Common failure analysis and solution of control valve


Control valves are widely used in oil refining and chemical industries. They are a common regulating device used to control fluid flow, pressure and liquid level in the production process. In the application of control valve, calculation and selection are the prerequisites, installation and debugging are the key, and use and maintenance are the purpose. If the control valve is improperly selected, improperly installed or poorly debugged, it will not be able to adjust. Therefore, it is very important to be familiar with the working principle and maintenance skills of the control valve. Today, the small valve will introduce the common faults and treatment methods of the pneumatic control valve.

Control valve principle

The pneumatic control valve uses compressed air as the power source, uses the cylinder and the membrane as the actuator, and uses the electric valve positioner, converter, solenoid valve, position retaining valve and other accessories to realize the switching value or continuous adjustment, and receives the control signal of the DCS. Adjust the flow, pressure, temperature and other process parameters of the medium. Pneumatic valves are mainly divided into two types: air-open type and air-close type, which are characterized by simple control, fast response, intrinsic safety, and no need for explosion-proof measures.
Control valve failure forms and causes
The main failure modes of pneumatic control valve are: the internal leakage valve does not act, the opening degree fluctuates greatly, the jamming phenomenon occurs, the action is normal but the flow rate does not change. The main reasons are: valve body failure, cylinder failure, positioner failure, DCS failure, control cable failure, inadequate maintenance, valve quality problems, selection or design reasons, and pneumatic component failures.
01. Valve body failure and countermeasures
common error
1) The metal seat of the butterfly valve is damaged, causing the valve body to be stuck; the fixed pin of the butterfly valve falls off or the connecting bolt of the hemi-ball valve core and the valve stem are sliding, mainly because the valve shaft seems to have moved but the actual valve core has not moved.
2) The internal wear of the ball valve body causes the valve to be easily stuck or the valve action is slow; the damage to the soft sealing gasket of the ball valve causes the valve to leak or be stuck.
3) The damage to the valve shaft sleeve of the hemispherical valve makes the valve difficult to operate.
4) The steam valve stem seals due to high temperature aging, resulting in steam leakage; the steam valve muffler plate falls off, causing the valve to be often stuck and the pipeline noise is abnormally loud.
5) The internal wear or corrosion of the valve stem causes internal leakage of the valve; the seal at the valve stem packing is damaged and leaks.
Cause countermeasures
1) The cause of the leakage of the valve core and seat is that the surface of the valve core and valve seat is worn or corroded, the force of the actuator is too small, and the thread of the valve seat is worn and loosened. Troubleshooting method: Improve the grinding joint surface, adjust the connection between the actuator and the valve stem, tighten the bolts or replace the valve core and valve seat.
2) The causes of leakage between the seat ring and the valve body are that the tightening torque is too small, the surface finish of the valve body is poor, the gasket is not suitable, and the valve body has small holes or burrs. Troubleshooting method: increase the tightening force, re-clean and handle the valve body, replace the appropriate gasket, and re-weld the valve body.
3) The causes of packing leakage are poor stem finish or bending, the packing cover is not pressed tightly or the packing type is unreasonable, the packing layer is stacked too high, the packing has corrosion pits or the packing cover is deformed and damaged. Troubleshooting method: Polish the valve stem and straighten it, re-tighten or replace the packing; install the spacer ring, reduce the height, change the packing with good performance, repair and replace the gland and related flange bolts, etc.
4) The valve stem and valve vibrate and cause the valve stem connection to be disconnected or broken. The reason is that the torque is too large and the pin connection is not suitable. Troubleshooting method: use the valve core valve stem integral part or welded valve core, connect the connecting parts firmly, and eliminate the vibration factor. The cause of valve vibration is the leakage of the sealing packing, the bypass is not adjusted properly, the gain of the positioner is too high, and the support is not good and there is a vibration source. Troubleshooting method: lubricate the packing and adjust, re-adjust the bypass, select a low-gain positioner, and fix the valve firmly to avoid the vibration source.
02. Causes and countermeasures of cylinder failure
common error
1) The limit bolt of the actuator falls off, causing the cylinder to leak air, causing the valve to fail to move or move slowly, failing to reach the predetermined opening.
2) The damage to the O-ring of the cylinder leads to air leakage, which causes the valve to operate difficult or unable to operate.
3) The deformation of the cylinder piston leads to an increase in the friction force, and the loosening of the cylinder bolts causes the cylinder to leak air, which makes the valve difficult to operate.
4) The linkage mechanism that the cylinder drives the valve body to move is loose or rusted, which makes the valve difficult to move.
5) The cylinder capacity is too small or the cylinder spring is faulty, which makes the valve difficult to operate.
Cause countermeasures
1) The cause of leakage at the piston seal is that the cylinder has poor finish; the assembly sealing of the piston ring is not enough, the use temperature of the seal ring is too high, and the seal is damaged for a long time. Troubleshooting method: Grind the cylinder and re-adjust the cylinder limit bolts, re-select the sealing ring according to the operating temperature, and install it correctly when replacing.
2) How to deal with the loosening of the cylinder limit bolts: Re-tighten the bolts.
