Failure Analysis and Maintenance of Control Valve

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Failure analysis of control valve

Various faults occur during the normal operation of the control valve, which can come from the actuator, adjustment mechanism or connected accessory device. Fault analysis is used to locate the fault and determine the solution.

Failure analysis of the actuator

■ Failure caused by the air tightness of the actuator

The failure caused by the air tightness of the actuator is manifested as an increase in response time and a sluggish valve stem action. analyse as below.

  1. The diaphragm of the pneumatic diaphragm actuator is not compressed. The diaphragm is not compressed or the force is not evenly caused to leak the input air signal, which makes the response of the actuator to the signal change sluggish, and the response time increases. If a valve positioner is installed, its influence will be reduced. The troubleshooting method is to brush with soapy water to check and eliminate the leakage point.
  2. The piston seal ring of the pneumatic piston actuator is worn. As a result, the control valve cannot respond quickly and the valve stem is not sensitive. The troubleshooting method is to replace the sealing ring and check whether the inner wall of the cylinder is worn.
  3. The diaphragm of the pneumatic diaphragm actuator is damaged. It shows that the valve stem is not sensitive and the sound of gas leakage can be heard. The troubleshooting method is to replace the diaphragm and check whether the limit device or the tray has burrs.
  4. The connecting pipeline is leaking. As a result, the valve stem is not sensitive and the response time increases. The troubleshooting method is to paint the connecting pipeline with soapy water, check for leaks, and replace or weld.

■ Failure caused by unbalanced force

The failure caused by unbalanced force is manifested as unstable operation of the control valve and loose closing. The failure analysis is as follows.

  1. Improper flow. Improper installation of the control valve causes the actual fluid flow direction to be inconsistent with the marked flow direction of the control valve, and changes the unbalanced force. For example, the flow-off control valve is installed to flow-on. The troubleshooting method is to reinstall.
  2. The actuator does not match. Insufficient thrust or thrust torque results in insufficient control valve action. The troubleshooting method is to replace the actuator.

■ Failure caused by packing

The faults caused by the packing are manifested as increased leakage, increased friction and the beating of the valve stem. analyse as below.

  1. The packing material is inappropriate. The fault caused by improper packing material is mainly the increase of external leakage and increased friction. For example, in high-temperature applications, PTFE packing is used. The troubleshooting method is to replace the packing.
  2. Improper design of packing structure. o In the packing cavity, the packing and related accessories are not installed properly, and the packing height is not suitable. The troubleshooting method is to install the packing and related accessories according to the requirements of the product manual.
  3. The packing is not installed properly. For example, the graphite packing adopts spiral installation, which causes uneven packing pressure and misalignment of the center. The troubleshooting method is to install in layers to make the pressing force uniform.
  4. There is debris in the filler. The debris in the packing caused the valve stem to scratch. The troubleshooting method is to clean the filler and remove debris
  5. Improper installation of the upper valve cover. Improper installation of the upper valve cover causes uneven stress on the packing. The troubleshooting method is to reinstall the gasket of the upper valve cover, and evenly tighten the upper valve cover fixing bolts diagonally.

■ Failure of electric actuator

In addition to common circuit short-circuits or open circuits, the faults of electric actuators also include faults such as servo amplifiers and motors. Common failure analysis is as follows.

  1. The connectors are loose or the wiring is open or short-circuited. Cause poor contact, and increase or decrease the impedance of the relevant line. The troubleshooting method is to check and move the connecting wires, re-plug and insert the connectors.
  2. The mechanical transmission parts of the reducer. Check whether the operation is normal and whether the gear meshes well. The troubleshooting method is to replace or repair the broken gear and add lubricant.
  3. Power supply. Check whether the fuse is blown, whether the servo amplifier position feedback has smoke and special odors, such as the burning of the insulation layer of the transformer housing and the scorching smell. The troubleshooting method is to replace the damaged components.

Failure analysis of adjustment mechanism

■ Failure caused by mismatched flow characteristics

The flow characteristics of the control valve are used to compensate for the different characteristics of the controlled object. If the selected flow characteristics are inappropriate, the control quality of the control system will deteriorate. For example, the sensitivity of the control system is different when the flow is small and large.

