You have to write a procedure for each loop. Where is the change over made (an existing marshaling cabinet, junction box etc.)?
Verify both the pneumatic and electrical signal failure state for the actuator installation. Some parallel spare equipment installations or multiple equipment arrangements permit alternating the service. Swapping parallel equipment operation may be the easiest method.
Control Valves
Often control valves are manifolded with a bypass valve. In these easier cases the operator can begin opening the bypass valve manually. The automatic control will begin closing the control valve. Switch the signal cable then reverse the process by closing the bypass valve.
Control and Shutdown or Blowdown Valves
Without a bypass the technicians rig a fitting to apply an external air source to the actuator. If done in operation this would cause a brief signal upset. Such fittings can be installed when that valve is already in the failure position during operation or easier when the unit is down. Depending upon the actuator plumbing it may be possible to apply an external air source to maintain the actuator signal without losing the air signal while swapping signals. If only the electrical signal is being changed then jumpers can apply the requeired source (24 Vdc, 120 Vac etc.). If in a hazardous area the unit would require gas detection to varify suitable for applying the power with explosion-proof enclosures open etc.
