In an automated control system, the regulating valve controls the operation of the regulating valve by receiving signals to achieve adjustment of the process flow. As a terminal device of the automatic adjustment system, the sensitivity of the adjustment valve determines the quality of the adjustment system. The regulating valve is one of the most prone to failure of the regulating system. Based on years of accumulated experience, it is easy to analyze several fault causes and solutions that are most likely to occur in the regulating valve:
1. Card plugging Regulating valves with high adjustment accuracy are generally straight stroke type. Most of the valve body structure is inverted S type, and there is a dead angle of circulation, which is prone to jamming. Especially in the initial stage of new installation pipeline or overhaul, because there is a large amount of welding slag, rust or other debris in the pipeline, it is easy to cause media blockage at the throttle and guide. In addition, during the overhaul of the regulating valve, if the packing is too tight, the friction between the valve stem and the packing will be increased, causing the small signal to be inoperable and the large signal to move too far.
Solution: The sub-line or regulating valve can be quickly opened and closed, and the debris is discharged through the medium pressure. Or use a pipe wrench to clamp the valve stem, and under the applied signal pressure, the valve stem will rotate forward and backward through the card. It is also possible to increase the pressure of the air source repeatedly up and down to achieve a loose jam. If the above methods are invalid, they can only be disassembled.
2, leakage
Phenomenon I. Leakage in the valve body The function of the regulating valve is mainly to play the role of regulation, and is not suitable for cutting. Therefore, due to structural design and other factors, the general regulating valve is allowed to have a certain amount of leakage. However, when the regulating valve is closed, the internal leakage is serious, generally due to the length of the valve stem is not suitable: the air opening regulating valve, when the valve stem is too long, the up and down movement distance is not enough, causing the valve core and the valve to have a gap If the valve stem of the valve is too short, there is a gap between the valve core and the valve seat, which may not be fully contacted, resulting in a tight seal and internal leakage.
Solution: Adjust the valve stem of the regulator to make the stem length suitable and close normally.
Phenomenon 2, packing leakage Many valves have the phenomenon of packing leakage. Because the valve is assembled, the packing is loaded with the axial packing by the gland. Due to the plasticity of the packing, it produces radial forces and is in intimate contact with the valve stem, but this process does not guarantee uniform distribution of the packing. There are some areas where the contact is tight, and some parts are loose or not in contact. During the operation of the regulating valve, there is relative axial movement between the valve stem and the packing. With the temperature, pressure and other factors, the interface between the valve stem and the packing of the regulating valve may be leaked. The highly permeable medium also causes the medium to leak outward along the filler fibers through the pressure of the textile filler. When the filler contact pressure is attenuated and the filler itself ages, the pressure medium leaks outward along the contact gap between the filler and the valve stem.
Solution: In order to facilitate the filling of the packing, chamfer the top of the stuffing box and place a metal guard ring with a small erosion-resistant gap at the bottom of the stuffing box (the contact surface with the packing cannot be inclined) to prevent the packing from being pressed by the medium. roll out. The metal surfaces of the parts of the stuffing box and the contact part of the packing are finished to improve the surface finish and reduce the wear of the packing. The filler is made of flexible graphite because of its good air tightness, low friction, small change after long-term use, small wear and tear, easy maintenance, no friction change after the gland bolt is retightened, pressure resistance and heat resistance. Good toughness, free from internal media, and pitting or corrosion of metals in contact with the stem and stuffing box. In this way, the sealing of the stem packing box is effectively protected, and the reliability and long-term sealing of the packing are ensured.
Phenomenon 3, valve core, valve seat deformation leakage valve core, valve seat leakage is mainly caused by medium corrosion, flashing, cavitation (about flashing, cavitation, etc. can refer to Shanghai Baofeng valve official website valve knowledge introduction) and many more Some reasons can be solved by the material selection and production process of the valve core and the valve seat, while others need to be eliminated by the pipeline design. Corrosion mainly exists in the form of erosion or cavitation. When the corrosive medium passes through the regulating valve, it will cause erosion and impact on the valve core and the valve seat material, so that the valve core and the valve seat are elliptical or other shapes. As time passes, the valve core and the valve seat are not formed. Matching, there is a gap, and the leak is not strict.
Solution: This solution can only be started from the production source of the valve manufacturer, and the selection of the material of the valve core and the valve seat and the production process should be improved. Choose corrosion-resistant materials and resolutely eliminate defects in products such as pitting and trachoma. Although the high-quality valve core and valve seat can not completely solve the problems of flashing, cavitation, etc., it can play a role in prolonging the service life.
3. Oscillation There are several reasons for the oscillation of the regulating valve: First, when the spring stiffness of the regulating valve is insufficient and the output signal of the regulating valve is unstable, it is easy to cause the regulating valve to oscillate. Second, the frequency of the valve selection is the same as the system frequency. Third, the pipe and the base are violently vibrated, so that the regulating valve vibrates accordingly. 4. Improper selection, the valve has a sharp flow resistance, flow velocity and pressure change at a small opening. When the valve stiffness is exceeded, the stability deteriorates and oscillation occurs when it is severe.
Solution: According to the specific cause of the oscillation of the regulating valve, the corresponding solution. For slight vibration, increase the spring stiffness to eliminate it. For example, choose a large stiffness spring and use the piston to perform the structure. The vibration of the pipe and the base can eliminate the vibration interference by increasing the support. The frequency of the valve is the same as the system frequency. Valves with different structures; the oscillation caused by working at a small opening degree is the selection of the improper flow capacity C value, the re-selection of the flow capacity C value is small or the split control or the sub-valve valve is used to overcome the control valve Work at a small opening.
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