Diagnostics and troubleshooting: "Hunting" and oscillation of control valves in industrial systems

1. Description of the problem and scope of application

"hunting" (slow, steady oscillation) and oscillation (rapid, high-frequency fluctuations) control valve problems are critical indicators of process instability and can lead to significant operational losses, equipment damage, and reduced product quality. This manual is intended to identify, diagnose, and eliminate the root causes of such malfunctions in various types of control valves, including ball, diaphragm, gate, and disk valves, used in industrial flow, pressure, level, and temperature control systems.

Problem symptoms:

• Hunting: Slow, cyclical changes in a process output parameter (for example, pressure, flow, level) around a set value, accompanied by slow but constant movement of the valve stem. Usually indicates problems with control loop setup or actuator size.
• Oscillation (Oscillation): Rapid, often high-amplitude fluctuations of the output process parameter and/or valve stem position. Can be caused by excessive controller gain, valve friction, improper valve size, or problems with the medium's dynamics (eg, cavitation).

Types of equipment covered by this guide:

• Control valves with pneumatic, electric or hydraulic actuators.
• Valve positioners (pneumatic, electropneumatic, digital intelligent).
• Sensors and transducers integrated into the control circuit.

Severity Classification:

• Critical: Uncontrolled fluctuations that cause extreme deviations in process parameters, a safety hazard, equipment destruction (eg rupture of pipelines), complete loss of control, or unplanned production shutdown. Requires immediate intervention.
• Significant: Constant "hunting" or oscillations leading to reduced product quality, increased valve and associated equipment wear, increased energy consumption, or significant degradation of process efficiency. Needs urgent repair planning.
• Minor: Periodic or low-amplitude oscillations that do not critically affect the process, but may be early signs of future problems. Monitoring and diagnostics during scheduled maintenance are recommended.

2. Precautions

CRITICAL IMPORTANT: All standard safety procedures must be strictly followed before beginning any diagnostic or repair work on control valves or related systems. Failure to follow these instructions could result in serious injury, death, or significant equipment damage.

• Lockout/Tagout (LOTO): Before any intervention, make sure that all energy sources (electrical, pneumatic, hydraulic) to the valve and actuator are isolated and blocked/tagged in accordance with the internal rules of the enterprise and the requirements of DSTU OHSAS 18001:2010.
• Protection against stored energy: Release all residual pressure from pneumatic and hydraulic actuator systems. Spring return valves can contain significant potential energy - handle them with care.
• Personal Protective Equipment (PPE): Always use appropriate PPE: safety glasses, protective gloves (chemical resistant, heat resistant as needed), safety shoes, hearing protection (if there is noise), protective clothing.
• Hazardous environments: Always check the type and properties of the process environment. Hot, cold, aggressive, toxic or flammable substances require special safety measures, including the use of breathing apparatus, gas analyzers and appropriate specialized equipment.
• Work at height: When working at height, use only certified platforms, scaffolding or safety systems according to DSTU EN 363-2007.
• Safety Distance: When testing the valve under pressure or during process operation, maintain a safe distance and use appropriate barriers to avoid exposure to environmental emissions.

3. Necessary diagnostic tools

Effective diagnosis of control valve instability requires a set of specialized tools. Below is a table with recommended equipment.

4. Initial assessment checklist

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 range of possible causes and avoid unnecessary waste of time.

5. Systematic diagnosis: Sequence of actions

This section presents a step-by-step diagnostic algorithm that will help systematically identify the cause of "hunting" or oscillations of the control valve.

