Highly Concealed! Review of Deaerator Control Valve Fault: Insufficient Opening & Feedback Deviation (ABB Positioner)

Pneumatic control valves serve as the core actuating components of a unit's control system, acting effectively as the equipment's "hands and feet." During routine operation and maintenance, issues such as distorted valve position feedback, restricted opening, and unexpected switching from automatic to manual mode are common; troubleshooting efforts often prioritize suspected positioner malfunctions or valve body binding. However, the recent failure of the deaerator water level control valve on Unit 4 presented a classic, highly deceptive, and obscure fault: the valve passed no-load full-stroke tests, the problem persisted even after replacing the positioner with a new one, and there were no signs of air leakage on the cylinder exterior. Yet, the valve continued to suffer from tracking failures, a maximum opening of only 20%, and spontaneous closing, creating significant troubleshooting challenges.

This article provides a comprehensive review of the troubleshooting process, delves into the root cause, and summarizes key lessons and standardized maintenance strategies to offer practical guidance for peers handling similar pneumatic control valve issues.

01 Fault Overview: Sudden Operational Anomaly

The faulty equipment was the deaerator water level control valve for Unit 4, equipped with an ABB pneumatic positioner. It operates on an "air-to-open" basis and is critical for regulating the unit's water level.

02 A Complex Troubleshooting Journey: Multiple Misjudgments and Systematic Elimination

The defining characteristics of this fault were its obscure nature, the presence of multiple confounding factors, and a high risk of misdiagnosis. Specialists from two disciplines conducted repeated cross-checks and overturned initial conclusions; the process was progressive and offers significant value for post-incident analysis. The complete resolution process is outlined below:

Step 1: Initial Suspicion of Valve Binding—Ruled Out

Based on symptoms such as insufficient valve opening and a failure to track the position signal, maintenance personnel initially suspected valve body binding. Mechanical staff manually operated the valve handwheel to force the position to a target of 25%, but upon releasing the handwheel, the valve quickly dropped back to 18%. Visual inspections and leak tests revealed no air leakage on the cylinder exterior, effectively ruling out valve binding or external cylinder leakage and shifting the focus of suspicion to the ABB valve positioner.

Step 2: Recalibrate the positioner.

After confirming normal operation during the no-load test, thermal control personnel properly issued a maintenance work order, implemented safety isolation measures, and closed the manual isolation valves upstream and downstream of the control valve. A comprehensive inspection of the air supply system was conducted, confirming that the air pressure reducing valve, supply piping, and pneumatic amplifier were operating correctly with no leaks and stable, compliant air supply pressure.

Subsequently, a precise full-stroke calibration of the ABB positioner was performed, followed by on-site open/close testing in coordination with operations personnel. Under no-load conditions (without process medium), the valve operated smoothly across its full stroke; command and feedback signals were perfectly synchronized with no deviation, and all parameters met specifications.

Step 3: Trial run with process medium; fault recurrence leads to an impasse.

After maintenance was completed and system isolation measures were lifted, a trial run with the process medium was conducted. The fault recurred: under pressurized operation, the valve's maximum opening reached only 20%, failing to achieve the required operating range.

At this stage, the equipment presented a highly misleading scenario: the positioner's calibration parameters were within limits, no-load operation was normal, the valve body showed no signs of sticking or damage, and the air supply system exhibited no leaks or pressure anomalies. Conventional troubleshooting methods failed to pinpoint the fault location, bringing maintenance work to a standstill.

Step 4: Replacement with a new positioner; fault persists.

To completely rule out potential positioner issues, thermal control personnel replaced the unit with a brand-new ABB positioner of the same model and performed a full-stroke calibration again. However, after manually opening the valve to 50% on-site, the valve failed to maintain its position and slowly closed on its own.

The fault persisted despite the new positioner; the operations and maintenance team preliminarily identified internal cylinder leakage as the root cause. However, mechanical personnel conducted repeated, meticulous leak checks on the cylinder's exterior and connections without finding any leak points, causing troubleshooting efforts to stall once more.

