The Factors Influencing the Normal Operation of Control Valve and Countermeasures
June 25 19:16:53, 2021
1 Foreword In a high degree of automation of the chemical control system, regulating valve as the terminal automatically adjusts the implementation of the device, to accept the control signal to achieve the chemical process regulation. Its motion sensitivity is directly related to the quality of the regulation system. According to the actual statistics about 70% of the fault from the control valve. Therefore, the routine maintenance of the analysis of the factors that affect the safe operation of the control valve and countermeasures. 2, card blocking valve Frequently the problem is stuck, often appear in the new commissioning system and overhaul early investment, due to the welding slag, rust, etc. in the throttle, the leading parts of the blockage caused by poor circulation of media, Or overhaul the valve overhaul, resulting in increased friction, resulting in small signals do not move over large signal action phenomenon. Troubleshooting: Can quickly open, close sub-line or control valve, so that dirt from the secondary line or regulating valve at the media run. Another way to clamp the valve stem with the pipe clamp, in the case of external signal pressure, positive and negative rotation of the stem, so that the spool flashed over the card office. If you can not increase the pressure of the gas source to increase drive power to move up and down repeatedly several times, you can solve the problem. If still not action, you need to disintegrate. 3, leakage 3.1 valve leakage, stem length discomfort. Gas valve open, the valve stem is too long (or down) the distance is not enough, resulting in a gap between the valve core and valve seat, can not be fully exposed, leading to lax closed and internal leakage. The same gas off the valve stem is too short, resulting in a gap between the valve core and the valve seat, can not fully contact, leading to lax closed and internal leakage. The solution: Should shorten (or extend) the valve stem to adjust the length of the valve so that it no longer internal leakage. 3.2 packing leakage. After the stuffing is loaded into the stuffing box, axial pressure is applied to it via the gland. Due to the plasticity of the packing, which generates radial forces and comes into close contact with the valve stem, this contact is not very uniform. Some parts of the contact loose, tight contact with some parts, and even some parts are not in contact. Valve in use, the valve stem with the filler there is a relative movement, this movement is called axial movement. In use, with the high temperature, high pressure and permeability of the fluid medium, regulating valve stuffing box is also the place where leakage occurs more. Leakage caused by the filler is the main reason for the interface leakage, for textile packing will appear leakage (pressure medium along the gap between the filler fibers to the outside leakage). Leakage of the interface between the stem and the packing is caused by the gradual attenuation of the contact pressure of the packing and the aging of the packing itself, and the pressure medium leaks outwards along the contact gap between the packing and the valve rod. To solve the problem: In order to facilitate the filling stuffing, chamfering at the top of the stuffing box, placed at the bottom of the stuffing box erosion-resistant gap smaller metal guard ring (contact with the filler can not be inclined) to prevent the packing media pressure roll out. Stuffing box Each part of the metal surface in contact with the filler to be refined to improve the surface finish and reduce filler wear. Filler use of flexible graphite, because of its good air tightness, friction, small changes after long-term use, small wear and tear wear and tear, easy maintenance, gland bolts re-tighten the friction does not change, pressure and heat Good quality, free from erosion of internal media, pitting or corrosion free from metal in contact with the stem and stuffing box. In this way, the seal of the stem stuffing box is effectively protected, ensuring the reliability and long-term sealing of the packing. 3.3 spool, seat deformation leak. Leakage of the core, seat is mainly due to the cast valve or forging defects in the production process can lead to increased corrosion. The corrosive medium through the erosion of the fluid medium can also cause the regulator valve leakage. Corrosion mainly in the form of erosion or cavitation. When the corrosive medium passes through the regulating valve, it will produce the valve core, seat material erosion and impact of the valve core, the valve seat into an oval or other shape, with the passage of time, resulting in spool, valve seat is not Supporting, there is a gap, the gap is not strict leak. Solution: the key to good spool, valve seat material selection, the quality of the relevant. Select corrosion-resistant materials, pitting, trachoma and other defects of products resolutely removed. If the valve core, seat deformation is not serious, can be fine sandpaper grinding, eliminate traces, improve seal finish, to improve the sealing performance. If the damage is serious, the new valve should be replaced. 4, the oscillation control valve spring stiffness is insufficient, the control valve output signal instability and rapid changes can easily cause the control valve oscillation. Also said that the frequency of the election valve and the same system frequency or pipe, base violent vibration, so that the control valve vibration. Improper selection, the control valve work in a small opening there is a sharp flow resistance, flow rate, pressure changes, when the valve stiffness exceeds stability deterioration, serious oscillation. Solution: As there are many reasons for the oscillation, so the specific problems of specific analysis. Slight vibration of the vibration, can increase the stiffness to eliminate. Such as the use of large stiffness spring, switch to the implementation of the piston structure. Pipelines, the base of the strong vibration by adding support to eliminate vibration interference; the frequency of the valve and the system frequency is the same, then replace the valve of different structures; work in a small opening caused by the oscillation, Must re-select the flow capacity C value is smaller or the use of sub-control or sub-valve to overcome the control valve work in a small opening. 5, the valve locator fault 5.1 ordinary locator mechanical balance principle work, nozzle baffle technology, the main existence of the following types of faults: 1) due to the principle of mechanical balance of force, the movable parts are more susceptible to Temperature, vibration, resulting in fluctuations in the control valve; 2) nozzle baffle technology, because the nozzle hole is very small, easy to be blocked by dust or dirty gas source, the locator does not work; 3) the use of force The principle of balance, the spring coefficient of elasticity in the harsh changes in the scene, resulting in control valve non-linearity led to the decline in quality. 5.2 intelligent locator by the microprocessor (cpu), A / D, D / A converter and other components, its working principle and the ordinary locator is completely different. Comparison of the given value and the actual value is pure electric signal, no longer force balance. It is therefore possible to overcome the disadvantage of the conventional positioner force balance. However, when used in emergency parking situations, such as emergency shut-off valve, emergency vent valve. These valves are required to rest in a certain position and only need to be reliably actuated when an emergency situation arises. Prolonged stay in a position easy to make the electrical converter out of control caused by small signal does not move the dangerous situation. In addition, the position sensor for the valve potentiometer due to work in the field, the resistance value changes easily lead to small signals do not move, the danger of large signals fully open. Therefore, in order to ensure the reliability and availability of smart positioners, they must be frequently tested.
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