Symptom
The valve stops at a position short of the command, in one direction or both; the position feedback disagrees with the command; the actuator stalls, hums, or trips on torque; the process does not respond as the command changes near the ends of travel.
The short answer
Valve Will Not Reach Position
A valve that stops short of the commanded position has a problem in one of three places: the command, which may not span the full range because of scaling or a signal fault; the actuator, which may be at its torque or thrust limit, out of air, mis-calibrated in its positioner or its limit switches, or mechanically restrained by a stop or an engaged handwheel; or the valve, whose stem may bind in its packing, whose disc or gate may meet debris or a damaged seat, or whose stem may be bent. The checks separate them: measure the command at the actuator across its range, stroke the actuator with the valve disconnected or with a local command, and stroke the valve by hand with the actuator declutched. The fix follows the finding: rescale or repair the signal, recalibrate or resupply the actuator, or clear, repack, or repair the valve.
Key points
- Three suspects: the command, the actuator, the valve. One check each tells which.
- Measure the command at the actuator terminals across its range; a signal that stops at 90 percent moves a valve to 90 percent.
- An actuator that trips on torque or stalls at the same point every time is meeting something: debris, a seat, a stop, or binding.
- A valve that will not move by hand with the actuator declutched is a valve problem, not an actuator problem.
- Limit switches and positioner spans that were set with the valve partly open keep it partly open forever.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Command signal not spanning the full range | Measure the milliamps or the volts at the actuator input at 0 and 100 percent command; scaling in the controller or a loop fault that limits the signal moves the valve short. |
| Positioner or actuator span calibrated wrong | A positioner calibrated with the valve not fully seated at the ends; the actuator local display shows 100 percent while the valve is at 90. Recalibrate at the true ends of travel. |
| Limit switches or travel stops set inside the true travel | Electric actuators with limit switches set short of the seat stop early. Reset per the manual with the valve at its mechanical ends. |
| Obstruction, debris, or a damaged seat | The actuator stalls or trips on torque at the same position each time; the valve moves freely elsewhere. Inspect the seat and the disc; clear the debris. |
| Stem binding in the packing or a bent stem | Hard to turn by hand with the actuator declutched; packing overtightened, corroded stem, or a bend. Loosen the packing, clean the stem, or repair. |
| Actuator torque or thrust insufficient | Trips on torque in normal operation at high differential pressure; an actuator sized for a lower pressure. Check the sizing against the valve torque at the service conditions. |
| Pneumatic supply pressure low or restricted | A pneumatic actuator that moves partway and stops; the supply gauge below the required pressure; a clogged filter or regulator. |
| Handwheel engaged or a manual override left in | The declutch lever or the handwheel engaged after manual operation; the motor cannot drive the valve. Return the override. |
| Mechanical stop or a broken coupling | A stop set to limit travel; a coupling or a key sheared so the actuator turns and the stem does not. Inspect the drive train. |
Separate the three
- 1
The command
At the actuator or positioner input, measure the signal at a 0 percent and a 100 percent command from the controller. It should span 4 to 20 milliamps, or the configured range, exactly. A short signal is the controller scaling, the output card, or the loop.
- 2
The actuator
With a local command at the actuator, or a calibrator injecting the full signal, stroke it. It should reach both ends and the local indication should agree with the feedback. An actuator that stops short on a full signal has a calibration, a limit, a supply, or a torque problem.
- 3
The valve
Declutch the actuator and stroke the valve by hand through its full travel, feeling for binding, a hard spot, or a stop. A valve that will not move by hand is the valve.
- 4
Fix and recalibrate
Repair the finding, then recalibrate the positioner or the limits at the true mechanical ends, and verify the feedback against the local indication and the process at 0, 50, and 100 percent.
Patterns
| Pattern | Likely cause |
|---|---|
| Stops short in both directions by a constant amount | Positioner or feedback span; scaling |
| Stops short in one direction only | A limit switch or a stop in that direction; debris at that seat; the command range clipped at one end |
| Stops at the same point each time and trips or stalls | Obstruction, seat damage, or a hard spot on the stem |
| Stops at varying points; worse under flow | Torque insufficient against the differential pressure; low air supply |
| Reaches position slowly and then does not | Supply pressure or volume; a stalling motor |
| Actuator moves, valve does not | Coupling, key, or stem failure; a declutched actuator |
| Worked until maintenance | Handwheel engaged; packing overtightened; limits disturbed |
Calibrating at the ends
A positioner or an electric actuator learns where closed and open are from where the valve is when it is calibrated. If the valve was calibrated with debris on the seat, or with the limit switches set inside the travel, the actuator believes 100 percent is where it stopped, and the feedback reads 100 percent with the valve short of its seat. Calibration is done with the valve stroked by hand or by the actuator to its true mechanical ends first, confirmed by the local indicator and, for a closing valve, by a shutoff test, and the limits and the span set from there.
Frequently asked questions
- The feedback says 100 percent but the valve is clearly not closed.
- The span was calibrated short, or the feedback linkage has slipped. Compare the mechanical indicator with the feedback at both ends; recalibrate the positioner or the feedback at the true ends of travel, and check the linkage.
- The actuator trips on torque at the seat every time it closes.
- Either the valve should be position seated and the actuator is set to torque seat, or the torque setting is below what the valve needs at the seat, or something is on the seat. Check the seating mode and the torque setting against the valve manufacturer data, then inspect the seat.
- Why does the valve reach position when the pump is off but not when it is running?
- The differential pressure across the valve rises when the pump runs, and the actuator does not have the torque or the thrust to move the valve against it. The actuator is undersized for the service, or its supply pressure has fallen. Check the sizing and the supply.
- Can I just move the limit switch so the feedback reads right?
- If the valve reaches its mechanical end and the limit is the only thing short, yes, that is the recalibration. If the valve is stopping short because of an obstruction and the limit is moved to match, the valve now reports closed while it leaks, which is worse than the original fault.
Related topics
- Actuator Torque FaultAn electric actuator that trips on torque: at the seat because the seating mode or setting is wrong, in mid-travel because something is in the way or the stem binds, or under flow because the differential exceeds the sizing. The checks before a reset.
- Position Feedback WrongThe HMI says one position and the valve is at another: a feedback potentiometer worn or miscalibrated, a linkage that slipped, scaling that does not match, a reversed signal, limits set wrong, or noise. Comparing indicator, feedback, and process.
- Valve Slow to RespondA valve that takes far longer to move than it should: a pneumatic supply restricted or leaking; a positioner tuned slow; an electric actuator whose speed is fixed; a stem binding in cold or packing; or a rate-limited output. Measuring the stroke time.
- Analog Scaling: Raw Counts to Engineering UnitsThe arithmetic that turns an analog card reading into feet, psi, or gallons per minute — plus the range mismatches that silently corrupt it.
- Outputs Not EnergizingThe program says the output is on and the equipment is not running. Where the signal gets lost between the output image and the field device, and how to find the point in a few measurements.
- FiltrationControlling gravity and pressure filters: effluent rate and level control, head loss and turbidity on every filter, the backwash triggers and sequence, filter-to-waste and ripening, the turbidity rules that filter monitoring answers to, and the operator view.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.