Choosing the wrong type of valve can be detrimental to your operation. There are a few different types of butterfly valves on the market, including the zero offset, double offset, and triple offset. I enlisted the help of Scott Telin, one of Crane's application engineers and a member of our valves team, to help differentiate between the three offsets. If you aren't familiar with all the terms mentioned below, be sure to download 'The Ultimate Glossary of Valve Terminology'.
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Zero Offset
Also known as a concentric design, this is common in general service 'resilient seated' or 'rubber seated' butterfly valves. The stem and disc are centered in the seat of the valve, all of which are centered in the body of the valve. The disc is in constant contact with the seat.
An advantage of this valve is the media going through the valve will not contact the body, since the seat is constructed as a sleeve or cartridge inside of the body.
Zero offset butterfly valves are used in basic and specialty liquid and gas applications up to 200 PSI and up to 400 deg F. The zero offset butterfly valve can handle chemicals, coatings, food, some solids, and abrasive applications.
The zero offset butterfly valve is available in epoxy coated cast iron and stainless steel with multiple seat materials such as Buna, EPDM, VITON, Teflon, Natural rubber and others.
Double Offset Butterfly Valve
This popular valve is also known as a high performance butterfly valve. This valve has two stems offset from the center. This creates a cam action during the application's process.
- 1st offset: The axis of the shaft is behind the centerline of the sealing point of the disc to seat.
- 2nd offset: The axis of the shaft is eccentric to the center of the valve/pipe line.
The double offset butterfly valve is generally used in:
- Power generation
- Pulp and paper
- HVAC
- Chemical
- Oil and gas
- Water and wastewater treatment
- Shipbuilding
Double offset butterfly valves are available in higher pressures than the zero offset style. The double offset is up to ANSI Class 600 rating in carbon steel, stainless steel, Hastelloy and other body material, with variations of Teflon seats and also Fire Safe design with metal back up seats.
Triple Offset Butterfly Valve
With the triple offset butterfly valve there are three offsets. Two are placed in a similar position to the double offset butterfly valve, while the third offset is the geometry of the seating surface, creating a type of cone shape of the disc and seat. This cone angle, along with the two eccentric shaft offsets, allows the disc to seal against the seat with no friction.
This seat design also allows for uniform sealing, and thus a tight shutoff in a metal seat design. This design is a lower cost, lower torque option (easier to automate), than alternative style metal seated valves.
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Triple offsets are generally used in applications similar to gate valves, where a metal seat is required, and tight shutoff and/or quarter turn actuation is desired. Applications like high pressure steam (over 150 PSI), superheated- steam, high temperature gases and oils, high temperature applications are good for this type of valve because a metal seat is required over a soft seat.
The triple offset valve is used in similar industries as the double offset butterfly valve, but in more demanding applications.
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What's the difference between zero, double and triple offset butterfly valves?
Butterfly valve offset is a term used to describe the distance between the centerline of the valve stem and the centerline of the valve disc. This offset creates a camming action when the valve is opened or closed, which can affect the torque required to operate the valve and the flow characteristics through the valve.
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There are two types of butterfly valve offsets: zero offset and eccentric offset. In a zero offset valve, the stem and disc are centered in the valve body, and the valve rotates around its central axis. In an eccentric offset valve, the stem is positioned off-center, which creates an offset between the disc and the stem.
Eccentric offset butterfly valves can be further classified into two types: double offset or high performance and triple offset. In a double offset valve, both the stem and disc are offset from the center. This creates a camming action that causes the disc to move away from the seat as it is opened, which helps to reduce wear on the seat and disc. In a triple offset valve, the stem is positioned off-center for the first offset, the second offset is the disc being positioned off-center and at an angle to the stem and the third offset is the geometry of the seating surface, which creates a cone shape of the disc and seat. This creates a camming action that causes the disc to move away from the seat and to pivot away from the flow path as it is opened, which helps to reduce wear on the seat and disc and minimizes flow turbulence and pressure drop across the valve. Triple offset valves are classified as zero leak valves while double offset or high performance valves are classified as Class VI shut off valves.
The type of offset used in a butterfly valve depends on the specific application and the desired performance characteristics of the valve. Zero offset valves are commonly used in HVAC applications where pressure & temperature requirements are low, while eccentric offset valves are often used in applications where flow control is important, such as in the chemical, food and beverage, and pharmaceutical industries.
Fluid Flow offers a large selection of butterfly valves from top manufacturers. Contact our team of experts to learn more.
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