日期: 2019-03-20

What are the influencing factors and treatment measures for the faults of control valves

浏览次数: 13

In recent years, with the continuous development of science and technology in China, the automation level of enterprise production has been continuously improved. Regulating valve is an important part of ensuring the automation of production process. Regulating valve is the terminal executing device in the automatic control system, which adjusts the production process by receiving the control signal. The control valve has the advantages of simple structure, high stability, low price, fire-proof and explosion-proof. It can be used with pneumatic and electric regulating instruments, and has a high degree of automation.

Advantages of Regulating Valve in Use

(1) Be agile and able to complete all adjusting orders in time;

(2) When used with large cylinder, it has greater momentum driving force.

(3) In harsh working environment, the performance is stable and can operate normally.

(4) High safety performance. Normal operation and agility of the control valve will have a direct impact on the quality and efficiency of production.

Therefore, it is particularly important to analyze and solve the influencing factors of the failure of the control valve in the use process.

1. Common Faults of Regulating Valve and Its Influencing Factors

1.1 card plugging

Blockage is a common problem in control valves, which often occurs in the new operation system and in the period of re-operation after overhaul. This is due to the accumulation of impurities such as welding slag and iron scraps in the throttle and guide parts of the pipeline, resulting in blockage. After blockage occurs, the medium flow will be blocked or the filling of the control valve will be too tight in the overhaul, so that the friction force will be increased, resulting in the phenomenon of small signal inactivity and large signal overaction.

1.2 Regulating Valve Leakage

Regulating valve leakage generally includes leakage caused by valve internal leakage, packing leakage, valve core and seat deformation. (1) Valve leakage is due to the inappropriate length of the valve stem, the length of the valve stem of the air-opened valve is too long, so that the distance between the valve stem up or down is too short, leading to the gap between the valve core and seat, there is no complete contact, resulting in valve leakage. (2) Packing leakage, filling into the packing box, will exert axial pressure on the control valve. Because of the plastic deformation of filler, radial pressure will be generated, which is in close contact with the valve stem, but the contact is not uniform. During the working process of the control valve, there will be axial movement between the stem and the packing. Under the high temperature and high pressure environment, it is easy to leak filler due to the influence of high permeability fluid medium. (3) The deformation and leakage of the valve core and seat are mainly due to the strong corrosion in the process of casting or forging and the erosion of the corrosive medium through the fluid medium during the production of the control valve. Corrosion is mainly caused by erosion and cavitation. When the valve passes through the highly corrosive medium, the spool and seat will be corroded and impacted, and the spool and seat will be deformed. With the passage of time, there will be a mismatch between the spool and the seat, and there will be a gap between them, which will lead to looseness and leakage.

1.3 Control Valve Oscillation

The reasons for the oscillation of the control valve are the insufficient spring stiffness of the control valve, the rapid change of the output signal of the control valve, the same frequency of the control valve and the system, and the violent vibration of the pipeline and the base. There are also inappropriate selection of control valve. When the small opening of the control valve works, drastic changes in flow resistance, velocity and pressure will occur. When this change exceeds the stiffness of the control valve, the stability of the control valve will be reduced, and in serious cases, the control valve will oscillate.

1.4 Regulating Valve Not Acting

The failure phenomena of non-operation of control valve include: (1) no signal and no air source. The causes are that the air source is not opened or the air source is malfunctioning, the air duct is blocked or the pressure relief valve is blocked because of failure, the compressor has a fault lamp; (2) there is air source, no signal. The reason is that the locator fails because of the leakage caused by the breakdown of bellows, or the converter fails; (3) the locator has no gas source. The causes are blockage of filters and vacuum reducers and failure; (4) the locator has air source but no output. The reason is that there is a blockage in the throttle hole of the locator and the fault occurs; (5) there is a signal, no action. The reason is that the spool drops off; the stem bends or breaks.

1.5 unstable operation of control valve

The reasons for unstable operation of control valve are: (1) the compressor capacity is too small, resulting in the failure of pressure relief valve, resulting in unstable air pressure; (2) the regulator is unstable, the time constant of control system is not appropriate, resulting in unstable signal pressure; (3) the friction resistance of valve stem in motion is very large, and the phenomenon of blockage occurs in the contact process; (4) The ball valve of the amplifier in the positioner is not tightly closed by the abrasion of dirt, and the output oscillation will occur when the gas consumption is especially increased; the nozzle baffle of the amplifier in the positioner is not parallel, and the baffle can not cover the nozzle.

1.6 Regulating Valve Acting Slowly

Pneumatic film breaks and leaks in operation, which makes the stem slow in unidirectional action; weak positioner response will lead to the slow action of the control valve; there are blockages such as rust in the control valve, which causes the slow action of the control valve in axial motion; poor quality of the filler or too tight filling of the filler. The friction resistance increases, which leads to the phenomenon of slow action of the control valve.

