double acting diaphragm actuators

[helpyou]

hi gents

Question please. how double acting diaphragm actuators hold last position when air is vented from both open and close actuator chambers during power or air loss. for example, when valve is closed, air is filled in close chamber, when power or air is lost, air is vented, how close position is remained, valve will not move to open position when air is vented from actuator close chamber ?

your inputs please.. thanks

 

[btrueblood]

Double acting actuators don't have a spring that acts to return the piston to one end of the cylinder. Thus, they will tend to fail in place. However, if the valve itself is capable of creating a stem force or torque when flowing (or shut), that force may be sufficient to move the actuator...I have seen butterfly valves creep slowly shut from these flow-generated forces when not properly locked down.