Common faults and reasons analysis of pneumatic diaphragm control valve

Pneumatic membrane valve in the soda ash industry is extremely common, supporting the use of other instruments, enabling the production process flow, level, pressure, temperature and other process parameters and other media such as liquid, gas, steam, etc. automatically adjust and remote control . With the gradual improvement of the degree of automation of enterprises, the application of distributed control system (DCS) and other smart meters in the field of automation has become more and more common. Through the optimized control of computers, the production will get the most benefit.

In the optimization, the main faults of the control system are also concentrated on the regulating valve which is the terminal executing device of the regulating system. The regulating valve receives the control operation signal during the control of the fluid flow and regulates the flow rate according to the control law. Its action or not, is directly related to the quality of the entire control system. According to the control system in the soda ash industry statistics, about 80% of the fault control system from the control valve. Therefore, how to ensure the pneumatic diaphragm valve in our factory production of reliable and accurate operation, we need to explore is a very important issue.

1, the status quo investigation

Our factory has a total of 237 sets of pneumatic diaphragm control valve, especially in the whole plant's core positions heavy alkali plant is particularly widely used, of which carbonized three gas flow control all use pneumatic diaphragm control valve.

Soda ash in the production process, due to the serious corrosion of ammonia salt, ammonium bicarbonate in the following 25 degrees Celsius easy to crystallize the nature of the valve in the operation because of the body wall scarring, crystallization, scaling lead to the valve card, not Action or slow action, the system can not be automatic adjustment of the phenomenon is more common, accounting for 50% of the total number of valve failure, resulting in greater impact on production; by the valve aging filler, hardening valve slow or from the valve stem Leakage and other failures up to 15%; due to the diaphragm damage caused by leakage or hard core fragmentation valve can not be adjusted up to 12% of the phenomenon; due to locators, VALVES, actuators and other corrosive valve failure caused by the phenomenon of 10%; other Cause the probability of fault valve control accounted for 13%.

2, the failure analysis

According to the soda ash production site for many years the use of pneumatic diaphragm control valve failure analysis can be summarized as a common fault and its causes are as follows:

2.1 valve does not move

1) due to regulator failure, so that no signal conditioning valve.

2) Leakage of air source manifold, so that the valve positioner no source or lack of air pressure.

3) Locator bellows leak, the locator no air source output.

4) Regulating valve diaphragm is damaged.

5) As the constant orifice of the amplifier in the positioner is blocked, the compressed air is water-filled and accumulates at the GLOBE VALVE of the amplifier, causing the positioner to have a source of air but no output.

6) Due to the following problems, the control valve has a signal but a gas source, but the valve still does not work: (1) the valve core and the bushing or valve seat are stuck; (2) the valve core is off (the pin is broken); the valve stem is bent or broken ; ④ actuator failure: ⑤ reactive type actuator leak ring; ⑥ valve foreign body block.

2.2 valve action instability

1) Due to the failure of the filter regulator, the pressure of the air supply changes frequently.

2) Locator amplifier ball valve by the particles or garbage wear, the ball valve is not strict, the air consumption will increase when the output oscillation.

3) Locator amplifier nozzle baffle is not parallel, the baffle cover nozzle.

4) Output pipeline leakage.

5) The actuator stiffness is too small, the thrust of the fluid pressure changes caused by insufficient.

6) stem wear power.

7) pipeline oscillation or vibrations around.

2.3 valve slow action

1) When the stem reciprocating stroke slow action: ① valve body with mud or viscous medium, so that the valve plugging or scaling; ② Teflon hardened, or graphite asbestos packing oil has dried.

2) Stem unilateral action slow action: ① diaphragm leakage and damage; ② actuator "O" ring leak.

2.4 full closure of the valve leak

1) The spool is corroded and worn.

2) The outer thread of the valve seat is corroded.

2.5 valve can not reach the fully closed position

1) medium pressure is very large, the actuator stiffness is too small.

2) there is foreign body in the valve body.

3) Bushing burned.

2.6 packing parts and valve seal leakage

1) packing cap did not compress, did not flatten.

2) Dry the graphite asbestos packing oil.

3) PTFE as a filler, PTFE aging deterioration.

4) The gasket is corroded.

3, the establishment of valve pre-inspection mechanism

In the daily production process, the maintenance of the control valve is only limited to the failure of the valve, with little regular adjustment and regular maintenance, in the company's measurement management procedures do not have strict requirements, in fact, the valve Failure stems from the accumulation of a number of unstable factors, accumulated to a certain extent, the formation of failure, therefore, in the valve failure before the formation of these instability factors ruled out in the bud, not only can extend the life of the valve, but also to avoid the valve The failure to produce serious impact. This requires the establishment of valve pre-inspection mechanism or regular maintenance mechanism. Taking the heavy gas-carbonized three-position gas flow control valve as an example, before the pre-inspection mechanism was established, due to the characteristics of easy crystallization, easy scaling and scarring in the soda ash process medium, the resistance of the movable part of the valve body increases, causing the actuator to move Inflexible, sluggish until the control valve spool and bushing or seat stuck can not move after the problem occurs, on the one hand need to stop the tower to disassemble the valve overhaul, affecting the production is inevitable, on the one hand need to prepare spare parts, Spare parts can not find temporary spare parts to take emergency measures have occurred, resulting in failure can not be completely resolved. After the establishment of pre-inspection mechanism, there may be sufficient time to prepare spare parts, spare parts, and valves can be fully under the maintenance of the valve, thereby improving valve performance and service life.

4, conclusion

Through the establishment of pre-inspection mechanism, not only can increase the service life of the control valve, reduce the control valve failure and reduce the failure rate of the instrument, but also can stabilize the production of the enterprise, reduce the cost and increase the benefit, play a positive role in promoting, meanwhile, can also optimize the process Operation, to ensure long-term stable operation of the production plant.