Solenoid valves are often used in some applications in which a control valve under certain conditions requires where it quickly driven to the fail position.
Solenoid valves are usually electrically actuated on-off or three-way valves installed into the air system and designed to take a specific action when tripped.
The solenoid can be thought of as an on-off switch for a pneumatic system.
The signal to the solenoid controls the action of the internal valve assembly, allowing flow through the valve in One position and in other position isolating or venting the air.
Why we require Single SOV on a Control Valve
For example, there is a control valve which requires quick action in fail safe condition in emergency situations, then a 2 way SOV is installed between Positioner output and Actuator inlet.
When the SOV is energized condition then SOV inlet connected to outlet and allowing air to flow from Positioner output to the actuator.
When SOV is de-energized condition then output is connected with vent (Exhaust port) and its venting the control valve air from exhaust port quickly and moving the valve to the fail-safe position.
Why we require Dual SOV on a Control Valve
In some cases it has been found out that due to SOV failure (SOV may failure due to many reasons like Coil open, fuse blown ,Plunger stuck up , passing in SOV etc) control valve goes to fail safe condition and its leads to trip the plant due to closing /opening the valve or may cause some unwanted process disturbances.
As shutdown of plant is goes very expensive, so dual SOV philosophy comes for redundancy purpose. In dual SOV if one SOV is goes fail then plant will not disturb.
Dual SOV circuits: Dual SOV pneumatic schematic shows in below Fig. Suppose that Air continuously going to the actuator in normal condition.
Positioner output is connected in both SOV inlet, SOV-1 outlet is connected to SOV-2 exhaust and final output goes from SOV-2 to actuator.
Case 1: When SOV-1 become failure in this condition then what will happen:
If SOV-1 is goes failure then SOV-1 output port is connected with exhaust and air is going from SOV-2 because SOV-2 is energized and inlet is connected to output. Air Flow indications shown in the above figure with the arrows.
Case 2: When SOV-2 becomes failure in this condition then what will happen:-
If SOV-2 becomes failure then SOV-2 outlet is connected with exhaust and SOV-1 is energized then air goes from SOV-1 inlet to outlet then outlet is connected with SOV-2 exhaust which is connected with SOV-2 outlet in fail condition and air is goes to actuator. Air Flow indications shown in the above figure with the arrows.
Case-3: When both SOV become de-energies
When both SOV is de energies then both SOV outlet is connected with exhaust and block the inlet then actuator air is release from SOV-2 outlet to exhaust then exhaust is connected with SOV-1 outlet and SOV-1 outlet connected with exhaust and air is released.
Air Flow indications shown in the above figure with the arrows.
How to check SOV Healthiness in running plant:
Check monthly surface temperature of SOV by pyrometer. if both SOV are from same company then surface temperature is almost equal. (Max difference between both SOV is 5 deg c). If temperature difference is more then check the coil resistance.
Check physically the exhaust port that air is blocked or in normal condition.