2. Troubleshooting Measures of Regulating Valve and Maintenance of Regulating Valve

2.1 Control Valve Fault Handling Measures

(1) After the blockage problem occurs, the control valve should be switched on and off quickly, and the blockage can be rushed away by using fluid medium; the stem can also be clamped by pipe pliers, and under the action of external signal pressure, the stem can be dazzled by positive and negative forces, so that the valve core flashes through the blockage place. If the plugging phenomenon can not be solved at this time, the external pressure and driving power can be increased appropriately, and repeated movement can be carried out. If the action can not be restored, professionals need to deal with the disintegration of the control valve.

(2) After the internal leakage of the control valve, the length of the valve stem should be adjusted appropriately so that the length of the valve stem is suitable and no internal leakage will occur; after the leakage of the filler, in order to make the filler convenient, it is necessary to chamfer the top of the filler box, and add a metal protection ring with a small gap at the bottom of the filler box. The contact surface between the metal protection ring and the filler can not make the inclined plane, so as to prevent the filler from being pushed out under the medium pressure. The surface of the contact part between the filler box and the filler should be finely machined, which can improve the surface finish and reduce the wear of the filler. The best filler is flexible graphite, which has high air tightness, low friction, low wear after long-term use, easy maintenance, high pressure and heat resistance. Deformation and leakage of the valve core and seat should be strictly selected, the material of the valve core and seat should be strictly selected, the corrosion-resistant high material should be selected, the defective products such as pitting and trachoma should be put out, so that the valve core and seat have a high sealing finish. When the wear and damage of the valve core and seat is serious, a new valve should be replaced.

(3) Control valve oscillation, when the oscillation is relatively minor, can increase the spring stiffness, such as the control valve to choose a spring with high stiffness; when the pipeline, the base vibration, to increase support to eliminate vibration interference; valve frequency and system frequency are the same to replace the control valve.

(4) When the control valve is not in operation, the pressure relief valve should be adjusted so that the pressure can reach the prescribed gas source pressure; the leakage points of the locator joints and pipelines should be checked, tightened or replaced; the enlargement and throttle holes of the locator should be checked and repaired.

(5) The operation of the control valve is unstable. The signal pipeline, joint, sealing ring and other places should be inspected to eliminate leakage; the loose parts should be found and tightened; and lubricating oil should be added to the parts with greater friction.

(6) The control valve moves slowly, checks the parts with larger friction force, and reassembles them; adjusts the packing cap and replaces the packing.

Maintenance of 2.2 Regulating Valve

The failure of the control valve in use will cause great harm to the normal and stable operation of the system. Therefore, the maintenance of the control valve should be carried out to reduce the failure rate of the control valve.

(1) Passive maintenance. When the control valve fails, the control valve should be repaired to find out the cause of the failure, and timely maintenance.

(2) Preventive maintenance requires regular cleaning of the control valve, keeping its hygiene and integrity and durability of its components, regular flushing of the pipeline, paying attention to blockages and garbage in the pipeline, and regular inspection of the fixed fittings of the control valve to ensure the corrosion resistance and lubricity of the components. When installing the control valve, the phenomenon of hard pull and hard bend in the installation can be reduced, the pressure can be reduced and the deformation can be reduced. Keep the distance between the control valve and the compressor, and reduce the vibration from the vibration source. Regular inspection of the control valve should be carried out. Close attention should be paid to the operation of the control valve, whether the filler leaks or not, and lubricating oil should be added to the filler regularly.

(3) Predictive maintenance, through intelligent instruments or other diagnostic equipment to obtain the operation information of the control valve, strengthen the control quality and diagnostic function of the control valve. Through the diagnosis of intelligent instrument or other diagnostic equipment, the problems of the control valve can be found in time, and the solutions can be found in time for maintenance. Increase the development and use of intelligent valve positioner.

3 conclusion

With the continuous development of science and technology in our country, the automation degree of enterprise production is constantly improving. Regulating valve is an important part of ensuring the automation of production process. Regulating valves play an important role in the production process and are the basis of production process automation. Regulating valve's normal use plays an important role in guaranteeing the quality and safety of production, and is an important guarantee to maintain the normal operation of process. The advantages of the control valve are simple structure, high stability, low price, fire and explosion-proof. It can be used with pneumatic and electric regulating instruments, and has a high degree of automation. The failure of control valve in use will directly affect the safe and stable operation of the whole system. There are many reasons for the failure of the control valve. It is necessary to analyze the different reasons for the failure and to maintain the control valve in accordance with the actual working conditions. It plays an important role in ensuring the safe and stable operation of the equipment and safe production. It also provides guidance for the management of the control valve in enterprises.

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