What is the difference between a pressure control valve, a temp control valve, and a flow control valve?

Offcourse temp control valve controls temp, and pressure control valve controls pressure....and so on. I am wondering, what are the mechnical differences in the valves body and how are they operated in terms of process control.What is the difference between a pressure control valve, a temp control valve, and a flow control valve?
In principle all valves are flow control valves. What changes is the variable they are controlling.





A temperature control valve controls a flow to heat or cool another fluid.





A pressure control valve, controls a flow to maintain the pressure in a vessel.





As pointed earlier, valves can be self contained, i.e., the sensor is part of the control mechanism, usually a diaprhagm which moves the valve stem. and a spring to adjust a set point.





Or, they can be pneumatic or electronic, in which case there is an external power source, transducer and controller to adjust set point, gain, reset and rate. This gives additional flexibility, if needed, depending on the process to be controlled.


.What is the difference between a pressure control valve, a temp control valve, and a flow control valve?
There are two basic types of control valves. Self contained valves and valves that are operated by a separate controller.





Self contained valves would be like:





back pressure regulator, pressure reducing valve or a thermostatic valve





You can provide the same functions with a valve and a separate controller. In a simple system, the controller senses the variable to be controlled (temperature, pressure, flow, level, etc.) and sends a signal (usually electric or pneumatic) to the valve and tells it to move to maintain the variable at the desired set point.





With valve and separate controller there are many variations from an all pneumatic system to an all electronic system with combinations in between.





Independent control valves can have various charteristics to aid the designer in coming up with the optimum control system.





Here is a link that will give you some other information:





http://www.documentation.frco.com/groups鈥?/a>