Troubleshooting: Overshoot and Oscillation of the Control Valve

1. Description of the Problem and Scope of Application

This diagnostic and troubleshooting manual is intended for maintenance personnel, reliability engineers, and maintenance managers who encounter erratic control valve operation. Overshoot (hunting) and oscillation (oscillation) of the control valve are critical modes of operation that can lead to a significant deterioration in the quality of process control, increased equipment wear, increased energy consumption and, in some cases, to complete system failure.

This manual covers a wide range of control valves, including but not limited to ball, diaphragm, gate and rotary valves, used in such industries as the chemical industry, oil and gas, food, energy and metallurgical industries of Ukraine, according to the national standards of DSTU and international standards EN, ISO.

Symptoms

• Hunting: Slow but constant oscillations of the process output parameter around a given set point, indicating that the valve cannot find a stable position.
• Oscillation (Oscillation): Rapid and often intense cyclical changes in valve position or output parameter, which may be caused by mechanical faults, misalignment of the positioner, or interaction with process dynamics.

Types of Affected Equipment

• Control valves with pneumatic, electric or hydraulic drives.
• Valve positioners (pneumatic, electropneumatic, digital).
• Process control systems (ASUTP, PLC).
• Sensors and transducers associated with the control loop.

Classification of Severity

• Critical: Valve instability leads to unsafe operating conditions, risk of accident, significant production losses, or product non-conformity (for example, exceeding critical temperatures or pressures). Immediate intervention is mandatory.
• Significant: The problem affects process efficiency, increases wear and tear on equipment, but does not pose an immediate threat to safety or critical losses (eg minor level fluctuations or costs). Requires planned elimination as soon as possible.
• Minor: Symptoms are barely noticeable, do not affect production, but indicate a potential future problem (for example, periodic micro-oscillations in valve position that do not show up on a process parameter). Requires monitoring and inclusion in the next planned repair.

2. Precautions

CAUTION: Before starting any diagnostic work or maintenance work on control valves, it is necessary to ensure complete safety of personnel and equipment. Failure to follow these precautions could result in serious injury, death, or significant equipment damage.

• LOCKOUT AND TAGING OUT (LOTO): Always perform LOTO procedures in accordance with the company's internal standards and the requirements of DSTU EN 1037:2006. Disconnect the valve actuator power sources (pneumatic, electric, hydraulic) and lock them in a safe position.
• PERSONAL PROTECTIVE EQUIPMENT (PPE): Use appropriate PPE: safety glasses (DSTU EN 166:2017), protective gloves (DSTU EN 388:2017), protective clothing, protective shoes. Depending on the process, respiratory or face protection may be required.
• STORED ENERGY: Valve actuators and pneumatic/hydraulic lines can hold a significant amount of stored energy even after a power outage. Make sure all power is reset to a safe level. Slowly open the drain valves and observe the pressure gauges.
• HAZARDOUS SUBSTANCES: Valves may contain hazardous, toxic, corrosive or high temperature liquids/gases. Make sure the system is isolated, pressure is relieved, and residual substances are removed and neutralized before disassembly. Check the absence of dangerous gases with a gas analyzer.
• WORKING AT HEIGHT: When working at height, use appropriate means of access (ladders, lifts) and fall prevention systems in accordance with DSTU EN 358:2015, DSTU EN 361:2017.

3. Necessary Diagnostic Tools

To effectively diagnose the instability of control valves, a set of specialized tools is required.

4. Initial Evaluation List

Before starting a detailed diagnosis, it is critical to gather as much information as possible about the operating conditions and history of the malfunction. This will help narrow down the potential causes.

5. Systematic Flow of Diagnostics (Decision Diagram)

This section presents a step-by-step diagnostic algorithm for identifying the root cause of valve overshoot and oscillation.

