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What Are Common Faults of Pneumatic Control Valves?

Views: 201     Author: Site Editor     Publish Time: 2021-06-25      Origin: Site

The pneumatic control valve is one of the instruments widely used by petrochemical enterprises. Its accurate and normal operation is of great significance to ensure the normal operation of the process equipment and safe production. Therefore, it is necessary to strengthen the maintenance of the pneumatic actuated ball valve.


43.valve actuator mounting bracket


The Control Valve Does Not Operate

First, we need to confirm whether the air source pressure is normal and find the air source failure. If the air source pressure is normal, we judge whether the amplifier of the positioner or the electric/pneumatic converter has output. If there is no output, we need to block the constant orifice of the amplifier or compress the moisture in the air to make it accumulate at the amplifier ball valve.


We can use the small steel wire to dredge the constant orifice, remove dirt or clean the air source. If all the above are normal, there is a signal but no action, which means that the actuator is faulty or the valve stem is bent, or the stainless ball valve core is stuck. In this case, the ball valve with drain must be removed for further inspection.



Blocked Control Valve

Most of the jamming faults of the union ball valve appear in the newly put into operation system and the initial stage of an overhaul. Due to welding slag and rust in the pipeline, the choke and guide parts are blocked. This makes the medium flow poorly, or the packing is too tight during the overhaul of the lockable ball valve, resulting in increased friction, which will eventually lead to the phenomenon that small signals do not operate and large signals operate excessively.


In such situations, we can quickly open and close the auxiliary line or regulating valve, so that the stolen goods will be washed away by the medium from the auxiliary line or regulating valve.



43.union valve


The spring stiffness of the regulating valve is insufficient, and the output signal of the regulating valve is unstable and changes sharply, which can easily cause the regulating valve to oscillate. There is also violent vibration of the pipe and base, which causes the control valve to vibrate. There are drastic changes in flow resistance, flow rate, and pressure when the regulating valve works at a small opening. When the rigidity of the valve is exceeded, the stability becomes worse, and oscillation occurs in severe cases.


Since there are many reasons for the oscillation, specific problems should be analyzed in detail. For slight vibration, we can increase stiffness to eliminate it. If we choose a regulating valve with a large stiffness spring, we need to switch to a piston actuator structure. If the pipeline and base vibrate violently, the vibration interference can be eliminated by adding valve actuator mounting bracket. When the frequency of the valve is the same as the frequency of the system, the regulating valve of a different structure is replaced. Oscillation caused by working at a small opening is caused by improper selection. Because the flow capacity of the valve C is too large, it must be re-selected. We need to choose a pneumatic ball valve with a small flow capacity C, adopt split-range control, or adopt a sub-mother valve to overcome the oscillation caused by the regulating valve working at a small opening.



When the union valve uses graphite-asbestos as the packing, lubricating oil should be added to the packing once about three months to ensure that the regulating valve is flexible and easy to use. If the packing pressure cap is found to be very low, the packing should be supplemented. If the PTFE dry packing is found to be hardened, it should be replaced in time. The staff should pay attention to the operation of the regulating valve during the patrol inspection, and check whether the valve position indicator and the regulator output are consistent. In addition, the staff should always check the air source for the regulating valve with the positioner, and deal with the problem in time. The most important thing is to keep the segmented ball valve clean and all parts intact and easy to use.

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