The failure analysis is as follows

  1. The controlled object has saturated nonlinear characteristics (for example, temperature control system). When the flow rate is small, the control system can operate normally, but when the flow rate is large, the control system is sluggish. Or when the flow rate is small, the control system is extremely sensitive, even oscillating and unstable, but when the flow rate is large, the control system can operate normally. The cause of the failure is the selection of a linear or quick-open flow characteristic control valve. The troubleshooting method is to replace the valve trim or control valve of the control valve, or install a valve positioner to make the control valve meet the requirements of equal percentage or parabolic flow characteristics.
  2. The controlled object has linear characteristics (for example, the flow follow-up control system). When the flow rate is small, the control system operates normally, and when the flow rate is large, the control system oscillates or becomes unstable. Or the control system is sluggish when the flow is small, and the control system can operate normally when the flow is large. The cause of the fault is the selection of equal percentage or parabolic flow characteristic control valve. The troubleshooting method is to replace the internal parts of the control valve or the control valve, or install a valve positioner to make the control valve meet the requirements of linear flow characteristics.
  3. Improper selection of the rated flow coefficient of the control valve. The selected rated flow coefficient is too large or too small, so that the adjustable minimum or maximum flow rate of the control valve becomes larger or smaller, which cannot meet the operating requirements of the process production process. The control valve works in a small opening or a large opening position, and the control quality becomes poor. The troubleshooting method is to recalculate the flow coefficient of the control valve and install a control valve that meets the requirements. For example, selecting a control valve directly according to the diameter of the process pipeline causes the rated flow coefficient to be too large, and the rated flow coefficient is too small due to the expansion of the production scale.

■ Failure caused by improper flow design and installation

Faults caused by improper design or installation of the control valve flow path are manifested as increased noise, dirt is likely to accumulate inside the valve body, and the control valve is not tightly closed, increased leakage or stuck. The failure analysis is as follows.

  1. The leakage of the double seat valve increases. The double-seat valve does not adopt an integrated design, resulting in a different expansion coefficient of the valve trim when the temperature changes, which increases the leakage. The troubleshooting method is to use an integrated double-seat valve or a sleeve valve with balance function.
  2. When the three-way valve is used for merging, the leakage will increase due to the different temperatures of the two fluids that merge. The troubleshooting method is to change the confluence of fluids to split flow control, and install a three-way valve before the heat exchanger to ensure the fluid temperature is consistent.
  3. Improper flow direction causes increased noise. For example, flow-open control valves are used in flow-close situations, causing increased noise at low flow rates. The troubleshooting method is to check the flow and reinstall.
  4. Improper installation of upstream and downstream shut-off valves and bypass valves. Cause dirt, condensate or non-condensable gas can not be discharged. The troubleshooting method is that the blowdown valve is installed at the lowest point of the control valve group, and the vent valve is installed at the highest point of the control valve group.
  5. Improper installation of guide bushing. Cause the center to be misaligned, increase friction, and jam the valve stem. The troubleshooting method is to reinstall the guide bushing.

■ Failure caused by leakage

Internal leakage causes the adjustable ratio to drop, and in severe cases, the control system cannot meet the process operation and control requirements. External leakage causes environmental pollution and increases costs. The failure analysis is as follows.

