Solve the cause and phenomenon of the malfunction of the pneumatic control valve

When the regulating valve is made of graphite-asbestos, the lubricating oil should be added to the packing for about three months to ensure the flexibility of the regulating valve. If it is found that the packing pressure cap is pressed very low, the filler should be replenished. If the polytetrafluoroethylene dry packing is found to be hardened, it should be replaced in time; the operation of the regulating valve should be paid attention during the inspection, and the valve position indicator and regulator should be checked. Whether the output is consistent; check the gas source for the regulating valve with the positioner, and find the problem in time; always keep the health of the regulating valve and the complete and easy use of all components.

The common faults of the pneumatic control valve and the causes are that the regulating valve does not operate, the regulating valve is unstable, the regulating valve vibrates, the regulating valve is slow, the leakage of the regulating valve is increased, and the flow adjustable range is reduced.

a) The regulating valve does not operate . The symptoms and causes are as follows:

1. No signal, no air source. 1 The gas source is not open, 2 Because the gas source water freezes in winter, the air duct is blocked or the filter and the pressure reducing valve are blocked, 3 compressor failure; 4 gas source manifold leakage.

2. There is a gas source and no signal. 1 regulator failure, 2 signal tube leakage; 3 positioner bellows leakage; 4 regulation mesh diaphragm damage.

3. The positioner has no air supply. 1 filter blocked; 2 pressure relief valve failure; 3 pipeline leakage or blockage.

4. The positioner has a gas source and no output. The orifice of the positioner is blocked.

5. There is signal, no action. 1 spool is detached, 2 spools are stuck with the valve seat; 3 stem is bent or broken; 4 valve seat spool is frozen or coke block dirt; 5 actuator spring is rusted due to long-term use.

b) The operation of the regulating valve is unstable . The symptoms and causes are as follows:

1. The gas source pressure is unstable. 1 The compressor capacity is too small; 2 The pressure reducing valve is faulty.

2. The signal pressure is unstable. 1 The time constant of the control system (T=RC) is not appropriate; 2 The regulator output is unstable.

3. The air supply pressure is stable and the signal pressure is stable, but the action of the regulating valve is still unstable. 1 The ball valve of the amplifier in the positioner is not tightly worn by the dirt, and the output oscillation occurs when the gas consumption is particularly increased; 2 the nozzle flapper of the amplifier in the positioner is not parallel, the baffle cover cannot cover the nozzle; 3 output tube The line is leaking; 4 the rigidity of the actuator is too small; 5 the frictional resistance in the valve stem is large, and there is a blockage in the contact part.

c) Adjust the valve vibration . The symptoms and causes are as follows:

1. The regulator valve vibrates at any opening. 1 support is unstable; there is vibration source near 2; 3 valve core and bushing wear seriously.

2. The regulator valve vibrates as it approaches the fully closed position. 1 The regulating valve is selected to be large, and is often used at a small opening; 2 The flow direction of the single seat valve is opposite to the closing direction.

d) The adjustment valve is slow . The symptoms and causes of dullness are as follows:

1. The valve stem is slow to operate in only one direction. 1 diaphragm breakage leakage in pneumatic diaphragm actuator; 2 "O" seal leakage in the actuator.

2. The valve stem is dull when reciprocating. 1The valve body is stuck with glue; 2Teflon filler metamorphic hardening or graphite-asbestos filler lubricating oil is dry; 3The packing is too tight, the frictional resistance is increased; 4The frictional resistance is large due to the stem is not straight; 5 The positioner's pneumatic control valve can also cause slow movements.

5) The leakage of the regulating valve is increased . The reasons for the leak are as follows:

1. The leakage is large when the valve is fully closed. 1 The valve core is worn, the internal leakage is serious, and the 2 valves are not adjusted well.

2. The valve does not reach the fully closed position. 1 The pressure difference of the medium is too large, the rigidity of the actuator is small, the valve is not tight; 2 the valve has foreign matter; 3 the bush is sintered.

Six) The flow adjustable range becomes smaller . The main reason is that the valve core is corroded and becomes small, so that the adjustable small flow of zui becomes large.

Understand the failure phenomenon and cause of the pneumatic regulating valve, and take measures to solve the problem.