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What are Butterfly valves?
We have been talking about various types of valves including gate valves, globe valves and so on used on various types of ships and now we will proceed to learn about another type of valves commony used in shipping namely butterfly valves. Butterfly valves are flow regulating, pivoted disc valves. The circular disc or plate, having the same radial dimensions as the pipe is pivoted exactly in the center of the pipe, perpendicular to the direction of the flow.The disc is connected to an actuator outside the valve with the help of a rod.The valve plate when pivoted at the center and moved with the help of an actuator, resembles butterfly wings and therefore they are known as butterfly valves.
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Butterfly valves belong to a family of valves known as "quarter turn valves". This is because this valves are quick acting valves which fully opens or closes the valve with only quarter of a turn. Though the valve gives an unrestricted flow of fluid, it induces a pressure drop in the flow as the disc is always present within the flow.The pressure drop can be reduced by providing the valve with a streamlined disc profile which will also give excellent flow characteristics along with low pressure drop.
Butterfly control valves are best suited for regulating the fluid flow. The actuator outside the valve can be fixed to an automatic mechanism or a remote control mechanism which can control the desired flow of the liquid. The actuator or the valve spindle can also be attached to a lever for manual control of the valve. The valves are available in the sizes ranging from 6 mm to 1000 mm bore.
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Types of Butterfly valves
Butterfly valves are mainly distributed into three types on the basis of pressure and usage.
Resilient Butterfly Valve - These types of valves are generally used for low pressure applications. They use a flexible rubber seat and therefore doesn't have high sealing ability. They can operate under working pressure up to 232 Psi.
High Performance Butterfly Valve - This valves are used in systems requiring higher pressure resistance. The disc though positioned in the center of the pipe is arranged in a peculiar way in order to increase sealing ability and robustness.They can work up to 725 working pressure.
Tri-centric butterfly valve -This valve uses a metal seat instead of a rubber one therefore can withstand high pressure conditions. They can work up to 1450 Psi working pressure.
One more type of butterfly valve known as the Diverter butterfly valve, is used to bypass coolers in order to attain fine control of cooling water temperature. This valve has two legs or pipes connecting to it. A pneumatic actuator controls the opening and closing of the valve on the basis of a signal provided by a temperature sensor. This valve delivers precise control of flow rate in both the branches. A provision of manual control is also provided in case of failure of the pneumatic system.
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Working & Uses of Butterfly Valves
The Working of butterfly valves is very much similar to that of ball valves. Rotating of the handle, manually or with the help of a pneumatic system, turns the plate either perpendicular or parallel to the flow of the fluid. Butterfly valves can also be used for regulating the flow by adjusting the lever and by keeping a close watch on the desired output flow.
Butterfly valves are used on all types of ships and firefighting apparatus. They are also widely used for domestic piping purposes. On ships they are used for large capacity lines such as forward and aft sea water suction lines or lines from various tanks to their respective pumps. Lube oil pipelines also preferably have butterfly valves fitted in them due to obvious reasons as discussed above.
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Marine Auxilary Machinery 7th Edition, 1995 by McGeorge