Fault Diagnosis of Regulating Valves
In the industrial field, process parameters such as medium flow, pressure, temperature and liquid level are just like several meridians in the production process, which are very important, and regulating valves are important acupoints distributed on these meridians. Once there is a problem with that parameter, it is necessary to mobilize these regulating valves for conditioning.
The regulating valve, also known as the control valve, is the final control element in the field of industrial automation process control, which changes the process parameters such as medium flow, pressure, temperature and liquid level by receiving the control signal output by the regulating control unit. In many systems, the operating conditions such as temperature, pressure, corrosion and pollution of the regulating valve are more serious than those of other components.
Malfunction of the air supply system
1. The instrument wind line is blocked
Because the ball valve has a throttling function at the end of the wind line of the instrument branch, the stolen goods in the wind line are easy to accumulate and block here. As a result, the instrument air pressure is too low, and the regulating valve cannot be fully opened and closed, or even the regulating valve does not act.
2. Failure of air filtration pressure reducing valve
There are too many stolen goods in the long-term use of the air filter pressure-reducing valve, and the pressure-reducing valve leaks air. The set output pressure of the pressure-reducing valve is too low, which makes the output instrument air pressure less than the specified pressure. As a result, the regulating valve is slow to move, unable to fully open and close or even not move.
3. Copper pipe connection failure
Air leakage from aging copper pipe, loose joint or stolen goods blocking copper pipe make the instrument signal wind down, resulting in that the regulating valve does not move and cannot fully open and close, and the unstable valve position in the manual state produces regulating oscillation.
4. Malfunction of the instrument air system
The air compressor station is abnormal, the purification air tank of the device is abnormal, the airline is frozen due to untimely water cutting, and the airline of the instrument leaks or is blocked by stolen goods, resulting in low air pressure or even no wind in the instrument of the device.
5. The valve of the instrument wind branch line is not opened, which causes the regulating valve not to move. It often occurs during the start-up period after overhaul and modification of the plant.
Power system failure
1. Loose, short-circuited, falling off and reverse polarity connection at the power cord terminal
Due to vibration in the field, loose connection caused by the weak connection or poor contact caused by too much dust, the signals of the control room arriving at the field are sometimes absent, resulting in chaotic movement of the control valve and adjustment oscillation.
Due to wiring errors, water ingress or damp of equipment, etc., the wiring of the power cord is short-circuited, so that the signal received by the regulating valve is lower than that of the regulator, resulting in that the regulating valve cannot be fully opened and closed.
2. The middle connector or the middle injured part of the power cord is faulty.
The power cord is pulled by environmental vibration and external force, the insulation performance of the insulation tape fails and the joint is baked at high temperature due to water ingress, which makes the power cord joint loose or seemingly broken, short circuit between power cords or short circuit to ground, and the connector or power cord break. As a result, the action of the regulating valve is discontinuous, unable to fully open and close, and does not act. During the maintenance, the middle connector of the power cord is connected reversely, which causes the regulating valve not to act.
3. The control valve is not controlled by the regulator.
During the start-up process after the overhaul and reconstruction of the device, the power cord is incorrectly connected or the configuration in the control room is wrong, resulting in the control valve not being controlled by the regulator.
Electrical converter failure
1. Zero point and measuring range are not allowed.
The zero point and measuring range of the output signal of the converter are inaccurate due to inaccurate installation and debugging, on-site vibration, temperature change and other reasons. As a result, the regulating valve cannot be fully opened and closed, and the leakage is large and limited.
In the field calibration of the converter, the accuracy of the indication of the converter signal meter should be ensured first. The signal meter should be maintained normally.
2. The orifice is blocked.
The instrument wind and dirt blocked the throttle orifice. Causing the regulating valve not to act.
3. The output is not linear.
Due to the aging of coils and components in the converter, or the influence of field vibration and ambient temperature, the output of the converter is not linear, which makes it unable to reach the required value in the process of adjusting it is zero point and range, and the regulating valve is not linear and cannot be fully opened and closed.
Valve positioner failure
Electric valve positioner
1. Zero point and measuring range are not allowed.
The minimum opening and maximum opening of the regulating valve are inconsistent with the signals in the control room due to inaccurate debugging during the installation of the positioner, field vibration, temperature change, valve stem stroke change and feedback rod position change. As a result, the signal output by the valve positioner cannot make the regulating valve fully open and closed, resulting in large leakage and limited leakage.