1. Are there fluctuations in the process output parameter?
   • IF YES:
     1. Is the oscillation stable and symmetrical?
        • IF YES (rapid fluctuations):
          1. Check the PID controller settings in ASUTP.
             • Trial action: Reduce the proportionality factor (P) by 10-20%.
             • Trial action: Increase integration time (I) by 10-20%.
             • Trial Action: Reduce differentiation time (D) by 10-20% (if used).
             • IF the problem disappears or decreases: Probable cause: Incorrect setting of the PID controller. Go to section 7.1.
             • IF the problem persists: Go to section 1.1.1.2.
          2. Check valve positioner.
             • Check air supply pressure. Norm: 4-6 bar (according to the manufacturer's specification).
               IF the pressure is unstable or low: Probable cause: Insufficient supply pressure. Go to section 7.2.
             • Perform the Positioner Auto Setup/Calibration procedure (if available).
             • Reduce the gain of the positioner by 10-20%.
             • Increase the damping (Damping) of the positioner by 10-20%.
             • IF the problem disappears or decreases: Probable cause: Incorrect positioning of the positioner. Go to section 7.3.
             • IF the problem persists: Go to section 1.1.1.3.
          3. Check the valve and actuator mechanics.
             • Perform a "Step Test": apply step signal changes to the positioner (eg 25%, 50%, 75%, 100%) and record the response time, overshoot and stabilization. Evaluate linearity and repeatability.
               IF response is non-linear, with delays or hysteresis >2%: Probable cause: Mechanical friction or backlash. Go to section 7.4.
             • Measure the air pressure on the actuator at different valve positions.
               IF there is a significant pressure difference to maintain one position (especially when changing direction): Probable cause: Excessive friction or jamming. Go to section 7.4.
             • Perform a "Dead Band Test": increase and decrease the input signal in small steps (0.1%, 0.2%, 0.5%) and record the minimum change in the input signal that causes the valve to move.
               IF dead zone >1% of full stroke: Probable cause: Mechanical friction, backlash, or poor positioner sensitivity. Go to section 7.4 or 7.3.
             • Check for external vibrations with a vibration analyzer.
               IF vibration exceeds 4.5 mm/s (RMS) on the actuator or stem: Probable cause: Mechanical problem or resonance. Go to section 7.4.
        • IF NO (uneven or asymmetric oscillations):
          1. Check interaction with the process.
             • Are there other regulators in the cascade that could affect this valve?
             • Are there pressure or flow fluctuations in the inlet flow?
             • Are phase transitions (eg, boiling) occurring in the fluid controlled by the valve?
             • IF a significant process effect is found: Probable cause: Interaction of control loops or process dynamics. Go to section 7.5.
             • IF not detected: Repeat the diagnosis of the positioner and mechanics (paragraphs 1.1.1.2 and 1.1.1.3), the symptoms may have been masked.
   • IF NO (no process parameter fluctuations, but valve instability):
     1. Check the valve position reading.
        • Are there small but constant valve position fluctuations that are not reflected in the process parameter?
        • IF YES: Probable cause: Excessive positioner sensitivity, slight friction or position feedback. Go to section 7.3 or 7.4.
        • IF NO: Possible valve position sensor malfunction or signal transmission problem. Check the feedback signal with a calibrator.

6. Malfunction-Cause matrix

7. Root Cause Analysis for Each Malfunction

7.1. Incorrect setting of the PID controller

Explanation: The PID controller (Proportional-Integral-Differential) is the heart of most control systems. If its parameters (P, I, D) are set too aggressively for the dynamics of the process, the regulator will overreact to the slightest deviations from the set point, which will lead to oscillation of the output parameter and, accordingly, to the constant movement of the control valve. Too large a proportionality factor (P) causes a fast but unstable response. Too small an integration time (I) leads to an accumulation of integral error and constant overshoot. The differentiation factor (D) is rarely the cause of oscillation, but setting it incorrectly can exacerbate the problem.

How to confirm: Observing the graphs of the process parameter and the output signal of the regulator. A typical indicator is symmetrical, rapid fluctuations of the process parameter around a set point, which disappear or are significantly reduced when manually decreasing the coefficient P or increasing I. Performing a "Step test" on the process followed by analysis of the response curves. For confirmation, use the ACS software to record trends. If the P-band is too narrow, even small perturbations will cause the valve to fully open/close.

Consequences if not corrected: Constant wear on valve, actuator and positioner due to continuous movement. Decrease in the quality of final products. Increase in energy consumption. Equipment failure due to overload is possible. Failure to meet the requirements of product quality standards (for example, DSTU ISO 9001:2015).

