Diagnostics and elimination of surge and oscillations of control valves

1. Description of the problem and scope of application

This manual is intended for diagnosing and troubleshooting control valve surges or oscillations in industrial systems. Valve pumping is the erratic, often cyclical movement of the valve stem caused by internal valve/positioner malfunctions or process interaction. Oscillations are rapid, often irregular changes in valve position. These phenomena can lead to:

• Significant wear of internal valve components, seals and actuator.
• Loss of control over the technological process, which affects product quality and production efficiency.
• Increased energy consumption due to inefficient regulation.
• Damage to associated equipment (pumps, compressors, pipelines) due to hydraulic shocks or pressure changes.
• Increasing the risks of emergency situations.

Types of equipment affected: All types of control valves, including ball, diaphragm, gate, butterfly, and gate valves, used in the oil and gas, chemical, food, and power industries.

Severity Classification:

• Critical: An uncontrolled surge that leads to an emergency shutdown of production or a direct threat to safety. Immediate intervention is critical.
• Significant: A constant surge or fluctuation that leads to a significant decrease in product quality, increased resource consumption, and accelerated wear and tear of equipment. Requires immediate attention.
• Minor: Intermittent or small fluctuations that cause small deviations in the process, but do not threaten safety or shutdown. Requires planning of corrective actions.

2. Precautions

🛡 ⚠ SAFETY WARNING ⚠ 🛡
Before starting any work on control valves, the following steps must be taken:

• Lockout and marking (LOTO): Isolate the valve from energy sources (electrical, pneumatic, hydraulic) and lock it in a safe position according to internal company procedures and DSTU EN requirements ISO 14118.
• Personal Protective Equipment (PPE): Always use appropriate PPE, including safety glasses, gloves, hard hat and protective clothing.
• Stored Energy: Be careful with drive springs and other items that contain stored energy. Follow the manufacturer's instructions for safe disassembly and assembly.
• Decompressing the system: Ensure that the system in which the valve is installed is completely depressurized and drained to avoid release of hazardous substances or injury from high pressure.
• Hazardous substances: Check the type of liquid/gas being transported and take extra precautions if it is corrosive, toxic, flammable or has a high temperature.

Failure to follow these instructions could result in serious injury or death.

3. Necessary diagnostic tools

4. Initial evaluation checklist

Before starting a detailed diagnosis, you should collect the following information and conduct a visual inspection. This will help localize the problem and speed up further analysis.

5. Systematic diagnostics (decision making scheme)

1. Symptom: Control valve surges or oscillates.
   1. Check positioner:
      1. Is the positioner calibrated?
         • NO: Perform a full positioner calibration using a calibrator.
         • YES: Go to the next step.
      2. Do the positioner settings (GAIN, DAMPING, FILTER) meet the process conditions?
         • NO: Optimize the settings. Gradually reduce GAIN, increase DAMPING, if necessary, increase filter time.
         • YES: Check the mechanical condition of the positioner.
      3. Does the positioner work correctly (no backlash, leaks, dirt)?
         • NO: Consider repairing or replacing the positioner.
         • YES: Go to drive check.
   2. Actuator Check:
      1. Does the actuator size match the valve and process requirements (required force/torque)?
         • NO: Perform the calculation and, if necessary, replace the actuator with an appropriately sized actuator.
         • YES: Go to the next step.
      2. Is the actuator diaphragm or seal intact with no leaks?
         • NO: Replace the diaphragm/seal.
         • YES: Check the actuator springs.
      3. Do the drive springs have the correct stiffness and are they not damaged?
         • NO: Replace the springs.
         • YES: Go to valve friction check.
   3. Valve friction diagnosis:
      1. Is there excessive friction in the stuffing box or guide rod?
         • NO: Go to next step.
         • YES: Reduce stuffing box tension (if possible without leaking) or replace stuffing box/rings. Check and, if necessary, replace the guides.
      2. Is there any evidence of corrosion, deposits, or mechanical damage to the valve stem/plunger?
         • NO: Go to Process Interaction Analysis.
         • YES: Repair or replace valve internals.
   4. Analysis of interaction with the process:
      1. Are the parameters of the PID controller set optimally for this circuit?
         • NO: Retune the PID controller, starting with low values ​​of the coefficients, gradually increasing them until stability is achieved.
         • YES: Check process dynamics.
      2. Is there instability arising from the process itself (eg, pulsations, fluid phase change, unstable flow meter)?
         • NO: Consider a combination of several small causes, or sensor malfunctions.
         • YES: Solve the problem at the process level or consider using more advanced control algorithms (eg, adaptive PID).

