How to Troubleshoot a Control Valve Passing Problem after Overhauling: Complete Root Cause Analysis

Process industries sometimes face the uncomfortable problem of a control valve that starts to leak or pass immediately following overhauling. When this occurs, maintenance teams frequently believe that the cause is mechanical wear, incorrect installation, or damage to the actuator, but the true cause is frequently more hidden.

In this detailed guide, we explain how to troubleshoot a control valve passing problem after overhauling, using a real-world case involving a globe valve in a dirty water application. You will learn about the technical checks, probable causes, and most importantly, how to prevent control valve leakage after service jobs through precise procedures.

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A globe-type control valve, operated with a diaphragm actuator (air-to-close), was used in a dirty water service line. Due to noticeable internal leakage, the maintenance team planned a complete overhaul, which included:

  • Dismantling and replacing all hard parts (plug, seat, etc.)
  • Replacing soft parts in the actuator
  • Auto-calibration using a digital valve positioner
  • Stroke testing and Class IV leakage testing at the workshop

Post-workshop, the valve was reinstalled at a different location and taken online. Despite successful test results in the workshop, passing was observed immediately after re-commissioning.

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Unwanted fluid leaking through the valve seat and plug when the valve is closed is referred to technically as a passing control valve. One possible cause of this is mechanical damage (worn seat or plug). This could be due to:

  • Mechanical damage (worn seat or plug).
  • Failure of the actuator (inadequate closing force)
  • The valve assumes a full stroke at 50% closure, which is a calibration issue.
  • Between the plug and the seat, there is foreign material.

Such leakage can result in major problems in regulated process loops, such as inadequate control or malfunctioning safety interlocks.

After almost ten years of use, the valve was overhauled. However, new hard parts were fitted during overhauling. Class IV leakage tests conducted after calibration revealed no obvious leaks. Therefore, mechanical damage was not likely to be the problem.

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The diaphragm actuator’s soft components were changed. During leak testing, no passage was seen from the actuator port, and the datasheet’s stated air supply pressure was confirmed. Actuator problems were excluded.

Auto-calibration was used for calibration in the workshop, and stroke verification at the bench was successful. However, field behavior is not always reflected in workshop calibration, particularly in services that include polluted fluid or strong backpressure.

Using lifting equipment and avoiding sudden shocks, the valve was moved with caution and under observation. There was little chance of mechanical misalignment due to transport.

After installation, the actuator’s air pressure was checked once again. Pressure was in line with the design specifications. Consequently, the cause was not actuator underperformance brought on by a lack of air.

The valve was brought offline for a second inspection when it was observed to be passing once more. This time, a crucial disparity was found: even though the controller required complete closure, the plug remained slightly raised since the valve stroke in the field was less than 100%.

Because of this incomplete stroke, the plug was never completely seated, allowing flow to continuously leak through; as a result, the control valve was “passing.”

Although calibration and leakage testing were successful at the workshop, the field conditions were different. Something had affected the valve’s travel after installation.

Upon questioning the site engineer and technician, the following sequence came to light:

  • After installation, the control valve showed 2%–3% hunting during operation.
  • Thinking it was a calibration issue, the engineer performed a second auto-calibration while the valve was still online.
  • This was done without isolating the process line.

Since the valve was in a dirty water service, particles present in the line likely entered the seat area during calibration.

As a result:

  • Consequently, the movement of the valve plug was limited by these foreign particles.
  • This limited stroke was mistakenly recognized by auto-calibration as the entire stroke.
  • Although the valve was technically still slightly open, the controller believed it was closing completely.
  • Uncontrolled leakage returned after the particles gradually drained downstream, creating a space between the seat and stopper.

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The issue was resolved through a systematic process:

Prevent more contaminants from entering the valve by isolating it from the process.

Clean water flushing was done to remove residual dirt and prevent further contamination.

With pure air and no process interference, the valve was auto-calibrated once more.

Physical measurements of the stroke length were made and compared to the valve datasheet’s design specifications.

The valve was brought back online once a tight shutdown and complete stroke were confirmed. Even when the design pressure was at 100%, no passing was seen.

Several important lessons for field personnel and engineers are highlighted by this real-world example:

To verify real movement, use an actuator travel indication or stroke scaleDon’t rely solely on the positioner feedback.

Always isolate the valve from the line before performing any calibration. Solid particles can cause false calibration data.

Even new pipelines can carry welding debris, scale, or rust. Flush thoroughly with clean water or air before putting any valve into service.

