A Fisher control valve and on the front of the actuator is a table listing "initial spring settings (PSI)" for various valve configurations. What does this mean? Does this mean that the actuator spring has been tightened to the point where it takes that much pressure to start the actuator moving? Other sources refer to this as the "Bench setting" This is the preload that you have to put on the spring before the valve is installed so that when it is in service at the specified pressure, it starts to open at 3 psi. You probably have an EZ or another unbalanced valve. The flow comes in under the plug and tries to force the plug to open. 100 psi differential against a 4" plug would make a force of 1256 pounds, and you need another 200 lbf per inch of seat:2513 pounds (total=3769) so in a typical actuator with 150 square inches effective diaphragm you would need a preload of about 25 psi to get class IV shutoff. It is necessary to have either a HUGE actuator or a substantial spring preload to force the valve closed against the process pressure. Balanced valves have substantially lower requirement for spring preload. Rotary valves such as Ball valves have almost no process pressure effect on actuator force required. Butterfly valves handle high differentials easily when closed, but make an enormous dynamic torque when open as a function of velocity....and velocity is a function of differential pressure.

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