The operating principles of self actuated control valves
In automatic control industries, self-actuated control valves are used for regulating the variables including flow rate of medium, pressure, temperature, liquid level, etc. by regulating the opening degree of valve in automatic system according to the control signals.
- Heating type
Temperature control valves work under Pascal's principle of thermal expansion and contraction. When the temperature of the controlled object is lower than the default temperature, the liquid inside temperature sensing bulb contracts, the force acting on the valve stem reduces, valve opens by the spring force, and closes when the object's temperature reaches to the set point by heat transferring from media like vapor and hot oil. And as the temperature falls, valve opens again. This is for maintaining stability of the medium's temperature.
- Cooling type
The operating principle of cooling type control valve can refer to that of heating type, the only difference is that the valve stem of cooling type opens as the temperature increases. Via cooling medium, it is mainly used in cooling system control.
- Self actuated pressure control valves
When the controlled medium enters the valve, the initial pressure (P1) changes to Ps(downstream pressure) because of the throttling valve. △P, the change in pressure, is considered as the effective pressure. The relative positions of valve trim and valve body are fixed due to an equilibrant force. (△F=Spring force, △F=△PS, S=area of diaphragm) △P goes up as the flow rate increases, P1 and Ps impacting on the downstream and the upstream of the control valve push the valve trim to valve body, changing the flow area between the two and increasing Ps. When reaching another equilibrium in a new position, the aim of controlling flow achieve.
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