In the on-site calibration of the positioner, it should be ensured that the regulating valve works well, the feedback system is firmly installed and works well, and then it is adjusted by standard signals. Make the stroke of the regulating valve consistent with the control signal.
2. The orifice is blocked.
Clogging the dirt orifice. The positioner has no output signal, which leads to the non-action of the regulating valve.
3. There are stolen goods between nozzles and baffles.
Affected by the field environment, a layer of dust will adhere to the positioner after being used for a period of time, which will affect the backpressure of the nozzle baffle, thus affecting the output of the positioner. The regulating valve is unstable and oscillated.
4. Poor sealing
All kinds of fastening nuts and sealing gaskets of locators used for a long time are prone to lose and aging, which causes air leakage of locators. The regulating valve cannot be fully opened and closed, and the valve position is unstable, resulting in regulating oscillation.
5. The feedback rod is faulty.
During long-term operation, the fastening nut of the feedback rod gradually loosens or even falls off, which causes the feedback rod to become loose, skewed, jammed with the fixed parts and falls off. It is difficult to stabilize the controlled parameters, especially in temperature control where the action of the regulating valve requires accuracy.
6. The fixing nut is loose.
The fixing nut of the positioner is loose due to improper installation, which causes the positioner to be skewed, affects the action of the feedback rod, and causes the jamming phenomenon. Make the regulating valve move unsteadily, resulting in limit and other phenomena.
The fastening screws of various springs in the positioner are loosened under the vibration environment, which changes the pre-tightening amount of the spring and affects the tension and state of the spring. The zero range of the positioner is changed, and the positioner is not linear so that the regulating valve cannot be fully opened and closed, and the action of the regulating valve is not linear.
7. The position of the permanent magnet changes.
Due to the external force, the positions of the two magnets changed, which changed the position of the magnetic field. The coil was unbalanced in force and the positioner output was non-linear, which led to the non-linear action of the regulating valve. Magnets adsorb impurities such as iron pins, which form a jam that hinders the movement of the baffle, making the output of the positioner inaccurate, thus making the action of the regulating valve inconsistent with the control signal.
Intelligent locator
1. The feedback rod is faulty.
The fastening nut of the feedback rod is loose or even falls off, which causes the feedback rod to be loose, skewed, jammed with the fixing piece and fall off. The regulating valve moves slowly, fluctuates frequently, limits the regulating valve and even loses control.
If the positioner is not firmly fixed, it will be skewed and loose, which will affect the activity of the feedback rod and cause the jamming phenomenon to limit the regulating valve.
The limit spring on the feedback plate falls off, or the feedback rod escapes from it, resulting in poor contact between the feedback rod and the feedback plate, resulting in hysteresis and frequent actions of the regulating valve. It is difficult to stabilize the controlled parameters, especially in temperature control where the action of the regulating valve requires accuracy.
2. The positioner is not well adjusted.
The middle position of the calibration was not found properly, the regulating valve was not fully closed during manual output, and the selection of air opening and closing was not equal. The regulating valve cannot be fully opened and closed, resulting in large leakage and limit.
3. Because the adjustment of the intelligent positioner is complicated and takes a long time, and it needs to be fully opened and closed many times, which has great fluctuation to the process, the adjusting valve should be cut out during adjustment, especially when adjusting the valve to control the temperature, so it must be adjusted offline.
Failure of regulating valve itself
1. The leakage of the regulating valve is large, and there is a gap between the valve core and the valve seat when the regulating valve is fully closed, which causes a large flow of medium when the valve is fully closed, and the controlled parameters are difficult to stabilize.
2. Improper adjustment of valve stroke or long-term use of valve core during adjustment of valve core causes wear and corrosion of valve core head. Generally, the valve stem is adjusted downward to reduce the clearance to reduce the leakage.
3. The corrosion around the valve core is serious, and the valve core is scratched by welding slag, rust and slag in the medium, resulting in scars. The valve core should be taken out for grinding, and a new valve core should be replaced in severe cases.
4. The valve seat is seriously corroded by the medium, or the welding slag, rust, slag and other scratches in the medium cause scars, and the seal between the valve seat and the valve body is damaged. The valve seat should be taken out for grinding, and the sealing gasket should be replaced. In severe cases, a new valve should be replaced.