7.2. Insufficient/Unstable Supply Air Pressure

Explanation: For pneumatic actuators, which are the most common, stable and sufficient supply air pressure is critical. The positioner uses supply air to move the actuator. If the pressure is too low, the actuator will not have enough power to overcome the frictional forces and fluid pressure in the valve, resulting in slow response, "sticking" and subsequent oscillation. Instability in the supply pressure (for example, due to clogged filters, a faulty pressure reducer or insufficient capacity of the air network) is directly transmitted to the valve actuator, causing it to move involuntarily.

How to confirm: Measure the supply air pressure directly at the positioner inlet with a high-precision pressure gauge (eg Testo 510i). Compare with the passport data of the valve (usually 4-6 bar, ±0.5 bar). Observe pressure stability during valve operation. Check for leaks in the pneumatic system using a soapy solution. Check the pressure at the outlet of the reducer. Air quality standards: DSTU ISO 8573-1:2010.

Consequences, if not eliminated: Inaccurate adjustment, increased wear of actuator and positioner, constant instability of the process, inconsistency of process parameters. It is possible to block the valve in one position at a critical pressure drop.

7.3. Incorrect Positioner Setup

Explanation: A positioner is a device that ensures accurate positioning of the valve according to the input signal from the regulator. Modern digital positioners have their own PID parameters (often called Gain, Damping, or Stiffness) that control the speed and stability of the valve's response. If the positioner's gain is too high, it will react too aggressively to the slightest deviation, causing the rod to oscillate rapidly. Insufficient damping will cause the valve to "overshoot" the target position. Also, the cause may be incorrect calibration of the range or zero point of the positioner.

How to confirm: Follow the automatic positioner setup procedure if available. Reduce the gain and/or increase the damping of the positioner and observe the behavior of the valve. Perform a "Step Test" and a "Dead Zone Test" with a calibrator (Fluke 754) to evaluate positioner response. Digital positioners allow you to get graphs of internal work, which facilitates diagnostics. Check the certification of the positioner for compliance with CE, UkrSEPRO.

Consequences if not eliminated: Similar to the consequences of improper PID adjustment, but with an emphasis on mechanical wear of the valve and actuator. Excessive wear of seals, rod, bearings, leading to increased backlash and friction. Premature failure of components.

7.4. Mechanical Friction or Backlash

Explanation: Mechanical friction in the stuffing box, guides, valve stem, or actuator is one of the most common causes of unstable operation. High friction leads to the fact that the valve "sticks" in one position, and then, when the drive force increases and exceeds the force of friction, it sharply "jumps" to a new position ("stick-slip" phenomenon). This creates a large "dead zone" - the range of the input signal to which the valve does not respond. Backlash in joints (eg between rod and actuator, in levers) can also cause instability as the positioner tries to compensate for uncontrolled movements. The causes of friction can be: oil seal damage, stem corrosion, deposits on the inside of the valve, improper assembly, bearing wear, and an overtightened oil seal.

How to confirm:

• Manual test: Isolate the valve (LOTO!), disconnect the actuator and try to manually move the stem. Assess the smoothness of the movement. Any uneven resistance or "sticking" indicates friction.
• "Dead Zone Test": Perform this with the calibrator as described in section 5. Dead zone over 1% is critical, over 2-3% is unacceptable.
• Vibration Analysis: Using a vibration analyzer (SKF Microlog) on ​​the actuator and rod can reveal increased levels of vibration, especially at low frequencies, indicating friction. Нормальний рівень вібрації для більшості клапанів менше 4.5 мм/с (RMS) згідно з ISO 10816.
• Thermal imaging inspection: A camera (FLIR T540) can detect localized overheating in areas of oil seals or bearings, indicating excessive friction. A temperature difference of more than 10°C relative to the surrounding areas is abnormal.
• Gland inspection: Visual inspection for damage, wear, improper installation.

Consequences, if not eliminated: Inaccurate control, increased wear of seals and stem, damage to the actuator, increased leakage due to worn seals, complete failure of the valve, risk of loss of tightness of the system.

7.5. Improper Selection of the Drive (Oversizing) or Interaction with the Process

Explanation:

• Oversizing actuator: If a valve actuator is too large for a particular application, it will have excess power. This can lead to an overly fast and aggressive response, even with a minimal signal from the positioner causing oscillation. A large actuator can also be heavy, increasing the inertia of the system.
• Process interaction: The valve does not operate in isolation. It is part of a large technological process. Other regulators in the cascade, long delays in the process, non-linear process dynamics (eg phase transitions, two-phase flows), or significant perturbations in the input flow can cause instabilities that the control valve tries to compensate for, but without success.