6. Malfunction-cause matrix

7. Analysis of the root causes of malfunctions

7.1. Incorrect positioner setting

Why this happens: Positioners have parameters such as GAIN, DAMPING and FILTER. Incorrectly setting them can lead to excessive sensitivity (high GAIN) or too slow response (high DAMPING). Aggressive settings lead to rapid, uncontrolled movements (pumping), while excessive damping can mask other problems.

How to confirm: Use a positioner calibrator or specialized software (eg ValveLink™) to check the settings. Perform signal step test and analyze valve response (overshoot, settling time, stability).

Damage if not repaired: Accelerates wear of seals, stem, plunger, valve seats due to constant movement. Noise and vibration are increasing. May cause internal component damage and leaks.

7.2. Excessive friction in the gland assembly or guide rod

Why this happens: Over time, the stuffing box can wear out, harden, or be over-tightened. Dirt, crystalline deposits, or corrosion may also accumulate on the stem and guides. This creates variable friction that is difficult for the positioner to compensate for, leading to instability and surge.

How to confirm: After isolating and decompressing the valve, disconnect the actuator from the stem and try to manually move the stem. Any binding, uneven resistance, or significant effort indicates a friction problem. Use a dynamometer to quantify the friction force. Permissible friction usually does not exceed 10-15% of the nominal force of the drive.

Damage, if not eliminated: Accelerates the wear of the rod, stuffing box, guide bushings. The likelihood of leaks through the stuffing box increases. Adjustment accuracy decreases.

7.3. Improper drive (actuator) size

Why this happens: The actuator may be too small or too large for the valve and process conditions. An actuator that is too small cannot provide enough force to overcome frictional forces and pressure drop. A drive that is too large can be too sensitive or create too much momentum, contributing to surge, especially with a suboptimal positioner setting.

How to confirm: Recalculate the required force/torque of the actuator, taking into account the maximum pressure drop, oil seal friction and other loads. Compare with the rated effort of the installed drive. Check the pressure in the actuator chambers during operation - if the pressure is unstable or fluctuates excessively, this may indicate a mismatch.

Damage if not repaired: The drive is working at the limit of its capabilities, resulting in accelerated wear. Failure of a valve to reach or maintain a set position affecting process control.

7.4. Positioner defect

Why this happens: Mechanical damage (wear of levers, position sensor), contamination of pneumatic channels, air leaks through membranes or seals, electronic failures. This results in incorrect valve position determination or inaccurate control of air pressure to the actuator.

How to confirm: Carry out a full diagnosis of the positioner using a calibrator, check all its functions, calibration, linearization. Visually inspect for leaks and wear. Check the electrical signals of the position sensor.

Damage if not repaired: Complete loss of valve control, unpredictable movement, high wear on valve and actuator.

7.5. Instability of the control loop (PID controller)

Why this happens: If the parameters of the PID controller (proportional, integral, differential) are set too aggressively or do not correspond to the dynamics of the process, the controller can overreact to the slightest deviations, causing the valve to constantly move and causing oscillations in the circuit.

How to confirm: Analyze the trends of technological parameters (PV - current value, SP - set value, OP - controller output signal) during a certain period. Fluctuations in PV correlated to OP indicate a problem with the PID controller. Perform a signal step test by changing SP or OP and evaluate the system response.

Damage if not repaired: Decreased product quality, increased energy consumption, accelerated valve and actuator wear due to continuous operation.

