Analysis of Common Fault Causes and Treatment of Pneumatic Control Valves
2024-03-10 09:55:53
Pneumatic control valves are one of the widely used instruments in the power industry. They play an essential role in various process flows of thermal power plants and are an important part of the automatic control system in power plants. A pneumatic control valve uses compressed air as the power source and a cylinder as the actuator. The valve intelligent positioner receives a control signal of 4-20mA, converts the weak electrical signal into an air pressure signal through the positioner, inputs the compressed air into the cylinder to drive the valve, realizes linear adjustment of the valve, and receives remote control signals from the control system to adjust the flow rate and pressure of the medium in the pipeline, thereby changing process parameters such as temperature. The valve intelligent positioner is one of the important accessories of the pneumatic control valve and plays the role of valve positioning.
The advantages of pneumatic control valves are: (1) They act quickly and can complete adjustment commands rapidly; (2) They can achieve a large torque driving force when combined with a large cylinder; (3) They can operate safely and stably for a long time under various harsh working conditions; (4) They are intrinsically safe.
1. Maintenance and Overhaul of Control Valves
The control valve is directly installed on the process pipeline and is often used in high-temperature and high-pressure environments. Its condition directly affects the quality of regulation. Practice has proved that the condition of each link in the control system has a direct impact on the system. Therefore, the control valve must be regularly maintained and overhauled, especially in harsh operating conditions, and regular overhaul work should be given more attention.
1.1 When the control valve is overhauled during the unit shutdown, the key inspection and maintenance parts mainly include the following aspects:
(1) After disassembling the valve, check whether the valve core is worn. If it is worn, the valve core needs to be replaced.
(2) Check whether the valve stem is deformed or corroded, and whether the thread is in good condition. The valve stem should be kept straight, free from corrosion, and the thread should be intact, with a bending degree < 0.05mm.
(3) Check the sealing surface of the valve core and valve seat. If the erosion depth of the valve core sealing surface exceeds 0.2mm, put it on a lathe and turn off the damaged layer at the original angle; use a special tool to grind the valve seat sealing surface to eliminate defects, and apply grinding paste between the valve core and the valve seat sealing surface for grinding; apply red lead to check the tightness of the sealing surface.
(4) Check the packing and replace it according to the situation.
(5) Check each bolt and nut, and replace them if they are worn.
1.2 The daily maintenance of the control valve is a preventive inspection and maintenance before the valve fails, which includes the following aspects:
(1) Keep the control valve clean and all its components in good condition. Regularly inspect the fixed connectors of the control valve and conduct anti-corrosion and lubrication inspections.
(2) The following points should be noted during daily inspections: Check whether the actuator and each connecting pipeline are leaking air; check whether the appearance of the actuator is intact, and whether the connecting rod is deformed or worn; check whether the valve position feedback is consistent with the output of the regulator; check whether the filter pressure reducing valve is in good condition and available; check whether there is leakage at the valve seat and whether the valve stem moves smoothly; establish a control valve account, record each fault handling phenomenon and the handling process, and summarize the experience. This is to analyze the fault causes of the control valve.
2. Common Fault Components of Control Valves
The pneumatic control valves in thermal power plants are mainly composed of an actuator, a valve, and an intelligent positioner. Therefore, the main fault components of the control valve are also composed of these three parts.
2.1 Main Fault Components of the Actuator
2.1.1 Diaphragm
For the diaphragm-type pneumatic actuator, the diaphragm is the most important component. When the air source output of the positioner is normal, if the actuator does not move or the cylinder leaks air during operation, it is necessary to check whether the diaphragm inside the cylinder is damaged. In addition, the diaphragm has a certain service life. After a long service life, the aging of the material will also affect its use. Therefore, the diaphragm should be replaced after a certain period.
2.1.2 Piston and Sealing Parts
The piston of the pneumatic actuator generates the driving force. Therefore, whether the piston and the sealing condition are good has a great impact on the operation of the actuator. During the major and minor overhauls of the unit, it is necessary to check whether the piston and the sealing parts are in good condition and available. The sealing parts should be replaced after being used for a certain number of years.
2.1.3 Pneumatic Pipeline and Push Rod
This is the passage connecting the actuator. Therefore, during daily inspections, it is necessary to check whether there is air leakage, and whether the push rod is bent, deformed, or detached. Check whether the connection between the push rod and the valve stem is firm, and adjust the position.
