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1 Pneumatic membrane valve selection 1.1 According to the requirements of the selection Pneumatic valve diaphragm and valve body (including the valve seat) consists of two parts, according to different requirements into straight through the single-seat valve, two-seat control valve and Pressure regulating valve. Leakage through the single-seat valve is small, the thrust of the fluid on the single-seat valve unbalanced force formed by a large, so it is suitable for small leakage requirements and smaller diameter of the valve before and after the smaller occasions. Through two-seater valve body has two upper and lower valve spool, due to fluid acting on the spool in the opposite direction and generally offset, so the two-seat valve unbalance force is small, allowing a larger pressure difference before and after the valve. However, due to the complicated flow path of the two-seated valve body, under the conditions of high pressure difference, the damage to the valve body is more serious and should not be used for medium with high viscosity and containing suspended particles or fibers. In addition, due to the processing conditions, the upper and lower two-seater valve spool is not easy at the same time off, so the closure of the leakage, especially in the case of high temperature or low temperature, due to different thermal expansion coefficient of the material is more likely to cause serious Leakage. Angular high-pressure valve body for the right angle, the flow path is simple, small resistance, high-speed fluid erosion is also small, especially suitable for high pressure, high viscosity and suspended solids containing particles of fluid can also be used to handle gas and liquid Mixed phase, easy flash cavitation occasions, to avoid coking, bonding and clogging, easy to clean and self-purification. 1.2 According to the safety performance selection Pneumatic membrane valve with air valve and gas valve closed in two forms, the selection should be based on the signal pressure is interrupted when the control valve is open or closed position on the size of the hazardous process due to the size. If the valve is in a closed position, the harm is less, the gas valve is used instead, and the gas closed valve is used instead. 1.3 According to the selection of flow characteristics In the design process of the control system, the choice of pneumatic diaphragm control valve should focus on the flow characteristics. Typical ideal features are linear flow characteristics, equal percentage flow characteristics (logarithmic flow characteristics), fast open flow characteristics and parabolic flow characteristics of four kinds. When the relative flow rate changes linearly with the same linear flow characteristics, the relative change of the flow rate is large when the flow rate is small, and the relative change value of the flow rate is small when the flow rate is large. Therefore, the linear flow control valve in the small opening (small load) the case of poor regulation performance is not easy to control, often produce oscillation, it should not be used for small opening of the case, it is not suitable for larger load regulation system , Only applies to the load is relatively stable, little change regulation system. The regulating valve with percentage flow characteristics is weak in adjusting action under small load and strong in adjusting action under large load. It has weak regulating action when close to close, stable work and gentle adjustment when it is close to full open, and is sensitive and effective in work. Therefore, Large changes in load occasions, both in full load and half load production can play a good regulatory role. 1.4 The choice of regulating valve diameter should be based on the known fluid to calculate the required flow coefficient CV, and then select the appropriate technical parameters of the product valve diameter. When calculating CVs, pay attention to the differences between liquids, gases, steam and other vapors. 2 Pneumatic membrane valve installation When installing the valve should pay special attention to the following points. (1) The control valve should be mounted vertically on the horizontal pipe, such as in special circumstances require horizontal and inclined installation, the general to add support. (2) When the nominal diameter of the selected control valve is different from the pipe diameter of the process, reducer should be added to connect. (3) The installation site should have better environmental conditions, the ambient temperature should be -25 ~ +55 ℃. (4) try to avoid the installation in a vibration of the occasion, or should take the necessary anti-vibration reinforcement measures. (5) During installation, the direction of the arrow on the valve body or on the flange must be in the direction of the medium. (6) Prior to installation, it is necessary to carefully clean the welding slag and other debris in the pipe. After installation, the valve core should be at the maximum opening and the pipe and regulating valve should be cleaned again to prevent the debris from jamming and damage Flow pieces. (7) Pneumatic control valve should be installed in a convenient location for maintenance and repair. (8) The straight pipe valve should be as long as possible. (9) Application of export pipe 3 to 5 times the diameter of the pipe straight section. (10) In order to ensure that the regulator and control system failure does not affect the production and safety accidents, the general need to install bypass and bypass valve. Bypass valve can not be installed directly above the control valve to prevent leakage of caustic medium bypass valve to the control valve. Before and after the control valve to be installed shut-off valve, for high temperature, high pressure, easily frozen, viscous medium, but also to install the pilot valve. 