A guide to finding and correcting the causes of oscillations and dimensional instability of the control valve

1. Problem description and scope

This guide is intended for finding and correcting the causes of fluctuations and dimensional instability of the control valve (valve hunting). These symptoms can occur in various production systems such as boiler rooms, chemical plants, gas supply and industrial processes. They are considered critical because they can lead to system instability, reduced efficiency, and danger to personnel.

2. Protective measures

Use personal protective equipment (PPE): gloves, glasses, protective weapons.
Ensure Energy Blocking and Tagging: Before taking any action, energy blocking and tagging the system elements.
Disable energy storage: disable pneumatic/hydraulic systems and check for residual energy before starting diagnostics.
Be careful when working with chemical liquids: use protective equipment against chemical substances.

3. Necessary diagnostic tools

4. Check before starting diagnostics

5. Systematic diagnostic scheme

1. Symptom: the valve oscillates when the signal from the positioner changes.
   1. Check: measure valve vibration.
      1. Result: vibration > 4.5 mm/s — probable cause: incorrect setting of the positioner.
      2. Result: vibration ≤ 4.5 mm/s — probable cause: a problem in the subsystem.
   2. Check: check the stability of the signal from the positioner.
      1. Result: the signal is stable - probable cause: a problem in the subsystem.
      2. Result: the signal is unstable - probable cause: incorrect setting of the positioner.
2. Symptom: valve does not open/close stably.
   1. Check: check the valve opening size.
      1. Result: the size of the opening changes - probable cause: a problem in the subsystem.
      2. Result: opening size is stable - probable cause: problem in subsystem.
   2. Check: check valve vibration.
      1. Result: vibration > 4.5 mm/s — probable cause: incorrect setting of the positioner.
      2. Result: vibration ≤ 4.5 mm/s — probable cause: a problem in the subsystem.
3. Symptom: valve is dimensionally unstable when the pressure changes.
   1. Check: check the system pressure.
      1. Result: pressure above 10 bar — probable cause: incorrect setting of the positioner.
      2. Result: pressure below 10 bar — probable cause: a problem in the subsystem.
   2. Check: check valve vibration.
      1. Result: vibration > 4.5 mm/s — probable cause: incorrect setting of the positioner.
      2. Result: vibration ≤ 4.5 mm/s — probable cause: a problem in the subsystem.

6. Matrix of causes of defects

7. Root cause analysis for each defect

7.1 Incorrect setting of the positioner

Reason: The positioner does not match the system parameters, which causes the valve to oscillate. This can occur due to incorrect configuration or using the wrong algorithm.

Confirmation: Measure the vibration of the valve and check the stability of the signal from the positioner. If the vibration is > 4.5 mm/s or the signal from the positioner is unstable, this confirms this cause.

Consequences: Unstable valve operation can result in loss of process control, reduced efficiency, and personnel hazards.

7.2 Incorrect subsystem parameters

Reason: The subsystem does not match the valve parameters, resulting in oscillations. This can be caused by improper pressure or vibration.

Confirmation: Check system pressure and valve vibration. If pressure > 10 bar or vibration > 4.5 mm/s, this confirms this cause.

Consequences: Unstable valve operation can result in loss of process control, reduced efficiency, and personnel hazards.

7.3 Insufficient torque due to action

Reason: The torque from the action is insufficient, which leads to oscillations of the valve. This can be caused by using the wrong actuator.

Confirmation: Check system pressure and valve vibration. If pressure > 10 bar or vibration > 4.5 mm/s, this confirms this cause.

Consequences: Unstable valve operation can result in loss of process control, reduced efficiency, and personnel hazards.

7.4 Residual energy in the system

Reason: Residual energy in the system can cause the valve to oscillate. This can occur due to improper shutdown of the system.

Confirmation: Check for residual energy in the system. If residual energy is present, this confirms this cause.

Consequences: Unstable valve operation can result in loss of process control, reduced efficiency, and personnel hazards.

8. Step-by-step correction procedures

8.1 Incorrect setting of the positioner

1. Measure valve vibration. If the vibration is > 4.5 mm/s, adjust the positioner.
2. Check the stability of the signal from the positioner. If the signal is unstable, adjust the positioner.
3. Perform a valve stability test. If the vibration is < 4.5 mm/s, the problem is fixed.

8.2 Incorrect subsystem parameters

1. Check the pressure in the system. If the pressure is > 10 bar, adjust the subsystem.
2. Measure valve vibration. If vibration > 4.5 mm/s, adjust the subsystem.
3. Perform a valve stability test. If the vibration is < 4.5 mm/s, the problem is fixed.

8.3 Insufficient torque due to action

1. Check the pressure in the system. If the pressure is > 10 bar, adjust the actuator.
2. Measure valve vibration. If the vibration is > 4.5 mm/s, adjust the actuator.
3. Perform a valve stability test. If the vibration is < 4.5 mm/s, the problem is fixed.

8.4 Residual energy in the system

1. Check for residual energy in the system.
2. Perform a complete shutdown of the system.
3. Measure valve vibration. If the vibration is < 4.5 mm/s, the problem is fixed.

9. Prevention methods

10. Replacement parts and components

Visit our e-catalog to order the required parts: https://www.unitecd.com/e-catalog/

11. Links

• Standards: DSTU 3010-95, EN 12981, ISO 10791
• Manufacturer's printed manuals
• Reference materials UNITEC-D