Butterfly valves are rotary valves wherein a disc-shaped closure element rotates approximately ninety degrees to open or close a flow passage. The original butterfly valve is a rudimentary, non-sealing pipeline damper. This valve remains an essential member of the family of butterfly valves.
The advent of elastomers has accelerated the development of butterfly valves with a tight shut-off in which the elastomer functions as the sealing element between the disc's rim and the valve body. Originally, these valves were used for water.
As more chemically-resistant elastomers became available, the adoption of butterfly valves in the process industries expanded. These elastomers must not only be corrosion-resistant, but also abrasion-resistant, dimensionally stable, and resiliency-retentive, i.e., they must not solidify. In the absence of any of these characteristics, the elastomer may be inappropriate. Manufacturers of valves can provide guidance regarding the selection and limitations of elastomers for a given application.
In an effort to surmount some of the limitations of elastomers, PTFE seats were incorporated into butterfly valves. Other endeavours led to the development of butterfly valves with metal seats that have a tight shut-off.
As a result of these advancements, butterfly valves are now available for a wide range of pressures and temperatures, based on a number of different sealing principles.
When entirely open, butterfly valves offer little resistance to flow and sensitive flow control when open between 15 and 70 degrees. Depending on the vapour pressure of the liquid and the downstream pressure, severe throttling of liquids may inevitably result in cavitation.
Any tendency of the liquid to cavitate as a result of throttling can be mitigated in part by sizing the butterfly valve smaller than the pipeline so that throttling occurs at a position close to half-open, and/or by allowing the pressure decrease to occur in stages using multiple valves. Also, if the butterfly valve is closed too quickly in liquid service, excessive waterhammer may result.
By carefully closing the butterfly valve, it is possible to avoid excessive water hammer. Since the disc of butterfly valves rotates into the seat with a wiping motion, the majority of butterfly valves can handle fluids with suspended particulates and, based on the quality of the seatings, powders and granules. In horizontal pipelines, butterfly valves must have their stems affixed horizontally. In addition, when the valve is opened, the bottom of the disc should separate from any particles that may have accumulated on the upstream side of the disc.
Problems in Butterfly Valves
The sealing efficacy of butterfly valves is a potential issue. The disc and valve body's sealing surfaces may deteriorate over time, resulting in leakage or diminished performance. In addition, the disc may become jammed in the valve body, preventing the valve from opening or closing correctly. Maintenance and the choice of high-quality valves can prevent these problems.