3 common fault control valve and on-site processing 3.1 card blocking valve is a common problem blocking the card, usually in the early start-up and overhaul new system put into operation early. Cause plug the card for the following reasons: (1) due to slag within the pipeline, iron and other stop in the throttle, causing the guide parts of the block. (2) In the process of overhauling, the filling valve of the regulating valve is too tight, causing the friction to increase, causing the small signal not to move and the large signal to move too far. Troubleshooting: (1) can quickly open and close the secondary line or control valve, so debris from the secondary line or control valve was washed away. (2) Clamp the valve stem with the pipe wrench and rotate the valve stem back and forth with the signal pressure applied so that the valve spool flashes over the card. If invalid, then increase the gas pressure to increase the drive power, repeated several times up and down to solve the problem. If you still can not move, you need to disassemble. 3.2 does not move in daily production operation, the control valve is not the action is more common fault. Caused by the valve does not move the cause and the corresponding treatment in Table 1. Table 1 does not move the valve causes and the corresponding treatment 3.3 leaks (1) filler leakage caused by filler leakage is mainly due to interface leakage. Interfacial leakage is usually caused by the gradual decrease of the contact pressure of the packing and the aging of the packing itself, in which case the pressure medium leaks outwards along the contact gap between the packing and the valve stem. Solution: In order to make packing convenient, chamfered at the top of stuffing box, placing a metal guard ring with small clearance on the bottom of stuffing box (the contact surface with stuffing can not be beveled) to prevent the stuffing from being damaged by medium pressure Introduced; stuffing box parts of the contact surface with the filler to be finishing, in order to improve the surface finish of the metal to reduce the wear of the filler; filler selection of flexible graphite, because of its good air tightness, small friction, long-term use after the change Small wear and tear, wear and tear is small, simple maintenance, gland bolts re-tighten the friction does not change, pressure and heat resistance is good, free from internal media erosion, the bottom of the metal filler ring does not occur pitting corrosion advantages . (2) valve core, valve seat deformation Leak valve seat, valve seat leakage is mainly due to the control valve in the production process of casting and forging defects led to increased corrosion, and the passage of corrosive media, fluid medium scouring Can cause valve leakage, corrosion mainly in the form of erosion or cavitation. Solution: Good spool, valve seat material selection, quality, should choose corrosion-resistant materials; If the valve core, seat deformation is not serious, can be fine sandpaper to eliminate traces of grinding to improve the sealing surface finish, Improve the sealing performance; if the valve is seriously damaged, you should replace the new valve. 3.4 Oscillation valve spring stiffness is insufficient to regulate the output signal instability and rapid changes, easy to cause the regulator valve oscillation. When the vibration frequency of the control valve is the same as the system frequency, the base of the pipeline will vibrate violently and the valve will vibrate. When the valve is operated under a small opening, the flow resistance and pressure will change rapidly. When the change exceeds the valve stiffness, the stability of the valve decreases, resulting in oscillation. Solution: For slight vibration, can be eliminated by increasing the stiffness of the valve, you can also use the higher stiffness of the spring, the use of piston actuators, pipe and base oscillation can be increased by increasing the support to eliminate interference; if the valve Vibration frequency and the same system frequency, the replacement of different structures of the valve; work in a small opening caused by the oscillation, it is due to the valve flow value is too large, this case must reselect the flow rate and process similar Large) control valve, or the use of split-step adjustment, or use the child valve to meet the valve in the small opening of the case of work. Causes of oscillation and treatment methods in Table 2. Table 2 causes of oscillation and processing methods 3.5 slow action In general, for emergency parking occasions, such as emergency shut-off valve, emergency vent valve, in the event of an emergency need reliable action, otherwise it will give the system Production poses a serious threat. The main reasons for the sluggishness of the regulating valve are as follows: ①the diaphragm in the pneumatic diaphragm actuator is damaged and leaked; ② the O-ring leaks in the actuator; ③ the clogged debris in the valve body; ④ the deterioration and hardening of PTFE filler or graphite - asbestos packing lubricant drying; ⑤ packing pressed too tight, increased frictional resistance; ⑥ due to the valve stem does not lead to increased frictional resistance. Solution: ①Replace the damaged diaphragm; ② Replace the O-ring; ③ Clean the debris in the valve body; ④ Replace the packing; ⑤ Properly adjust the packing gland; ⑥ Replace the valve stem. 3.6 valve positioner ordinary positioner mechanical balance principle work, nozzle baffle technology, the main problems are as follows: ①because of its use of mechanical balance of work principle, moving parts too much, easily affected by temperature, vibration Influence, cause the fluctuation of the regulating valve; Adopt the nozzle baffle technology, because the nozzle hole is very small, it is easy to be clogged by the dust in the air, so that the locator can not work normally; Long-term work, the elastic coefficient is easy to change, resulting in poor linearity locator, valve control quality decline. As the valve positioner working position sensor potentiometer work in the field, easy to change the resistance value, resulting in small signal does not move, the danger of large signals fully open. In order to ensure the reliability and availability of intelligent positioners, valve positioners must always be tested. Conclusion; through the analysis of the cause of the control valve failure, to take appropriate treatment, improve the method can greatly reduce the instrument failure rate and improve the utilization rate of the valve to effectively improve the stability of the control system and extend the operational life of the production plant, the right Improve the production efficiency and economic efficiency of enterprises and reduce energy consumption has an extremely important role.
February 02, 2023