After years of  research and development, the new generation Butterfly valve - a precision engineered product is designed capable of being used effectively in a wide range of applications that were previously only considered suitable for gate, globe or ball valves - and at a fraction of their cost.

This article outlines the development of the Butterfly valve and the state of the art Triple Offset metal sealed design.

In the 1950's and 60's the typical maximum operating pressure of butterfly valves would be about 10 bar with only uni-directional sealing at temperatures close to ambient and the usual design would be single or double offset using elastomeric seals or body liners. Over the years this design was refined and improved and termed as 'High Performance' butterfly valves.

In some applications this design of valve was found to suffer from problems concerning wearing / erosion of the soft sealing material at high flow rates and where sand was flowing with the fluid; seat failure due to high and low temperatures and seat damage when used on throttling duty for part of it's life. In addition there was a requirement for these valves to be operable after a fire, usually when used in firewater main systems.

About 25 years ago some valve manufacturers started to investigate a more robust valve design where both the seat and seal were metallic and two basic types were developed which can be classified as follows:

• Seat and seal contact area was energized by mechanical deformation of either the seat or seal component. This design would be called Double Offset.

• Seat and seal contact force produced by the operating mechanism (gearbox, actuator) without significant deformation of the components. This design would be Triple Offset.

The Triple Offset design has the following significant advantages over the deformable, double-offset design:

• For the Triple Offset valve, torque (not position) is the important criterion for tight shut off. It does not require accurate location of the actuator closed limit stop (except that this stop should never be in contact).

• The broad seal contact width of Triple Offset valve is less susceptible to leakage resulting from minor damage to the sealing surfaces than the deformable type which usually only have line contact.

• Some deformable designs have cavities around the seat or seal that can fill with solids present in the pipeline fluid and impair the sealing performance - the Triple Offset is free of such cavities.

Triple offset valves were first used in offshore process lines in 1990 with pressure ratings up to 600# Class and have demonstrated a proven sealing capability over an extended time span and are now being increasingly used in place of ball valves.

Gate valve replacement: gate valves for isolation are being replaced by ball valves and butterfly valves and butterfly valves are now available with the same face to face dimension as gate valves so they can be retrofitted into existing pipelines without changes to the pipe work.

Cryogenic applications: Triple Offset Butterfly valves have been successfully adapted for cryogenic service down to - 196 C complete with cryogenic extensions and butt-weld top-entry body style

Steam jacketed: for applications where the pipeline product must be kept heated to prevent it from solidifying.

Bellows sealing of gland: although butterfly valves with conventional packed glands meet the usual requirements for gland emissions (i.e EPA Method 21) , glands fitted with bellows are now available with virtually zero leakage.

Weld end/Top entry: for valves welded into the pipeline a top entry feature is available from some suppliers which allows access for seat/seal replacement without removing the valve from the pipeline.

Block and Bleed: process applications requiring double isolation can benefit from the double block & bleed facility which is now available in some Triple Offset Butterfly valves.

Control: globe valves have traditionally been used as they have a high recovery which tends to minimize flashing and cavitation when compared to rotary valves but for less critical control applications butterfly valves have proven to be successful and in particular the metal sealed design.