1. Confirming the symptom and its characteristics:
   1. Is there oscillation or "hunting"?
      • If not → The problem is not related to this manual.
      • If yes → Record amplitude and frequency of PV oscillations and valve position.
   2. Is there noise or vibration from the valve?
      • If so → Use a vibration analyzer and noise meter. Probable cause: cavitation, flashing, friction, loose components.
2. Checking the control loop (DCS/SCADA):
   1. Put the loop into manual mode (MAN).
      • Set the constant position of the valve (for example, 50%).
      • If PV stabilizes: Probable cause: Incorrect setting of PID controller, gain (P) too high. Go to item 7 "Controller Settings".
      • If PV continues to fluctuate: Probable Cause: Mechanical valve problems, positioner or actuator problems, process problems. Go to step 3.
3. Positioner and input signal diagnostics:
   1. Measure the input current signal (4-20 mA) at the positioner terminals.
      • Use a multimeter (current measurement mode) or a current loop calibrator.
      • Expected result: A stable signal corresponding to the controller output (CO) from DCS/SCADA. Permissible fluctuations: ±0.1 mA.
      • If the signal is unstable or noisy: Probable cause: Problems with wiring, signal source, DCS output module. Check wiring, grounding. Eliminate the source of the noise.
      • If the signal is stable but the valve oscillates: Go to point 3b.
   2. Positioner calibration check:
      • If the positioner is digital → Connect a computer with software (HART modem) and perform full diagnostics and autocalibration.
      • If the positioner is pneumatic/analog → Perform manual calibration: apply reference signals of 4 mA, 12 mA, 20 mA and check the compliance of the valve position (0%, 50%, 100%).
      • Expected result: The valve accurately works the given signal, linearity within ±1% of the full stroke. The zone of insensitivity (dead band) is no more than 0.5% (for general purpose) or 0.2% (for high-precision).
      • If calibration fails or insensitivity zone is too large: Possible cause: Positioner malfunction (nozzle clogging, gasket wear, position sensor malfunction) or excessive valve/stem friction. Go to point 4.
4. Diagnostics of the executive mechanism (actuator) and pneumatic system:
   1. Checking the supply air pressure:
      • Measure the air pressure at the positioner inlet (use an accurate manometer).
      • Expected result: Stable pressure within the limits specified by the manufacturer (usually 4.0-7.0 bar). Permissible fluctuations: ±0.1 bar.
      • If the pressure is unstable or low: Probable cause: Problems with the compressor, dryer, filter, reducer or supply air line. Check the pneumatic network.
   2. Checking the output pressure of the positioner:
      • Measure the air pressure supplied by the positioner to the actuator.
      • Expected result: The pressure should change smoothly according to the input signal and valve position.
      • If the pressure is unstable or does not match the signal: Probable cause: Positioner failure or clogged pneumatic lines to the actuator.
   3. Actuator size check:
      • Compare the actual actuator size with the valve manufacturer's recommended size for the given process conditions (pressure drop, stroke).
      • If the actuator is too small: It will not be able to overcome friction or pressure drop, resulting in incomplete travel or oscillation. Probable cause: Insufficient actuator effort. → Go to point 5d.
      • If the actuator is too big: It can react too quickly, causing oscillations. Probable cause: Excessive actuator force.
   4. Checking volume boosters/relays (if installed):
      • Check for leaks, blockages, correct operation.
      • Expected result: The booster should provide fast supply/discharge of air without significant delays.
      • If the booster is faulty: Probable cause: Increased delay or insufficient reaction speed.
5. Mechanical diagnostics of the valve:
   1. Checking the friction in the stem seal (gland seal):
      • Put the valve into manual mode (MAN). Apply discrete signal steps (eg 1%, then 2%, 3%, etc.) from 0 to 100% and back while watching the rod move. Measure the pressure in the actuator.
      • Expected result: The rod should move smoothly, without jerks or sticking. The pressure in the actuator should change smoothly. Friction can be estimated by the difference in actuator pressure required to initiate up and down motion. Normal friction: no more than 1-3% of the stroke range.
      • If friction is excessive ("stick-slip"): Probable cause: Gland seal overtightened, worn/damaged packing, rod corrosion, improper lubrication. Use thermal imager to detect gland overheating.
   2. Inspection of stem attachment and connections:
      • Visually inspect all connections between valve stem, actuator and positioner. Check for backlash, loosening of bolts, wear of levers and rods.
      • Expected result: All connections must be tight, without backlash.
      • If there is play or wear: Probable cause: Loose mechanical connections.
   3. Inspection of internal components of the valve (if necessary and after full LOTO):
      • If possible and with appropriate safety procedures, perform defection of the plunger, seat, cage.
      • Expected result: Absence of significant wear, erosion, corrosion, clogging, foreign objects.
      • If damage is found: Probable cause: Mechanical wear, erosion, cavitation, clogging.
6. Process interaction analysis:
   1. Cavitation or flushing check:
      • Estimate valve pressure drop (ΔP) and downstream pressure (P2). Compare P2 with the saturated vapor pressure of the liquid at the operating temperature.
      • Symptoms: Sharp noise, vibration, erosion of internal parts of the valve, significant pressure drop.
      • If P2 ≤ saturated vapor pressure: Probable cause: Flushing (part of the liquid evaporates).
      • If P2 > saturated vapor pressure, but there is noise: Probable cause: Cavitation (formation and destruction of vapor bubbles).
   2. Problems with multiphase flow:
      • If the valve operates with gas-containing liquids or liquid-containing gases.
      • Symptoms: Unstable pressure drop, noise, vibration.
      • Probable cause: Incorrect valve selection for multiphase flow leading to instability.
   3. Wrong valve size:
      • If the valve is too big → Works at very small openings (0-20%), where its characteristic is non-linear and the gain changes dramatically.
      • If the valve is too small → Operates fully open without providing the necessary control or creates excessive pressure drop causing cavitation.
      • Probable Cause: The valve is operating outside of its optimal control range (typically 20-80% open).
7. Regulator settings (PID parameters):
   1. Trend graph analysis:
      • If the circuit was transferred to MAN and it stabilized (point 2b), then the problem is in the PID regulator settings.
      • Symptoms: Excessive gain (P), too short integration time (Ti), too long differentiation time (Td).
      • Probable cause: Aggressive setting of the PID controller.
   2. Execution of the "step test":
      • Make a small discrete change in the output signal of the regulator (CO) (for example, by 5-10%) in manual mode. Record the PV response.
      • Use tuning methods (eg, Ziegler-Nichols or improved adaptive methods) to calculate PID parameters.
      • Probable cause: Incompatible PID parameters.

