often using resilient seats and rated according to the manufacturer’s specified cold working pressure (CWP).
API 609 Category B: More commonly associated with offset seat designs and may use either eccentric or concentric disc configurations. These valves are rated according to ASME Class and the corresponding pressure-temperature ratings.
EN 593: Does not use the API 609 Category A / Category B classification. Its scope can cover concentric butterfly valves as well as different eccentric designs, including single-offset, double-offset, and triple-offset configurations.

When to Choose an API 609 Butterfly Valve?

An API 609 butterfly valve is generally more suitable when the project follows the API / ASME piping standards system or when API 609 is specifically required in the technical specification.

Common situations include:

The pipeline uses ASME flange standards
The project uses ASME Class pressure ratings
The project requires inspection and testing according to API standards
The valve is intended to replace an existing valve in an API / ASME piping system

When to Choose an EN 593 Butterfly Valve?

An EN 593 butterfly valve is generally more suitable when the project follows the EN piping standards system, especially when the flange, face-to-face, and testing requirements are based on European standards.

Common situations include:

The pipeline uses EN flange standards
The project uses PN pressure ratings
EN 558 face-to-face dimensions are required
EN 12266-1 testing is required
The valve is intended to replace an existing valve in an EN piping system

EN 593 butterfly valve

What Information Should Buyers Specify When Ordering a Butterfly Valve?

When purchasing butterfly valves, providing only the valve size and pressure rating is usually not enough.

For a more accurate quotation and technical confirmation, buyers are recommended to provide the following key standards and operating requirements:

Item	Example
Valve Type	Butterfly Valve
Design Standard	PI 609 / EN 593
Standard Edition	As required by the project
Size	DN300 / NPS 12
Pressure Rating	PN16 / Class 150
Connection Type	Wafer / Lug / Flanged
Valve Design	Concentric / Double Offset / Triple Offset
Face-to-Face Dimensions	EN 558 / API 609 project requirement
Flange Standard	EN 1092 / ASME flange standard
Testing Requirements	EN 12266-1 / API project requirement
Body Material	Ductile Iron / WCB / Stainless Steel
Disc Material	DI / SS304 / SS316
Seat Material	EPDM / NBR / PTFE / RPTFE / Metal
Operation	Lever / Gearbox / Pneumatic / Electric

Providing this information helps the manufacturer confirm the correct valve configuration more accurately and reduces the risk of mismatches in flange drilling, face-to-face dimensions, pressure rating, and materials.

API 609 and EN 593 are used within different standard systems, so the final selection in actual projects should be based on the technical specification and the requirements of the existing piping system.

Union Valve can provide butterfly valves designed to meet API 609 or EN 593 project requirements, with the valve configuration confirmed according to the required pressure rating, face-to-face dimensions, flange connection, and testing standards.

If you are working on butterfly valve selection, replacement, or a project inquiry, you can send us the datasheet, drawing, or technical specification. Our technical team can help review the relevant standard requirements and recommend a suitable butterfly valve configuration for your project.You can contact us at any time.

FAQ

What Is the Main Difference Between API 609 and EN 593?

API 609 divides butterfly valves into Category A and Category B and is mainly used within the API / ASME piping standards system.
EN 593 does not use this classification and is mainly associated with the European EN standards system.

Can EN 593 Butterfly Valves Use ASME Class Pressure Ratings?

Yes.
EN 593 is not limited to PN pressure ratings. EN 593:2026 also covers Class 150, Class 300, Class 600, and Class 900.

About the Author

Serena Zhang

Sales Manager · Tianjin Union Valve Co., Ltd

With 4 years of valve export experience and dedicated service, Serena ensures a smooth and reliable order process. Quick replies 24/7. Welcome your inquiries!