  1. Increased leakage due to cavitation and cavitation. The valve core and valve seat are damaged due to cavitation, flashing and cavitation, and the increase of the leakage of the control valve appears as an increase in gas or liquid dynamic noise. The troubleshooting method is to check the internal parts of the valve, replace or grind the valve core, valve seat, and valve core surfacing cemented carbide, reduce the pressure drop at both ends of the control valve, eliminate noise sources, and adopt low-noise control valves.
  2. Increased leakage due to the controlled fluid containing debris. In the start-up phase, impurities are often entered into the control valve due to irregular operations in which the control valve is not removed during pipe purge, or during operation, the debris entrained by the controlled fluid accumulates inside the valve body. These debris cause the valve The sealing surface of the core and the valve seat is damaged, which increases the leakage. The troubleshooting method is to grind the valve core and valve seat, remove the control valve when the pipeline is purged, and install a filter device upstream of the control valve for the controlled fluid containing particles, and install the control valve group at a higher position, and regularly Perform sewage.
  3. The connection between the actuator and the adjustment mechanism is improper. The troubleshooting method is to reinstall and perform a leakage test.
  4. Improper installation of packing. Due to improper installation of the packing, friction increases or the valve stem is deformed. The troubleshooting method is to reinstall the packing and reshape the deformed valve stem.
  5. Improper flange installation. The uneven force causes external leakage. The troubleshooting method is to reinstall the connecting flange and gasket, and evenly press the connecting flange.
  6. The wear of the valve core and valve seat caused by fluid flow. The troubleshooting method is to grind the valve core and valve seat.
  7. Improper installation of the packing causes increased friction, and loose control valve closing causes an increase in external leakage. The troubleshooting method is to reinstall the packing to reduce friction.
  8. Improper flow direction causes increased leakage. Improper selection of flow direction will increase the unbalanced force and increase the leakage. The troubleshooting method is to check the design drawings and reinstall.

■ Failure caused by spool falling off

Before the spool falls off, the control valve will present greater mechanical noise. After the fault occurs, the control system cannot adjust normally, and the controlled variable suddenly rises or falls. The failure analysis is as follows.

  1. The flow path design of the control valve is unreasonable, causing the valve core to oscillate and receive shearing force. During long-term operation, the connecting pin between the valve core and the valve stem is broken, and the valve core falls off. The troubleshooting method is to check the control valve flow path and replace the pins.
  2. The spool connecting pin is not installed firmly, causing the spool to fall off. The troubleshooting method is to reinstall the pins and tighten them.

■ Failure of valve positioner

The malfunction of the valve positioner deteriorates the characteristics of the cascade secondary ring. Since the valve positioner is in the secondary loop of the cascade control system, it has a certain adaptability. The failure of the valve positioner is manifested as the control system is unstable and stuck.

Failure caused by improper valve positioner cam

The malfunction caused by improper valve positioner cam is similar to the malfunction caused by improper flow characteristics of the control valve. It makes the control system unstable or sluggish at different operating points. The troubleshooting method is to select the appropriate valve positioner cam according to the characteristics of the controlled object and the flow characteristics of the control valve, and debug after installing the cam.

Failure caused by valve positioner amplifier

The faults of the valve positioner amplifier include clogged orifice and excessive amplifier gain. The former makes the output change slowly, and the latter makes the control system resonate. Therefore, the troubleshooting method is to check and dredge the throttling hole of the amplifier. When the amplifier gain is too large, the elastic force of the reed pressing the steel ball can be reduced or the amplifier can be replaced.

Failure caused by mismatched valve positioner detection rod

The mismatch of the valve positioner detection rod causes the dead zone to increase, and the feedback signal of the valve position cannot be accurately reflected in time. Therefore, the control quality of the control system deteriorates. The troubleshooting method is to check and reinstall the feedback detection lever.

■ Common faults of pneumatic system

The poor quality of the air supply of the pneumatic control valve is the most common pneumatic system failure.

Analysis of the influence and failure caused by moisture

Moisture is formed when the compressor absorbs moist air and cools down. Moisture makes the components of the pneumatic device rust and affects the action of the pneumatic components. The effects of moisture are as follows.

  1. Piping. It causes rust inside the pipeline; corrosion of the pipeline causes air leakage and rupture of the container; moisture retention at the bottom of the pipeline causes insufficient air flow and increases pressure loss.
  2. Components. The rust of the pipeline accelerates the filter mesh blockage, making the filter inoperable; the rust debris in the pipe enters the valve, causing malfunction and air leakage; the rust debris makes the components bite and cannot operate smoothly; directly affects the parts of the pneumatic components, Causes poor conversion, air leakage and unstable operation; water droplets invade the valve body and cause poor operation; water droplets enter the components and make it impossible to run smoothly; water droplets flush lubricating oil, causing poor lubrication, valve action failure, and unstable operation of the actuator ; Water droplets in the valve cause insufficient flow and increase pressure loss; water hammer phenomenon causes component damage.
  3. Environment. The discharged water discharged from the exhaust port pollutes the environment. The fault treatment method that can be used to deal with the fault caused by moisture is to remove water, that is, the compressor outlet temperature drops to the point that the water contained in it can analyze and remove water droplets. For this reason, coolers and separators should be installed and installed after the compressor, and air filters should be installed in the compressor population. The horizontal pipeline has a certain slope, and a drain valve is installed at the lower end. Install a dryer at the outlet.

