Introduction:

Control valves are essential components in regulating the flow of fluids within industrial processes. However, they can experience various problems that disrupt their operation. In this article, we will explore common control valve issues and their causes, as well as provide solutions to address them. Understanding and effectively resolving these problems are crucial for maintaining efficient and reliable systems.

1. Control Valve Not Responding

• Cause 1: Lack of air supply or insufficient air pressure.
• Solution: Start by checking the air supply (instrument air) for obstructions and ensure that the air pressure meets the valve's operational requirements.
• Cause 2: Air supply is present, but there is no output signal pressure.
• Solution: For mechanical controllers or positioners, replace the mechanical pressure controller or positioner. For control valves with mechanical actuators, inspect the signal collection pipeline for any open or severely leaking valves. Address any anomalies promptly.
• Cause 3: Output signal pressure is normal, but the valve remains unresponsive.
• Solution: Examine the pneumatic diaphragm actuator for significant air leakage. If leakage is present, replace the diaphragm and related sealing components. If the diaphragm is intact, the valve core may be stuck within the seat, and disassembly and cleaning are required. In cases of severe valve stem bending or deformation, disassemble the main valve and take appropriate actions. Check for leaks in the signal air supply lines and address them. If the signal amplifier is malfunctioning or improperly adjusted and the main air supply does not pass through the amplifier into the diaphragm cavity, adjust or replace the amplifier promptly.

2. Control Valve Operational Instability and Oscillations

• Cause 1: Significant fluctuations in air supply pressure or malfunctioning filters and pressure reducers.
• Solution: Examine the compressed air system's operational status. Check and replace filters or pressure reducers as necessary.
• Cause 2: Air supply pressure is stable, but the signal pressure is inconsistent.
• Solution: After replacing an intelligent positioner, if instability persists, conduct PID parameter tuning. For mechanical controllers or positioners, replace them and calibrate the controllers. If instability continues after changing the mechanical controller or positioner, inspect and calibrate the signal air supply amplifier.
• Cause 3: Both air supply and signal pressure are stable, but the control valve remains unstable.
• Solution: Inspect the airtightness of the pneumatic diaphragm actuator. Carefully check for slight air leakage in the drive rod seal. If leakage is present, disassemble and replace the seal. Inspect the diaphragm for scratches or punctures causing minor leaks, and replace it if necessary. Check the connection points between the positioner and the main valve for gaps and refasten or replace relevant connections. Thoroughly inspect the signal air pipeline for minor leaks using soapy water and address any issues promptly. If the signal air supply amplifier is improperly adjusted, adjust the balance screw.
• Cause 4: Excessive resistance in the control valve's actuation mechanism (including the main valve and pneumatic diaphragm actuator).
• Solution: Disassemble the main valve, inspect, and replace any faulty components, including the valve core, seat, valve stem, stem seal, and drive rod. Check whether the pressure balance spring inside the pneumatic diaphragm actuator is damaged or has undergone fatigue deformation. If anomalies are present, replace the pressure balance spring.

3. Delayed Control Valve Operation

• Cause 1: Accumulation of sticky substances within the valve body.
• Solution: Disassemble the main valve body and clean out the sticky substances.
• Cause 2: Deterioration or hardening of valve stem packing material or dry lubrication of graphite or asbestos packing.
• Solution: Disassemble the main valve body and replace the valve stem packing.
• Cause 3: Packing is tightened too much, leading to increased friction.
• Solution: Loosen the valve stem packing nut and perform multiple reciprocating actions on the main valve. Adjust the valve stem packing nut to an appropriate torque.
• Cause 4: Valve stem bending causing significant friction.
• Solution: Disassemble the main valve and replace the valve stem.
• Cause 5: Minor air leakage in the pneumatic diaphragm actuator or signal air supply lines.
• Solution: Replace the diaphragm of the pneumatic diaphragm actuator and address any leaks in the signal air supply.

4. Control Valve Operates Normally, but Process Control Parameters Are Abnormal

• Cause 1: Displacement of the control valve core.
• Solution: Disassemble the main valve and rectify the dislocated valve core.
• Cause 2: Relatively shifted connection between the control valve core and stem but not detached.
• Solution: Disassemble the main valve and restore the valve core's position.
• Cause 3: Control valve stem breakage.
• Solution: Disassemble the main valve and replace the valve stem.
• Cause 4: Clogging of the control valve core.
• Solution: Disassemble the main valve and clean the clogged material from the valve core.
• Cause 5: Other control parameters of the controlled medium system are outside the specified design range, such as flow or pressure not meeting design requirements.
• Solution: Adjust the system's control parameters within the specified design range.