This is called choked flow, it happens when the pressure at the valve vena contracta(the smallest actual flow area) drops belob vapor pressure, this causes formation of bubbles which restrict liquid flow; in the case of gases, it happens when gas flow reaches sonic flow velocity.
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ANSI has 6 classes to classify valve leakage performance, where Class I is the least strict and Class VI is for the tightest.
Class I requires no testing while Class VI requires that leakage throught the valve don´t exceed certain amounts depending of the size of the valve, Test is...
Why don´t you use a differential pressure transmitter with a simple PID controller, like C100 From ABB.
Differential pressure transmitter sends a signal to the controller proportional to Pressure drop between the 2 fluids and it´s more accurate than using 2 sensors.
This series of...