How to confirm:

• Oversizing: Analysis of calculated valve and actuator characteristics. If the valve operates most of the time in the opening range of <20% or >80% to maintain the setpoint, this may indicate an incorrect selection.
• Interaction with the process: Trend analysis of all related process parameters. Temporarily disconnecting other control circuits (with safety!) to isolate the impact. Study of the dynamics of the process, possible pressure drops or changes in the phase state of the working environment. Consultation with a technologist.

Consequences, if not eliminated: Low control efficiency, increased equipment wear, potential safety problems due to uncontrolled changes in process parameters, inability to achieve optimal plant operation.

8. Step-by-Step Troubleshooting Procedures

8.1. Adjusting the PID controller settings

1. Identify the controller: Determine which PID controller controls the problem valve (ASUTP, local controller).
2. Save current settings: Always save or export current P, I, D values ​​before making changes. This will allow you to return to the initial configuration in case of a worsening of the situation.
3. Proportionality Reduction (P): Reduce the P factor by 10-20% of the current value. Observe the reaction of the process. If the oscillation decreases but the response becomes too slow, gradually increase P to an acceptable compromise.
4. Increasing integration time (I): Increase the value of I (or integration time) by 10-20%. This will reduce the accumulation of integral error.
5. Differentiation correction (D) (if used): The D component is usually not the main cause of the oscillation. If the oscillations are very fast, you can try to reduce D a bit, but be careful as this can degrade the response to fast perturbations.
6. Perform a "Step Test": Apply a small step change in the setpoint (eg ±5%) and analyze the response curve. The goal is to get fast, stable feedback with minimal overshoot.
7. Documentation: Record final settings and observations.

8.2. Restoration of Stable Supply Air Pressure

1. Isolation and LOTO: ALWAYS perform LOTO for the pneumatic line.
2. Source check: Make sure that the compressor and dehumidifier are working correctly and provide the required pressure and air quality according to DSTU ISO 8573-1:2010 (class 3.4.4 or better).
3. Checking the pressure reducer:
   • Check the reading of the pressure gauge at the outlet of the reducer.
   • Try to adjust the reducer to the desired value (usually 4-6 bar for actuators).
   • If the reducer does not hold pressure or the pressure is unstable, it may be faulty and needs to be replaced or repaired.
4. Filter Check: Inspect the filter regulator (if applicable) for contamination. Clean or replace the filter element.
5. Finding Leaks: Using a soapy solution, check all connections, fittings, hoses, and seals on the pneumatic line from the reducer to the positioner and from the positioner to the actuator for leaks. Eliminate leaks (replace fittings, hoses, FUM tape).
6. Flow capacity check: Ensure that the diameter of the pneumatic lines is sufficient to provide the required air flow to the actuator. Tubes that are too thin can restrict flow.
7. Verification: Restore air supply, check pressure stability under load.

8.3. Adjustment and Calibration of the Positioner

1. Isolation and LOTO: ALWAYS perform LOTO for valve and actuator.
2. Manual control: Put the valve in manual mode (if possible) or disconnect the signal from the control system.
3. Span Calibration:
   • Using a calibrator (Fluke 754), apply minimum (eg 4 mA) and maximum (20 mA) signals to the positioner input.
   • Make sure the positioner is calibrated for full valve travel (eg 0% to 100% open). Perform the calibration procedure according to the positioner manufacturer's instructions.
4. Parameter settings (Gain, Damping):
   • If the auto-tuning function is available, run it.
   • If manual adjustment: gradually reduce the gain of the positioner. Usually start at the default value and then decrease until the oscillations stop or are greatly reduced while still maintaining an adequate response rate.
   • Increase the damping (Damping) if the oscillation is caused by "jumping" the target position valve.
5. Testing: Perform the "Step Test" and "Dead Zone Test" to evaluate the behavior of the positioner. Dead zone should be <1%, hysteresis <2%.
6. Verification: Return the valve to automatic mode and observe its operation.