8. Step-by-step troubleshooting procedures

8.1. Correction of positioner settings

1. Isolation and Security: Perform the LOTO procedure, provide access to the valve.
2. Connecting the calibrator: Connect a positioner calibrator (eg Beamex MC6) to the signal terminals and pneumatic ports of the positioner.
3. Zero Point and Span Calibration: Perform automatic or manual 0% and 100% valve travel calibration. Make sure that the "dead zone" is no more than 0.2-0.5% of the full stroke.
4. GAIN Optimization: Start with the GAIN value recommended by the manufacturer or a low value. Gradually increase the GAIN until slight oscillations appear, then decrease it by 10-20% of this value.
5. Optimizing DAMPING: Start with the DAMPING value recommended by the manufacturer or a low value. Gradually increase DAMPING to reduce overshoot, but avoid slowing down the response too much.
6. FILTER settings: Apply filter (increase filter time) only if the input signal of the valve is affected by high-frequency noises that cause small oscillations.
7. Linearization Check: Perform a linearization test by checking that the control signal matches the actual rod position over the full range. The deviation should not exceed ±1% of the full stroke.
8. Verification: Remove the calibrator, return the system to working condition (after removing LOTO). Monitor valve and process stability.

8.2. Elimination of friction

1. Isolation and Security: Perform the LOTO procedure, system decompression.
2. To disconnect the actuator: Disconnect the valve stem from the actuator.
3. Friction Rating: Manually move the valve stem through the entire travel range. Estimate the required effort (a dynamometer can be used). Permissible friction: <10-15% of the nominal drive force.
4. Gland packing/ring replacement: If friction is high, carefully loosen the gland assembly. If this does not help, replace the stuffing/rings according to the valve manufacturer's instructions. Use high-quality materials that meet the operating conditions (temperature, environment).
5. Inspection of guides: Inspect guide bushings and rod for wear, scratches, corrosion. Replace damaged items. Ensure proper clearances between stem and guides.
6. Cleaning and Lubrication: Clean the rod of deposits, apply the appropriate lubricant recommended by the manufacturer (if provided).
7. Assembly and verification: Assemble the valve, connect the actuator. Check the movement of the rod with the actuator. Recheck the positioner.

8.3. Replacing the drive (actuator)

1. Isolation and Security: Perform the LOTO procedure, system decompression.
2. Determining Required Force: Based on the maximum pressure drop across the valve, packing friction, and other factors (using the manufacturer's data), calculate the required force or torque for the actuator.
3. Actuator Selection: Choose an actuator with the appropriate effort margin (25-50% margin recommended) and response speed. Pay attention to the type (pneumatic, electric, hydraulic) and type of action (direct, reverse).
4. Dismantling the old actuator: Carefully disassemble the existing actuator, following the manufacturer's instructions and taking into account the stored energy (springs).
5. Installing the new actuator: Install the new actuator, ensuring the valve stem and actuator are properly aligned. Tighten the fasteners to the recommended torque.
6. Connection and calibration: Connect the positioner to the new actuator and perform a full calibration of the positioner with the new range and characteristics.
7. Verification: Return the system to working state (after removing LOTO). Check the stability of the valve operation over the entire range.

8.4. Optimization of PID controller settings

1. Trend Analysis: Collect process behavior data (PV, SP, OP) before and during fluctuations.
2. Manual mode: Put the controller in manual mode.
3. Adjusting the proportional factor (P): Start with a low value of P. Gradually increase P by observing the PV response to the OP step. Stop when the PV starts to fluctuate. Reduce P by 30-50% of this value.
4. Integral time (I) settings: Gradually decrease I (increase integral action) to eliminate static error but not cause slow oscillations.
5. Differential Timing Adjustment (D): Apply D carefully to reduce overshoot and speed up response, but avoid being overly sensitive to noise.
6. Verification: After each step, observe the system's response to SP changes or external disturbances. Aim for quick setup without over-adjustment or wobble.

9. Precautions

10. Spare parts and components

Critical spare parts must be available for effective troubleshooting and prevention. UNITEC-D GmbH offers a wide range of components for control valves.

Look for these and other components in our online UNITEC-D catalog.

11. Links

• DSTU EN ISO 9001:2018 Quality management systems. Requirements
• DSTU EN ISO 14118:2023 Machine safety. Prevention of unexpected start.
• DSTU EN 60534-2-1:2020 Industrial control valves. Part 2-1. Bandwidth.
• Operation and maintenance manuals from valve manufacturers (eg Emerson, Samson, Fisher, KOSO).
• UNITEC-D: Other maintenance manuals.