1, an overview Control valve as an automatic control system in the implementation of the executive part, in modern industrial production has been widely used, the control and communication with the computer and bus technology has undergone a fundamental change, greatly improving the control Accuracy and reliability. However, during the process of high temperature and high pressure, the pipeline fluid often causes cavitation due to equipment structure design, improper installation or process parameter design, etc., causing serious damage to the regulating valve trim and causing vibration and noise of the entire system. Seriously affect the life of the control valve and control system accuracy, to bring great risks to industrial production. 2, cavitation mechanism Cavitation is a phenomenon of hydraulic flow, the direct cause of cavitation pipeline fluid due to a sudden change in resistance generated flash and cavitation. Regulating valve in the process system is throttling components, starting from the role of variable resistance element, the core of which is a movable valve flap and the valve seat formed between the throttle window, change the valve position can change the control valve Resistance characteristics, and then change the resistance characteristics of the entire process system. In the high pressure difference (△ p> 2.5MPa), the adjustment process of the control valve is a sudden change of resistance, and the process is prone to cavitation. For ease of analysis, the throttling process of the control valve is modeled as an orifice adjustment (Figure 1). The formation of the cavitation in Figure 1 shows that when the inlet pressure is p1 and the fluid with a velocity of V1 flows through the orifice, Suddenly a sharp increase in the flow rate, according to the law of conservation of fluid energy, the static pressure will suddenly increase the flow rate decreased. When the outlet pressure p2 reaches or falls below the saturated vapor pressure pv of the fluid, part of the liquid is vaporized to form a gas-liquid two-phase coexistence phenomenon, which is not only the formation of flash. If flash does not occur, p2 is not maintained below the saturated vapor pressure. After leaving the orifice, the flow rate decreases as the cross-section of the flow channel increases. The increased pressure compresses the bubble created by flashing, the bubbles change from circular to elliptical, and then the bubble upstream surface that reaches the critical size begins to flatten, then burst suddenly. All the energy concentrated in the rupture point, have a huge impact, its strength up to several thousand Newtons. This impact collides with the valve disc, the valve seat and the valve body, resulting in plastic deformation of the surface to form a rough cellular slag hole, which is the formation of cavitation process. Cavitation is not only found in high pressure differential valves, but is found in many areas of industrial production. 3, the measure to prevent cavitation 3.1, the type of choice can be seen from the analysis, cavitation occurs because of the occurrence of cavitation, and cavitation occurs because the throttle caused a sudden change in pressure, it should avoid cavitation. The resulting pressure of the cavitation of the pressure that is blocking the formation of pressure differential △ pT △ pT = FL2 (p1-pvc) where FL --- pressure recovery factor in the process conditions allow the use of △ p <△ pT Of the valve, the choice of small pressure coefficient of recovery valve, such as ball valve or butterfly valve. If the process conditions must be △ p> △ pT, two regulating valve can be used in series, so that each regulator valve pressure △ p less than △ pT, cavitation will not occur. If the valve pressure difference △ p less than 2.5MPa, generally does not produce cavitation, even if there is cavitation will not cause serious damage to the valve. In addition, the choice of angle valve can also reduce the flash destructive power. Because the medium in the angle valve directly flows to the center of the downstream pipe inside the valve body instead of directly impacting the body wall, the number and times of saturated bubbles in the body wall of the impact valve body are reduced, and correspondingly, the occurrence of cavitation is reduced. 3.2 Material Selection From the results of cavitation analysis, the material hardness can not resist the impact of bubble rupture and the release of one of the main causes of damage, but can withstand cavitation for a long time little material, expensive, both at home and abroad Commonly used materials Stellite alloy (including cobalt, chromium, tungsten alloy, 45HRC), hardened tool steel (60HRC) and tungsten steel (70HRC) and so on. But the hardness of the material processing molding inconvenient, easy to brittle fracture, processing costs, the commonly used method is to build a stainless steel substrate welding or welding Division Lai Lai alloy (Figure 2), formed in the erosion of fluid cavitation Hardened surface. When hardened surface damage occurs, you can make the second surfacing or spray welding, so that both to increase the service life of the equipment, but also reduce the maintenance costs of the device. 3.3. The result of structure selection analysis shows that cavitation is caused by the sudden change of pressure, and the pressure drop required by the system can not be reduced. Therefore, a large pressure mutation is decomposed into several stages of multi-stage valve disc structure (Figure 3) , This structure of the valve can divide the total pressure difference into several small differential pressure step by step, so that each level does not exceed the critical pressure. Or designed as a special structure of the valve flap and valve seat, such as the labyrinth disc and laminated disc, etc., can make high-speed fluid through the disc and valve seat, the pressure at each point are higher than at this temperature Of saturated vapor pressure, or the liquid itself collide with each other, resulting in a high degree of turbulence between the channels, so that the kinetic energy of liquid due to mutual friction and become thermal energy, can reduce the formation of bubbles. Figure 1 Different structure of the valve has its own cavitation coefficient δ where H1 --- valve (outlet) pressure, MPa H2 --- atmospheric pressure and temperature corresponding to the difference between the saturated steam pressure, MPa Δp --- Differential pressure before and after the valve, MPa Various valves due to different structures, allowing the cavitation coefficient δ is also different, such as the calculated cavitation coefficient is greater than the allowable cavitation coefficient, the cavitation does not occur. Butterfly valve to allow cavitation coefficient of 215 as an example for illustration. When δ> 2.5, cavitation does not occur. When 2.5> δ> 1.5, slight cavitation occurs. When δ <1.5, vibration is generated. When δ <0.5, if the valve is used continuously, the valve and the downstream pipe will be damaged. It can be seen from the calculation that cavitation is related to the outlet pressure H1 of the valve, and increasing H1 will change the situation. The improvement of many ways, such as the valve installed in the pipeline at a lower point, or in the valve pipe after the orifice plate to increase resistance, but also the valve outlet directly connected to the reservoir, so that the bubble burst of space increased, reduced cavitation. 4 Conclusion Cavitation phenomenon by the valve valve material, fluid, mechanics, structure and media and other factors, through the rational choice, accurate calculation, as well as the new valve technology and new structure applications, cavitation Will be better solved in production.
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