Troubleshooting Guide: Diagnosis and Resolution of Oscillation and 'Hunting' in Industrial Control Valves

1. Problem Description and Scope

This technical guide addresses the identification and resolution of instabilities known as oscillation and hunting in industrial control valves. Such phenomena compromise the stability of the controlled variable, resulting in inconsistent product quality, increased energy consumption, excessive mechanical stress on the valve and actuator components, and a drastic reduction in the useful life of the equipment. Oscillation is characterized by cyclical and sustained variations of the process variable, while 'hunting' describes a continuous and erratic movement of the valve stem in search of an equilibrium point that is never reached in a stable manner.

Symptoms of oscillation and 'hunting' can affect a wide range of industrial equipment where control valves are employed, including systems controlling flow (fuels, water, chemicals), pressure (steam, gases, liquids), level (tanks, reactors) and temperature (heat exchangers, furnaces). Early detection and accurate diagnosis are critical to avoid unscheduled downtime and optimize operational efficiency.

• Severity Rating:
• Critical: Uncontrolled oscillation that prevents process operation or causes imminent safety hazards. Requires immediate intervention.
• Major: Significant degradation of product quality, substantial increase in energy consumption, or accelerated wear of components. Considerable economic impact.
• Minor: Intermittent or small-amplitude instability that, although it does not cause immediate failure, indicates a condition of degradation and can evolve into more serious problems if not corrected.

2. Safety Precautions

CRITICAL WARNING: Before beginning any diagnostic or maintenance procedure on a control valve, it is mandatory to completely isolate the valve and actuator from all sources of energy and pressure. Strictly follow the Lockout/Tagout (LOTO) procedure established by NBR 5410 and NR-10 (Safety in Electrical Installations and Services). Failure to comply with this guideline could result in serious injury or death.

DANGER FROM ACCUMULATED ENERGY: Pneumatic and hydraulic actuators can contain energy stored in springs or fluids under pressure, even after isolation of the supply line. Be sure to release all residual pressure from the actuator and valve body, using the appropriate relief or drain valves, before any disassembly. Check with pressure gauges that the pressure is zero.

MANDATORY PPE: Always use Personal Protective Equipment (PPE) appropriate for the task, according to NR-6. This includes, but is not limited to: safety glasses with side protection, protective gloves (mechanical, thermal or chemical, depending on the risk), helmet, hearing protection (if the environment is noisy) and safety shoes with steel toes. For corrosive or high temperature fluids, wear full face protection and chemical/heat resistant clothing.

HOT AND/OR DANGEROUS FLUIDS: Always check the temperature and chemical nature of the process fluid. High temperature fluids can cause serious burns. Toxic, flammable or corrosive liquids or gases require adequate ventilation and specific containment procedures. Consult the Chemical Product Safety Data Sheet (MSDS) for each substance present in the system.

3. Required Diagnostic Tools

The accuracy of the diagnosis depends on the use of appropriate and calibrated tools. Below is a table with the essential instruments:

4. Initial Assessment Checklist

Before beginning the in-depth diagnosis phase, collect vital information to direct the investigation. This checklist helps you compile history and recent operating conditions.

5. Systematic Diagnosis Flowchart

This flowchart guides the technician through a logical sequence to identify the source of the oscillation or 'hunting'.

