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Diagnosis and elimination of vibrations and shocks in control valves: positioninger setup, actuator selection, analysis of friction and process

Technical analysis: Troubleshooting control valve hunting and oscillation: positioner tuning, actuator sizing, friction

Diagnosis and elimination of vibrations and shocks in control valves: positioninger setup, actuator selection, analysis of friction and process

1. Problem Description and Area of Application

This manual is intended for engineers and technical service personnel of industrial enterprises in Ukraine. It covers systematic diagnosis and elimination of malfunctions related to cyclic vibrations, hunting and self-oscillations of pneumatic and electric control valves.

The problem affects processes in the energy, chemical, oil and gas, and metallurgical sectors, where accuracy of flow, pressure or temperature regulation is critical for safety and product quality. Instability of the valve stem position causes stuffing box seal wear, piston and seat damage, as well as technological losses due to suboptimal control valve operation.

Classification of criticality level:

  • Critical (Class A): Auto-oscillation of the level control unit in the drum of the boiler, fuel gas consumption or main pressure. Causes tripping of protections, emergency shutdown of the unit or discharge of non-conforming product. Requires immediate intervention.
  • Significant (Class B): Fluctuations in auxiliary circuits (cooling water, superheated steam), leading to reduced process efficiency and accelerated valve wear. Resolution during the next scheduled shutdown window.
  • Minimum (Class C): Slight vibration of the positioning device or short-term fluctuations when changing the task. Does not directly affect the process, subject to monitoring.

2. Safety Measures

WARNING: Before starting any diagnostic or maintenance work on the control valve, perform the lockout/tagout (LOTO) procedure in accordance with current safety regulations.

WARNING — COMPRESSED AIR: Control valves operate under high working medium pressure (up to 250 bar and above) and under the influence of pre-compressed spring actuators. It is prohibited to remove the actuator cover or the stuffing box cover until the pressure of air (0 bar) and the working medium from the valve body is fully released.

PROTECTIVE EQUIPMENT (PPE): Mandatory use of safety goggles, helmets, heat-resistant gloves and safety footwear. In case of work with toxic or aggressive environments (acids, alkalis, pressurized gases) use autonomous respiratory protection devices and chemical-resistant suits.

3. Required diagnostic tools

Tool Name Specification / Model Measurement range Purpose during diagnostics
Industrial multimeter Fluke 87V or equivalent (True-RMS) 0 – 50 mA DC, 0 – 1000 V DC/AC Current signal measurement 4-20 mA at the positioner terminals.
Portable Process Calibrator Fluke 725 / Pressure DPI 620 0 – 24 mA, 0 – 10 bar (pneumatic module) Generation of reference signal and measurement of air supply pressure.
Vibration analyzer SKF Microlog / Emerson CSI 2140 10 Hz – 10 kHz, 0.1 – 50 mm/s Measurement of the valve body and pipeline vibration level to detect cavitation and mechanical resonances.
Thermovisor (Thermal imager) Flir E8 / Testo 883 -20°C to +500°C Detection of internal leaks (leakage through the valve) and overheating of the positioner electronics.
Pressure gauge set Class accuracy 0.6, with minimess connectors 0 – 10 bar (air), 0 – 400 bar (hydraulics) Control of air supply pressure stability directly at the input to the positioning unit.

4. Initial Assessment Checklist

Parameter / Event What to observe and record Norm / Acceptable Value Alarm Value
History of Development Did the vibrations appear suddenly (after repair/adjustment) or develop gradually? Reliable operation after commissioning. Rapid appearance after changing parameters of the PID-controller or mechanical interference.
Air supply pressure Pressure in the pneumatic network at the input to the regulator-filter of the positioner during rod movement. Stable pressure exceeding the maximum operating pressure of the drive by 1.0 – 1.5 bar. Pressure drop more than 15% during valve stroke, presence of moisture/oil in the line.
Control signal (4-20 mA) Presence of noise or jumps in the current signal from DCS (distributed control system). Signal noise < 0.05 mA. Signal fluctuations over 0.2 mA under a stable technological process.
Stroke frequency Period of full cycle of valve position oscillations (Feedback). Absence of cyclic movements (dead zone does not exceed 0.1% of the full stroke). High-frequency vibrations (> 2 Hz) or low-frequency self-oscillations (period 2–20 s).
Pressure drop across the valve ($\Delta P$) Difference between the inlet and outlet pressure of the working medium ($\Delta P = P_1 - P_2$). Within the calculated project range (according to the datasheet of the valve). Exceeding the maximum allowable $\Delta P$ for this type of valve (risk of cavitation or stalling).

