Zero, Double and Triple Offset Butterfly Valves

 

There are three butterfly valve designs offered with the difference being the location of the actuating stem in relation to the disc and internal bore of the valve body; they are zero, double and triple offset.

 

The most common butterfly valve is the zero offset style, most often referred to as concentric or "resilient seated".

 

All butterfly valves require a ¼ (90º) turn to open or close the flow; some designs have geared operators to enable precise opening for flow/pressure throttling applications located in-between "full open or closed".  Butterfly valves can be provided with manual, electric or pneumatic actuators.

 

Resilient seat butterfly valves have a rubbery material to which the internal disc seals against.  The most common materials are Buna-N, EPDM and FKM and they are chosen based upon fluid compatibility.  The inherent advantages of this design are:

 

1. The seat is deformable so it will seal if some debris becomes caught between the valve disc and seat.

2. The body can be made of less expensive materials such as ductile iron or PVC because it is not in contact with the process fluid.

3. The disc stem o-ring seal eliminates maintenance prone packing gland seals typical of ball valves.

4. The elastomeric seal generally requires less torque for actuation, which reduces the actuator size/complexity and thus reduces initial cost; assuming pressures are not too high.

 

Resilient seat butterfly valves are used mostly for water and industrial fluid applications.  They are best suited for fluids with few particulates because the valve disc is in constant contact with the seat (creates wear).

 

What is a double offset butterfly valve?

A double offset butterfly valve design is used for higher pressures and fluids with particulate because this design reduces contact between the valve disc and seat, thus prolonging the seat life. 

 

Referring to the valve images below (looking downwards from the operator), the first image illustrates a zero offset or resilient butterfly valve because the stem travels through the center of the disc and the valve is centered within the valve seat.

The double offset butterfly valve design is represented in the middle image, notice that the stem is offset from the centerline of the disc.  Although not represented well in this image, the valve disc isn't quite centered on the valve seat and this is why they will perform better for higher pressure applications.

 

Triple offset  butterfly valves are highly engineered to satisfy more complex process requirements such as high pressures, dirty fluids and bubble tight sealing performance normally associated with ball valves.  As illustrated, they are similar to a double offset design except they also have a tapered disc seat to eliminate rubbing against the valve seat altogether, thus enabling tight sealing with a metal-to-metal interface.  Due to their compact size, triple offset butterfly valves are sometimes a better alternative to gate valves due to reduced weight, cost and ease of operation (ergonomics).

 

If your application benefits from a double or triple offset style butterfly valve, you might also consider a ball valve design, we wrote an article Comparing Ball Valves to Butterfly Valves.