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:: Control Valve ::

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Control Valve   Types and Application Rotary Valve Eccentric Plug Or Segmented Ball Butterfly Valve Ball Valve Linear Motion Valve GLOBE VALVES ANGLE VALVES DIAPHRAGM VALVES PLUG VALVES GATE VALVES RISING STEM BALL VALVES Self-acting Regulators Body & Materials Valve Trim Valve Designs Special Application Valve Actuator Valve Accessories Others Intro

Introduction : CONTROL VALVES is A final controlling element, through which a fluid passes, which adjusts the size of flow passage as directed by a signal from a controller to modify the rate of flow of the fluid. Valve Flow Coefficient (Cv) - The number of US gallons per minute of 60°F water that will flow through a valve with a one pound per square inch pressure drop
 

Rated Cv - The value of Cv at the valve full-open position

Rated Travel - The linear movement of the valve plug from the closed position to the valve full - open position
Leakage - The quantity of fluid passing through an assembled valve when the valve is in the closed postion
On-Off Service - When the valve is used to start / stop the flow by being cycled to the full open or to full closed position
Modulating Service - When the valve is being used to throttle or regulate the flow by varying the opening between open and closed positions
Maximum shut-off Pressure - The pressure of the fluid flowing into the valve against which the valve will have to close Service Temperature - The maximum and minimum temperature of the media
Supply Pressure - The plant air supply pressure available to operate a pneumatic actuator
Stem Torque - The force required at the valve stem to open or close the valve against system pressure and service conditions
Capactiy - Rate of flow through the valve under stated conditions
Fail close - The condition wherein the valve port remains closed should the actuating power fail
Fail open - The condition wherein the valve port remains open should the actuating power fail
Fail last - The condition wherein the valve port remains in the last position should the actuating power fail

Application : A Control Valve is a power-operated device used to modify the fluid flow rate in a process system. Well, what happens if the power is cut off? When a Control Valve is sized or selected to do a particular job, one of the first questions you should consider is how that valve will respond in the event of a loss of signal or power. This is called its "fail-safe mode" and knowing the fail-safe mode is the key to troubleshooting it. In most applications (about 80%), it is desirable for valves to fail closed. In other applications, you might want a valve to fail open or fail in place. Safety concerns and process requirements will mandate the fall mode of the valve. When a valve is not sitting in its fail position, is is being told how and when to move by some external signal. By the comments one hears, you would be led to believe that control valves sit around and think up things to do on their own. Perhaps this will some day be true when all control valves are "smart." If a Control Valve is observed in an unstable condition or appears to not be responding correctly to an input signal, remember that something is telling the valve to behave that way.
A control valve is only as strong as its weakest link. When the 1965 Ford Mustang first appeared, it was powered by a 6-cylinder engine with a 3-speed transmission - but it had a 140 m.p.h.(225 k.p.h.) speedometer. The fact that it had a 140 m.p.h.(225 k.p.h.) speedometer did not mean it could actually travel that fast. In the same way, a control valve with a 600# rated valve body cannot throttle and shut off against 1440 pounds of pressure. There are two basic types of control valves: rotary and linear. Linear-motion control valves commonly have globe, gate, diaphragm, or pinch - type closures. Rotary-motion valves have ball, butterfly, or plug closures. Each type of valve has its special generic features, which may, in a given application, be either an advantage or a disadvantage