Standardize your control valve commissioning with a checklist including:

  • Air pressure verification
  • Offline calibration
  • Stroke and seat leak test
  • Signal feedback check
  • Loop tuning validation

Auto-calibration routines are helpful but not fail-proof. Reliable control requires an understanding of the actuator’s behavior in actual fluid conditions.

Although earlier parts concentrated on a particular underlying reason, the following other real-world situations frequently result in control valve passage following overhaul:

Incomplete closure may result from even a small misalignment between the actuator stem and valve stem. If the actuator linkage is not centered or tightened to spec, the actuator may not transmit full travel force to the plug causing a control valve to pass under load.

Digital valve positioners can store multiple configuration parameters like travel span, signal range, zero calibration, and fail-safe actions. If the configuration file used during re-installation is incorrect, it may command incorrect stem travel. This is especially critical for split-range or non-standard signal setups.

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A simple tubing swap between supply and control signal lines can reverse the actuator logic (air-to-close becoming air-to-open). In such a case, the valve may remain partially or fully open even when commanded to close mistakenly seen as a passing issue.

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Some teams test control valves at atmospheric pressure conditions, which doesn’t replicate the actual system pressure or media. Because of fluid viscosity and thermal expansion, a valve that passes a leaking test on a test rig may leak when exposed to actual differential pressure or fluid temperature.

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How to Troubleshoot a Control Valve Passing Problem after Overhauling: Complete Root Cause Analysis

Check the actuator force at shutoff in addition to the stroke length during calibration. For certain applications to satisfy Class IV or V leakage requirements, a strong sitting force is required. To make sure there is enough closure pressure, use an actuator torque specification or force gauge.

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Choosing the right seat and plug material is very important in dirty, rough, or corrosive services. For long life and a tight shutdown, you could need stainless steel, hardened trim, or ceramic inserts. After an overhaul, trim material that doesn’t match can wear out soon and cause immediate passing.

Always record down the valve configuration settings, the dates of calibration, the checks of air pressure, and any problems you see. A clear trail of commissioning data makes it easier to find problems and prevent guesswork.

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Control valves work in changing conditions, notably in utilities and wastewater processes where foreign particles, pressure spikes, and dirty media are typical. Overhaul workshops are great for rebuilding and calibrating since they are clean and regulated, but they don’t always look like the real line circumstances.

The main lesson from this example is that valve service in a workshop must include field-level validation stroke inspections, consistent air pressure, and offline calibration under clean conditions. If not, hidden factors like dirty process media, wrong reinstallation, or calibration under stress might make tests seem to work when they really don’t, and then cause real problems in the field.

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Control valve passing problems after overhauling are not always caused by mechanical failure or installation errors. In this case, a simple auto-calibration at the wrong time and condition triggered the entire issue.
If you have a control valve that is passing, you need to do more than simply look at the valve itself; you need to take a more general approach. Every part of the valve loop must be checked, from how it works and how it is set up to how it is contaminated and how it is calibrated.

This scenario shows how important it is not to jump to conclusions regarding broken parts or damaged actuators. Instead, systematic diagnostics, isolating field influences, and careful commissioning can save your control valves from needing extra maintenance cycles and make sure they work as they should after an overhaul.

By applying root cause analysis and verifying calibration, stroke, and process cleanliness, the engineering team avoided unnecessary rework and restored proper functionality.

There are ways to find control valve passing, such as acoustic monitoring, which uses ultrasonic sensors to pick up the sound of a leak, and infrared thermography, which shows temperature changes produced by fluid moving through a closed valve. Another way to find irregular flow is to evaluate the pressure and flow downstream while the valve is closed. During shutdowns, checking the seat and plug by hand is a straightforward way to make sure there is a leak.

Internal leakage is a typical problem with control valves. This can happen when the seat or plug wears down, the actuator doesn’t work right, or the valve is misaligned. If there is debris stuck in the valve, it may not close all the way. Depending on the valve class, small leaks are okay. However, too much leakage affects control performance and usually means that the valve has to be fixed or recalibrated.

 Routine checks, oiling moving parts, and cleaning to get rid of dirt are all ways to stop valve passage. When you do maintenance, it’s vital to replace worn seats and seals. Regular checks of the actuator stroke and calibration assist make sure the valve closes all the way. Using positioners and monitoring tools can also assist find problems before they get worse.

Passing a valve can make a process unstable, waste energy, and put people in danger. Common results include bad control, pollution, and damage to equipment downstream. Worn sealing surfaces, debris, and broken actuators are some of the things that can cause this. Taking care of these problems during regular maintenance helps make sure that the system works well and is reliable.

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