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.
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.
