control valve

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:

• Globe valve: this is the classic control valve. They are usually actuated by a spring-diaphragm actuator given a direct 3-15 psig pneumatic signal without the use of a positioner. However, with the ability to produce higher pneumatic air supply levels and the advent of the 4-20 mA electric control signal, other kinds of valves are now in use.
• Ball valve: although these valves are considered erroneously to perform poorly, thier inherent equal percentage gain characteristic matches perfectly to linearize the pressure vs. flow relationship.
• V-ball valve: these are specially designed ball valves where the ball is actually a segment or half ball in shape and the leading edge is not circular but is made into a V shape hence the name. These valve also have the option of anti-noise or anti-cavitation trim for severe services.
• Butterfly valve: these valves will give reasonable control in undemanding applications. However, an effect seen in tougher applications is flutter, where the disc is destabilized by hydrodynamic forces caused by the media rushing across the surface of the open disc.

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, temperature, consistency, pH, 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 transmitters, solenoid valves, limit switches, etc.