Control valves are valves that throttle, or move to a partially open postion from on or off, to control the flow
of media passing through the valve. The movement is done by an actuator
which can be powered pneumaticly, hydraulicly or electrically. For pneumaticly actuated valves, there is often a valve positioner
which varies the air pressure to the actuator in accordance with a control signal
There are different types of control valves:
Control valves are referred to by the element that they are designed ultimately to control. For example a level control valve
is designed to control the level of a vessel
. This is achieved by monitoring the level and letting out more fluid when the level is high and letting out less fluid when the level is low. Other control valves control pressure
, as well as the obvious flow
. These conventions are used for clarity in P&ID drawings (i.e. FCV-100)
The following aspects are considered when the process is designed and the control valves are selected:
- Cavitation: this phenomena happens when the effective pressure at the vena contracta drops below the vapor pressure of the liquid and the liquid forms vapor bubbles which are collapse further down the line when the pressure recovers. This is generally to be avoided as the collapse of the bubbles can be violent enough to mark and wear away the insides of the valve and pipe.
- Flashing: this occurs when the outlet pressure of the valve is below the vapor pressure of the liquid and the liquid boils off entirely into vapor. A common problem at pump inlets.
- Sizing: A sizing calculation based on the valve co-efficient (or Cv) and the particular process conditions should always be performed to ensure that the valve capacity is neither too great or too small. A rule of thumb was once used that the control valve should be 1 nominal pipe size smaller than the pipe. In today's world, that would be a starting point for the engineer's first sizing calculation.
- Selection of valve type. In addition to those listed above, there are a host of specialty valves developed to solve particular process problems.
Optional items that may appear on control valves are: position transmitter
s, solenoid valve
s, limit switch