Pneumatic diaphragm regulating valve common faults and solutions

Pneumatic diaphragm regulating valve is widely used in the soda ash industry. It can be used with other instruments to realize automatic adjustment and remote control of process parameters such as flow, liquid level, pressure and temperature during production. Other media such as liquid, gas and steam. . With the gradual improvement of the degree of automation of enterprises, the application of distributed control system (DCS) and other intelligent instruments in the field of automation has become more and more common, and the optimal control of the computer will maximize the production. At the same time of optimization, the main fault of the control system is concentrated on the terminal actuator of the regulating system, that is, the regulating valve. In the working process of controlling the fluid flow, the pneumatic diaphragm regulating valve receives the control operation signal and realizes the flow according to the control law. adjust. Whether its action is sensitive or not is directly related to the quality of the entire control system. According to the application statistics of the control system in the soda ash industry, about 80% of the faults in the regulation system come from the regulating valve. Therefore, how to ensure the reliable and accurate operation of pneumatic diaphragm regulating valve in our factory production is a very important issue that we need to explore.
In the production process of soda ash, due to the serious corrosiveness of ammonia brine, the ammonium hydride is easily crystallized below 25 ° C, which makes the pneumatic membrane regulating valve in operation due to crusting, crystallization and scaling of the valve body wall. The phenomenon that the system is unable to perform automatic adjustment is relatively common, accounting for 50% of the total number of failures of the regulating valve, which has a great influence on the production; the valve is slow or the valve is slow due to aging and hardening of the regulating valve packing. The failure of the rod leakage is up to 15%; the valve cannot be adjusted due to the diaphragm damage or the hard core fragmentation; the phenomenon of valve failure due to corrosion of the positioner, pressure reducing valve, actuator, etc. accounts for 10%. The probability of malfunction of the pneumatic diaphragm regulating valve is 13% due to other reasons.
Analysis of the causes of the failure of the pneumatic diaphragm regulating valve According to the fault analysis of the pneumatic diaphragm regulating valve used in the soda production site for many years, the common faults and their causes can be summarized as follows:
A. The pneumatic diaphragm regulating valve does not operate. 1) The regulator valve has no electric signal due to the regulator failure.
2) Due to the leakage of the gas source manifold, the valve positioner has no air source or insufficient air supply pressure.
3) The positioner bellows leaks, so that the positioner has no air source output.
4) The diaphragm of the pneumatic diaphragm regulating valve is damaged.
5) Due to the blockage of the constant orifice of the amplifier in the positioner, the compressed air contains water and accumulates at the ball valve of the amplifier, the positioner has a gas source but no output.
6) Due to the following problems, although the regulating valve has signal and air source, the valve still does not work: 1 the valve core is stuck with the bushing or the valve seat; 2 the valve core is off (the pin is broken); 3 the pneumatic diaphragm regulating valve The rod is bent or broken; 4 actuator failure: 5 reaction type actuator sealing ring leakage; 6 valve has foreign body block.
B. The operation of the pneumatic diaphragm regulating valve is unstable. 1) Due to the failure of the filter pressure reducing valve, the pressure of the air source often changes.
2) The amplifier ball valve in the positioner is subject to wear by particles or garbage, which makes the ball valve not tight, and the output oscillation occurs when the air consumption is particularly increased.
3) The nozzle flaps of the amplifier in the positioner are not parallel, and the flap cover does not cover the nozzle.
4) The output line leaks.
5) The rigidity of the actuator is too small, and the change in fluid pressure causes insufficient thrust.
6) Pneumatic membrane adjustment valve stem has large wear force.
7) The pipeline oscillates or there is a vibration source around it.
C. The operation of the pneumatic diaphragm regulating valve is sluggish 1) The pneumatic diaphragm adjusts the valve stem to be slow in reciprocating stroke: 1 the valve body has mud or viscous medium to block or scale the valve; 2 PTFE filler metamorphic hardening, or The lubricating oil of the graphite asbestos packing has been dried.
2) The valve stem is slow in one direction: 1 diaphragm leaks and breaks; 2 "O" seal leaks in the actuator.
D. Leakage when the valve is fully closed 1) The valve core is corroded and worn.
2) The thread on the outer ring of the seat is corroded.
E. The pneumatic diaphragm regulating valve does not reach the fully closed position. 1) The medium pressure difference is large, and the rigidity of the actuator is too small.
2) There is foreign matter in the valve body.
3) The bushing is burnt.
F, leakage of the packing part and the sealing part of the valve body 1) The packing cover is not pressed and is not flattened.
2) Dry with lubricating oil at the graphite asbestos packing.
3) When polytetrafluoroethylene is used as the filler, the polytetrafluoroethylene deteriorates.
4) The gasket is corroded.