10-12-2012, 01:34 PM
Valves
Valves Instumentation and Control.ppt (Size: 366.5 KB / Downloads: 43)
Definition
A valve is a mechanical device that controls the flow of fluid and pressure within a system or process. A valve controls system or process fluid flow and pressure performing any of the following functions
Functions
Stopping and starting fluid flow
Varying (throttling) the amount of fluid flow
Controlling the direction of fluid flow
Regulating downstream system or process pressure
Relieving component or piping over pressure
Parts of a Valve
1. Closure member: part of the valve that closes flow (disk, ball, gate, etc.).
2. Actuator: means of operating the valve – hand, gear, chain wheel, motor, solenoid, pressure and flow of the media, air pressure.
3. End fitting: must be specified when buying the valve - butt weld end, compression flange, pipe thread, quick disconnect
4. Material: closure member, housing, seat – stainless steel
5. Packing/seals: seals stem, replaced
6. Seat: where the closure members seals against the valve housing
Ball Valve
A ball valve is a rotational motion valve that uses a ball-shaped disk to stop or start fluid flow.The ball, performs the same function as the disk in the globe valve. When the valve handle is turned to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet. When the valve is shut, the ball is rotated so that the hole is perpendicular to the flow openings of the valve body and the flow is stopped.
Most ball valve actuators are of the quick-acting type, which require a 90° turn of the valve handle to operate the valve. Other ball valve actuators are planetary gear-operated. This type of gearing allows the use of a relatively small handwheel and operating force to operate a fairly large valve. Some ball valves have been developed with a spherical surface coated plug that is off to one side in the open position and rotates into the flow passage until it blocks the flowpath completely. Seating is accomplished by the eccentric movement of the plug. The valve requires no lubrication and can be used for throttling service.
Butterfly Valve
A butterfly valve, illustrated in is a rotary motion valve that is used to stop,
regulate, and start fluid flow. Butterfly valves are easily and quickly operated because a 90 rotation of the handle moves the disk from a fully closed to fully opened position. Larger butterfly valves are actuated by handwheels connected to the stem throughgears that provide mechanical
advantage at the expense of speed.
Low pressure, large diameter lines where leakage is unimportant
Advantages – low pressure drop, small and light weight
Disadvantages – high leakage, high actuation forces so limited to low pressures
Operation
Butterfly valves are especially well-suited for the handling of large flows of liquids or gases at relatively low pressures and for the handling of slurries or liquids with large amounts of suspended solids.
Butterfly valves are built on the principle of a pipe damper. The flow control element is a disk of approximately the same diameter as the inside diameter of the adjoining pipe, which rotates on either a vertical or horizontal axis. When the disk lies parallel to the piping run, the valve is fully opened. When the disk approaches the perpendicular position, the valve is shut.
Intermediate positions, for throttling purposes, can be secured in place by handle-locking devices.