Being the Final Control Element in a system is not an easy job. To start with, you are blamed for any and all problems that crop up in the process. You are subjected to corrosion, high velocity, cavitation, flashing liquids, cryogenic temperatures, high temperatures, abrasion, and thermal shock. You are expected not only to throttle along through all this, but most likely, you are also being asked to act as a block valve and shut off tight. As you work with control valves always keep in mind that a control valve only does what it is told to do.
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.**source