Maintenance skills of VDC regulating valve

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In industrial automation instruments, VDC pneumatic control valve is widely used and plays an important role in the production of enterprises with high degree of automation, 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 regulating valve installed on the key process pipeline, due to the great impact on production and the great psychological pressure of instrument maintenance personnel, the problem of rush 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 overhauling 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 into the tower, including 5 direct flow control valves and 5 circulating water control valves. In recent years of operation, the leakage problem at the valve rod and valve body gland of the control valve has not been eliminated. On the one hand, the leaked water causes serious corrosion on the surface of the valve body, affects the service life of the valve and pollutes the floor of the heavy alkali process; On the other hand, the leakage rate group ranks first in the assessment of each team of the instrument installation and maintenance company, which affects the team construction

2. Causes of leakage of control valve

the maintenance method of the seal between the valve stem and valve body of 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 buying the equipment back, we 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 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 circles 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 widely used 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 elasticity due to rust, while the PTFE 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 can 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 increase the sealing grease method: for the valve without sealing grease, it can be considered to increase the 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 to valve stem, the method of adding packing can be adopted. It is usually in the form of double-layer and multi-layer mixed fillers. Simply increase the number and increase the number from 3 to 5, but the effect is not obvious

③ replacement of graphite packing method: because the working temperature of tetrafluoro packing widely used is in the range of - 20 ~ 200 ℃, when the temperature changes greatly at the upper and lower limits, its sealing performance will be significantly reduced, aging is fast and its service life is short. Flexible graphite filler can overcome these shortcomings and has long service life. Therefore, some factories change all the PTFE packing into graphite packing, and even the newly purchased regulating valve will replace the PTFE packing with graphite packing. However, the action of the regulating valve with graphite packing is large return difference, and some crawling phenomena occur at the beginning, which must be considered

④ change the flow direction and place P2 at the valve stem end: when △ P is large and P1 is large, 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 seal P2 shall be considered for bellows

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, resulting in 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 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

according to the above analysis, during the maintenance, we 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 regulating valve to improve the lubrication and sealing performance of the valve stem. The characteristics of this maintenance mode are: maintain the original sealing mode and structure of the valve, but the material branch required for maintenance has been unable to solve it, 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 mode. Open the valve gland, take out the gasket in the stuffing box, then hook out the "V" 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 good pre tightening force, and it will not lock the valve rod when it is pressed very tightly. Moreover, 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 the 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 the VDC regulating valve, according to the actual working conditions of the regulating valve of the carbonization tower, we created a simple and easy maintenance method with local materials, comprehensive utilization, which eliminates the leakage of the regulating valve and reduces the leakage rate of the team; It also saves a lot of manpower, material resources and maintenance time. The successful application of this maintenance skill has obtained valuable experience for our maintenance and adjustment in the future, and has good economic benefits and popularization value

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