Butterfly valves are valves that can be used for regulating, isolating, stopping, or starting the fluid flow. These valves are mostly used as control valves in applications where the pressure drops required of the valves are relatively low. These valves can be used as either shutoff valves or as throttling valves. Butterfly valves are rotary motion valves, and it can be easily operated because of the handle which can rotate ninety degrees and this rotation of the handle can fully open the disk. Butterfly valves are mostly small in size, larger butterfly valves can be actuated by a hand wheel and this handwheel will be connected to a stem by gears. Butterfly valves are lightweight, small in size quick-acting and it provides positive shut-off and it can also be used for throttling. Butterfly valves can handle a large flow of liquids or gases at low pressure and it can also handle slurries or liquids that have a huge amount of suspended solids.
Construction of the butterfly valve could vary but, but the disk which is of butterfly type and certain sealing are common to all butterfly valves. In the above figure of the butterfly valve, it shows parts like a resilient seat, butterfly type disk, stem packing, a body, and a notched positioning plate and handles. When the resilient seat is mounted in the valve body then it is under compression. The compression causes a seal to form around the edge of the disk and both the upper and lower points where the stem passes through the seat. In order to close the valve, the handle must be turned a quarter of a turn to rotate the disk 90 degrees. The resilient seat will exert positive pressure against the disk, and because of this tight shut off can be achieved. In larger size butterfly valves electrical or mechanical gear-driven mechanism is used to open or close the valve. These valves are constructed according to the principle of the pipe damper. Flow control is done with the help of a disk and the disk would have the same diameter of the adjoining pipe, which rotates either in a vertical or horizontal axis. If the disk lies parallel to the piping run then the valve is fully opened and when the disk approaches the perpendicular position then the valve is shut.
The major difference is that in a butterfly valve a disk is used which is used as the opening and closing part, but in a ball valve, a ball is used. Butterfly valves are smaller in size so it has a certain advantages over ball valves like it can be used for many applications because of its size and less maintenance is required for butterfly valves because of fewer parts.
Butterfly valves consist of a disc that rotates in bearings. In the open position, the disc is parallel to the pipe valve, which allows full flow through the valve. In the closed position, it is rotated against a seat and perpendicular to the pipe wall. Mostly butterfly walls are used for low temperature and pressure applications due to soft seats. To overcome these drawbacks valves with higher temperature seats or high quality machined metal to metal seats are available. The fluid which flows through the butterfly valve will create a low-pressure drop, and the valve shows little resistance to flow when open. Sealing action of these valves is achieved by rotating the disk that has the same diameter as the pipe from a position in line with fluid flow to a position perpendicular to the flow. if resilient seats or piston rings on the disk are used, these valves can be sealed by relatively low opening torque on the valve stem. This sealing action is assisted by the fluid pressure distribution that tends to close the valve. This same hydraulic unbalance requires that a latching device or worm gear be installed to prevent the unwanted closure of manually operated valves.
Butterfly valves are classified into wafer type valves and lug style valves
Wafer style butterfly valve
The wafer style butterfly valve is designed to maintain a seal against bi-directional pressure differential to prevent to prevent backflow in systems designed for unidirectional flow. It accomplishes this by a tightly fitting seal, gasket ring, precision machined, and a flat valve face on the upstream and downstream sides of the valve.
Lug style butterfly valve
Lug style valves have threaded
Lug style valves have threaded inserts at both sides of the valve body, and this will allow them to be installed into a system using two sets of bolts. The valve is installed between two flanges using a separates set of bolts for each flange. This setup permits either side of the piping system to be disconnected without disturbing the other side.