Causes of failure in pneumatic control valves

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This post was last edited by A Le on 2016-11-23 at 13:53. Pneumatic control valves are widely used in the soda ash industry; when used in conjunction with other instruments, they enable the automatic regulation and remote control of process parameters such as flow rate, liquid level, pressure, and temperature during production, as well as of other media like liquids, gases, and steam. As enterprises become more automated, Distributed Control Systems (DCS) and other intelligent instruments are being used more and more widely in the field of automation. Optimized control through computers enables production to achieve maximum efficiency. During optimization, the main faults of the control system tend to concentrate in the terminal actuator of the control system, namely the control valve. In the process of controlling fluid flow, the control valve receives control signals and adjusts the flow rate in accordance with the control rules. Whether its response is sensitive or not directly affects the quality of the entire control system. According to statistics on the application of control systems in the soda ash industry, about 80% of the failures in such systems occur in the control valves. Therefore, how to ensure the reliable and accurate operation of pneumatic diaphragm control valves in our factory’s production is a very important issue that we need to address. 1 Current Situation Survey: In the soda ash production process, due to the high corrosiveness of ammonia salts, and the tendency of ammonium bicarbonate to crystallize at temperatures below 25°C, control valves often experience problems such as sticking, failure to operate, or delayed operation as a result of scarring, crystallization, and scaling on the inner walls of the valves. This leads to an inability of the system to carry out automatic adjustments, and such issues account for 50% of all control valve failures, having a significant impact on production ； 15% of the failures are caused by the aging and hardening of the control valve packing, resulting in sluggish valve operation or leakage at the valve stem ； 12% of the cases involved a failure of the valve to regulate, caused by membrane damage and air leakage or fragmentation of the hard core ； Valve failures caused by corrosion of positioners, pressure relief valves, actuators, etc. account for 10% ； Other reasons account for 13% of the cases of control valve failures. 2 Fault Cause Analysis Based on the analysis of faults in pneumatic diaphragm control valves used in soda ash production over the years, the common faults and their causes can be summarized as follows: 2.1 Unstable valve operation 1) Faults in the pressure reducing valve cause the air supply pressure to fluctuate frequently. 2) The amplifier ball valve in the positioner is worn by particles or debris, resulting in poor sealing of the ball valve; this leads to increased gas consumption and output oscillations. 3) The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle does not cover the nozzle. 4) Air leakage in the output pipeline. 5) The actuator has too low rigidity, resulting in insufficient thrust due to changes in fluid pressure. 6) The valve stem experiences high wear force. 7) Pipeline oscillation or presence of vibration sources in the vicinity. 2.2 Slow valve operation 1) Slow movement of the valve stem during its back-and-forth motion: ① Mud or highly viscous fluids inside the valve can cause blockages or scaling ； ② The PTFE packing has hardened due to deterioration, or the lubricant in the graphite-asbestos packing has dried out. 2) Slow response when the valve stem moves in one direction: ① Diaphragm leakage and damage ； ② There is a leak in the \"O\"-ring seal in the actuator. 2.3 Valve does not operate 1) Due to a regulator failure, the control valve receives no electrical signal. 2) A leak in the main air supply line results in the valve actuator having no air supply or insufficient air pressure. 3) The bellows of the locator is leaking air, resulting in no air output from the locator. 4) The diaphragm of the control valve is damaged. 5) The locator has a gas supply but no output due to the blockage of the constant-throttle orifice in the amplifier, moisture in the compressed air, and its accumulation at the amplifier ball valve. 6) Due to the following issues, the control valve fails to operate even though there is a signal and air supply: ① The valve stem is stuck to the sleeve or seat ； ② Valve core detachment (pin broken) ； ③ The valve stem is bent or broken ； ④ Actuator failure: ⑤ Leakage in the seal ring of the reaction-type actuator ； ⑥ There is a foreign object blocking inside the valve. 2.4 The valve cannot reach the fully closed position: 1) The pressure difference of the medium is too large, and the actuator has insufficient rigidity. 2) There is a foreign object inside the valve body. 3) The bushing is burned. 2.5 High leakage when the valve is fully closed 1) The valve core is corroded or worn. 2) The threads on the outer ring of the valve seat are corroded. 2.6 Leaks in the packing area and the valve body sealing area 1) The packing cover is not tightened or pressed flat enough. 2) Dry the lubricant at the graphite asbestos gasket. 3) When polytetrafluoroethylene is used as a filler, it ages and deteriorates. 4) The gasket is corroded. 3 Establish a pre-maintenance mechanism for valves. In the daily production process, the maintenance of pneumatic control valves is limited to dealing with faults; regular calibration and maintenance are rarely carried out, and there are no strict requirements regarding this in the company’s measurement management procedures. In fact, valve failures arise from the accumulation of various unstable factors, and when these factors accumulate to a certain extent, failures occur. Therefore, addressing these unstable factors at an early stage, before failures occur, can not only extend the service life of the valves but also prevent the serious disruptions that valve failures can cause to production. This requires establishing a pre-maintenance mechanism for valves, or in other words, a regular maintenance mechanism. Taking the three-gas flow control valve in the heavy alkali carbonation unit as an example, before the pre-maintenance mechanism was established, the fact that the soda ash process medium tends to crystallize, form scale, and deposits led to increased resistance in the movable parts of the valve body. This resulted in inflexible and sluggish operation of the actuator, until the valve stem became stuck against the bushing or seat and could no longer move. Once such problems occurred, it was necessary to shut down the unit in order to disassemble and repair the valve, which inevitably affected production. Additionally, it was required to have spare parts on hand; since these parts were not always available immediately, emergency measures had to be taken, which prevented the faults from being completely resolved. By establishing a pre-maintenance mechanism, sufficient time is available to prepare spare parts, and the valves can be thoroughly maintained based on their usage conditions, thereby improving their performance and service life. 4 Conclusion By establishing a pre-maintenance mechanism, it is possible to not only extend the service life of control valves, reduce their failures, and lower the failure rate of instruments, but also play a positive role in stabilizing enterprise production, reducing costs, and improving efficiency. Additionally, it helps to optimize process operations and ensure the long-term stable operation of production facilities.