1. Identify the Symptom: Oscillation or 'Hunting' in the Process Variable (PV) and/or Valve Output (OP/MV).
2. Problem Isolation: Valve Manual Mode
   1. Place the valve in manual mode (via DCS/SCADA or positioner, if applicable).
   2. Keep the valve position fixed (e.g. 50% opening).
   3. NOTE:
      1. If the Process Variable stabilizes: The probable cause is in the control loop (PID Controller) or in the interaction with the process. Proceed to 6.3.
      2. If the Process Variable continues to oscillate or the valve stem presents 'hunting' even in manual mode: The likely cause is in the Positioner, Actuator, or in the Valve itself (body/trim). Proceed to 5.1.
3. Positioner Diagnosis (If instability persists in manual):
   1. Calibration Check:
      1. Use the HART communicator or manufacturer's software.
      2. Perform a full calibration test (0-100% input vs. position output).
      3. RESULT:
         1. Out of specification calibration: Adjust the calibration. If the problem persists, proceed to 6.1 (Inadequate Positioner Tuning).
         2. Correct calibration: Proceed to 5.1.2.
   2. Positioner Tuning Check (Gain and Damping):
      1. With the positioner in service/test mode, apply a step signal (ex: 40% to 60% and vice versa).
      2. Monitor valve stem response (speed, overshoot, stabilization).
      3. RESULT:
         1. Fast response with excessive overshoot (fast oscillation): The gain may be too high. Reduce gain or increase damping.
         2. Slow response or 'hunting' with small variations: Low gain or insufficient damping. Adjust according to manufacturer's guidelines.
   3. Internal Leaks Check (Positioner):
      1. With the air supply and input signal applied, block the air outlet to the actuator.
      2. Monitor the positioner output pressure to the actuator with a pressure gauge.
      3. RESULT:
         1. Rapid pressure drop: Indicates internal leakage in the positioner. Consider positioner repair or replacement.
4. Actuator Diagnosis (If the positioner is OK, or the problem persists):
   1. Check Friction on the Actuator Stem and Seal:
      1. Apply a very small step signal (ex: 0.5% variation) to the positioner and observe the stem response.
      2. Alternatively, remove the positioner and move the rod manually (if possible safely and without process pressure).
      3. RESULT:
         1. Irregular movement, 'stick-slip' (stick-and-peel), or difficulty initiating movement: Indicates excessive friction. Proceed to 6.2.
   2. Actuator Diaphragm Leak Check:
      1. With the actuator pressurized and in a stable position, disconnect the supply air line and listen for leaks.
      2. Soap water can be applied to the joints to identify microleaks.
      3. RESULT:
         1. Audible or visible leak: Damaged diaphragm. Requires diaphragm replacement or actuator repair.
5. Valve Diagnosis (Body/Trim – If actuator and positioner OK, or problem persists):
   1. Trim Wear Check (Seat, Plug, Cage):
      1. If possible, visually inspect the trim after isolating and draining the process.
      2. RESULT:
         1. Excessive wear, erosion, visible cavitation: Trim is no longer controlling flow effectively. It could be a sizing or cavitation issue. Proceed to 6.3 or 6.5.
   2. Check for Obstructions in the Valve Body:
      1. Internal visual inspection (if possible and safe).
      2. RESULT:
         1. Presence of debris, scale: Cleaning and inspection.
   3. Valve Sizing Check:
      1. Compare actual operating conditions (flow, differential pressure) with valve design data.
      2. Calculate the Cv required for current conditions and compare with the nominal Cv of the valve.
      3. RESULT:
         1. Nominal CV too high for the actual flow (oversized valve): The valve operates at very small openings, making control difficult and causing instability. Proceed to 6.3.
         2. Very low rated hp (undersized valve): The valve operates 100% open with no controllability. Proceed to 6.3.
6. PID Controller Diagnosis and Interaction with the Process (If the PV stabilized in manual mode):
   1. PID Controller Tuning Check:
      1. Analyze the trend curves of the PV (Process Variable), MV (Manipulated Variable) and the Setpoint (SP) in the DCS/SCADA.
      2. RESULT:
         1. Slow, cyclical oscillation, with MV following PV with delay or excessive gain: PID tuning is inadequate (Proportional Gain too high, Integral Time too low). Proceed to 6.1.
   2. Loop Interaction Check:
      1. In complex systems, one control loop can influence another, causing instability.
      2. RESULT:
         1. Unstable adjacent loop: Optimize tuning or reconfigure adjacent loop.
   3. Process Disturbances:
      1. Significant and rapid fluctuations in load, supply pressure, temperature, or fluid composition that the controller cannot compensate for.
      2. RESULT:
         1. Evidence of external disturbances: Identify and stabilize the source of the disturbance. Consider implementing feedforward control.

6. Failure-Cause Matrix

This table presents a consolidated view of symptoms, probable causes, diagnostic tests and expected results.

7. Root Cause Analysis for Each Failure

Understanding the underlying cause is essential for a lasting solution and implementing preventative measures.

7.1. Inadequate Positioner Tuning

• Why It Occurs: The tuning parameters (gain, integral time, derivative time) of the positioner are not adjusted for the specific dynamics of the actuator and valve. Too high a gain can cause over-response and rapid oscillation, while too low a gain can result in slow response and 'hunting'. Incorrect damping parameters also contribute to instability.
• How to Confirm: Perform a dynamic response test on the positioner, applying a step signal (ex: 40% to 60% and 60% to 40% of the control range) and recording the position of the rod over time. A poorly tuned positioner will exhibit excessive overshoot/undershoot, sustained oscillation, or a very long settling time.
• Damage if Unresolved: Premature wear of the positioner (due to excessive movements), actuator and valve trim. Persistent instability of the process variable, compromising product quality and plant efficiency. Increased instrumentation air consumption.

