A piston ACTUATOR piped up double acting and operating with full supply pressure of about 500 kPa is very stiff and can normally operate satisfactorily with the flow direction either under the plug or over. This ENABLES the flow direction to be chosen to assist with the fail action, which means that only a small bias spring is necessary inside the actuator to start initial MOVEMENT in the right direction in the event of air failure. In the CASE of diaphragm actuated valves, the stiffness is much lower and so the flow direction MUST always be under the plug, resulting in the need of a heavy spring to give fail closed action. This cannot be fitted inside the actuator.

A piston actuator piped up double acting and operating with full supply pressure of about 500 kPa is very stiff and can normally operate satisfactorily with the flow direction either under the plug or over. This enables the flow direction to be chosen to assist with the fail action, which means that only a small bias spring is necessary inside the actuator to start initial movement in the right direction in the event of air failure. In the case of diaphragm actuated valves, the stiffness is much lower and so the flow direction must always be under the plug, resulting in the need of a heavy spring to give fail closed action. This cannot be fitted inside the actuator.