The available water removal measures are as follows.

  1. Adsorption and water removal method: use materials with strong adsorption capacity to absorb water, such as silica gel, aluminum glue and molecular sieves
  2. Pressure dehumidification method: increase the pressure, reduce the volume and lower the temperature, so as to precipitate water droplets.
  3. Mechanical dewatering: Dewatering with mechanical blocking, cyclone separation, etc.
  4. Freezing and dewatering: Use refrigeration equipment to cool the air below the dew point, so that water vapor condenses into water and precipitates.

Influence and failure analysis caused by oil

Compressor lubricating oil is mixed into the compressed air in the form of oil mist, and is sent out with the compressed air under heat, which is the reason why the compressed air contains oil. The influence of oil content is as follows.

  1. The sealing ring is deformed. The sealing ring shrinks, the air leakage valve fails to operate, and the output force of the actuator is insufficient; the sealing ring swells with oil, the friction increases, the valve cannot move or the output force of the actuator is insufficient; the sealing ring is hardened, the friction surface is worn, and the air

Increased leakage; increased friction, and poor valve and actuator actions.

  1. Environment. The place where industrial raw material chemicals directly contact the air causes the nature of the raw chemicals to change; industrial furnaces and other places that directly come into contact with flames cause fire hazards; air measuring instruments fail due to clogging of nozzles; extremely oily environments are required due to valves and actuators The leaking oil in the sealing part causes environmental pollution, and the oil removal method is to use a degreasing filter. For example, use a centrifugal filter to remove oil mist particles, use activated carbon to adsorb or use a porous filter element to remove oil.

Analysis of influence and failure caused by dust

The compressor sucks in dusty air and flows into the pneumatic device, causing friction, damage and increasing friction of pneumatic components. The effects of dust are as follows.

  1. Control components. The control elements are rubbed, worn and stuck, and the action fails and cannot be reversed; it affects the stability of the pressure regulation.
  2. Actuator. Actuating elements are rubbed, worn and stuck, causing malfunction; reduce output force.
  3. Pneumatic components with throttling parts such as amplifiers. Block the orifice of the nozzle baffle and cause it to malfunction due to oil pollution. The dust removal method is to install an air filter at the compressor suction port, and then filter it with an air filter before entering the pneumatic device. Clean the filter regularly or replace.

Maintenance of control valve

The control valve must be maintained and maintained after normal operation. As a part of the automation control system, the control valve should be maintained at the same time as the automation instrument and other equipment.

The maintenance of control valves is similar to that of general instruments, and can be divided into passive maintenance, preventive maintenance and predictive maintenance. Passive maintenance is a maintenance method in which equipment such as control valves fails. Maintenance is only due to equipment failures, which often results in shutdowns during the production process, or even equipment damage or casualties in severe cases. Passive maintenance is maintenance that is not desired in the production process, and preventive maintenance is a maintenance method that is based on past operating experience and time-based maintenance. For example, the commonly used regular maintenance is preventive maintenance, which formulates corresponding maintenance schedules according to the operating conditions of different equipment, and performs maintenance before the equipment fails. Since maintenance is carried out without failure, the probability of failure can be greatly reduced. However, this maintenance method does not analyze the actual situation of the currently used control valve, and often disassemble and inspect the control valve that can be used for a certain period of time, which wastes time and resources. Predictive maintenance starts from the data analysis of the currently used control valve and predicts the state of the control valve, so that the control valve can be used to the maximum.

