Application of pneumatic diaphragm control valve in Soda Industry

Pneumatic membrane regulating valve is widely used in the soda industry It can be used 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 optimization control of computer, the production will achieve great benefits of Zui At the same time, the main fault of the control system is concentrated on the terminal executive device of the control system, that is, the control valve In the process of controlling the fluid flow, the control valve receives the control operation signal and realizes the flow regulation according to the control law Its sensitivity 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 failures of the control system come from the control valve Therefore, how to ensure the reliable and accurate operation of the pneumatic diaphragm control valve in our factory is a very important problem we need to explore 1 According to the investigation, there are 237 pneumatic membrane control valves in our plant, which are widely used in the heavy alkali workshop, especially in the core posts of the whole plant Among them, pneumatic membrane control valves are used for the three air flow control of carbonization   In the process of soda ash production, due to the serious corrosiveness of ammonia salt water and the crystallization of ammonium bicarbonate below 25 ℃, it is common that the valve is stuck, inactive or slow in operation due to the scaling, crystallization and scaling on the inner wall of the valve body, which makes the system unable to conduct automatic regulation, accounting for 50% of the total failures of the control valve and has a relatively large impact on production Large; 15% of the failures are caused by aging and hardening of the packing of the regulating valve, resulting in sluggish action of the valve or leakage from the valve stem; 12% of the failures are caused by air leakage due to diaphragm damage or hard core fragmentation; 10% of the failures are caused by corrosion of the positioner, pressure reducing valve, actuator, etc.; 13% of the failures are caused by other reasons According to the failure analysis of pneumatic diaphragm control valve used in soda ash production for many years, the common failures and their causes can be summarized as follows: 2.1 valve does not act; 1) the control valve has no electrical signal due to regulator failure 2) due to the leakage of the main air source, the valve positioner has no air source or the air source pressure is insufficient 3) the bellows of the positioner leaks, so that the positioner has no air output 4) the diaphragm of regulating valve is damaged 5) because the constant orifice of the amplifier in the positioner is blocked, the compressed air contains water and accumulates at the ball valve of the amplifier, the positioner has air source but no output 6) although the control valve has signal and air source, the valve still does not operate due to the following problems: ① the valve core is stuck with the bushing or the valve seat; ② the valve core falls off (the pin is broken); ③ the valve stem is bent or broken; ④ the actuator fails: ⑤ the seal 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 source pressure changes frequently due to the failure of the filter pressure reducing valve 2) the ball valve of the amplifier in the positioner is worn by particles or garbage, which makes the ball valve not tightly closed, and the output oscillation will occur when the air consumption is especially increased 3) the nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle 4) the output pipeline is leaking 5) the rigidity of the actuator is too small, and the change of fluid pressure causes insufficient thrust 6) the valve rod has great wear force 7) the pipeline oscillates or there is vibration source around 2.3 sluggish action of the valve 1) sluggish action of the reciprocating stroke of the valve rod: ① there is mud or viscous medium in the valve body to block or scale the valve; ② the polytetrafluoroethylene packing is deteriorated and hardened, or the lubricating oil of graphite asbestos packing has dried 2) when the valve rod acts in one direction, it acts slowly: ① the diaphragm leaks and is damaged; ② the "O" ring in the actuator leaks 2.4 when the valve is fully closed, the leakage is large 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 large, and the actuator rigidity is too small 2) there are foreign matters in the valve 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 pressed and flattened 2) dry the lubricating oil with graphite asbestos packing 3) when polytetrafluoroethylene is used as packing, it will be deteriorated 4) the sealing pad is corroded 3 Establish valve pre inspection and repair mechanism In the daily production process, the maintenance of the control valve is limited to the troubleshooting 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 failure of the valve is caused by the accumulation of several unstable factors, which will cause the failure to a certain extent Therefore, before the failure of the valve is formed, the maintenance of the control valve is limited to the troubleshooting of the valve Removing the unstable factors in the bud can not only prolong the service life of the valve, but also avoid the serious impact on the production caused by the valve failure This requires the establishment of valve pre inspection mechanism or regular maintenance mechanism Taking the three gas flow regulating valve of the 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 was increased due to the characteristics of easy crystallization, scaling and scarring of the process medium of soda ash, resulting in the inflexible and sluggish action of the actuator until the valve core and bushing or valve seat of the regulating valve were stuck and unable to act After the problem occurred, on the one hand, the tower needs to be shut down for the valve to enter It is inevitable that physical examination and repair will affect production On the one hand, spare parts and spare parts need to be prepared The failure can not be solved completely due to the phenomenon that no spare parts can be found temporarily and emergency measures can be taken After the pre inspection and repair mechanism is established, sufficient time can be available to prepare spare parts, and comprehensive maintenance can be carried out for the valve according to the service condition of the valve, so as to improve the service performance and service life of the valve Conclusion: through the establishment of the pre inspection and repair mechanism, not only can the service life of the control valve be increased, the failure of the control valve be reduced, the failure rate of the instrument be reduced, but also can stabilize the production of the enterprise, reduce the cost and improve the benefit, and optimize the process operation to ensure the long-term and stable operation of the production device.