Diagnostic Manual: Vibration and Oscillation of Control Valves

This diagnostic manual has been prepared for maintenance technicians and reliability engineers. The goal is to provide a structured approach to diagnosing and solving vibration and oscillation problems in control valves, which lead to instability in process variables and increased wear of components.

1. Problem description & Scope

Control valve oscillation, often referred to as 'hunting', occurs when the valve cannot stably hold its desired position, resulting in continuous movement around the set point. This manifests itself as a fluctuating process variable (e.g. pressure, flow, temperature, level) and/or an unstable valve position. The consequences include reduced process quality, accelerated wear of valve components (gaskets, seats, stem), increased energy consumption and potential damage to other process equipment.

This manual addresses problems with various types of control valves, including globe, butterfly, ball and diaphragm valves, equipped with pneumatic, hydraulic or electric actuators and positioners. The severity of this failure can range from critical (direct impact on process safety or product quality) to major (significant wear and loss of efficiency) or minor (slight inefficiencies that will worsen over time).

2. Safety measures

WARNING: Before performing any work on control valves or their actuators, it is critical to follow all relevant safety procedures. Non-compliance can lead to serious injuries or fatalities.

• Lockout/Tagout (LOTO): Ensure that the energy sources to the valve and actuator (electrical, pneumatic, hydraulic) are fully isolated and locked/tagged. This includes the process piping if the valve is installed in a hazardous medium.
• Personal Protective Equipment (PPE): Always wear the required PPE, such as safety glasses (EN 166), hearing protection (EN 352), safety gloves (EN 388/EN 374) and safety shoes (EN ISO 20345).
• Stored Energy: Pneumatic and hydraulic actuators can store significant energy even after isolation from the supply. Always bleed the actuator and check for residual pressure. Spring return actuators contain compressed springs; be extremely careful when disassembling.
• Process hazards: Check process conditions. High temperatures, high pressures, toxic, corrosive or flammable liquids/gases may be present. Ensure adequate ventilation and compliance with ATEX guidelines where applicable.
• Lock: Prevent unintentional movement of the valve and actuator during inspection or disassembly.

3. Diagnostic Tools Required

For an effective diagnosis of valve oscillation, the following tools are essential:

4. Initial Assessment Checklist

Before beginning detailed diagnostics, perform an initial assessment to gather context and rule out potential causes:

5. Systematic Diagnosis Flow Chart

Follow this decision diagram to isolate the cause of the valve oscillation:

1. Symptom: Valve or process variable oscillates.
   1. Is the oscillation cyclical and consistent?
      • YES: Go to step 1b.
      • NO: Go to step 1d.
   2. Is the oscillation noticeable in the valve position itself (mechanical movement)?
      • YES:
        1. Check positioner diagnostics (with HART/Fieldbus communicator). Are there any error messages or warnings?
           • YES: Positioner defective, recalibrate or replace. End of diagnosis.
           • NO: Go to step 1c.
      • NO (only process variable oscillates, valve position appears stable):
        1. Verify valve feedback. Is the feedback accurate and stable?
           • YES: Process-related problem (see step 1d).
           • NO: Check positioner feedback circuit (wiring, sensor). End of diagnosis.
   3. Check the pneumatic/hydraulic supply to the positioner and actuator.
      1. Measure the supply pressure with a precision pressure gauge. Is it stable and within specifications (e.g. 5.5-6.0 bar for pneumatic)?
         • YES: Go to step 1c(ii).
         • NO:
           1. Cause: Insufficient or fluctuating supply pressure.
           2. Resolution: Check air supply unit (filter, pressure reducing valve), compressor capacity, pipe diameters.
           3. End diagnosis.
      2. Check the output pressure from the positioner to the actuator. Does it also oscillate?
         • YES:
           1. Possible Cause: Aggressive positioner tuning or positioner defect.
           2. Resolution: Perform auto-tune or manually adjust the P/I/D parameters. If problem persists, replace positioner.
           3. End diagnosis.
         • NO: Go to step 1c(iii).
      3. Perform a mechanical inspection on the valve and actuator.
         1. Are there any signs of excessive friction (sticky valve stem)?
            • YES:
              • Possible Cause: Excessive friction in stem packing, guide, or actuator.
              • Resolution: Lubricate spindle, adjust gasket (NEN-EN 12266-2), replace gasket. Check for bent spindle or damage.
              • End diagnosis.
            • NO: Go to step 1c(iv).
      4. Check the size of the actuator. Is this sufficient to move the valve against the process forces and friction?
         • YES: Go to step 1d.
         • NO:
           1. Cause: Actuator is undersized.
           2. Resolution: Replace actuator with a correctly sized model. Consult UNITEC-D e-catalog for correct specifications.
           3. End diagnosis.
   4. Process-related oscillation (when valve movement does not appear to be the primary cause).
      1. Analyze the process control parameters (PID controller) in the DCS. Are the P, I, D values ​​correctly matched for this process?
         • YES: Go to step 1d(ii).
         • NO:
           1. Cause: Incorrect PID tuning of the process controller.
           2. Resolution: Perform systematic PID tuning.
           3. End diagnosis.
      2. Are there other control loops that interact with this valve or process variable?
         • YES:
           1. Cause: Interaction between multiple control loops.
           2. Resolution: Analyze the interactions, adjust the tuning of the relevant circuits or consider disconnection if possible.
           3. End diagnosis.
         • NO:
           1. Possible Cause: Process instability (e.g. cavitation, flashing, unstable flow regime).
           2. Resolution: Analyze process design and operating conditions.
           3. End diagnosis.

6. Error Cause Matrix

This matrix ranks likely causes based on their frequency and presents associated diagnostic tests and expected results.

7. Root Cause Analysis for Each Error

7.1 Positioner Tuning (Too Aggressive or Incorrect)

Explanation: The positioner is a controller that controls the valve position based on an input signal (usually 4-20 mA). Most positioners use PID algorithms. If the gain (P component) is too high, the positioner will react too quickly and too strongly to deviations, leading to overshoot and oscillation. Too short an 'integration time' (I component) can also cause instability by amplifying small deviations. An incorrect 'derivation' (D-component) can lead to 'overshoot'.

Confirmation: Perform a step test. If the valve overshoots the desired position and oscillates several times before stabilizing, this indicates overly aggressive tuning. Positioner diagnostics via HART can show graphs of the control behavior.

Damage if left unresolved: Increased wear of all moving parts of the valve and actuator, especially gaskets and stem guides. This significantly shortens the lifespan of the equipment and increases maintenance costs. Process variables remain unstable, which affects product quality.

7.2 Excessive Friction in Valve/Actuator (Stick-Slip)

Explanation: Stick-slip is a phenomenon in which the valve rod becomes stuck and suddenly loosens, leading to jerky movements. This is caused by excessive friction in the stem gasket, a bent stem, insufficient lubrication, corrosion in the guides or an incorrectly mounted valve. The positioner tries to move the valve, but due to friction nothing happens until the force becomes large enough. The valve then shoots through its target, after which the positioner tries to adjust again and the process repeats itself.

Confirmation: Manual movement of the valve stem (after LOTO) feels jerky, or requires unusually high starting force. Vibration analysis can detect peaks at low frequencies indicative of mechanical binding. A thermal camera can reveal elevated temperatures (above 50°C or a ΔT greater than 5°C relative to ambient) at the stuffing box.

Damage if left unresolved: Accelerated wear of the valve stem, gasket and guides. This can lead to process fluid leaks, reduced valve functionality and even complete mechanical seizure. Unstable process control and increased energy consumption.

7.3 Insufficient or Fluctuating Pneumatic/Hydraulic Supply Pressure

Explanation: Positioners and actuators require stable and sufficient supply pressure to function properly. Insufficient pressure results in insufficient force to accurately position the valve, especially against process forces. Fluctuations in the supply pressure (due to a defective regulator, overloaded compressor or too thin pipes) cause direct variations in the actuator output and therefore in the valve position.