Daily maintenance of control valves

■ Skill requirements of control valve maintenance personnel

The skill requirements of control valve maintenance personnel and instrument maintenance personnel are similar. Maintenance personnel should have the following conditions:

  1. Familiar with the maintenance procedures of control valves and the technical requirements of control valve product manuals.
  2. Familiar with the control valve structure, working principle and technical requirements for disassembly and assembly.
  3. Understand the role of the corresponding control valve in the process production process, and be familiar with the requirements of the process production process for the control valve o
  4. Familiar with the use of instruments and meters required for maintenance of control valves.

■ Daily maintenance of control valve

The daily maintenance work of the control valve is divided into two parts: tour inspection and regular maintenance. The tour inspection work is as follows.

  1. Learn about the operation of the control valve from the process operator on duty.
  2. Check the supply energy of the control valve and related accessories (air source, hydraulic oil or power source)
  3. Check the operation of the hydraulic oil system.
  4. Check the static and dynamic sealing points of the control valve for leakage.
  5. Check the control valve connection pipeline and joints for looseness or corrosion.
  6. Check the control valve for abnormal sound direction and large vibration, and check the supply situation.
  7. Check whether the action of the control valve is flexible and whether it changes in time when the control signal changes
  8. Listen to the valve core and valve seat for abnormal vibration or noise.
  9. If problems are found, contact them in time.
  10. Make a record of the touring inspection and file it.

The contents of regular maintenance are as follows:

  1. Clean the outside of the control valve regularly.
  2. Regularly adjust the control valve stuffing box and other sealing parts. If necessary, replace the sealing parts to maintain the tightness of the static and dynamic sealing points.
  3. Regularly add lubricating oil to the parts that need to be lubricated.
  4. Periodically drain and clean the air source or hydraulic filter system.
  5. Regularly check the connection and corrosion of each connection point, and replace the connection if necessary.

■ Periodic calibration of control valves

Units that have not yet carried out predictive maintenance of control valves should conduct regular inspections of control valves. Periodic calibration work is preventive maintenance work.

According to different production processes, the periodic calibration of control valves should have different calibration cycles. The information provided by the manufacturer can be combined to determine the period of regular calibration of each control valve. It can usually be carried out at the same time as the overhaul is carried out during the production process. When some control valves are used in high pressure, high pressure drop or strong corrosive occasions, the inspection cycle should be shortened.

The content of the inspection is mainly the static performance test of the control valve. Corresponding test items can be added when necessary, such as the test of the flow characteristics of the control valve. Periodic calibration requires related test equipment and instruments, as well as replacement parts. Therefore, the manufacturer can usually be entrusted to complete it.

■ Maintenance of control valve

Control valve maintenance is divided into emergency maintenance, regular maintenance and predictive maintenance. Emergency maintenance is the maintenance when the control valve fails and cannot meet the requirements of process operation. Regular maintenance usually includes daily maintenance and maintenance that is carried out simultaneously with process shutdown overhaul. Predictive maintenance is based on the analysis results of predictive maintenance, targeted maintenance of relevant control valve components. Emergency maintenance is the maintenance after the failure of the control valve, and regular maintenance and predictive maintenance are the maintenance before the failure of the control valve.

■ Main contents of daily maintenance

The main contents of daily maintenance of control valves and accessories are as follows.