5. Systematic Diagnostic Block Diagram

Use this step-by-step algorithm for fault localization:

  • Крок 1. Перевірка стабільності електричного та пневматичного живлення
    • Виміряйте мультиметром напругу та струм живлення на клемах позиціонера.
      • ЯКЩО сигнал 4-20 мА коливається без причини з боку DCS $ ightarrow$ Перевірте вихідний модуль DCS та заземлення екрану кабелю.
      • IF signal is stable $ \rightarrow $ proceed to Step 2.
    • Check the air supply regulator with a manometer.
      • IF the manometer needle is fluctuating or the pressure drops when the valve moves $ ightarrow$ Faulty regulator-filter (clogged filter element or damaged diaphragm).
  • Крок 2. Діагностика позиціонера та налаштування контуру позиціонування
    • Перевірте стан механічного зв'язку (тяга, хомут, ричаг зворотного зв'язку).
      • ЯКЩО є люфт у шарнірних з'єднаннях або знос кулачка $ ightarrow$ Механічний люфт зворотного зв'язку. Усуньте люфт або замініть комплект тяг.
      • IF mechanical connection is rigid $ \rightarrow $ perform the autotune procedure for the positioner.
    • Analyze the PID regulator parameters inside the digital positioner (Kp, Ki, Kd or gain coefficients).
      • IF the gain parameters are too high $ \rightarrow $ the valve overshoots and starts low-frequency oscillation. Reduce the positioner gain.
  • Крок 3. Аналіз механічного тертя та стану сальника
    • Зробіть діагностику характеристик тертя за допомогою сервісного ПЗ позиціонера (наприклад, ValveLink, FieldComm Group, AMS Device Manager) шляхом побудови петлі гістерезису.
      • ЯКЩО гістерезис перевищує 3-5% або спостерігається «stick-slip» (ефект залипання-ривка) $ ightarrow$ Надмірне тертя в сальниковому вузлі або пошкодження штока.
      • IF friction is within normal limits $ \rightarrow $ proceed to Step 4.
  • Крок 4. Аналіз взаємодії з технологічним процесом (Process Interaction)
    • Порівняйте частоту коливань клапана з частотою коливань технологічних параметрів (витрата, тиск).
      • ЯКЩО частота коливань клапана збігається з частотою коливань ПІД-регулятора DCS $ ightarrow$ Конфлікт налаштувань зовнішнього та внутрішнього контурів (неузгодженість часів релаксації). Виконайте розгонку параметрів налаштування контурів (detuning).
      • IF a high pressure drop $\Delta P$ is detected with signs of cavitation or water hammer $ ightarrow$ Hydrodynamic process problems (incorrect valve selection or insufficient lift at the valve).

6. Matrix "Symptom - Cause - Diagnosis - Expected Result"

Symptom Likely causes (by likelihood) Diagnostic test Expected result upon confirmation of the cause
High-frequency piston vibration (1–5 Hz), unstable position signal 1. Too high gain (Kp) of the positioner.
2. Electrical interference on the analog input.
3. Play in the actuator rod.		 1. Reduce the amplifier of the positioner by 20-30%.
2. Check with an oscilloscope for the presence of induced AC voltage on the 4-20 mA cable.
3. Check the rod coupling.		 Elimination of high-frequency noise, stabilization of the rod in the desired position.
Low-frequency self-oscillations (period 5–30 s), "hunting" of the valve 1. High friction in the seal (stick-slip).
2. Incorrect settings of the outer loop PID-controller DCS.
3. Too large pneumatic actuator volume without booster.		 1. Friction profile and hysteresis build-up in the service program.
2. Conversion of the DCS loop to manual mode (Manual).
3. Analysis of air motor fill time.		 When the DCS loop is switched to manual mode, oscillations stop (indicates a conflict in PID settings).
Spring-loaded valve movement (flow break / sticking) 1. Stem seal wear.
2. Valve stem bend.
3. Salt or sludge deposit on piston/saddle.		 1. Loosening of the stuffing box nuts by 1/4 turn (under observation).
2. Measurement of rod runout with a dial indicator (DI).		 Reduction of breakaway force and smooth operation without jerks.
Valve vibration during change of process operating load 1. Insufficient flow capacity of the pneumatic distributor / positioner (slow response time).
2. Incorrect flow characteristic (logarithmic instead of linear or vice versa).		 1. Measurement of full stroke time (Stroke Time) in forward and reverse directions.
2. Checking the curve of flow rate calculation relative to stroke.		 Run time exceeds the valve passport's normative values (requires installation of a pneumatic booster).

