If the positioner power fail on a PST valve while the unit is in running condition and the valve is at fully open condition, what would happen will the valve close or stay in the same position?
Valve will remain in same position (open), positioner is not linked to the valve in normal operation, valve will operate only through sov. If the pst command was given and if there is any failure in the loop or positioner, then the valve will remain open for safety reason since it is in normal process operation.
in a positioner we can configure power fail action, normally it will be open when power fails because they are using reverse relay output.