5. The valve is blocked with welding slag, rust, slag and other stolen goods, which makes the regulating valve unable to be fully closed. The regulating valve should be disassembled for cleaning, and at the same time, observe whether the valve core and valve seat are scratched and worn.
6. The sealing gasket between the spool and the valve seat of the sleeve valve is damaged, and the sealing ring of the disc valve is damaged, so that the throttle clearance is relatively large when the regulating valve is fully closed.
7. Packing failure of regulating valves
The friction between the valve stem and the packing makes it difficult for the regulating valve to move with a small signal, while the large-signal jumps and vibrates, resulting in large fluctuation of the regulating valve and difficulty in stabilizing its parameters. When the friction force is large, the regulating valve acts unidirectionally or even does not move. Lubricating oil or grease should be added regularly in daily maintenance. If the packing is seriously aged and leaked, the packing should be replaced.
8. The valve stem and fittings are loose or fall off. Due to the field vibration or loose fastening nut of fittings, the valve stem is too low to connect with fittings. In operation, the valve stem and the push rod of the actuator are not synchronized or fall off, which affects the action or even failure of the regulating valve.
9. The valve seat is stuck or blocked by foreign matter.
10. Regulating failure of the valve membrane head
The corrugated diaphragm of regulating valve is aged and deteriorated after long-term use, and its elasticity becomes smaller, its airtightness becomes worse, and even cracks and air leakage are serious. The elastic coefficient of compression spring changes or even breaks after aging.
11. Setting of PID parameters in the control system of regulating valves
Improper PID setting affects the action of the regulating valve and even causes the regulating valve to oscillate and adjust, which affects the service life of the valve.
12. Confirmation of process status
When the leakage of the regulating valve is large, confirm whether the secondary valve is fully closed; when the regulating valve is limited, confirm the valve opening degree before and after the regulating valve. Confirm whether there is a big fluctuation in the process flow when the controlled parameters change frequently.
13. When repairing the fuel oil regulating valve of the heating furnace, it is best to cut the regulating valve out of the putting-in-service line for operation, so as not to affect the production. If you don't cut out the valve that can be opened a little, you must make sure that the furnace is not turned off because the regulating valve is fully closed during maintenance.
The regulating valve, also known as the control valve, is the final control element in the field of industrial automation process control, which changes the process parameters such as medium flow, pressure, temperature and liquid level by receiving the control signal output by the regulating control unit. In many systems, the operating conditions such as temperature, pressure, corrosion and pollution of the regulating valve are more serious than those of other components.
Malfunction of the air supply system
1. The instrument wind line is blocked
Because the ball valve has a throttling function at the end of the wind line of the instrument branch, the stolen goods in the wind line are easy to accumulate and block here. As a result, the instrument air pressure is too low, and the regulating valve cannot be fully opened and closed, or even the regulating valve does not act.
2. Failure of air filtration pressure reducing valve
There are too many stolen goods in the long-term use of the air filter pressure-reducing valve, and the pressure-reducing valve leaks air. The set output pressure of the pressure-reducing valve is too low, which makes the output instrument air pressure less than the specified pressure. As a result, the regulating valve is slow to move, unable to fully open and close or even not move.
3. Copper pipe connection failure
Air leakage from aging copper pipe, loose joint or stolen goods blocking copper pipe make the instrument signal wind down, resulting in that the regulating valve does not move and cannot fully open and close, and the unstable valve position in the manual state produces regulating oscillation.
4. Malfunction of the instrument air system
The air compressor station is abnormal, the purification air tank of the device is abnormal, the airline is frozen due to untimely water cutting, and the airline of the instrument leaks or is blocked by stolen goods, resulting in low air pressure or even no wind in the instrument of the device.
5. The valve of the instrument wind branch line is not opened, which causes the regulating valve not to move. It often occurs during the start-up period after overhaul and modification of the plant.
Power system failure
1. Loose, short-circuited, falling off and reverse polarity connection at the power cord terminal
Due to vibration in the field, loose connection caused by the weak connection or poor contact caused by too much dust, the signals of the control room arriving at the field are sometimes absent, resulting in chaotic movement of the control valve and adjustment oscillation.
