Valve Passing: Causes & Solutions
A 'passing' valve leaks when it should be fully closed. This guide covers why valves pass — from seat wear to debris — and how to diagnose and fix each cause.
What Does 'Passing' Mean?
A valve is 'passing' when it allows fluid to flow through even when fully closed. This is different from a leak (which occurs through the valve stem or body gasket) — passing means the internal seat is not sealing. The fluid goes through the valve the way it's supposed to, but the valve can't shut it off.
A passing valve can be a safety issue (isolation not achieved during maintenance), an environmental issue (leaking hazardous fluid), or an operational issue (process can't be shut down, energy waste from steam or compressed air leakage).
Causes by Valve Type
| Valve Type | Common Cause of Passing |
|---|---|
| Ball valve | Scratches on ball or seat from abrasive particles; worn PTFE seats |
| Gate valve (wedge) | Debris preventing full closure; wedge worn or corroded; gate not reaching full travel |
| Globe valve | Wiredrawn seat from high-velocity erosion; worn disc; misaligned disc-to-seat |
| Butterfly valve | Worn elastomer liner; disc not reaching full closed position; shaft play |
| Needle valve | Worn needle tip; scratched seat from particulates |
| Check valve | Broken hinge pin; worn seat; debris preventing closure |
| Control valve | Wiredrawn seat from cavitation or high-velocity flow; worn plug; actuator not reaching full closed position |
Diagnosis
- Verify the valve is fully closed: Check the position indicator and the actuator stroke. A pneumatic actuator may not be reaching full closed due to insufficient air pressure or a worn actuator spring.
- Measure the leak rate: Install a pressure gauge downstream and isolate all other sources. The rate of pressure rise indicates the leak rate.
- Listen: Use an ultrasonic probe to detect the high-frequency sound of fluid leaking through a closed valve. This is the most sensitive detection method.
- Check for debris: If the valve can be opened slightly and then closed again, the debris may have been flushed. If the leak stops, debris was the cause.
- Inspect internals: If the valve can be isolated and opened, inspect the seat and closing member for wear, erosion, or damage.
Solutions
- Clean debris from the seat — open and close the valve several times to flush, or remove and clean internals
- Lap the seat — for globe and needle valves, lapping compound can restore a smooth seat surface
- Replace the seat or disc — if the seat is wiredrawn or eroded, replacement is the only fix
- Replace elastomer seats — for ball and butterfly valves, replace worn PTFE or rubber seats
- Fix the actuator — if the valve isn't reaching full closed, adjust the actuator travel or increase air pressure
- Install a strainer upstream — prevents debris from damaging the valve seat in future
- Consider a different valve type — if the valve repeatedly passes due to erosion, a harder seat material (Stellite) or a different valve type may be needed
Safety Warning
Never assume a valve is fully isolated based on the handle position alone. Always verify double-block-and-bleed isolation before breaking any flange or opening any equipment. A passing isolation valve can cause serious injury.