Control valve passing
A passing valve leaks when it is asked to shut. The controller drives the output onto the seat — 0% on a fail-closed valve — and the process variable never arrives. The loop looks saturated. The cause is not a missing 2% of travel; it is flow through a seat that does not seal.
Tuning cannot close a leak. Slowing the PID leaves the output on the seat for longer. The PV still does not come down. The repair is the valve, the bypass, or whatever else is still passing when the controller is doing everything it can.
Which way is passing
Output at 0% and PV still high (or still flowing) is a valve that will not shut. Output at 100% and PV still low can be the opposite — a valve that will not open enough — which is undersizing or a blocked path, not passing. Passing is the shut-off failure.
Confirm there is no bypass cracked open, no parallel valve leaking, and no minimum-flow recirculation that was left lined up. The controller is reporting a real leak. Find where the flow is going.
Do not confuse it with a clamp
A configured low output limit of 10% is not passing. Read the limits. If the output is truly at the seat and flow continues, it is the trim, the seat, or a path around the valve.
What causes it, most likely first
Seat leak or worn trim
most common tuning cannot fix this
The valve is on the seat and still passing enough to hold the PV off setpoint.
- How to confirm it
- Output at 0% (or at the mechanical stop), isolation confirms flow downstream, bypass closed. A stroke test may still look fine — passing is a shut-off problem, not a positioner problem.
- What to do
- Repair or replace the trim. Tuning will not seal it.
Bypass or parallel path open
common tuning cannot fix this
The control valve is shut. Something else is still feeding the process.
- How to confirm it
- Walk the bypasses and parallel valves. Trend any that have position.
- What to do
- Close the path. Then see whether the loop has authority again.
Wrong fail action or reversed valve
less common tuning cannot fix this
The controller is sitting at 0% believing that is shut, and 0% is actually open — or the opposite.
- How to confirm it
- Compare fail action, air-to-open versus air-to-close, and the actual stem position.
- What to do
- Correct the action. Do not invert the PID as a workaround.
Questions that come up
The output is at 5%, not 0%. Is that still passing?
It can be. Some valves leak badly near the seat. If a few percent open still holds the PV off setpoint in the shut-off direction, treat it as a passing or oversized-near-seat problem, not as a need for more gain.
Related
Tuning guides
Free tools
Last reviewed 2026-09-10.