6. Matrix of malfunctions and causes

This table presents the most common symptoms of control valve instability, their likely causes, diagnostic methods, and expected results.

7. Root cause analysis for each malfunction

Understanding the root cause is critical to fixing the problem, not just the symptom.

7.1. Incorrect setting of the PID controller

• Explanation: Excessive gain (P) makes the controller too sensitive to error, causing fast response and oscillation. Too short an integration time (Ti) leads to the accumulation of the integral component, which causes overregulation.
• How to confirm: Switching the circuit to manual mode stabilizes the PV. Analysis of the trend graphs shows the correlation between the PV fluctuations and the controller output signal.
• Damage if not removed: Increased valve and actuator wear due to constant motion, reduced product quality, increased energy consumption, process instability.

7.2. Excessive valve/stem friction

• Explanation: Heat in the gland seal or clogging of the internal parts of the valve causes the stick-slip effect. The valve remains stationary until the pressure in the actuator reaches a level sufficient to overcome the friction, at which point it jerks to readjust.
• How to confirm: A manual valve travel test with actuator pressure monitoring shows pressure spikes and stem jerks. The thermal imager detects local overheating of the oil seal.
• Damage if not eliminated: Increased wear of packing, rod, bushings, plunger. Possible leaks of the working environment. Damage to the actuator and positioner.

7.3. Malfunction or inaccurate calibration of the positioner

• Explanation: Clogged injectors/nozzles, worn seals, faulty position sensor or incorrect calibration can result in excessive dead band, slow response or non-linear response. Digital positioners may have internal software errors or electronic malfunctions.
• How to confirm: Linearity test and dead zone of the positioner. Diagnostics using software for digital positioners.
• Damage if not removed: Imprecise process control, increased valve wear, increased energy consumption (for pneumatics).

7.4. Problems with the executive mechanism (actuator)

• Explanation: Improper actuator size (too small or too large), diaphragm/piston leaks, worn springs, or defective actuators can cause under- or over-force, slow response, or instability.
• How to confirm: Check air/feed pressure and positioner output pressure. Visual inspection of the sources. Calculation of the required actuator effort for given process conditions.
• Damage if not corrected: Inability to control the valve, complete stoppage of the process, damage to the valve.