Step 5: Pinpointing the hidden defect and eliminating the fault.

To break the troubleshooting impasse, thermal control and mechanical teams conducted a joint, detailed inspection. They ultimately identified the core hidden fault: the seal at the connection between the cylinder and the valve stem had aged and deteriorated, causing a minute internal leak.

This was an internal equipment leak with no external signs; it could not be detected via conventional visual inspections or external leak-detection methods, making it a classic example of a hidden defect. Maintenance personnel immediately replaced the seal with a brand-new one, completely eliminating the risk of internal leakage. After the equipment was returned to service, the valve’s full-stroke switching and regulating actions were precise and smooth, and operation under actual process conditions was stable; the fault was thoroughly resolved.

03 In-depth Analysis of the Root Cause

Many O&M colleagues might wonder: why does the valve function perfectly during no-load testing, yet frequently malfunction once operating under actual process conditions and load? In fact, the root cause of all these anomalies lies in internal leakage at the cylinder valve stem seal. The underlying mechanism is straightforward:

The core issue is internal leakage at the cylinder valve stem seal, as explained below:

1. No-load/No-pressure-differential—defect remains hidden: Without process media, there is no counter-pressure acting on the valve plug. The air loss from slight internal cylinder leakage is negligible and sufficient air pressure remains to meet operational needs; consequently, calibration and full-stroke tests appear normal.
2. Loaded/With-media—defect becomes apparent: During operation with process media, the pressure from the deaerator exerts a counter-force on the valve plug, requiring the cylinder to provide sufficient thrust to overcome the pressure differential. Internal leakage at the valve stem seal causes a loss of air supply pressure and insufficient output force, making it impossible to maintain the target valve position. This leads to a series of issues, such as restricted opening, valve position drift, and significant discrepancies between command and feedback.
3. Highly deceptive nature of the fault: Since the leak occurs inside the cylinder, there are no external signs of air leakage. Conventional external leak detection and visual inspections fail to spot the issue, making it easy to misdiagnose the problem as a fault with the positioner or valve body, resulting in ineffective repairs and repeated rework.

04 O&M Lessons Learned & Preventive Measures

This incident serves as a classic case study in the O&M of pneumatic control valves at power plants, highlighting blind spots in routine maintenance. Key practical takeaways include:

1. Break away from conventional maintenance mindsets: When faced with valve position drift, insufficient opening, or large feedback deviations, do not blindly proceed to calibrate or replace the positioner! Normal no-load test results do not guarantee satisfactory operational performance; prioritize inspecting concealed components such as internal cylinder seals and valve stem seals.
2. Add a dedicated "valve position holding" test: Following the maintenance of pneumatic control valves, a position-holding test must be performed. This involves setting the valve to a specific opening, cutting off the air supply, and monitoring for any drift or drop in valve position to precisely detect internal micro-leaks.
3. Strictly enforce trial runs under actual operating conditions: After maintenance and calibration, control valves must undergo trial runs under load and with the process medium flowing. Passing a no-load test is merely a baseline requirement; normal operation under load is the critical standard for placing the equipment into service.
4. Focus on hidden components prone to aging: Cylinder and valve stem seals are highly susceptible to aging and failure due to constant reciprocating motion and temperature fluctuations, yet such faults are often concealed. These components must be included in the priority inspection list during routine maintenance and replaced regularly to proactively mitigate risks.

Pneumatic control valve malfunctions in power plants are never simple issues that can be resolved merely by swapping parts and recalibrating. Many hidden defects lie within the blind spots of routine inspections, testing the logical reasoning and troubleshooting expertise of operations and maintenance personnel.

This review of the deaerator control valve failure aims to help you avoid similar maintenance pitfalls, build experience in troubleshooting hidden faults, and strengthen the safeguards for the safe and stable operation of the generating unit.