Manual override may be specified if it will disengage automatically when the remote signal for the actuator is activated to trip the valve to the safe position. Unfortunately, only electric and pneumatic motor actuators and thermally-activated, coil spring actuators have this capability.
Piston and diaphragm actuated EBVs should not have manual overrides, because the valve cannot be tripped after it has been opened manually by an override.
If an EBV is designed properly, tested at the factory, and tested regularly at least every three months, it should respond whenever a demand is placed on a system. Overrides, therefore, should not normally be required nor used for quarter-turn or diaphragm valves.
There are three basic types of manual overrides:
• Gear box
Gearbox manual overrides are the most dependable, maintenance free, and physically compact. Unfortunately, they often cost as much as the actuator.
Hydraulic overrides depend on the operation of a hydraulic pump and eventually need maintenance. They are less costly than a gear box override.
Jackscrew overrides are inexpensive but should have a handwheel attached. Unfortunately, the exposed threads rust over time. Also, the operator expends significant amounts of time and effort to move the valve, which is impractical in an emergency. Pneumatic wrenches can facilitate testing jackscrew overrides.
To operate the manual override, the force required at the rim of the handwheel should not exceed 80 lbs. Check the size of the handwheel and the number of turns required to open or close the valve fully to make sure the handwheel is neither too large nor the number of turns required too many.
The handwheel must also be accessible. If the actuator is not at grade but up a pipeway; then provide safe, unobstructed access for the operator to reach and operate the handwheel.
For an electric or pneumatic motor actuator used on an EBV, it should be impossible to lock the motor in the manual or hand position.