7. Root Cause Analysis of Each Fault

7.1. Excessive stuffing of the stuffing box (Stick-Slip Effect)

Cause: The stuffing box (graphite, PTFE) is overly tightened to prevent environmental leakage. When the signal changes, the stem does not move due to stiction. The positioner accumulates maximum pressure in the actuator until the pressure force exceeds the friction force. At this moment, the stem breaks free and overshoots the desired position. The positioner then attempts to return it back, creating cyclic jerks.

How to confirm: Run the valve diagnostic test (Step Response or Valve Signature). On the graph showing the relationship between air pressure and piston stroke, a characteristic "steplike" (deadband zone) and sharp pressure spikes at the beginning of movement will be observed.

Наслідки: Прискорений знос штока, руйнування ущільнень, втрата герметичності затвора та неможливість точного регулювання малих витрат.

7.2. Mismatch between the dynamics of the positioner and the external PID-controller DCS

Why it occurs: The response time of the positioner's internal loop is much faster (less than 1 second) than the response time of the technological process (for example, heating of the furnace or level in the tank). If the gain in the positioner is set too aggressively, it reacts to every micro change in the signal from the DCS, creating high-frequency oscillations that superimpose on the process.

How to confirm: Switch the external regulator DCS to manual mode (Manual). If the valve oscillations stop immediately — the problem is in the PID regulator settings or in the conflict between "fast actuator - slow process".

Наслідки: Перегрів котушки електропневматичного перетворювача, швидкий знос рухомих елементів реле позиціонера, нестабільність технологічних параметрів.

7.3. Incorrect selection of drive volume and absence of a booster valve (Volume Booster)

Why it occurs: Large diaphragm or piston actuators require a large amount of air for fast movement. The standard built-in pneumatic module of the positioner has limited flow capacity (Cv usually 0.05–0.12). Air flows slowly, the positioner "cannot keep up" with the signal and overloads the output, resulting in overshoot and oscillation.

How to confirm: Measure the time for the full stroke of the valve under load. If the stroke time of a valve with a volume of more than 5 liters exceeds 10 seconds without technological necessity — the flow capacity of the positioner is critically low.

Наслідки: Затримка спрацьовування захисних контурів, перегрів і вихід з ладу вихідних пневмомодулів позиціонера.

8. Step-by-step troubleshooting procedures

Procedure 1: Adjustment and Replacement of the Seal Unit

  1. Perform the LOTO procedure: remove air pressure from the drive, close the shut-off valves around the valve, release the medium pressure to 0 bar, and cool the system to a safe temperature (< 40°C).
  2. Remove the protective rod cover and disconnect the feedback rod.
  3. Loosen the stuffing box flange bolts evenly (diagonally) by one full turn.
  4. Check the stem condition using a micrometer. Radial runout shall not exceed 0.05 mm. If there are scratches, polish the stem with a grinding paper of grit P600–P1000 with mineral oil.
  5. Replace the seal rings with new ones (recommended combination PTFE + graphite according to the standards TA-Luft / ISO 15848).
  6. Tighten the gland nuts with a torque wrench to the torque recommended by the manufacturer (usually 15–25 Nm for DN50–DN100), avoiding over-tightening.
  7. Assemble the unit, apply air pressure and perform 5 full valve strokes for seal break-in.
  8. Perform a re-diagnosis of hysteresis: total hysteresis should not exceed 1.5% of the full stroke.
    9. </s

    Procedure 2: Digital Positioner Recalibration (Auto-tuning and Tuning)