7.2. Excessive Friction on Actuator or Stem Seal

• Why It Occurs: Friction in the valve stem or actuator is a major contributor to hunting. It can be caused by: lack of stem lubrication, hardened, overtightened or damaged gasket seal, bent valve stem, worn guide bushings or misalignment. Friction creates a 'dead-band' where the positioner needs to apply significantly greater force to initiate rod movement, resulting in a 'stick-slip' movement rather than a smooth, proportional movement.
• How to Confirm: Perform a 'dead-band' test on the positioner or apply a very small step signal (ex: 0.5% to 1%) and observe whether the rod moves smoothly or in jumps. A slow step-response test (for example, varying the valve position from 20% to 80% in 20 seconds) may reveal points of increased resistance. Vibration analysis on the actuator may indicate excessive friction.
• Damage if Unresolved: Loss of fine control of the valve, resulting in oscillation and 'hunting'. Accelerated wear of the stem, seal and bushings, leading to external leaks and mechanical failure. Greater air consumption to overcome resistance.

7.3. Incorrect Valve Sizing

• Why It Occurs: A control valve is considered poorly sized when its flow coefficient (Cv) is not suitable for the operational conditions of the process.
• Oversized valve: The Cv is very high, causing the valve to operate most of the time at very low openings (e.g. below 10-20%). In this range, valve authority (ability to influence the process) is poor and fine control is extremely difficult, leading to instability.
• Undersized valve: The Cv is very low, causing the valve to constantly operate fully open (e.g. above 80-90%), without the ability to modulate the flow to meet process variations. Although it does not cause 'hunting' in itself, it impedes effective control.
• How to Confirm: Review valve design and specifications. Calculate the Cv required for real operating conditions (maximum and minimum flow, inlet and outlet pressures) and compare with the nominal Cv of the installed valve. Analyze valve position history in DCS; if it consistently operates at extremes, scaling is likely root cause.
• Damage if Unresolved: Chronic process instability. For oversized valves, wear concentrated at small openings is common. Cavitation and 'flashing' can be exacerbated if pressure conditions are marginal. Need to replace valve or trim.

7.4. Instrumentation Air Problems

• Why It Occurs: The quality and stability of instrumentation air are crucial for the operation of valves with pneumatic actuators and positioners. Common problems include: contamination by water (high dew point), oil or particles (rust); insufficient air pressure or excessive fluctuations; saturated filters; inoperative air dryers; or defective pressure regulators. Fluctuations in supply pressure can simulate an erratic control signal.
• How to Confirm: Continuously monitor the air pressure at the positioner inlet with a calibrated pressure gauge. The pressure must be stable within the range specified by the manufacturer (generally 1.4 to 7 bar). Drain the regulator filter and observe the presence of water or oil. Collect air samples for quality analysis (dew point, oil content).
• Damage if Unresolved: Premature positioner failure due to plugged orifices and internal corrosion. Erratic and unstable valve response. Reduction in the useful life of pneumatic components.

7.5. Cavitation and 'Flashing'

• Why It Occurs: These phenomena are caused by an excessive pressure drop across the valve.
• Cavitation: Occurs when the pressure inside the valve drops below the vapor pressure of the fluid, causing the formation of vapor bubbles that subsequently collapse violently as the pressure recovers downstream.
• Flashing: Occurs when the pressure downstream of the valve remains below the vapor pressure of the fluid, resulting in continuous vapor formation.
• Both phenomena generate vibration, noise and can induce oscillation, in addition to causing severe damage.
• How to Confirm: Visual inspection of the valve trim will reveal characteristic erosion, such as a 'sanded look' or pitting. Noise analysis with a decibel meter will show high noise levels (generally above 85 dB, reaching up to 120 dB) with a 'gravel' (cavitation) or 'jets' (flashing) sound. The thermal camera can identify abnormal cold (flashing) or hot (cavitation erosion) spots.
• Damage if Unaddressed: Severe and accelerated erosion of trim, valve body and downstream piping. Intense vibration of the piping and valve, which can lead to component fatigue. Excessive noise, representing an occupational hazard. Loss of valve controllability.

7.6. Inadequate PID Controller Tuning

• Why It Occurs: Similar to the positioner, the tuning (proportional gain P, integral time I, derivative time D) of a PID (Proportional-Integral-Derivative) controller in a process control system may be inadequate. A gain P that is too high or a time I that is too low often results in slow and sustained oscillations of the process variable, as the controller overreacts to the variations or is unable to eliminate the error in time.
• How to Confirm: Analyze the trends of the Process Variable (PV), Setpoint (SP) and Manipulated Variable (MV) in the DCS/SCADA system. If the PV and MV exhibit slow, cyclical oscillations without the PV stabilizing around the SP, controller tuning is the likely cause. Setpoint step tests can be used to evaluate the response of the control loop.
• Damage if Unresolved: Continuous fluctuations in the process variable, impacting the quality of the final product, energy efficiency and raw material consumption. Unnecessary stress on downstream equipment.

8. Step-by-Step Resolution Procedures

The following corrective actions must be implemented after identifying the root cause, always with appropriate safety precautions (LOTO and PPE).

