What should I do if the pneumatic control valve fails?
The regulating valve does not work. First check whether the gas source pressure is normal and find the gas source fault. If the air source pressure is normal, determine whether the amplifier of the positioner or the electric / pneumatic converter has an output; if there is no output, the constant orifice of the amplifier is blocked, or the moisture in the compressed air accumulates at the amplifier ball valve. Use a small thin wire to clear the constant orifice, remove dirt or clean the air source.
If all of the above are normal and there is no signal, the actuator is malfunctioning or the valve stem is bent or the valve core is stuck. In this case, the valve must be removed for further inspection.
If the reciprocating stroke of the valve stem is slow, there may be a sticky substance in the valve body, clogging or over-tightening of the packing, or PTFE packing aging, and the valve stem bending and scratching. Most of the blocking faults of the regulating valve occur in the newly put into operation system and in the initial stage of overhaul. Due to the blockage of the welding slag and rust in the pipeline at the throttle and the guide, the medium cannot flow smoothly, or the packing has been filled during the maintenance of the regulating valve. Tight, resulting in increased friction, resulting in small signal inaction, excessive signal movement.
In such cases, the auxiliary line or the regulating valve can be quickly opened and closed to allow the stolen goods to be washed away by the medium from the auxiliary line or the regulating valve. In addition, you can use a pipe wrench to clamp the valve stem. When the signal pressure is applied, you can turn the valve stem forward and backward to make the valve core flash over the card. If you cannot solve the problem, you can increase the pressure of the air source and increase the driving power to move up and down several times to solve the problem. If it still cannot move, you need to dismantle the control valve. Of course, this job requires strong professional skills, and it must be completed with the assistance of professional and technical personnel, otherwise the consequences will be more serious.
Valve Leakage Regulatory valve leaks are generally caused by internal valve leakage, packing leakage, and leakage caused by valve core and valve seat deformation, which are analyzed separately below.
1. The length of the leaking valve stem in the valve is unsuitable, the valve stem of the air-open 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 can not be fully contacted, resulting in inadequacy. leak. Similarly, the valve stem of a gas shut-off valve is too short, which can cause a gap between the valve core and the valve seat, which can not be fully contacted, resulting in inadequate closing and internal leakage. Solution: Shorten (or lengthen) the valve stem of the control valve to make the length of the control valve appropriate so that it no longer leaks inside.
2. Packing leakage After the packing is packed into the stuffing box, it is subjected to axial pressure through the gland. Due to the plastic deformation of the packing, it produces radial force and comes into close contact with the valve stem, but this contact is not very uniform, some parts are loose, some parts are tighter, and some parts are not in contact at all. During the use of the regulating valve, there is relative movement between the valve stem and the packing. This movement is called axial movement. During use, with the influence of high temperature, high pressure, and highly permeable fluid media, the stuffing box of the regulating valve is also the place where leakage occurs. The main cause of filler leakage is interface leakage, which also occurs for textile fillers (the pressure medium leaks outward along the tiny 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 and the aging of the packing itself. At this time, the pressure medium will leak outward along the contact gap between the packing and the valve stem.
In order to facilitate the packing, fillet the top of the stuffing box, and place a metal protection ring with a small erosion resistance gap at the bottom of the stuffing box. Note that the contact surface of the protection ring and the packing cannot be inclined to prevent the packing from being pressured by the medium. roll out. The surface of the stuffing box in contact with the packing should be finished to improve the surface finish and reduce the wear of the packing. Flexible graphite is used as the filler, because it has good air tightness, low friction, small change in long-term use, small wear and tear, easy maintenance, and the friction force does not change after the gland bolts are re-tightened. Pressure resistance and heat resistance Good performance, not affected by the internal medium, and the metal in contact with the valve stem and stuffing box will not pit or corrode. In this way, the sealing of the valve stuffing box is effectively protected, the reliability of the packing seal is guaranteed, and the service life is greatly improved.
3. Deformation and leakage of valve core and valve seat The main cause of valve core and valve seat leakage is due to the strengthening of corrosion caused by casting or forging defects in the production process of the regulating valve. The passage of the corrosive medium and the flushing of the fluid medium will also cause the leakage of the regulating valve. 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 material of the valve core and the valve seat, so that the valve core and the valve seat become oval or other shapes. Mismatches, gaps, leaks due to improper closing.
Close the material selection of valve core and valve seat. Select corrosion-resistant materials and resolutely remove products with defects such as pits and trachoma. If the valve core and valve seat are not deformed seriously, it can be ground with fine sandpaper to eliminate traces and improve the sealing finish to improve the sealing performance. If the damage is severe, the valve should be replaced with a new one.