Confirmation: Measure the supply pressure directly at the positioner with a calibrated pressure gauge. A pressure below the specified minimum operating pressure (e.g. < 5.0 bar for pneumatic systems) or fluctuating by more than ±0.2 bar indicates a supply problem. Check the compressor, air supply unit and pipe diameters.

Damage if unresolved: Reduced control accuracy, leading to process instability. The actuator cannot exert sufficient force, causing the valve to fail to reach the desired position, causing wear and potential process disruptions.

7.4 Undersized Actuator

Explanation: An actuator must be able to provide sufficient force (torque) to move the valve accurately over the entire working range, taking into account process pressure differences, friction and dynamic loads. If the actuator is sized too small, it will struggle to move the valve against these forces, leading to slow response, inaccurate positioning and oscillation, especially at higher process loads.

Confirmation: Analyze the valve data (Cv value, pressure drop) and actuator calculations. Observe the valve behavior under maximum process load. If the actuator visibly struggles or cannot fully open/close the valve under the most demanding conditions, it is likely undersized. An independent agency can perform a NEN-EN 60534-2-1 compliant actuator sizing calculation.

Damage if left unresolved: The valve will never provide optimal control, resulting in inefficiency, unstable process conditions and shortened life of both the valve and actuator due to overload.

7.5 Process PID Controller Tuning (Too Aggressive or Incorrect)

Explanation: The primary process PID controller (in the DCS or PLC) controls the process variable by changing the valve position positioner setpoint. If this controller is adjusted too aggressively (too high Proportional band or too short Integration time), it can itself cause oscillations in the process variable that are then monitored by the valve. This can often be recognized by valve movements that perfectly follow the oscillation of the process variable, while the valve itself functions mechanically correctly.

Confirmation: Analyze the trends of the process variable and the valve position in the process historian. If the valve appears to follow the process oscillation and the valve position oscillation is less than that of the process variable, this indicates a process PID tuning problem. Temporarily switch the controller to manual mode and observe. If the oscillation stops, the PID tuning is the cause.

Damage if unresolved: Unstable process conditions, variations in product quality, increased energy consumption. Although the valve itself will experience less wear than with positioner tuning problems, the process will suffer.

8. Step-by-Step Troubleshooting Procedures

Perform the following procedures considering the identified root cause.

8.1 Positioner Re-tuning

1. Security: Apply LOTO. Isolate the valve from the process if necessary.
2. Connection: Connect the HART/Fieldbus communicator to the positioner.
3. Diagnostics: Read the current positioner parameters and document them carefully.
4. Auto-tune: If available, starts the positioner's auto-tune routine. Follow the manufacturer's instructions.
5. Manual Tuning: If auto-tune is not available or successful, adjust the P (Proportional), I (Integral) and D (Derivative) parameters incrementally. Start with a lower P-gain and longer I-time. Gradually increase the P-gain until the desired response is achieved without overshoot. Adjust the I time to eliminate offset. The D component is rarely used for valve positioners unless there are slow process dynamics.
6. Verification: Perform a step test and check the stability of the valve position and process variable. The stabilization time must be minimal.
7. Documentation: Record the new tuning parameters and the observed response.

8.2 Reduction of Friction

1. Security: Apply LOTO. Isolate the valve from the process and ensure a safe working environment.
2. Disassembly: Disassemble the positioner, actuator and stem gasket according to the manufacturer's instructions.
3. Inspection:
   • Check the valve stem for bending, scratches, or corrosion. A deviation of more than 0.1 mm on a 100 mm length indicates a bent spindle.
   • Inspect the stuffing box and guides for wear, corrosion or dirt deposits.
   • Check the gasket itself for aging, hardness or damage.
   • Visually check for play in the actuator rod mounting or coupling.
4. Cleaning & Lubrication: Clean all parts thoroughly. Lubricate the valve stem with a lubricant approved by the manufacturer.
5. Gasket Replacement/Adjustment: Replace worn gaskets with a new set. Adjust the gasket correctly according to NEN-EN 12266-2, ensure a good seal without excessive compression that causes friction.
6. Installation: Install the valve and actuator. Check for free movement of the spindle.
7. Verification: Perform a manual stroke test and observe the smoothness of the movement.