7.5. Mechanical backlash or loose connections

• Explanation: Any play in the connections between the valve stem, actuator and positioner (eg worn levers, loose fasteners) creates non-linearity and delays that can cause oscillation.
• How to confirm: Visual inspection and manual check of all mechanical connections for play.
• Damage if not removed: Increased component wear, reduced control accuracy, possible complete disconnection.

7.6. Cavitation or flushing

• Explanation: Cavitation occurs when the liquid pressure inside the valve drops below the saturated vapor pressure, forming bubbles that then collapse abruptly in the high pressure zone. Flashing is the partial evaporation of a liquid due to a significant drop in pressure. Both phenomena cause noise, vibration and erosion of internal valve components, which can lead to instability.
• How to confirm: Characteristic noise ("cracking", "hissing", "sand"). Analysis of the pressure drop across the valve. Using a vibration analyzer and a thermal imager. A visual inspection of the valve internals (after disassembly) reveals erosion.
• Damage if not removed: Rapid wear and destruction of internal parts of the valve, seat, plunger, body. Damage to pipelines. Significant noise and vibration affecting associated equipment.

7.7. Incorrect valve size for process conditions

• Explanation: If the valve is too large, it will operate at very small openings (e.g. 0-15%) where its response is non-linear and the relative change in flow per unit stroke is very large. This makes the control loop unstable. If the valve is too small, it will not be able to provide the required flow or will cause an excessive pressure drop, which can lead to cavitation.
• How to confirm: Valve operating range analysis by trends (CO vs position). Comparison of the valve's actual Cv/Kv with that calculated for operating conditions.
• Damage if not corrected: Impossibility of precise control, excessive wear, cavitation (for valve too small).

8. Step-by-step troubleshooting procedures

These procedures are designed to address specific root causes of valve instability.

8.1. Adjustment of PID controller settings

1. CRITICAL: Put the control loop in manual mode (MAN) and isolate the valve (if possible without stopping the process).
2. Run a "step test": change the controller output (CO) by 5-10% and record the PV response.
3. Use DCS/SCADA software to calculate new PID parameters (P, I, D) based on collected data (eg Ziegler-Nichols or Cohen-Coon methods).
4. Enter new parameters in the regulator.
5. Return the circuit to automatic mode (AUTO) gradually, monitoring the PV.
6. Verification: The PV should stabilize without oscillation, reaching the setpoint with minimal overshoot.

8.2. Elimination of excessive friction in the gland seal

1. CRITICAL: Perform a complete Lockout/Tagout (LOTO) procedure for the valve and actuator. Decompress the pipeline.

2. Visually inspect the stuffing box for visible damage or excessive tightening.
3. Loosen the gland nuts. Carefully, evenly tighten them using a calibrated torque wrench to the valve manufacturer's recommended torque (typically in the 20-50 N·m range, see OEM documentation). Do not drag!
4. Check the stem for scratches, corrosion. If necessary, replace the rod or packing. Use quality packaging that meets the EN ISO 15848-1 (Low Emissions) standard.
5. Lubricate the rod with the appropriate lubricant (if provided by the design).
6. Verification: After restoring supply pressure and removing LOTO, perform a manual valve travel test. The movement of the rod should be smooth, without jerks. The zone of insensitivity should be normal (0.2-0.5%).

8.3. Positioner calibration or repair

1. CRITICAL: Perform a complete Lockout/Tagout (LOTO) procedure for the valve and actuator.

2. Disconnect pneumatic and electrical connections.
3. For digital positioners:
   1. Connect HART modem/FF interface and computer with software.
   2. Perform the auto-calibration and diagnostics procedure (Automatic Calibration, Auto-Tune).
   3. Check parameters such as dead zone, gain, damping. Restore factory settings if necessary.
   4. Check internal diagnostic trouble messages.
4. For pneumatic/analog positioners:
   1. Check air supply, filters. Clean the injectors/nozzles.
   2. Perform manual calibration: apply 4-20mA reference signals and adjust valve position compliance (0%, 50%, 100%) using adjustment screws.
   3. Adjust the gain and damping of the positioner according to the manufacturer's recommendations.
5. Verification: After installing the positioner, apply test signals and check the linearity of the response and dead zone.

