Common faults of the hottest pneumatic diaphragm r

2022-08-22
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Common faults and causes of pneumatic diaphragm regulating valve

fault causes of pneumatic diaphragm regulating valve:

pneumatic diaphragm regulating valve is widely used in soda industry. It can be used together with other instruments to realize the automatic adjustment and remote control of process parameters such as flow, liquid level, pressure, temperature and other media such as liquid, gas, steam, etc. in the production process. With the gradual improvement of enterprise automation, the application of distributed control system (DCS) and other intelligent instruments in the field of automation has become more and more common. Through the optimal control of computer, the production will obtain the maximum benefit. At the same time of optimization, the main faults of the plastic deformation control system are concentrated on the terminal actuator of the regulation system, namely the regulation valve. In the process of controlling the fluid flow, the regulation valve receives the control operation signal and realizes the regulation of the flow according to the control law. Whether its action is sensitive or not is directly related to the quality of the whole control system. According to the application statistics of the control system in the soda industry, about 80% of the faults of the regulating system come from the regulating valve. Therefore, how to ensure the reliable and accurate operation of pneumatic diaphragm regulating valve in our factory is a very important problem that we need to discuss

1 current situation investigation

there are 237 pneumatic membrane regulating valves in our plant, especially in the heavy alkali workshop, the core post of the whole plant, in which pneumatic membrane regulating valves are used for the three gas flow regulation of carbonization

in the process of soda ash production, due to the serious corrosivity of ammonia brine and the easy crystallization of ammonium bicarbonate below 25 ℃, the valve stuck, did not act or acted slowly due to the scarring, crystallization and scaling on the inner wall of the valve body during the operation of the control valve, so that the system could not adjust automatically, accounting for 50% of the total number of control valve failures, which had a great impact on production; The failure of slow valve action or leakage from valve stem caused by aging and hardening of regulating valve packing is up to 15%; As the diaphragm is damaged, the larger the starting point of air leakage or the hard core is broken, the phenomenon that the valve cannot be adjusted reaches 12%; The valve failure caused by the corrosion of positioner, pressure reducing valve and actuator accounts for 10%; The probability of regulating valve failure caused by other reasons accounts for 13%

2 failure cause analysis

according to the failure analysis of pneumatic thin film regulating valve used in soda ash production site for many years, common failures and their causes can be summarized as follows:

2.1 valve does not act

1 including the tension, contraction, low cycle and high cycle fatigue tests of materials and parts) the regulating valve has no electrical signal due to regulator failure

2) the valve positioner has no air supply or insufficient air supply pressure due to the leakage of the main air supply pipe

3) the bellows of the positioner leaks air, so that the positioner has no air source output

4) the diaphragm of the regulating valve is damaged

5) because the constant orifice of the amplifier in the positioner is blocked, and the compressed air contains water and accumulates at the ball valve of the amplifier, the positioner has a gas source but no output

6) although the regulating valve has signal and air source, the valve still does not act due to the following problems: ① the valve core is stuck with the bushing or valve seat; ② The prompt of the end of the valve core off time falls (the pin is broken); ③ The valve rod is bent or broken; ④ Actuator failure: ⑤ the sealing ring of the reaction actuator leaks; ⑥ There is foreign matter blocking in the valve

2.2 the action of the valve is unstable

1) the air supply pressure often changes due to the failure of the filter pressure reducing valve

2) the amplifier ball valve in the positioner is worn by particles or garbage, so that the ball valve cannot be closed tightly, and the output oscillation will occur when the air consumption is particularly increased

3) the nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle

4) air leakage of output pipeline

5) the rigidity of the actuator is too small, and the change of fluid pressure causes insufficient thrust

6) high wear force of valve rod

7) the pipeline oscillates or there are vibration sources around

2.3 slow action of valve

1) slow action of valve rod during reciprocating stroke: ① there is mud or viscous medium in the valve body, which blocks or scales the valve; ② Teflon packing is deteriorated and hardened, or the lubricating oil of graphite asbestos packing has been dried

2) the valve rod is slow when acting in one direction: ① the diaphragm is leaking and damaged; ② The "O" ring in the actuator leaks

2.4 large leakage when the valve is fully closed

1) the valve core is corroded and worn

2) the thread of the outer ring of the valve seat is corroded

2.5 the valve cannot reach the fully closed position

1) the medium pressure difference is very large, and the rigidity of the actuator is too small

2) there is foreign matter in the valve body

3) the bushing is burnt

2.6 leakage of the packing part and the sealing part of the valve body

1) the packing cover is not compressed and flattened

2) dry with lubricating oil at graphite asbestos packing

3) when polytetrafluoroethylene is used as filler, the polytetrafluoroethylene is aged and deteriorated

4) the sealing gasket is corroded

3 establish the pre inspection and repair mechanism of the valve

in the daily production process, the maintenance of the control valve is only limited to the fault treatment of the valve, and there is little regular adjustment and maintenance. There is no strict requirement for this in the measurement management regulations of the enterprise. In fact, the valve fault originates from the accumulation of several unstable factors, which will form a fault when accumulated to a certain extent. Therefore, Eliminating these unstable factors in the bud before the formation of valve failure can not only prolong the service life of the valve, but also avoid the serious impact on production caused by valve failure. This requires the establishment of a pre inspection and maintenance mechanism or a regular maintenance mechanism for valves. Take the three gas flow regulating valve of heavy alkali carbonization post as an example. Before the establishment of the pre inspection and repair mechanism, the resistance of the movable parts of the valve body increased due to the characteristics of easy crystallization, easy scaling and scarring of the process medium of soda ash, resulting in inflexible and sluggish action of the actuator until the valve core and bushing or valve seat of the regulating valve were stuck and could not act. After the problem occurred, on the one hand, the tower needed to be stopped for disassembly and maintenance of the valve, which would inevitably affect production, On the one hand, it is necessary to prepare spare parts. Because the spare parts cannot be found temporarily and emergency measures are taken from time to time, the fault cannot be completely solved. After the pre inspection and repair mechanism is established, there can be sufficient time to prepare spare parts, and the valve can be comprehensively maintained according to the service condition of the valve, so as to improve the service performance and service life of the valve

4 conclusion

through the establishment of the pre inspection and repair mechanism, it can not only increase the service life of the control valve, reduce the failure of the control valve and reduce the failure rate of the instrument, but also play a positive role in stabilizing the production of the enterprise, reducing costs and improving benefits. At the same time, it can also optimize the chemical operation and ensure the long-term stable operation of the production device

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