Due to wiring errors, water ingress or damp of equipment, etc., the wiring of the power cord is short-circuited, so that the signal received by the regulating valve is lower than that of the regulator, resulting in that the regulating valve cannot be fully opened and closed.
2. The middle connector or the middle injured part of the power cord is faulty.
The power cord is pulled by environmental vibration and external force, the insulation performance of the insulation tape fails and the joint is baked at high temperature due to water ingress, which makes the power cord joint loose or seemingly broken, short circuit between power cords or short circuit to ground, and the connector or power cord break. As a result, the action of the regulating valve is discontinuous, unable to fully open and close, and does not act. During the maintenance, the middle connector of the power cord is connected reversely, which causes the regulating valve not to act.
3. The control valve is not controlled by the regulator.
During the start-up process after the overhaul and reconstruction of the device, the power cord is incorrectly connected or the configuration in the control room is wrong, resulting in the control valve not being controlled by the regulator.
Electrical converter failure
1. Zero point and measuring range are not allowed.
The zero point and measuring range of the output signal of the converter are inaccurate due to inaccurate installation and debugging, on-site vibration, temperature change and other reasons. As a result, the regulating valve cannot be fully opened and closed, and the leakage is large and limited.
In the field calibration of the converter, the accuracy of the indication of the converter signal meter should be ensured first. The signal meter should be maintained normally.
2. The orifice is blocked.
The instrument wind and dirt blocked the throttle orifice. Causing the regulating valve not to act.
3. The output is not linear.
Due to the aging of coils and components in the converter, or the influence of field vibration and ambient temperature, the output of the converter is not linear, which makes it unable to reach the required value in the process of adjusting it is zero point and range, and the regulating valve is not linear and cannot be fully opened and closed.
Valve positioner failure
Electric valve positioner
1. Zero point and measuring range are not allowed.
The minimum opening and maximum opening of the regulating valve are inconsistent with the signals in the control room due to inaccurate debugging during the installation of the positioner, field vibration, temperature change, valve stem stroke change and feedback rod position change. As a result, the signal output by the valve positioner cannot make the regulating valve fully open and closed, resulting in large leakage and limited leakage.
In the on-site calibration of the positioner, it should be ensured that the regulating valve works well, the feedback system is firmly installed and works well, and then it is adjusted by standard signals. Make the stroke of the regulating valve consistent with the control signal.
2. The orifice is blocked.
Clogging the dirt orifice. The positioner has no output signal, which leads to the non-action of the regulating valve.
3. There are stolen goods between nozzles and baffles.
Affected by the field environment, a layer of dust will adhere to the positioner after being used for a period of time, which will affect the backpressure of the nozzle baffle, thus affecting the output of the positioner. The regulating valve is unstable and oscillated.
4. Poor sealing
All kinds of fastening nuts and sealing gaskets of locators used for a long time are prone to lose and aging, which causes air leakage of locators. The regulating valve cannot be fully opened and closed, and the valve position is unstable, resulting in regulating oscillation.
5. The feedback rod is faulty.
During long-term operation, the fastening nut of the feedback rod gradually loosens or even falls off, which causes the feedback rod to become loose, skewed, jammed with the fixed parts and falls off. It is difficult to stabilize the controlled parameters, especially in temperature control where the action of the regulating valve requires accuracy.
6. The fixing nut is loose.
The fixing nut of the positioner is loose due to improper installation, which causes the positioner to be skewed, affects the action of the feedback rod, and causes the jamming phenomenon. Make the regulating valve move unsteadily, resulting in limit and other phenomena.
The fastening screws of various springs in the positioner are loosened under the vibration environment, which changes the pre-tightening amount of the spring and affects the tension and state of the spring. The zero range of the positioner is changed, and the positioner is not linear so that the regulating valve cannot be fully opened and closed, and the action of the regulating valve is not linear.
7. The position of the permanent magnet changes.
Due to the external force, the positions of the two magnets changed, which changed the position of the magnetic field. The coil was unbalanced in force and the positioner output was non-linear, which led to the non-linear action of the regulating valve. Magnets adsorb impurities such as iron pins, which form a jam that hinders the movement of the baffle, making the output of the positioner inaccurate, thus making the action of the regulating valve inconsistent with the control signal.
