The hottest factors affecting the safe operation o

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Factors and Countermeasures Affecting the safe operation of the regulating valve (Part I)

1. Preface

in the chemical control system with a high degree of automation, the regulating valve, as the terminal executive device of the automatic regulating system, receives the control signal to realize the regulation of the chemical process. Its action sensitivity is directly related to the quality of the regulating system. According to the actual statistics on site, about 70% of the faults come from the regulating valve. Therefore, the factors affecting the safe operation of the regulating valve and their countermeasures are summarized and analyzed in the daily maintenance

2. Blockage

the frequent problem of the regulating valve is blockage, which often occurs in the new operation system and the initial stage of overhaul and operation. Due to the blockage of welding slag and rust in the pipeline at the throttle and guide parts, the medium circulation is not smooth, or the filler is too tight in the maintenance of the regulating valve, resulting in the increase of friction force, resulting in the phenomenon of small signal inaction and large signal over action

Troubleshooting: quickly open and close the auxiliary line or regulating valve, so that dirt can be washed away by the medium from the auxiliary line or regulating valve. Another way is to clamp the valve rod with a pipe wrench. Under the condition of external signal pressure, rotate the valve rod with positive and negative force to make the valve core flash past the card. If not, the problem can be solved by increasing the air source pressure and increasing the driving power by the end of 2020 and moving up and down repeatedly for several times. If it still doesn't work, it needs to be disassembled

3, leakage

3.1 the valve has internal leakage, and the length of the valve rod is inappropriate. When the valve is opened by air, the valve rod is too long, and the upward (or downward) distance of the valve rod is not enough, resulting in a gap between the valve core and the valve seat, which cannot be fully contacted, resulting in loose closing and internal leakage. Similarly, the valve rod of the air shutoff valve is too short, resulting in a gap between the valve core and the valve seat, which cannot be fully contacted, resulting in loose closing and internal leakage

solution: shorten (or extend) the valve rod of the regulating valve to make the length of the regulating valve appropriate so that it will no longer leak inside

3.2 packing leakage

after the packing is installed in the stuffing box, axial pressure is applied to it through the gland. Due to the plasticity of the packing, it produces radial force and is in close contact with the valve stem, but this contact is not very uniform. Some parts are loose, some parts are tight, and even some parts are not in contact. During the use of the regulating valve, there is a relative movement between the valve rod and the packing, which is called axial movement. In the process of use, with the influence of high temperature, high pressure and highly permeable fluid medium, the stuffing box of the control valve also has leakage, which is more than enough when the tension range is 100 Newton. The main cause of packing leakage is interface leakage. For textile packing, there will be leakage (the pressure medium leaks outward along the tiny gap between the packing fibers). The interface leakage between the valve rod 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 cause and troubleshooting of this kind of fault: (1) the force point blade is loose, and the contact gap between the packing and the valve rod is loose

solution: to make the packing easy to install, chamfer the top of the stuffing box, and place a metal protective ring with small erosion resistance gap at the bottom of the stuffing box (when it is placed at the highest position for the first time, the contact surface with the packing cannot be an inclined plane), so as to prevent the packing from being pushed out by the medium pressure. The metal surface of each part of the stuffing box in contact with the stuffing shall be finished to improve the surface finish and reduce the wear of the stuffing. Flexible graphite is selected as the packing because of its good air tightness, small friction, small change after long-term use, small wear and burning loss, easy maintenance, no change in friction after retightening the gland bolts, good pressure resistance and heat resistance, no erosion by internal media, and no pitting or corrosion of the metal in contact with the valve stem and the stuffing box. In this way, the sealing of the valve stem stuffing box is effectively protected, and the reliability and long-term sealing of the packing are guaranteed

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