Failure analysis of pneumatic diaphragm control valve

1 Valve does not operate

1) Due to the failure of the regulator, the regulating valve has no electrical signal.

2) The valve positioner has no air source or insufficient air source pressure due to the leakage of air source main pipe.

3) The bellows of the positioner leaks air, so that the positioner has no air source output.

4) The diaphragm of control 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) Due to the following problems, although the control valve has signal and air source, the valve still does not act: ① the valve core is stuck with the bushing or valve seat; ② The valve core falls off (the pin is broken); ③ The valve stem is bent or broken; ④ Actuator failure: ⑤ air leakage of the seal ring of the reaction actuator; ⑥ There is foreign matter in the valve.

The action of valve 2 is unstable

1) Due to the failure of the filter pressure reducing valve, the air source pressure often changes.

2) The amplifier ball valve in the positioner is worn by particles or garbage, so that the ball valve is not closed tightly, 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 leaks air.

5) The rigidity of actuator is too small, and the change of fluid pressure results in insufficient thrust.

6) The wear force of valve stem is large.

7) The pipeline vibrates or there are vibration sources around it.

The action of 3 valve is slow

1) The valve rod moves slowly in reciprocating stroke: ① there is mud or viscous medium in the valve body, which makes the valve blocked or scaling; ② Polytetrafluoroethylene packing is modified and hardened, or the lubricating oil of graphite asbestos packing is dried.

2) When the valve stem moves in one direction, the action is slow: ① the diaphragm leaks and is damaged; ② The O-ring in the actuator is leaking.

The leakage is large when 4 valve is fully closed

1) The valve core is corroded and worn.

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

Valve 5 cannot reach the full closed position

1) The medium pressure difference is very large, and the rigidity of the actuator is too small.

2) There is foreign matter in the valve body.

3) The bushing is scorched.

6 leakage of packing part and sealing part of valve body

1) The packing cover is not pressed or flattened.

2) Dry with graphite asbestos packing lubricating oil.

3) When polytetrafluoroethylene is used as filler, polytetrafluoroethylene will deteriorate. 4) The gasket is corroded.