2.1.4 Positioner
The positioner is the core component for controlling the valve action. Therefore, it is necessary to regularly check the working status of the positioner, mainly including checking whether the air intake and exhaust of the positioner are normal, whether the local status display is normal, and whether the valve action response time meets the requirements of the system operation, etc.; in addition, the positioner should be regularly calibrated and adjusted to ensure the regulation quality.
2.2 Main Fault Components of the Valve
(1) Valve Body: Regularly check the corrosion and wear conditions of the inner wall of the valve body, especially for valves used in harsh process conditions such as corrosive media and high pressure differences. It is necessary to ensure that their pressure resistance, wear resistance, and corrosion resistance meet the on-site requirements.
(2) Valve Core: Since the valve core plays the role of regulating and cutting off the fluid, it is a movable throttling element and is easily eroded, corroded, and collided by the medium. It is more likely to be damaged under high pressure differences. Therefore, it is necessary to check whether all parts of it are damaged, worn, or corroded, and repair or replace it according to the actual situation.
(3) Valve Seat: The joint surface of the valve seat is the key to ensuring the closure of the valve, and it is also a part prone to leakage and corrosion.
(4) Valve Stem: Check whether the connection between the valve stem and the valve core and the push rod is loose, and whether there is obvious deformation, cracks, or corrosion.
(5) Gasket and O-ring: Check whether they are damaged or aged, and replace them if necessary.
3. Fault Analysis and Treatment during the Use of Control Valves
The common problems of control valves on-site are that the valve does not move, vibrates, or moves sluggishly. The treatment methods are as follows:
(1) The Valve Does Not Move
No Air Source and No Signal: The air source is not opened or the air source pressure is too low; the air source contains impurities, resulting in the blockage of the air source pipe, filter, or pressure reducing valve; the filter pressure reducing valve is blocked or malfunctioning.
There is an Air Source but the Valve Cannot Move: There is no command signal from the DCS: Check the command line; the positioner has no display and no output: Replace the positioner; there is a leakage in the air path output of the positioner: Eliminate the leakage; there is an internal fault in the control valve cylinder: Check the inside of the cylinder; the valve stem or valve core is stuck or deformed: Handle or replace it according to the situation; the handwheel position is incorrect: Set the handwheel to the release position.
(2) The Control Valve Oscillates
The air source pressure is unstable; the command signal oscillates; when the air source pressure meets the requirements and the command signal is also stable, but the action of the control valve is still unstable: The positioner has not been adjusted for a long time: Readjust the positioner; the positioner malfunctions: Replace the positioner; there is air leakage in the output pipeline of the positioner: Eliminate the leakage; the valve stem does not move smoothly when in contact with other parts: Add lubricant or reinstall the valve stem.
(3) The Control Valve Vibrates
The installation base is unstable: Reinforce the base; it is caused by nearby vibrating equipment: Eliminate the vibration source; the valve core and the bushing are severely worn: Replace the bushing; the control valve is not properly selected: Replace it with a suitable valve; the flow direction of the valve medium is opposite to the closing direction: Change the installation direction of the valve.
(4) The Control Valve Moves Sluggishly
The diaphragm in the pneumatic diaphragm actuator is damaged and leaking: Replace the diaphragm; the O-ring in the actuator is damaged: Replace the O-ring; there is adhesive matter blocking inside the valve body: Eliminate the blockage; the valve stem is not straight, resulting in a large frictional resistance: Deal with the valve stem.
(5) The Control Valve Has a Large Leakage
The valve core is worn and there is serious internal leakage: Eliminate the internal leakage; the valve stem is of inappropriate length, and the valve is not adjusted properly and cannot be closed tightly: Adjust the valve stem and the valve; the sealing ring inside the valve body is damaged: Replace the sealing ring; the medium pressure difference is too large, and the actuator cannot close tightly: Increase the air source and improve the actuator; there is foreign matter inside the valve: Remove the foreign matter; the air source pressure is low or the joint air pipe is leaking: Adjust the air source and eliminate the leakage.
4. Conclusion
There are many types of control valves, with different installation positions, diverse operating conditions, and different fault causes. Therefore, for the maintenance of control valves, it is necessary to combine with the actual operation situation to promptly find out the fault causes and find practical and effective solutions.
Cited from (Journal) Technology Innovation and Application, Issue 3, 2016. Authors: Huang Xianping, Ai Penglong