    1. Connect the communicator or service PC with appropriate software to the digital positioner via the HART, Profibus PA or FOUNDATION Fieldbus protocol.
    2. Check the current microprogram version and input filter settings (Input Filter Time Constant — set the value within 0.2 – 0.5 s to suppress high-frequency noise).
    3. Launch the built-in self-calibration and auto-tuning (Autotune / Performance Tune) procedure in "Standard" or "Adaptive" mode. Warning: the process is accompanied by a series of rapid valve movements, ensure that this does not affect the operation of the current production.
    4. If after auto-tuning the oscillations persist, manually reduce the proportional gain (Kp) by 15–25% and increase the integral time (Ti) to make the positioner's response more damped.
    5. Save the configuration in the non-volatile memory of the positioner and make a backup copy of the configuration file on the enterprise server.
      6. </s

      Procedure 3: Installation of the Pneumatic Booster (Volume Booster)

      1. Select the booster model taking into account the drive volume (for example, a pneumatic relay with adjustable dead zone and bypass bypass for damping small fluctuations).
      2. Mount the booster directly on the drive housing using short connecting fittings and tubes of minimal internal diameter 8 mm to reduce capacitive resistance.
      3. Set the booster bypass valve (Deadband / Bypass needle valve): open the bypass globe valve by 1/2 – 1 turn to allow the positioner to precisely control small signal changes without triggering a large flow through the booster.
      4. Check the valve travel time: for DN100 valves, the full travel time under load should be 3 – 5 seconds.
        5. </s

        9. Preventive Maintenance

        Primary Cause Preventive strategy Monitoring method Recommended interval
        Contamination of compressed air (oil, moisture) Installation of mainline adsorption type dryers and three-stage filters-reducers on each valve (air purity class according to ISO 8573-1: class 2.4.2). Weekly inspection and condensate draining from filter vessels, checking of humidity indicators. Weekly (inspection), every 12 months (filter element replacement).
        Wear of moving pivot joints Use of non-contact position sensors (based on Hall effect or magnetoresistive) instead of mechanical levers. Hysteresis profile diagnostics using service software for the positioner. Every 6 months.
        Seal Degradation Application of spring-loaded packings with spring-loaded nuts (Live-loading packing), compensating for wear and thermal deformations. Thermal imaging inspection and monitoring of leaks through the stuffing box. Every 12 months during planned shutdown.

        10. Spare Parts and Components

        Description of the part Technical specification When to replace Category UNITEC-D
        Digital electro-pneumatic positioner HART / Profibus PA, two-wire, housing made of anodized aluminum / stainless steel 316L, IP66/67 In case of electronics failure, zero drift over 2% or mechanical damage to the pneumatic relay. Regulating valves and positioners
        Seal assembly repair kit PTFE with carbon filling / graphite rings according to DIN/EN ISO 15848 When visual leaks occur through the seal or the hysteresis exceeds > 3%. Valve Packing and Seals
        Pneumatic Booster (Volume Booster) Flow coefficient 1:1, working pressure up to 10 bar, temperature range -40°C to +85°C Stalling during operation of the drive with volume > 5 l or failure of the internal diaphragm. Pneumatic automation and drives
        Air supply filter-reducer With a stainless steel manometer, filtration accuracy 5 microns, maximum pressure 16 bar When the flow capacity decreases, the filter is contaminated or the shut-off valve is damaged. Air Preparation for KVPiA

        For selecting compatible spare parts and the complete catalog of industrial equipment, visit electronic catalog UNITEC-D.

        11. References

        • ISO 5234 / IEC 60534: Industrial Control Valves (Parts 1–8: Testing Methods, Flow Capacity, Acoustics).
        • ISO 8573-1: Compressed air — Part 1: Contaminants and purity classes.
        • EN 13463 / ISO 80079: Non-standard equipment for explosive atmospheres (ATEX).
        • Manufacturer References: Emerson Fisher, Metso FlowControl, Samson AG — technical instructions for setting up digital positioners of the FIELDVUE, DVC and TROVIS series.
        • Related Guides UNITEC-D: «Cavitation and Erosion Wear Diagnosis», «Methodology for Checking Valve Tightness According to Standard ISO 5208».

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