Non-Operation and Slow Response of Pneumatic Control Valve

 
In industrial control systems, pneumatic control valves play a crucial role in ensuring that the system operates smoothly and efficiently. However, sometimes these valves may exhibit non-operation or slow response during operation. Understanding the causes of these issues and mastering the corresponding solutions is essential for maintenance personnel. This article will delve into the common causes of non-operation and slow response in pneumatic control valves and their solutions.

Non-Operation of Pneumatic Control Valves

 
When a pneumatic control valve exhibits non-operation, the first step is to check whether the quality of the compressed air meets the standard requirements of the air source. The following conditions may lead to non-operation of the pneumatic control valve:

1. Air Supply Issues

 
Leakage in the air supply pipeline: This can cause a drop in supply pressure, affecting the normal operation of the control valve.
Pipeline blockage: This can affect the normal supply of the air source, preventing the valve from moving.
Filter or pressure regulator blockage: This can lead to insufficient air supply, failing to meet the working requirements of the control valve.
Moisture in the compressed air: This can affect the normal operation of the pneumatic system, resulting in non-operation.

2. Signal Transmission Issues

 
If the air supply is normal, further check if there is a signal being transmitted from the preceding stage. The cause of the fault can be determined by checking the input and output signals of the electrical valve positioner.
 
Input signal check: Measure the input current of the valve positioner with a multimeter. If the input current is normal, it indicates that the current circuit is functioning properly.
Output signal check: If the positioner has no output, check whether the air supply to the positioner has been interrupted, whether the air pressure is too low, whether the diaphragm is leaking, or whether the throttle hole of the amplifier is blocked.

3. Positioner Malfunction

 
Sometimes, moisture in the compressed air accumulates at the amplifier ball valve, which can also lead to a malfunction in the valve positioner where there is an air source but no output.

4. Mechanical Failure

 
The reasons for the positioner having output but the control valve not moving may include:
 
Rust on the spring of the pneumatic actuator: Springs may rust due to long-term disuse, preventing normal movement.
Damage to the diaphragm of the pneumatic actuator: Damage to the diaphragm can affect the normal operation of the actuator.
Deformation, bending, or breakage of the valve stem: Physical damage to the valve stem can prevent the control valve from moving properly.
Valve core and seat jamming: Friction or blockage between the valve core and seat can affect the valve's movement.
Over-tightened gland packing nut: An overly tight gland packing nut can cause excessive friction on the valve stem, jamming the control valve. In this case, slightly loosening the nut of the gland packing may help.

Slow Response of Pneumatic Control Valves

 
Slow response of pneumatic control valves is mainly caused by increased friction between the valve stem and the valve core. Here are the main causes and solutions for slow response of pneumatic control valves.

1. Common Causes of Slow Response

 
Increased friction between the valve stem and the valve core slows down the response of the control valve, preventing timely control of flow and pressure. The main causes of slow response in pneumatic control valves are as follows.
 
Increased friction between the valve stem and the valve core: Increased friction may be due to insufficient lubrication or wear.
Over-tightened packing gland nut: An overly tight packing gland nut increases the friction on the valve stem, leading to slow response.
Bending or deformation of the valve stem: Physical damage to the valve stem, such as bending or deformation, can affect its smooth movement.
Scratches or corrosion on the valve core guide surface: Scratches or corrosion on the guide surface of the valve core can cause jamming, affecting the sensitivity of movement.
Insufficient air supply pressure: Low instrument air supply pressure can lead to poor response performance of the valve positioner, resulting in slow response or slow full-stroke time of the control valve.

2. Basic Solutions for Slow Response

 
Check and adjust the packing gland nut: Check if the packing gland nut is overly tight. If so, loosen the nut for adjustment or apply some oil to lubricate the packing to reduce the friction on the valve stem.
Check and replace the valve stem: Check if the valve stem is bent, deformed, or scratched. If any of these issues are present, replace the valve stem.
Inspect and clean the valve core and seat: Check for scratches, corrosion, or blockages on the guide surface of the valve core. Clean the valve core and seat to remove obstructions and ensure normal operation.
Improve air supply pressure and quality: Increase the pressure and quality of the compressed air supply, or calibrate or replace the valve positioner to address poor response performance.
 
By providing a detailed analysis of the above issues and introducing solutions, maintenance personnel can more effectively diagnose and handle faults with pneumatic control valves, ensuring the normal operation of the system. Regular maintenance and inspection are key to preventing these issues, which can improve the efficiency and service life of the pneumatic control valves and thus ensure the stable operation of the entire industrial control system.