8.3 Stabilization of Supply Pressure

1. Safety: Apply LOTO to the air supply to the valve.
2. Control Air Supply Unit (LVE):
   • Inspect and clean the filter of the LVE. Replace the filter element if necessary.
   • Check the operation of the pressure reducing valve. Measure the pressure before and after the valve. The output pressure must be stable within ±0.1 bar of the set point. Replace defective pressure reducing valves.
   • Check the water separator; empty it if necessary.
3. Pipes & Connections: Check all pneumatic lines for kinks, damage or too small a diameter. Pressure loss across the pipes may not exceed 0.5 bar at maximum flow. Check all connections for leaks with a soapy solution.
4. Compressor system: Evaluate the capacity and stability of the main compressor. Any problems here can lead to systemic pressure fluctuations.
5. Verification: Monitor the supply pressure at the positioner over an extended period of time (minimum 1 hour) with a data logger to confirm stability.

8.4 Actuator Replacement/Upgrading

1. Security: Apply LOTO. Isolate the valve from the process.
2. Dimension Analysis: Consult UNITEC-D experts or the valve manufacturer to perform a correct actuator calculation based on the specific process conditions and valve characteristics (NEN-EN 60534-2-1).
3. Purchase: Order a correctly sized actuator via the UNITEC-D e-catalog.
4. Replacement:
   • Disassemble the existing actuator.
   • Install the new actuator according to the manufacturer's instructions, including proper coupling to the valve stem.
   • Ensure correct connection of pneumatic/hydraulic lines and electrical signals.
5. Calibration & Tuning: After installing the new actuator, calibrate the positioner and perform a tuning.
6. Verification: Test the valve over its full range and under representative process conditions to confirm stable and accurate response.

8.5 Process PID Controller Re-tuning

1. Safety: Communication with process operators is essential. Only switch the control loop to manual mode after consultation and with insight into the process impact.
2. Data analysis: Analyze the historical trends of the process variable, setpoint and valve position in the process historian.
3. Controller Mode: Switch the PID controller to manual mode.
4. Step test: Perform a step test by manually changing the valve position in a small increment (e.g. 5-10%). Record the response of the process variable.
5. Tuning Method: Apply an appropriate PID tuning method (e.g. Ziegler-Nichols, Cohen-Coon, or a model-based tuning) to determine the optimal P, I, and D values. The goal is a fast, stable response without overshoot.
6. Controller Mode: Switch the controller back to automatic mode.
7. Verification: Monitor the process variable and valve position for an extended period of time to confirm stability.
8. Documentation: Document the new PID parameters and the observed process response.

9. Preventive Measures

To prevent recurrence of valve oscillation, implement the following preventive strategies:

10. Spare Parts & Components

The timely availability of critical spare parts is essential for the rapid resolution of valve oscillation problems. Consult the UNITEC-D e-catalog for the correct specifications and ordering information.

For a complete overview of available parts and detailed specifications, visit our UNITEC-D e-catalog.

11. References

• NEN-EN 60534: Industrial process control valves
• NEN-EN 12266-2: Industrial valves - Testing of valves - Part 2: Pressure tests of valves not having pressure-retaining test requirements for the valve body
• NEN-ISO 6789: Mounting tools for screws and nuts - Torque wrenches and torque wrenches - Requirements and test methods for conformity of design and conformity of quality
• ISO 10816: Mechanical vibrations – Evaluation of machine vibrations by measurements on non-rotating parts
• ISA S75.05: Control Valve Flow Test Data
• Manufacturer specific manuals for valves, actuators and positioners.