Maintenance of pneumatic control valve

Maintenance of pneumatic control valve

Overhaul according to the working principle of pneumatic control valve

Pneumatic control valve has anti-corrosion characteristics and is widely used in chemical plants. Positive action control valve is widely used. The most intuitive way to distinguish whether it is gas on or gas off is to look at the regulating valve die. The die input gas above is generally gas off valve, the die input gas below is generally gas on valve, and the commonly used is gas off valve.

When the input gas pressure increases, the valve opening decreases. When the input gas decreases, the valve opening becomes larger. When there is no input gas, the valve opening is the largest. According to the process requirements, in order to ensure safety, the valve is fully opened when there is no air source.The air on regulating valve is opposite to the air off valve and is fully closed in case of gas loss.

Air source failure

When the regulating valve works, it needs standard air sources with different pressures (2.5kp). The regulating valve can not work normally if the air source pressure is too high or too low.

Too high pressure: closing the regulating valve with positive action gas will make the regulating valve unable to open or reach the upper limit and cannot work normally; Opening the regulating valve with positive action air will make the regulating valve not tightly closed or not close to the set value; The reaction valve is the opposite of the above.

Too low pressure: closing the regulating valve with positive gas will make the regulating valve not tightly closed or not close to the set value; The positive action air opening regulating valve will make the regulating valve unable to open or reach the upper limit, and can not work normally.

Adjust the valve setter, adjust the air supply pressure to reach the standard value, adjust the instrument pressure if it is too high or too low, and eliminate the fault after adjustment.

No air source: for various reasons, the air source cut-off regulating valve is at zero or maximum and cannot be adjusted normally.

If the air source is cut off artificially during maintenance, it can be judged whether it is mechanical failure or air source or power signal failure.

Valve seat failure

During the operation of the regulating valve, there will be residue and hard matter when various materials pass through. There are foreign matters stuck between the valve core and the valve seat, so that the valve can not be closed tightly. Turn off the air source and power signal. If there is leakage in the valve body, there is foreign matter in the valve body. Disassemble the valve body to remove the foreign matter and eliminate the fault.

The change of the mechanical position of the valve core will also produce the phenomenon of loose closing. After adjusting and changing the valve core position and fixing, the fault can be eliminated.

There is often material leakage at the packing at the dynamic sealing point of the valve stem and valve seat. The treatment method is to tighten the packing pressure shell. In serious cases, it can be disassembled to add and replace the packing. 

When the diaphragm head of the regulating valve fails, the regulating output standard gas signal is input to the diaphragm head air chamber of the regulating valve to drive the valve rod to work.

The troubleshooting is as follows:

Leakage of air chamber: when the valve positioner is normal and does not output air signal, the leakage point can be judged respectively. Remove the input connector of membrane head air chamber to judge whether the air outlet chamber is leaking. Causes of leakage: ① poor sealing of the joint, ② loose screws of the diaphragm head, and ③ leakage of the diaphragm itself. Solution: replace the diaphragm, seal the joint and tighten the screws.

Failure of diaphragm head spring: the inner spring of diaphragm head will deform, break and change its position after long-term use, so that the diaphragm cannot work. Solution: disassemble the membrane head, clean and replace it.

The membrane head air chamber will be flooded due to water in the air source or other reasons. Ice blocks will be generated in winter and cannot work. terms of settlement. Thawing and ice melting drainage.

Fault of the valuer the thread of the hand wheel lever of the valuer will be worn after being used for a period of time and can be replaced. Sometimes there will be leakage or blockage, which can dredge or tighten the seal.