  1. Replacement of diaphragm of pneumatic actuator. The diaphragm of the pneumatic film actuator is stretched during operation, so it is easy to fatigue and damage. The rubber diaphragm of the same specification should be used for replacement, and the diaphragm should be evenly stressed when tightening to prevent leakage and crushing of the diaphragm.
  2. Grinding. The valve core and the valve seat cause leakage after a certain period of operation, and the internal leakage between the piston and the cylinder body of the cylinder will also cause internal leakage. At this time, grinding should be carried out. Hand-grinding, mechanical grinding, coating treatment and inserting can be carried out. The emery grain size used for grinding should be suitable, and the grinding force should be uniform and suitable. After grinding, it should be polished to meet the required finish and precision requirements, and to meet the centering requirements of the valve core and the valve seat. After the final assembly, a tightness test is required.
  3. Replace the stuffing box. When replacing the stuffing box, the same type of stuffing box should be used. When replacing the stuffing box, you should carefully hook out the packing and remove the packing correctly to prevent damage to the valve stem. The installation of the new stuffing box should be in accordance with the requirements of the instructions. The incision should be misaligned to prevent the thread of the valve stem from scratching the packing. The packing should be evenly pressed against stress and increase friction.
  4. Replacement of transmission parts. If part of the transmission parts in the control valve and accessories are worn out, parts can be replaced and repaired. After replacement and repair, the transmission should be flexible and the transmission gap should be as small as possible.
  5. Cleaning of pneumatic amplifier. When the orifice of the pneumatic amplifier is blocked due to the dirt in the compressed air for the instrument, the orifice should be cleaned, and the appropriate steel wire can be used for dredging and cleaning. When reinstalling, the amplifier diaphragm should be evenly stressed to prevent blockage or leakage. The gain of the amplifier can be adjusted by adjusting the pressing force of the steel ball to prevent resonance.

Regular maintenance of control valves

Usually, the daily maintenance of the control valve is performed by the instrument maintenance personnel, and the regular maintenance performed at the same time as the overhaul is performed by the manufacturing technicians.

■ Routine maintenance of control valve

The routine maintenance work of the control valve includes the following.

  1. Eliminate stress. Various stresses caused by improper installation or combination. For example, high-temperature media produce thermal stress, which makes installation tight. Imbalance of solid force causes stress and so on. The imbalance of stress acts on the control valve, causing the control valve stem and guide to deform, failing to correctly align with the valve seat, causing leakage and increasing deterioration. Therefore, maintenance work to eliminate stress should be carried out in daily maintenance.
  2. Remove rust and dirt. Frequently check whether there is rust, welding slag, dirt, etc. in the connecting pipe of the control valve, and remove it in time after discovery. Because these dirt will cause the control valve spool and valve seat to wear and affect the normal operation of the control valve. Usually, filter devices such as filters can be installed in front of the control valve and cleaned regularly.
  3. Check the control valve support. The control vacuum valve support makes each part of the control valve in a position that is not affected by gravity. If the support is improper, the control valve stem and the valve seat will not be centered, which will increase the deterioration and reduce the sealing performance. Therefore, check whether the control valve support is appropriate.
  4. Remove the dirt that supplies energy such as air source and hydraulic oil. Air source and hydraulic source are the energy source for controlling valve operation. Impurities contained in the instrument compressed air and hydraulic oil can block the orifice and pipes, causing malfunctions. Therefore, it is very important to check the air source and hydraulic oil regularly, and to drain the filter device regularly.
  5. Inspection of gear transmission. The gear transmissions of the hand-wheel mechanism, electric actuator and hydraulic actuator should be checked regularly and lubricants should be added to prevent seizure. Check whether the brake and limit device are flexible and easy to use.
  6. Stuffing box inspection. The wear and compression force of the packing should be checked, and the stuffing box should be replaced regularly to ensure that the packing can seal while reducing the impact of its friction. Lubricating oil should not be added to the stuffing box without oil lubrication.
  7. Check for safe operation. For control valves and related accessories used in explosive hazardous locations, check their safe operation, such as whether the sealing cover is tightened, the operation of the safety barrier, and the power supply to ensure the safe operation of the control valve and related accessories.
  8. Transportation and storage. During transportation and storage, the control valve should be fixed with a special bracket to prevent loosening; related accessories installed on the control valve, such as valve positioner, handwheel mechanism, etc., should be firm, and the feedback lever connected to the control valve should be prevented from being subjected to external force Damage; each connection interface shall be covered with plastic film to prevent the intrusion of foreign objects; the connection port of the control valve can be sealed with matching flanges and blind plates, or adhesive paper can be used to seal to prevent foreign objects from intruding. A firm wooden box should be installed during transportation, and the impact of harsh transportation environmental conditions such as wind, sand, rain and dust should be taken. The environmental conditions for transportation and storage should meet the requirements of the product specification
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