Maintenance skills of the hottest VDC regulating v

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Maintenance skills of VDC control valve

1. Raising problems

in industrial automation instruments, VDC pneumatic control valve is widely used and plays an important role in the production of highly automated enterprises, but it is often ignored because of its simple structure. In the process of chemical production, the working conditions of instrument regulation are relatively complex. Once there is a problem with the pneumatic control valve installed on the key process pipeline, it will have a great impact on production, and the psychological pressure of instrument maintenance personnel is also large. The problem of hurry is difficult to find, which is often laborious and thankless. It also involves system operation, system safety, regulation quality, environmental pollution, etc, Therefore, the phenomenon of affecting the process production by repairing the regulating valve often occurs. The cooling system of 5 carbonation towers in the heavy alkali process of our plant adopts 10 pneumatic control valves to control the cooling water flow into the tower, including 5 direct flow control valves and 5 circulating water control valves. In recent years of operation, the leakage problem of the valve rod and the gland of the valve body of the control valve has not been eliminated. On the one hand, the leaked water seriously rusted the surface of the valve body, affecting the service life of the valve and polluting the floor of the heavy alkali process; On the other hand, the leakage rate group ranks first among the teams of the instrument installation and maintenance company, which affects the team construction

2. Cause of leakage of the control valve

the maintenance method of the seal between the valve stem and the valve body of the pneumatic control valve is as follows: first disconnect the valve actuator from the valve body, then open the upper gland, take out the gasket under the gland, hook out the "V" packing in the stuffing box, replace the new packing, and then install the gland to complete the maintenance. Generally, after you buy the equipment back, you should first understand its correct use method and the normal protection of the equipment, which can last for two to six months. Some even leak after the maintenance, and the gland cannot be pressed too tightly, otherwise it will affect the automatic adjustment of the valve. At the initial stage of maintenance, 4-6 rounds of "V" shaped packing are generally added. Later, due to the gradual weakening of spring tension, the added "V" shaped packing is gradually increased, which is everywhere in the high-end market; Our fixture has now increased to more than ten turns. From the above analysis, it can be concluded that the main reason for poor sealing is that the spring in the stuffing box loses its elasticity due to rust, while the tetrafluoro sealing packing we use is hard and pressed too tightly, which will lock the valve stem, resulting in inflexible valve action and too loose pressure, which will not play a sealing role

3. Maintenance of the control valve of the cooling system of the carbonization tower

there are six common maintenance methods for the pneumatic control valve:

① add sealing grease method: for the unused sealing grease valve, add sealing grease to improve the sealing performance of the valve rod

② method of adding packing: in order to improve the sealing performance of packing on valve rod, the method of adding packing can be adopted. Usually, it is in the form of double-layer and multi-layer mixed fillers, simply increasing the number from 3 to 5, but the effect is not obvious

③ replacement of graphite packing method: due to its working temperature within -20 ~ 200 ℃, when the temperature changes greatly at the upper and lower limits, the sealing performance of tetrafluoro packing used widely will be significantly reduced, aging faster and its service life will be short. Flexible graphite filler can overcome these shortcomings and has a long service life. Therefore, some factories have changed all tetrafluoro packing into graphite packing, and even the newly purchased regulating valve has changed the tetrafluoro packing into graphite packing for use. However, the action of the regulating valve with graphite packing is large return difference, and some of them also have crawling phenomenon at the beginning, which must be considered

④ change the flow direction and place P2 at the valve rod end: when △ P is larger and P1 is larger, it is obviously more difficult to seal P1 than P2. Therefore, the flow direction can be changed from P1 at the valve stem end to P2 at the valve stem end, which is more effective for valves with high pressure and large differential pressure. For example, the bellows should usually be sealed P2

⑤ adopt lens gasket sealing method: seal the upper and lower covers, and seal the valve seat with the upper and lower valve bodies. If it is a plane sealed rotor pump, the sealing performance is poor under high temperature and high pressure, causing leakage. At this time, the lens gasket can be used for sealing, and satisfactory results can be obtained. Imported metering pump

⑥ replace the sealing gasket: up to now, most of the sealing gaskets still use asbestos plate. Under high temperature, the sealing performance is poor and the service life is short, which is easy to cause leakage. In this case, spiral wound gaskets and "O" rings can be used, which have been adopted by many factories now

according to the above analysis, during the maintenance, we will disassemble the valve as a whole, replace the spring in the valve, then add packing, and add a small amount of (white oil) in the middle of each layer of "V" packing of the control valve to improve the lubrication and sealing performance of the valve rod. The characteristics of this maintenance mode are: maintain the original sealing mode and structure of the valve, but the material branch company has been unable to solve the problem, and the maintenance time is too long. Generally, one valve takes 1 to 2 days. In view of this situation, we have reformed the original maintenance method. Open the valve gland, take out the gasket in the stuffing box, then hook out the "V" type packing under the gasket for 2~3 circles, add our commonly used oily asbestos packing, press on 2~3 layers, cover the gasket and then put on the gland. It has a good pre tightening force, and it will not lock the valve rod when it is pressed tightly, and the oily asbestos packing has a good sealing effect on the regulating valve whose regulating medium is water. Using this sealing method, we have overhauled the pneumatic control valve of 1#~5# tower and achieved satisfactory results

4. Operation and effect of the control valve after maintenance

after more than 3 months of development and operation from imitation to independent renovation, all 10 pneumatic control valves in the cooling system of the carbonization tower have no leakage, the control valve is flexible, and the regulation quality fully meets the requirements of process operation

in the troubleshooting of the leakage fault of VDC control valve, according to the actual working situation of the control valve of the carbonization tower, we created a simple and easy maintenance method based on local materials, comprehensive utilization, which eliminated the leakage of the control valve and reduced the leakage rate of the team; It also saves a lot of manpower, material resources and maintenance time. The successful application of this maintenance technique has gained valuable experience for our future maintenance and adjustment. Do you want to try it?, It has good economic benefits and promotion value

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