Common faults and daily maintenance of pneumatic control valves

Pneumatic control valves are one of the instruments widely used in petrochemical enterprises. It is of great significance to ensure the normal operation and safe production of process equipment. Therefore, it is necessary to strengthen the maintenance of pneumatic control valves.

1. Key inspection parts during maintenance
Check the inner wall of the cavity: Under high pressure difference and corrosive substances, the inner wall of the valve body and the membrane of the diaphragm valve is often impacted and corroded by the substances. Be sure to check the pressure resistance and corrosion resistance. Check the valve seat. Due to the infiltration of substances during work, the inner surface of the thread of the fixed valve seat is easily corroded, which makes the valve seat loose;
Check the valve core: The valve core is one of the moving parts of the control valve and is seriously corroded by substances. During maintenance, carefully check whether all parts of the valve core are corroded and worn. Especially under high pressure difference conditions, the valve core is more seriously worn due to cavitation. Replace the severely damaged valve core;
Check the sealing packing and check whether the packing asbestos rope is dry. When using polytetrafluoroethylene packing, check whether it is aging and whether the mating surface is damaged: Check whether the actuator rubber membrane is aging and cracking.

2. Daily maintenance of pneumatic control valves and pneumatic black water control valves

When the control valve packing is made of graphite asbestos, lubricating oil should be added to the packing once every three months to ensure that the control valve is flexible and easy to use. If the pressure cap of the packing is found to be very low, the packing should be supplemented. If the polytetrafluoroethylene dry packing is found to be hardened, it should be replaced in time. Pay attention to the action of the control valve during inspection and check whether the valve position indicator is consistent with the output of the regulator. For control valves with positioners, the air source should be checked frequently and problems should be solved in time; the hygiene of the control valve and the integrity and ease of use of each component should be maintained regularly.

3. Common faults and causes
(1) The regulating valve does not work. The fault phenomena and causes are as follows:
The air source is not open;
The water in the air source freezes in winter, causing the air pipe to be blocked or the filter and pressure reducing valve to be blocked; if the compressor fails, the air source main pipe leaks; if the regulator fails;
The signal pipe leaks;
The positioner below leaks: the regulating net is damaged;
The filter is blocked;
The pressure reducing valve fails; the pipeline leaks or is blocked.
There is a signal, but no action.
The valve core falls off;
The valve core is stuck with the society or the valve seat;
The valve stem is bent or broken;
The valve seat and valve core are frozen;
The actuator spring is rusted after long-term use.

(2) The regulating valve is unstable.

The fault phenomena and causes are as follows:
The air supply pressure is unstable: the compressor capacity is too small; and the pressure reducing valve fails.
The signal pressure is unstable: the time constant of the control system (t=RC) is inappropriate; the output of the regulator is unstable
The air source pressure is stable, and the signal pressure is stable, but the regulating valve is still unstable.
The ball valve of the amplifier in the positioner is not closed tightly due to dirt and wear, especially when the gas consumption is increased, it will cause output vibration;
The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle;
The output pipeline leaks;
The rigidity of the actuator is too small;
The friction resistance of the valve stem during movement is large, and the contact part is blocked.
(3) The pneumatic diaphragm control valve vibrates.

The fault phenomena and causes are as follows:
The bracket is unstable; there is a vibration source nearby; the valve core and bushing are seriously worn.
When the regulating valve is large, it is often used at a small opening; the flow direction of the single valve is opposite to the closing direction.
(4) The regulating valve moves slowly.

The phenomenon and reasons for the darkening are as follows:
The diaphragm of the pneumatic diaphragm actuator is damaged and leaking; the "o" type seal of the actuator leaks; the valve body is blocked by sticky substances; the polytetrafluoroethylene packing is deteriorated and hardened or the lubricating oil of the graphite asbestos packing is dry; the packing is added too tightly, and the friction force is enhanced; the valve stem is not straight, and the friction resistance is large; the pneumatic control valve without a positioner will also cause slow movement.
(5) Increased leakage of the regulating valve.

The reasons for the leakage of the single-seat pneumatic diaphragm regulating valve are as follows:
① When the valve is fully closed, the leakage is large. The valve core is worn and the internal leakage is serious; the valve is not adjusted tightly and is not closed tightly.
② The pneumatic and dynamic cut-off valve cannot be large enough to fully close. The medium pressure difference is too large, the actuator stiffness is small, and the valve is not closed tightly; there are foreign objects in the pneumatic three-way regulating valve; 3 bushings are sintered.