Fail Lock

The question is too vague.

I am assuming you want to know how to make a control valve fail in "locked" mode (i.e. fail in place). This is done by installing a "lockup valve" inline with the pneumatic actuator's air line trapping air inside the actuator under "fail" conditions. When the lockup valve shuts, no air can enter or exit the control valve's actuator, which makes the control valve hold its position.