Instrumentation Piping Valves

Butterfly Valves

Butterfly valves have been in use since the invention of steam engines. The initial butterfly valves were available in large sizes (6" and above) and were used mainly for low pressure drop applications. The structure of a butterfly valve does not produce as much disturbance in the flow compared to other valves due to its simplicity. As can be seen in the figure, a butterfly valve essentially consists of a disc which blocks the flow this disc can be rotated about an axis passing through the plane of the disc. Rotation of the disc by 90 degrees can completely open or close the butterfly valve. Use of a smooth disc as the closure member allows the valve to be operated with low torque.

Due to the unique structure, these valves are quite smaller than all other types of valves. Moreover they have high flow capacities compared to other valves. Hence, use of butterfly valves allows the nominal valve size to be smaller as well. Thus use of butterfly valves can result in reduced weight, space and overall cost requirements.

Butterfly valves have only a couple of parts in contact with the process fluid. Variety of linings can be used to isolate the valve body from process fluids thus avoiding requirement of expensive materials for the valve body. This also contributes to cost reduction due to the use of butterfly valve over other valves.

Various butterfly valves can be clubbed into two groups - general purpose and high performance butterfly valves.

Advantages

As mentioned above, use of these valves can result in reduced costs thanks to the small size, small weight, lesser space requirements, lesser requirement of expensive materials etc. They have few parts, even fewer exposed to the fluids, making them easy to maintain. They can be easily and quickly operated by turning the disc by 90 degrees.  They are suitable for control purpose and can offer good rangeability. These valves are easily available in larger sizes.

Disadvantages

The use of these valves for control applications is limited to the valve opening range between 30 degrees to 70 degrees. Throttling using butterfly valves is limited to low pressure drop applications. Potential cavitation is a concern when butterfly valves are used. Also possibility of a choked flow is a concern.

Applications of butterfly valves

  • Throttling service requiring low pressure drop
  • Corrosive service needing lining in the valves.
  • Slurry or abrasive service.
  • High pressure and high temperature water or steam service.
  • Cooling water, fire water, circulating water etc.
  • Vacuum service

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