what is air to open and air to close control valve and
difference btwn this...and how to measure milli amp by
this...and what is the role of positioner and i/p converter in
this..plz help me
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Answer Posted / mohammed abdul rahman
This query is mostly related to pneumatic diaphragm actuated control valves. Air to open valves have the pneumatic input on the under-side of the diaphragm with a spring on the topside, which causes the valve to open on increase in control signal. Air to close valves will have the pneumatic input on the topside of the diaphragm with a spring on the under-side to provide counter force.
An i/p converter will convert the 4-20mA signal from control system to an equivalent 3-15psi pneumatic signal to actuate the control valve.
A positioner will have the i/p converter inbuilt and is connected to the moving part of the valve and automatically adjusts the signal to the actuator to maintain a position proportional to the signal. This introduces a loop inside a loop.
Please refer the control valve handbook from Fisher for reference. It is available as a free download.
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