Control Valve

A control valve works to restrict the flow of fluid a pipe at the command of an automated signal. Control valve design are intended

·        For discrete  (on/off) control

·        Throttle fluid flow somewhere between fully open and fully closed (shut).

Control Valve are comprised of two major parts :

• Valve body : contains all the mechanical component necessary to influence fluid flow
• Valve actuator : provides the mechanical power necessary to move the component within the valve body

1. Sliding Stem Valves

A sliding-stem valve body is one that actuates with a linear motion. Some examples of sliding-stem valve body design are shown here :

Most sliding-stem control valves are direct acting, which means the valve opens up wider as the stem is drawn out of the body. Conversely, a direct-acting valve shuts off (closes) when the stem is pushed into the body.

a.      Globe Valves

Globe valves restrict the flow of fluid by altering the distance between a movable plug and a stationary seat .

A set of three photographs showing a cut-away Masoneilan model 21000 globe valve body illustrates just how the moving plug and stationary seat work together to throttle flow in a direct acting globe valve.

A Variation of  stem guided globe Valve :

• Needle Valve :  where the plug is extremely small in diameter and usually fits well into the seat hole rather than sitting on top of it. Needle valves are very common as manually-actuated valves used to control low flow rates of air or oil.

• Port-guided valve : where the plug has unusual shape that projects into the seat. Thus the seat ring acts as a guide for the plug to keep the centerlines of the plug and seat always aligned, minimizing guiding stresses that would otherwise be placed on the steam.