8.4. Elimination of Mechanical Friction and Backlash

1. Isolation and LOTO: ALWAYS perform LOTO for valve and actuator. Depressurize the system.
2. Dismantling the actuator: Carefully disconnect the actuator from the valve stem. Note the orientation and position of all components.
3. Manual stem check: Move the valve stem manually. It should move smoothly, without "sticking", backlash or uneven resistance.
   If friction is found:
   • Gland inspection: Check the condition of the gland seal. Replace it if it is damaged, worn, or overtightened. Use the stuffing box material recommended by the manufacturer. Tighten the gland nuts evenly to the recommended torque. DSTU ISO 15848-1:2015 regulates leaks through oil seals.
   • Stem Inspection: Inspect the stem for corrosion, scratches, deposits, or bends. Clean rod, polish (if allowed) or replace if damage is significant.
   • Inspection of the guides: Check the condition of the guide bushings. Replace if worn.
   • Valve Internal Inspection: Whenever possible, inspect the valve internals for deposits or damage that could impede valve movement.
4. Backlash Check: Check the backlash in the joints between the valve stem and the actuator stem, in the levers. Eliminate excessive play by tightening fasteners or replacing worn bushings/hinges.
5. Lubrication: Lubricate the moving parts (stem, gland, hinges) according to the valve manufacturer's instructions. Use the recommended lubricant.
6. Assembly: Assemble the actuator and valve, making sure all components are correctly installed and the fasteners are tightened to the correct torque.
7. Verification: After assembly, perform positioner calibration and "Step test".

8.5. Analysis and Adjustment of Process Interaction / Drive Selection

1. Trend Analysis: Collect trend data from the control system over an extended period (several days or weeks) including: setpoint, process parameter, controller output, valve position, valve inlet and outlet pressure/flow, and other related loop parameters.
2. Correlation detection: Analyze data for correlations between valve instability and changes in other process parameters.
3. Optimization of loops: If interaction with other loops is detected, consider optimizing the settings of the PID controllers of these loops or changing their interaction logic (for example, changing the sequence of cascade regulation).
4. Valve/actuator selection re-evaluation:
   • Refer to valve specifications and design operating conditions.
   • If the valve is constantly operating at the limit of the range (very small or very large opening), consider replacing the valve with a valve with a more appropriate characteristic (eg, equal percentage instead of linear) or a smaller size.
   • If the actuator is excessively large, this may require replacing it with a smaller one or resetting the positioner to reduce its aggressiveness.
   • Carry out the hydraulic calculation of the valve according to EN 60534.
5. Technologist Consultation: Discuss with the technologists the possibility of changing the process mode, which can reduce the disturbance that causes the valve to oscillate.
6. Verification: After making changes, monitor the process and valve for stability.

9. Precautions

10. Spare Parts and Components

For quick and effective troubleshooting, it is recommended to have a certain list of spare parts in stock. Please refer to UNITEC-D's e-Catalog to order.

Find the spare parts you need in our extended e-catalog UNITEC-D.

11. Links

• DSTU EN 1037:2006 Machine safety. Unexpected Start Prevention (EN 1037:1995, IDT)
• DSTU EN 166:2017 Individual eye protection. Technical conditions (EN 166:2001, IDT)
• DSTU EN 388:2017 Protective gloves against mechanical damage (EN 388:2016, IDT)
• DSTU EN 358:2015 Individual equipment for protection against falling from a height. Restraint systems, belts and slings for restraint (EN 358:1999, IDT)
• DSTU EN 361:2017 Individual equipment for protection against falling from a height. Insurance ties (EN 361:2002, IDT)
• DSTU ISO 9001:2015 Quality management systems. Requirements (ISO 9001:2015, IDT)
• DSTU ISO 8573-1:2010 Compressed air. Part 1. Pollutants and purity classes (ISO 8573-1:2010, IDT)
• DSTU ISO 10816-1:2004 Mechanical vibration. Evaluation of machine vibration based on the results of measurements on stationary parts. Part 1. General requirements (ISO 10816-1:1995, IDT)
• DSTU ISO 15848-1:2015 Industrial pipeline fittings. Measurement, testing and qualification of leakage to atmosphere from valve stems and flanged connections. Part 1: Classification and qualification requirements for testing typical valve designs (ISO 15848-1:2015, IDT)
• EN 60534 Adjustable industrial pipeline fittings (IEC 60534)
• OEM operation and maintenance manuals for valves and positioners (eg Emerson Process Management, Siemens, Samson, Metso Automation).
• Related UNITEC-D Maintenance Manuals: (links to future or existing manuals).