Intelligent locator
1. The feedback rod is faulty.
The fastening nut of the feedback rod is loose or even falls off, which causes the feedback rod to be loose, skewed, jammed with the fixing piece and fall off. The regulating valve moves slowly, fluctuates frequently, limits the regulating valve and even loses control.
If the positioner is not firmly fixed, it will be skewed and loose, which will affect the activity of the feedback rod and cause the jamming phenomenon to limit the regulating valve.
The limit spring on the feedback plate falls off, or the feedback rod escapes from it, resulting in poor contact between the feedback rod and the feedback plate, resulting in hysteresis and frequent actions of the regulating valve. It is difficult to stabilize the controlled parameters, especially in temperature control where the action of the regulating valve requires accuracy.
2. The positioner is not well adjusted.
The middle position of the calibration was not found properly, the regulating valve was not fully closed during manual output, and the selection of air opening and closing was not equal. The regulating valve cannot be fully opened and closed, resulting in large leakage and limit.
3. Because the adjustment of the intelligent positioner is complicated and takes a long time, and it needs to be fully opened and closed many times, which has great fluctuation to the process, the adjusting valve should be cut out during adjustment, especially when adjusting the valve to control the temperature, so it must be adjusted offline.
Failure of regulating valve itself
1. The leakage of the regulating valve is large, and there is a gap between the valve core and the valve seat when the regulating valve is fully closed, which causes a large flow of medium when the valve is fully closed, and the controlled parameters are difficult to stabilize.
2. Improper adjustment of valve stroke or long-term use of valve core during adjustment of valve core causes wear and corrosion of valve core head. Generally, the valve stem is adjusted downward to reduce the clearance to reduce the leakage.
3. The corrosion around the valve core is serious, and the valve core is scratched by welding slag, rust and slag in the medium, resulting in scars. The valve core should be taken out for grinding, and a new valve core should be replaced in severe cases.
4. The valve seat is seriously corroded by the medium, or the welding slag, rust, slag and other scratches in the medium cause scars, and the seal between the valve seat and the valve body is damaged. The valve seat should be taken out for grinding, and the sealing gasket should be replaced. In severe cases, a new valve should be replaced.
5. The valve is blocked with welding slag, rust, slag and other stolen goods, which makes the regulating valve unable to be fully closed. The regulating valve should be disassembled for cleaning, and at the same time, observe whether the valve core and valve seat are scratched and worn.
6. The sealing gasket between the spool and the valve seat of the sleeve valve is damaged, and the sealing ring of the disc valve is damaged, so that the throttle clearance is relatively large when the regulating valve is fully closed.
7. Packing failure of regulating valves
The friction between the valve stem and the packing makes it difficult for the regulating valve to move with a small signal, while the large-signal jumps and vibrates, resulting in large fluctuation of the regulating valve and difficulty in stabilizing its parameters. When the friction force is large, the regulating valve acts unidirectionally or even does not move. Lubricating oil or grease should be added regularly in daily maintenance. If the packing is seriously aged and leaked, the packing should be replaced.
8. The valve stem and fittings are loose or fall off. Due to the field vibration or loose fastening nut of fittings, the valve stem is too low to connect with fittings. In operation, the valve stem and the push rod of the actuator are not synchronized or fall off, which affects the action or even failure of the regulating valve.
9. The valve seat is stuck or blocked by foreign matter.
10. Regulating failure of the valve membrane head
The corrugated diaphragm of regulating valve is aged and deteriorated after long-term use, and its elasticity becomes smaller, its airtightness becomes worse, and even cracks and air leakage are serious. The elastic coefficient of compression spring changes or even breaks after aging.
11. Setting of PID parameters in the control system of regulating valves
Improper PID setting affects the action of the regulating valve and even causes the regulating valve to oscillate and adjust, which affects the service life of the valve.
12. Confirmation of process status
When the leakage of the regulating valve is large, confirm whether the secondary valve is fully closed; when the regulating valve is limited, confirm the valve opening degree before and after the regulating valve. Confirm whether there is a big fluctuation in the process flow when the controlled parameters change frequently.
13. When repairing the fuel oil regulating valve of the heating furnace, it is best to cut the regulating valve out of the putting-in-service line for operation, so as not to affect the production. If you don't cut out the valve that can be opened a little, you must make sure that the furnace is not turned off because the regulating valve is fully closed during maintenance.
