A control valve is the final control element that is used to regulate or restrict the fluid flow through a flow channel. There are “automated” and “Control” valve, the distinction between the valves is usually considered to be the ability of the latter to “modulate,” i.e., to assume an infinite number of “throttling” travel positions during normal control service.
There are three basic components of a control valve:
Valve body subassembly: This is the working part and, in itself, a pressure vessel.
The actuator: This is the device that positions the throttling element inside the valve body.
Accessories: These are positioners, I/P transducers, limit switches, handwheels, air sets, position sensors, solenoid valves, and travel stops.
The valve consists of 3 main components namely body, valve and wrench. The body functions as a casing for all components of the valve and as a connector with the pipe. The wrench functions as a controller (hand rotation or electric motor) to regulate flow and valves to realize the movement of the player into a rotary or straight movement which will change the flow condition.
Other components include the sleeve to prevent friction between the valve and the body. Diaphragm to prevent shifting of valve position due to fluid pressure. The valve can be in the form of plug, ball, gate and others.
The control valve is the final control element. The prime concern of an operator of a process control loop is to have a loop that is stable. The final control element will influence the stability of a loop more than all the other control elements combined.
Besides the obvious, such as good quality workmanship, correct selection of materials, noise emission, etc., special attention should be paid to two areas:
Low dead band of the actuator/valve combination (with tight packing).
Tight shutoff, in cases of single-seated globe valves and some rotary valves (if required)
The biggest culprit here is “dead time.”l This is the time it takes for the controller to vary the output signal sufficiently to make the actuator and the valve move to a new position.
The valve itself should never have a dead band of more than 5% of span, that is, less than 0.6 psi for a 3 to 15 psi signal span or 0.8 mA for a 4-20 mA signal. The positioner/valve combination should have no more than 0.5% of signal span