8.1. Positioner Retuning

1. WARNING: Apply LOTO and release all pressure from the actuator before any mechanical intervention.
2. Connect the HART communicator or manufacturer's diagnostic software to the positioner.
3. Run the self-calibration or auto-tuning routine, if available.
4. If automatic tuning is not effective, manually adjust the positioner gain and damping parameters. Start with a lower gain and gradually increase it, observing the response to small steps of input signal. Add cushioning to smooth the response.
5. Check the tightness of the air lines leaving the positioner for the actuator and the calibration of the pressure gauges.
6. Recalibrate the positioner to ensure linearity between the input signal and the position of the rod.
7. Test the dynamic response of the valve over the entire operating range.

8.2. Excessive Friction Approach

1. WARNING: Apply LOTO and release all pressure from the actuator and process.
2. Inspect the valve stem and guide bushings for warping, corrosion or wear.
3. Check alignment between actuator and valve body.
4. Clean and lubricate the valve stem with lubricant compatible with the process fluid and seals.
5. Inspect and, if necessary, replace the gasket seals. Make sure the gasket is not overtightened. Use a calibrated torque wrench to apply the torque specified by the manufacturer when assembling the gasket, in accordance with ABNT NBR ISO 15848-1 for fugitive emissions.
6. Perform a 'dead-band' test and a slow 'step-response' to check the smoothness of the movement.

8.3. Valve Sizing Correction

1. WARNING: Apply LOTO and release all pressure from the actuator and process.
2. If the sizing analysis confirms that the valve is oversized for the operating conditions, the options are:
• Replace the valve with one with suitable Cv.
• Replace the valve trim with one with a lower CV.
• Install an upstream flow reducer (if appropriate and with engineering analysis).
3. If the valve is undersized, the only effective solution is to replace it with a valve with a higher Cv.
4. Consult process engineering to validate any sizing changes.

8.4. Improving Instrumentation Air Quality

1. Inspect the instrumentation air system upstream of the positioner.
2. Check operation of air dryers. Monitor the dew point (ideally below -40°C as per ISA S7.3).
3. Drain the coalescing and regulating filters. Replace the filter elements according to the maintenance plan.
4. Test the positioner supply air pressure. If fluctuations occur, inspect the pressure regulator and supply line for leaks or restrictions.
5. Ensure air lines are free from kinks or obstructions.

8.5. Cavitation Mitigation and 'Flashing'

1. WARNING: Apply LOTO and release all pressure from the actuator and process.
2. If inspection confirms cavitation or 'flashing', the following solutions can be applied:
• Replace the valve trim with an anti-cavitation or anti-flashing trim. These trims have geometries that dissipate the energy of the fluid in multiple stages, increasing the pressure and preventing the formation of vapor.
• Reduce the pressure differential across the valve, if process conditions allow (e.g. relocating the valve, changing pump setpoints).
• If it is a level control valve, consider changing the injection or drain point.
• In extreme cases, replacing the valve with a more robust type of valve or with specific technology for these conditions may be necessary.

8.6. PID Controller Retuning

1. WARNING: Make sure the control valve and positioner are operating stably before tuning the PID.
2. Use the DCS/SCADA system tuning tools.
3. Start with a classic tuning method (Ziegler-Nichols, Cohen-Coon, or auto-tune if available).
4. Adjust the parameters P (Proportional), I (Integral) and D (Derivative) step by step, monitoring the process response to small steps in the setpoint.
5. For slow oscillations, it is generally necessary to reduce the Proportional Gain (P) or increase the Integral Time (I).
6. Document the tuning parameters.

9. Preventive Measures

Prevention is the most effective strategy to avoid the recurrence of oscillation and 'hunting' problems, increasing the reliability and useful life of assets.

10. Spare Parts and Components

Having the correct replacement parts in stock is critical to minimizing downtime during troubleshooting.

To find the exact parts for your control valve and actuator, explore our complete online catalog:

Visit the UNITEC-D E-Catalog

11. References

• ABNT NBR ISO 9001: Quality management systems – Requirements.
• ABNT NBR 5410: Low voltage electrical installations.
• NR-10: Safety in Electricity Installations and Services (Ministry of Labor and Employment of Brazil).
• NR-12: Occupational Safety in Machines and Equipment (Ministry of Labor and Employment of Brazil).
• ISA S5.1: Instrumentation Symbols and Identification.
• ISA TR20.00.01: Specification Forms for Process Measurement and Control Instruments, Control Valves, and Regulators.
• Operation and Maintenance Manuals specific to the control valve manufacturer (e.g. Fisher, Emerson, Samson, KSB, Neles).
• Other UNITEC-D maintenance guides on instrumentation and control.