Pneumatic control valve is very common 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, temperature and other media such as liquid, gas, and steam in the production process. With the gradual improvement of the degree of enterprise automation, the application of distributed control systems (DCS) and other intelligent instruments in the field of automation has become more and more common. Optimal control of computers will maximize production benefits. While optimizing, the main faults of the control system are concentrated on the pneumatic control valves, which is the terminal actuator of the pneumatic control valves system. The pneumatic control valves receives the control operation signal during the working process of controlling the fluid flow, and realizes the regulation of the flow according to the control law. Whether it 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 regulating system are caused by the pneumatic control valves. Therefore, how to ensure the reliable and accurate operation of the pneumatic membrane control valve in production is a very important issue that we need to discuss.
A factory has a total of 237 pneumatic membrane control valves, which are especially widely used in core positions in the heavy alkali workshop. Among them, the three gas flow adjustments of carbonization all use pneumatic membrane control valves.
In the production process of soda ash, due to the severe corrosiveness of the ammonia brine, the ammonium bicarbonate is easy to crystallize at temperatures below 25°C, which causes the valve to be stuck and not blocked due to scarring, crystallization, and fouling on the inner wall of the valve during operation. The phenomenon that the system cannot be automatically adjusted due to slow action or action is common, accounting for 50% of the total number of control valve failures, which has a greater impact on production; the aging and hardening of the control valve packing leads to slow valve action or from the valve stem. Leakage and other failures accounted for 15%; the valve could not be adjusted due to diaphragm damage, air leakage or hard core fragmentation, up to 12%; valve failure due to corrosion of positioners, pressure reducing valves, actuators, etc. accounted for 10%; others The probability of failure of the control valve is 13%
Failure cause analysis
According to the failure analysis of the pneumatic membrane pneumatic control valve used in the production site of soda ash for many years, the common failures and their causes can be summarized as follows:
. The pneumatic control valve does not work
- The control valve has no electrical signal due to a malfunction of the regulator.
- The valve positioner has no gas source or insufficient gas source pressure due to the leakage of the gas source main pipe.
- The bellows of the positioner leaks air, so that the positioner has no air source output.
- The diaphragm of the pneumatic control valve is damaged.
- The positioner has air source but no output due to the blockage of the constant orifice of the amplifier in the positioner, the compressed air contains water and the accumulation of the amplifier ball valve.
- Due to the following problems, the control valve does not operate despite the signal and air supply:
- The valve core is stuck with the bushing or valve seat;
- The spool falls off (the pin is broken);
- The valve stem is bent or broken;
- Failure of the actuator:
- The sealing ring of the reaction type actuator is leaking;
- There is a foreign body blocking the valve.
The action of the pneumatic control valve is unstable
- Due to the failure of the filter pressure reducing valve, the air source pressure often changes.
- The amplifier ball valve in the positioner is worn by particles or rubbish, so that the ball valve is not closed tightly, and the output oscillation will occur when the gas consumption is particularly increased.
- The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle.
- Air leakage in the output pipeline.
- The rigidity of the actuator is too small, and the fluid pressure changes cause insufficient thrust.
- The valve stem has a large wear force.
- The pipeline oscillates or there are vibration sources around.
Slow action of pneumatic control valve
The action of the valve stem of the pneumatic control valve is slow during the reciprocating stroke:
- There is mud or a viscous medium in the valve body, which causes the valve to be blocked or fouled;
- The PTFE packing has deteriorated and hardened, or the lubricant of the graphite asbestos packing has dried.
When the valve stem of the pneumatic control valve moves in one direction, the action is slow:
- Leakage and damage of diaphragm;
- Leakage of O-ring seal in the actuator.
Large leakage when the pneumatic control valve is fully closed
- The valve core is corroded and worn.
- The threads of the outer ring of the valve seat are corroded.
Pneumatic control valve cannot reach the fully closed position
- The medium pressure difference is large, and the rigidity of the actuator is too small.
- There is a foreign body in the valve.
- The bushing is burnt.
Leakage of packing part and valve body sealing part
- The packing cover is not compressed or flattened.
- Use graphite asbestos packing to dry with lubricating oil.
- When PTFE is used as the filler, PTFE will deteriorate due to aging.
- The gasket is corroded.
Establish a pre-overhaul mechanism for pneumatic control valves
In the daily production process, the maintenance of the pneumatic control valve is only limited to the troubleshooting of the valve. Periodic adjustment and periodic maintenance are rarely carried out. There is no strict requirement for this in the metering management regulations of the enterprise. In fact, the valve The failure is due to the accumulation of several unstable factors, which will form a failure when they accumulate to a certain extent. Therefore, removing these unstable factors in the bud before the failure of the valve can not only prolong the service life of the valve, but also avoid the valve failure. The failure has a serious impact on production. This requires the establishment of a valve pre-overhaul mechanism or a regular overhaul mechanism. Take the three-gas flow control valve for heavy alkali carbonization posts as an example. Before the establishment of the pre-overhaul mechanism, the soda ash process medium has the characteristics of easy crystallization, easy fouling, and scarring, which causes the resistance of the movable parts of the valve body to increase, which leads to the action of the actuator. Inflexible and sluggish, until the valve core and bushing or valve seat of the pneumatic control valve are stuck and cannot move. After the problem occurs, on the one hand, the tower needs to be stopped for disassembly and repair of the valve. It is inevitable that the production will be affected. On the other hand, spare parts must be prepared. Spare parts, because emergency measures are sometimes not found temporarily, the failure cannot be completely resolved. After the pre-overhaul mechanism is established, there can be sufficient time to prepare spare parts and spare parts, and to perform comprehensive maintenance on the valve according to the use of the valve, thereby improving the performance and service life of the valve.
Conclusion The establishment of the pre-overhaul mechanism 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 the cost, and improving the efficiency. At the same time, it can also optimize the process. Operation to ensure the long-term stable operation of the production equipment.