A solenoid valve may not show external leakage but may allow a small gas flow when closed. This is called internal leakage or leakage through the seat.
Why it is important
Internal leakage can cause valve proving failure and, in severe cases, create unwanted gas presence downstream.
Possible causes
Contaminants on the seat, seal wear, mechanical damage, or foreign objects can prevent full closure.
Diagnosis
Diagnosis is performed using the procedure prescribed by the manufacturer or the valve proving system by monitoring the pressure change in the closed section.
The end switch is not enough
A "closed" feedback signal indicates position, not necessarily seat tightness.
After repair
The valve or assembly is retested before normal operation is permitted.