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A proportional-integral-derivative controller (PID controller) is a control loop feedback mechanism used in power and industrial control systems. PID controller calculates an error value as the difference between a measured value and a desired value. The controller attempts to minimize the error by adjusting the process through use of a manipulated variable.
PID controller
PID controller

How a PID controller control 3-way valve and maintain desired temperature?
Terms for description-
Process variable=PV
Set point/Desired value=SP
Manipulated variable=MV or Control variable=CV
Note: Manipulated variable defined as output of the controller (value of temperature) with the input to the process/system (valve position).
Error (set point and measured temperature difference, too cold or too hot and how much) = e
In general a controller can control any process which has a measurable output (PV), a known value for that output (SP) and an input to the process (MV) which affect the relevant MV or PV.
After calculating the error, the controller decides how to set the tap position. The method is proportional control i.e. the tap position is set in proportion to the current error.
In derivative action the rate of temperature change also –increasing the valve opening to add extra hot water on falling of temperature and closing the vale on rising temperature. Integral action uses the average temperature in the past to detect whether the temperature of the container is settling out too low or too high and set the tap proportional to the past errors i.e. opening or closing the valve.
Fig: LT cooling circuit (Wartsill 20v32 GD)
Fig: HT cooling circuit (Wartsill 20v32 GD)
Example: From the snaps below, for LT cooling water here the set value is 38 degree Celsius and the process value is 37.80 degree Celsius and valve opening 4%.
Here the error is 0.20
For HT cooling water here the set value is96 degree Celsius and the process value is 96 degree Celsius and valve opening 71%.
In HT cooling the error is zero.
3-way valve (From every direction)
Fig: PID controller 3-way valve cross sectional view

Fig: PID controller 3-way valve with pipeline

The 3-way control valves are especially suitable for actuation by pneumatic or electrical actuators.

Thanks to: Wikipedia, Armaturen, Powergen


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