The purpose of a “Control” valve is to control. It might control flow, level, pressure or temperature depending on the purpose of loop it is installed in. The valve itself is normally a “globe” type valve body but with an air or electro-pneumatic operator. This operator is controlled through a loop that includes a primary sensor and the control logic.
What is a control valve?
The normal globe type body control valve will be a single of double port configuration with a sliding “can” type plug with holes or tapered opening to allow fluid or gasses through.
A Control Valve is normally only one specific device in a “loop” (system) that controls a parameter affecting a process. There are four main parameters and a few minor parameters. The four main parameters are: Flow, Level, Pressure and Temperature. These four make up (maybe) 95% of the control parameters in any process plant. The other or minor parameters include: weight, thickness, humidity, density, Ph, color and viscosity to name just a few.
What are the parts that compose a control loop? What is their function?
A control Loop (system) will normally be made up of a sensor device (a) to read the desired control point, then a transmitter (b) to send data, then a control board unit to display(c) and or record the data (d), then a set point controller (e) with a relay followed by the control valve (f) itself.
So let’s say we are interested in the level of liquid in a specific tank when we are filling that tank. We would need some type of device (a) to read the level in the tank. Then we would need a transmitter (b) to send the level data to the control board. On the control board we would need a display gage(c) to indicate the current level. Then we would need a recorder (d) to record the level. Then we need to have a set point controller (e) because sometimes the process requires changes to the primary level. When the correct level reaches the “set point’ then a signal is sent to the control valve actuator to close the valve (f).
In the previous paragraphs I said “normally” when defining a control system. The exception is the “Self Contained Pressure Controllers.” These devices are a control valve that does not have the type of “Loop” described in the previous paragraph. They are in fact self-contained. They come in two types. One is made to control the pressure up-stream of the valve. The other is made to control the pressure downstream of the valve.
Different types of control valves
Control Valves come in many different “body” types and configurations and, as has been noted by others, can have different types of actuators. The most common body type is the “Globe” type body. There are also variations in the globe type body. There are single seat and double seated globe type bodies. There are also straight pattern bodies and angle pattern body control valves. Other valve types can be fitted with an actuator and thus become a control valve. These include the Butterfly valve, the Needle valve and the Diaphragm valve.
Typical Globe Type Control Valve with Pneumatic Actuator
The “Actuator” is the part that operates the valve. Actuators might be pneumatic, electric solenoid, electric motor or hydraulic.
This text is an extract from questions gathered from numerous queries of anonymous people in the Piping Engineering and Design profession. The answers or discussion of the topics were provided by James O. Pennock. Images obtained from www.coastalflange.com