Needle Valve Troubleshooting

The needle valve is a common type of valve used to control and cut off the flow of fluids, widely applied in various industrial fields. However, during its operation, needle valves may experience various faults, affecting their efficiency. This article ATO industrial automation will discuss the causes of common needle valve faults and methods for resolving them to help users restore normal operation as quickly as possible.

Regulating valve stuck

The common problem with regulating valves is blockage, which often occurs in the early stages of new and major maintenance systems. Due to the blockage of welding slag and rust in the throttle and guide parts of the pipeline, the medium flow is not smooth, or the filler is too tight during the maintenance of the regulating valve, resulting in an increase in friction force, leading to the phenomenon of small signals not acting and large signals acting excessively.

Solution: The auxiliary line or regulating valve can be quickly opened or closed, allowing dirt to be washed away by the medium from the auxiliary line or regulating valve. Another method is to clamp the valve stem with a pipe wrench, and under the external signal pressure, rotate the valve stem with both positive and negative force, allowing the valve core to flash past the clamp. If it is not possible, increase the air supply pressure and drive power by repeatedly moving up and down several times to solve the problem. If it still does not work, it needs to be disassembled for handling.

Leakage

1. The valve has internal leakage and the length of the valve stem is not suitable. The valve stem of the pneumatic valve is too long, and the upward (or downward) distance of the valve stem is not enough, resulting in a gap between the valve core and the valve seat, which cannot be fully contacted, resulting in incomplete closure and internal leakage. Similarly, the valve stem of an air shut-off valve is too short, resulting in a gap between the valve core and valve seat that cannot be fully contacted, resulting in incomplete closure and internal leakage.
   
   Solution: The valve stem of the regulating valve should be shortened (or extended) to make the length of the regulating valve suitable and prevent internal leakage.
   
   
2. Leakage of packing. After the packing is loaded into the stuffing box, axial pressure is applied to it through the gland. Due to the plasticity of the packing, it generates radial force and is in close contact with the valve stem, but this contact is not very uniform. Some parts are in loose contact, some parts are in tight contact, and even some parts are not in contact. During the use of the regulating valve, there is relative movement between the valve stem and the packing, which is called axial movement. During use, with the influence of high temperature, high pressure, and highly permeable fluid media, the control valve packing box is also a location where leakage occurs more frequently. The main reason for filler leakage is interface leakage, and for textile fillers, there may also be leakage (pressure medium leaks outward along the small gaps between the filler fibers). The interface leakage between the valve stem and the packing is caused by the gradual attenuation of the contact pressure of the packing, the aging of the packing itself, and other reasons. At this time, the pressure medium will leak outward along the contact gap between the packing and the valve stem.
   
   Solution: To facilitate the installation of fillers, chamfer the top of the stuffing box and place a metal protective ring with a small erosion resistance gap at the bottom of the stuffing box (the contact surface with the filler should not be inclined) to prevent the filler from being pushed out by medium pressure. The metal surfaces of each part of the stuffing box in contact with the packing should be precision machined to improve surface smoothness and reduce packing wear. Flexible graphite should be used as the filling material, as it has good air tightness, low friction, small changes after long-term use, minimal wear and burn, easy maintenance, and no change in friction after re tightening the gland bolts. It has good pressure and heat resistance, and is not corroded by internal media. The metal in contact with the valve stem and packing box does not undergo pitting or corrosion. In this way, the sealing of the valve stem packing box is effectively protected, ensuring the reliability and long-term performance of the sealing of the packing.
   
   
3. Deformation and leakage of valve core and valve seat. The main cause of valve core and valve seat leakage is the strengthening of corrosion caused by casting or forging defects during the production process of regulating valves. The passage of corrosive media and the flushing of fluid media can also cause leakage of the regulating valve. Corrosion mainly exists in the form of erosion or cavitation. When corrosive media passes through the regulating valve, it will cause erosion and impact on the valve core and valve seat materials, causing the valve core and valve seat to form an oval or other shape. Over time, it will result in mismatched valve core and valve seat, gaps, and leakage if not tightly closed.
   
   Solution: The key is to ensure the material selection and quality of the valve core and valve seat. Choose corrosion-resistant materials and resolutely eliminate products with defects such as pitting and trachoma. If the deformation of the valve core and valve seat is not too severe, it can be ground with fine sandpaper to eliminate traces and improve the sealing smoothness to improve sealing performance. If the damage is severe, the valve should be replaced with a new one.

Oscillation

The spring stiffness of the regulating valve is insufficient, and the unstable output signal of the regulating valve can easily cause oscillation. The frequency of the selected valve is the same as the system frequency, or the pipeline or base vibrates violently, causing the regulating valve to vibrate accordingly. Improper selection results in sharp changes in flow resistance, flow rate, and pressure during the operation of the regulating valve. When the valve stiffness is exceeded, the stability deteriorates, and in severe cases, oscillation occurs.

Solution: Due to the multifaceted reasons for oscillation, specific problems will be analyzed. For slight vibrations, stiffness can be increased to eliminate them, such as using a high stiffness spring and switching to a piston actuator structure. Severe vibration of pipelines and bases can be eliminated by adding support to eliminate vibration interference; If the frequency of the selected valve is the same as the system frequency, replace the valve with a different structure; The oscillation caused by working at a small opening is due to improper selection. If the flow capacity C value is large, it is necessary to select a new one with a smaller flow capacity C value or use split range control or sub mother valve to overcome the control valve working at a small opening.

By following the information provided in this article, users can better comprehend the causes of needle valve malfunctions and employ appropriate solutions to safeguard and maintain these critical devices, ensuring their safe and efficient operation.