Anti-cavitation pressure regulating valve selection tips

Usually PN ≥ 16MPa valve is called high-pressure valve. The main problem in the use of high-pressure valves is the life-span problem (cavitation, flashing, erosion and other physical phenomena on the valve parts under pressure drop). General structure of the valve in high pressure conditions can only be used for 1 to 2 months, in order to solve the problem of short life of high pressure valve, domestic and foreign manufacturers have made great efforts, respectively, in the material and structure to do a lot of exploration , Formed a wide range of products, mainly multi-stage high-pressure valve, labyrinth high-pressure valve and single-seat sleeve load-type high-pressure valve. Its purpose is only one, by reducing cavitation, erosion, flash and other physical phenomena on the destruction of the valve to improve the service life of high-pressure valve. Among the many structures, Hualin Company designs anti-cavitation high-pressure valves according to the principle of "simplicity is beauty". It not only has the advantages of simple structure and convenient maintenance, but also mainly solves the problem of short life of high-pressure valves and enjoys fame in China. First, the reasons for the short valve life caused by the short life of high-pressure valve there are two main reasons: cavitation, erosion. Flash, Cavitation If the pressure difference across the valve (P1-P2) is greater than the maximum saturated differential pressure (ΔPmax) of the medium, a flash occurs and vapor cavitation is re-created and the internal and adjacent Near the pipeline structure caused damage. The velocity of the medium is maximized when the medium passes through the orifice with the smallest cross section. The increase in flow velocity (or kinetic energy) is accompanied by a significant decrease in pressure (or potential energy). When the pressure is lower than the saturated vapor pressure of the medium, bubbles form in the medium. As the pressure at the orifice drops further, bubbles can form in large numbers. There is no essential difference between flash and cavitation at this stage, but the possibility of structural damage to the valve is certain. Flash, erosion: If the medium passes through the orifice, the pressure is still below the saturated steam pressure of the medium. Bubbles will remain in the stream behind the orifice, which we call flashing. Flashing on the valve spool will have serious erosion damage, which is characterized by the erosion of the surface smooth and polished shape. The area with the most erosion damage is usually the area with the highest flow rate, usually on or near the contact line between the valve plug and seat. Especially in the small opening, small throttle clearance, high velocity environment, the erosion damage is also the most serious. Therefore, high-pressure valve should try to avoid a small opening work. Even the best of the valve, if long-term work in a small opening, its life will be doubled. To avoid the small opening of the work, the key in the calculation, selection, which is Design Institute and users should pay more attention. Typical contour cavitation damage by flash evaporation: On the other hand, if the medium pressure returns to above the saturated vapor pressure of the medium after passing through the orifice, the bubble will burst or inwards explode, creating cavitation. The rupture of the steam bubble releases energy and produces a noise similar to sand flowing through the valve. If the bubbles burst near the solid surface inside the valve, the energy released will slowly tear the material, leaving honeycomb holes. The destructive effect of cavitation may extend to the adjacent downstream pipe. Obviously, the high recovery valve is more prone to cavitation from the pressure recovery factor because its post-orifice pressure is more likely to rise above the saturated vapor pressure of the medium. The typical shape of cavitation damage and the relationship between cavitation erosion, erosion: a small opening work, throttle flow faster, lower pressure, more bubbles in the media, so the flash destruction More serious, erosion damage is the main contradiction. The main contradiction when working at the opening ceremony is the cavitation damage caused when the system pressure recovers above the saturation pressure. Therefore, users should try their best to avoid the small opening when using high-pressure differential valve. The high-pressure valve manufactured by the manufacturer must have better anti-cavitation measures. Otherwise, the valve will soon be damaged by cavitation erosion and erosion. Second, a mature anti-cavitation high pressure valve Now the most high-pressure valve from the structure is divided into mainly multi-stage high pressure valve, labyrinth high pressure valve and single-seat sleeve high-pressure valve load valve and several other categories. From the use of the situation, they have their own advantages, but multi-level and labyrinth high pressure internal structure is more complex, spare parts preparation and replacement more difficult to use very convenient. In response to these circumstances, Hualin company developed a single-seat load-type fine anti-cavitation high pressure valve. The product spare parts is simple, easy to replace and repair, a good solution to the above problems. In 1992 the product won the national patent (patent number: ZL 92 2 20633.3). (This anti-cavitation high-pressure valve uses a three-stage throttling structure of "orifice plate + single-seat valve + sleeve valve": ● Once throttling-take-over constriction (①) is equal to orifice throttling, accounting for about Total pressure drop of 10% ● Secondary throttling - single-seat throttling (main throttling, ② Department) single-seat valve structure, accounting for about 30% of the total pressure drop ● throttling - Flow (③ Department) sleeve structure, accounting for about 60% of the total pressure drop Pneumatic membrane anti-cavitation high pressure valve "orifice + single seat valve + sleeve valve" three throttle structure, the pressure drop through the valve into Several smaller pressure drops, each with a smaller pressure drop, ensure that the pressure at its smallest throttling surface is greater than the saturated vapor pressure, avoiding or mitigating the damaging effect of the flash effect on the valve. Cylinder throttling piece instead of the conventional "guide bushing + valve seat" mode, can completely eradicate the easy-to-loose guide bush problem, while the orifice jet throttling structure can reduce the flow on the spool seat unilateral local damage , Reduce the throttle 喑 sound because the orifice injection mode to withstand the majority of the throttle pressure drop (about 60%), you can Erosion damage caused by the valve seat to move to the hole, protecting the sealing surface and improve the service life of the valve, up to 2 to 3 times the pressure of ordinary high pressure valve. Third, anti-cavitation measures (1) past the use of carbide Not desirable, the hardness and toughness should be better corrosion resistance, anti-erosion materials. (2) The 70's often used to set the valve plate orifice technology to reduce pressure drop, borrow this idea can be in the valve Set the resistance, to achieve better antihypertensive effect. (3) to prevent the work of a small opening to reduce the speed of flushing fluid. (4) Select the fluid to change as small as the valve, such as straight stroke angle valve to reduce the particle impact. (5) Deflection will cause serious local erosion, should be used to disperse the spoiler method to overcome. (6) The use of multi-stage throttle structure, the pressure drop through the valve into several smaller pressure drop to improve the use of the valve (7) increase the stem diameter, stem stiffness to improve, to prevent rupture (8) large diameter, large differential pressure, should be equipped with powerful piston-type actuator Fourth, the use of precautions (1) to prevent the small opening Work is off Key, the small opening work will multiply shorten the life.Selection of such cases, should be reduced DN or dg; (2) for large-diameter valve, you must carefully calculate the unbalanced force, select the actuator and the spring range, To prevent the valve is not in place; (3) on the small diameter, the pressure difference is particularly large valve, prepared throttling pieces, in order to timely replacement; (4) 95 years of chemical industry notified a chemical plant due to spool rupture , Flow closed high-pressure valve (side into the bottom) valve head automatically shut down, the pressure rises, resulting in explosion of equipment, causing serious casualties.Therefore, from the safety point of view, should be used open type Into the side).