8.4. Elimination of executive mechanism problems

1. CRITICAL: Perform a complete Lockout/Tagout (LOTO) procedure for the valve and actuator. Decompress the pipeline.

2. If air leaks from the actuator are detected → Replace the diaphragm or piston seal.
3. If the actuator is incorrectly selected (too small for the required force) → Calculate the required force (moment) for the actuator according to the actual process conditions and replace it with the appropriate size. This may require consultation with UNITEC-D specialists or the valve manufacturer.
4. Check the springs for wear or damage, replace if necessary.
5. Verification: Check the movement and effort of the actuator after repair.

8.5. Elimination of mechanical backlash

1. CRITICAL: Perform a complete Lockout/Tagout (LOTO) procedure for the valve and actuator.

2. Inspect all bolted connections, levers, rods, fingers, bushings.
3. Tighten the loosened bolts with a torque wrench to the recommended tightening torque (see the manufacturer's documentation).
4. Replace worn parts (hubs, fingers, levers) that create backlash.
5. Verification: Check the absence of backlash manually. Carry out a test on the linearity of the valve position.

8.6. Fight against cavitation and flashing

1. CRITICAL: If intervention is required inside the valve, perform a complete Lockout/Tag (LOTO) procedure and decompress the piping.

2. Process analysis: View working conditions. Is it possible to change the pressure drop across the valve (for example, by changing the pump mode, installing an additional pressure controller)?
3. Valve Upgrade: Consider installing anti-cavitation or low-noise trim. These are the internal components of the valve that change the flow characteristics, preventing the pressure from falling below the critical point. UNITEC-D offers a wide range of such components.
4. Valve Replacement: If retrofitting is not possible or insufficient, it may be necessary to replace the valve with a type more suited to cavitation/flushing conditions (eg special plunger designs, multi-stage trims).
5. Verification: Monitoring of noise, vibration, PV and pressure drop after intervention. Absence of characteristic noises and vibrations.

8.7. Adjusting the size of the valve

1. Carry out a careful engineering calculation of the required Cv/Kv value for the valve based on minimum, normal and maximum flow and the corresponding pressure drops.
2. Compare the calculated Cv/Kv with the Cv/Kv of the installed valve.
3. If the valve is too large → Replace with a smaller one that will work in the range of 20-80% of its stroke.
4. If the valve is too small → Replace with a larger one.
5. Verification: After replacing the valve, test in working mode. The valve should operate in the range of 20-80% opening, providing stable control.

9. Preventive measures

10. Spare parts and components

Timely replacement of worn or damaged components is key to maintaining the stability and longevity of control valves. You can find all necessary spare parts and accessories in the UNITEC-D electronic catalog.

Find all necessary spare parts and accessories for your control valves in the UNITEC-D electronic catalog!

11. Links

• DSTU EN 60534-1:2018 (EN 60534-1:2018, IDT) Industrial technological control valves. Part 1. Terminology and general technical characteristics.
• DSTU EN 60534-2-1:2018 (EN 60534-2-1:2018, IDT) Industrial technological control valves. Part 2-1. Bandwidth. Formulas for calculating the size of valves for technological environments.
• DSTU ISO 5208:2016 (ISO 5208:2015, IDT) Industrial fittings. Pressure testing of metal valves.
• DSTU EN ISO 15848-1:2020 (EN ISO 15848-1:2015, IDT) Industrial fittings. Measurement, testing and qualification of emissions of fugitive substances. Part 1. Classification of external leakage systems for valves.
• OEM operation and maintenance manuals (eg Emerson Process Management, Siemens, Samson, Metso).
• Related UNITEC-D Service Manuals: "Diagnosis and Troubleshooting of Pneumatic Control Systems", "Calibration and Maintenance of Digital Positioners".