3) The treatment method for the large friction force of the piston: re-apply lubricating oil, re-clean and grind the cylinder and piston.
4) The valve is difficult to move and cannot reach the fixed stroke, the spring of the actuator is not set enough or the spring is damaged, and the position of the limit block is inaccurate. Processing method: calibration, readjustment, spring replacement.
03. Locator failure causes and countermeasures
common error
1) The failure of the positioner circuit board causes the valve to act abnormally.
2) The small spool valve is damaged or worn, which causes the valve to operate abnormally. The reason is that the cleanliness of the instrument air is not high, or there is dirt entering the positioner, which causes the small spool valve to wear and cause the valve to operate abnormally.
3) The damage of the positioner proportional valve causes the valve to act abnormally.
4) The damage of the sliding rheostat leads to inaccurate positioning of the valve, causing abnormal valve action. The main reason is that the ambient temperature is too high and other liquids such as water enter, causing the rheostat to be damaged.
5) The O-ring seal of the air source interface of the positioner is broken, causing the broken O-ring seal to jam the valve stem of the small spool valve, making it unable to move.
6) The seal of the positioner’s glan head is not good, causing the circuit board of the positioner to be damaged by water.
7) Due to the excessive vibration of the pipeline, the high ambient temperature and the high humidity, the circuit board fails.
Cause countermeasures
1) The sealing problem at the Glan head causes water in the positioner to cause damage to the circuit board. Troubleshooting method: Improve the sealing performance of the Glen head, so as to avoid the circuit board damage caused by water entering inside the positioner.
2) Due to the high humidity and dust of the instrument air, the small slide valve enters the dirt and causes the small slide valve to wear, so that the valve action is abnormal. Troubleshooting: Add an air filter to improve instrument air quality.
3) The sliding rheostat and the circuit board are frequently damaged due to excessive ambient temperature, excessive humidity and excessive vibration. Troubleshooting method: separate the circuit board from the small slide valve and slide varistor to make a separate valve positioner.
4) Because the O-ring of the positioner is often broken, the stem of the small spool valve is stuck. The main reason is that the pressure of the air source is too large. It should be about 0.5MPa under normal conditions. When it is higher than 0.5MPa, the aging of the sealing ring will be accelerated. ,easily damaged. Troubleshooting method: keep the air source pressure at about 0.4MPa, and provide a better quality sealing ring.
04. Failures and countermeasures caused by process and design
common error
1) The valve cannot operate due to the change of process parameters, which causes the pressure difference in the pipeline to be too large.
2) The valve body is fouled due to the medium, causing the valve to act abnormally.
3) The valve cannot operate normally due to the blockage of the pipeline.
4) Due to the design and selection, the actuator is too small, the torque is not enough, the valve cannot operate normally, and the valve body material is not suitable, resulting in damage to the valve body.
Cause countermeasures
1) The pressure difference is too large, so that the control valve cannot be opened normally. Treatment method: install a bypass valve before and after the control valve.
2) The control valve cannot be opened normally due to the scaling of the valve body due to process reasons. Treatment method: increase the flushing water pipe, disassemble the valve regularly to clean the valve body, and polish the valve core if necessary.
3) The control valve cannot operate due to the blockage of the pipeline. Treatment method: optimize the operation method, try not to block the pipeline, and disassemble the pipeline flushing valve.
4) About design selection. Treatment method: try to optimize various process parameters in the design stage to ensure that there will be no problems in selection due to inaccurate process parameters.
05. Faults and countermeasures caused by DCS
common error
1) The I/O card is damaged and there is no signal output, so the control valve cannot operate.
2) The DCS output signal is unstable, causing the control valve opening to fluctuate, making the control valve act abnormally.
3) The control cable is damaged or the insulation is not good, and the wiring is loose, resulting in abnormal signal and abnormal control valve action.
4) Due to insufficient maintenance, the positioner has entered water, the cylinder is corroded and leaked, and the gas source pipe ferrule has fallen off and leaked, causing the control valve to operate abnormally.
5) Due to the lack of careful inspection, the hidden dangers were not discovered in time, which led to further expansion, and eventually caused the control valve to operate abnormally.
Cause countermeasures
Maintenance and inspection work should be done daily, and hidden dangers should be dealt with in time; when the opening of the control valve fluctuates, check whether the PID parameter settings are correct and adjust the parameters in time.
06. Failures and countermeasures caused by damage to pneumatic components
common error
1) The aging and damage of the air source hose or the leakage of the stainless steel air source pipe causes the air source pressure to be low, which makes the control valve act abnormally.
2) The air filter is broken or the filter element is too dirty, resulting in abnormal control valve action.
3) The pressure of the air source is low or the air source is not turned on, which causes the control valve to act abnormally.
Cause countermeasures
1) Replace the air source hose or air filter with a better quality; regular inspections, if the air source hose or air filter is found to be damaged and leaking, or the filter element is too dirty, you need to find an opportunity to replace it as soon as possible.
2) Adjust the air source pressure to meet the smaller air source pressure requirements of the pneumatic valve.

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