Revealing the selection of anti-cavitation high-pressure control valves

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This post was last edited by yinkuilin6868 on 2015-11-23 08:37. Valves with a PN value of ≥16 MPa are generally referred to as high-pressure valves. The main issue with high-pressure valves in use is their lifespan. Valves with a conventional structure can only be used for 1 to 2 months under high pressure differences. To address the issue of short lifespan in high-pressure valves, manufacturers both domestically and internationally have made significant efforts, conducting extensive research on materials and design, resulting in a variety of products such as multi-stage high-pressure valves, labyrinth-type high-pressure valves, and single-seat sleeve-loaded high-pressure valves. Its purpose is only one: to extend the service life of high-pressure valves by reducing the damage caused to them by physical phenomena such as cavitation, erosion, and flashing. 　　I. Reasons for short valve life The main reasons for the short service life of high-pressure valves are two: cavitation and erosion. 　　Vaporization, cavitation: If the pressure difference across the valve (P1-P2) exceeds the maximum saturated pressure difference of the medium (△Pmax), vaporization occurs, which in turn leads to cavitation and causes damage to the interior of the valve as well as to the adjacent pipe structures. The flow velocity is highest when the medium passes through the orifice with the smallest cross-section. An increase in flow velocity (or kinetic energy) is accompanied by a **decrease** in pressure (or potential energy). Bubbles form in the medium when the pressure is below its saturated vapor pressure. As the pressure at the throttle port further decreases, a large number of bubbles will form. At this stage, there is no essential difference between flashing and cavitation, but the possibility of structural damage to the valve does exist. 　　Vaporization, erosion: If the pressure of the medium remains below its saturated vapor pressure after passing through the throttle, Bubbles will remain in the flow stream behind the throttle, and we call this flashing. Vaporization causes severe erosion damage to the valve spools; the characteristic of such damaged surfaces is a smoothly polished appearance. The areas most severely affected by erosion damage are generally those with the highest flow velocity, usually located on or near the contact line between the valve core and the valve seat. Especially at low opening angles, where the throttling gap is small and the flow velocity is high, erosion damage is most severe. Therefore, high-pressure valves should avoid operating at low opening degrees as much as possible. No matter how good a valve is, if it operates at a low opening degree for an extended period, its lifespan will be significantly reduced. To avoid operating at low openings, the key lies in calculation and selection, which design institutes and users must pay close attention to. 　　A typical example of flash damage – external cavitation: On the other hand, if the pressure of the fluid returns above its saturated vapor pressure after passing through the throttle, the bubbles will burst or implode, thereby causing cavitation. The bursting of steam bubbles releases energy, producing a noise similar to that of sand and gravel flowing through a valve. If the bubble bursts near the solid surface inside the valve, the energy released gradually tears through the material, leaving behind honeycomb-like pores. The destructive effects of cavitation can extend to adjacent downstream pipes. Obviously, in terms of the pressure recovery coefficient, high-recovery valves are more prone to cavitation, as the pressure behind their throttle opening is more likely to rise above the saturated vapor pressure of the medium. 　　The typical pattern of cavitation damage – the relationship between flashing and cavitation, as well as erosion: At low flow rates, the velocity of the fluid at the throttle opening is higher and the pressure is lower; as a result, there are more bubbles in the fluid. Therefore, flashing damage is more severe at such times, with erosion damage being the main issue. When operating at high open degrees, the main issue is cavitation damage caused by the system pressure rising above the saturation pressure. Therefore, users should try to avoid operating the high-pressure differential valve at low opening degrees. High-pressure valves manufactured by producers must have effective anti-cavitation measures; otherwise, the valve will be damaged quickly due to cavitation and erosion. 　　II. A mature anti-cavitation high-pressure valve The high-pressure valves that are most widely used today can be classified structurally into several types, including multi-stage high-pressure valves, labyrinth-type high-pressure valves, and single-seat sleeve-loaded high-pressure valves. 　　In terms of usage, each type has its advantages, but the multi-stage and labyrinth-type high-pressure internal structures are relatively complex; it is difficult to prepare spare parts and carry out replacements, which makes them inconvenient to use. In response to these situations, Hualin Company developed a single-seat load-type compact anti-cavitation high-pressure valve. This product has simple spare parts, making replacement and maintenance easy, and it effectively solves the aforementioned problems. In 1992, this product was granted a **patent (Patent Number: ZL92 2 20633.3). (This anti-cavitation high-pressure valve features a three-stage throttling structure of \"orifice plate + single-seat valve + sleeve valve\": ● Primary throttling – the reduction in diameter of the connection pipe functions as an orifice plate throttling effect, accounting for approximately 10% of the total pressure drop.) 　　●Secondary throttling – the structure of a single-seat throttle valve, accounting for approximately 30% of the total pressure drop. 　　●Three-stage throttling – a sleeve throttling structure, accounting for approximately 60% of the total pressure drop. 　　The pneumatic diaphragm anti-cavitation high-pressure valve features a three-stage throttling structure of \"orifice plate + single-seat valve + sleeve valve\", which divides the pressure drop across the valve into several smaller drops. Each of these smaller drops ensures that the pressure at the thinnest part of the throttling surface is higher than the saturated steam pressure, thereby preventing or reducing the destructive effects of flashing on the valve. By replacing the conventional \"guide bushing + valve seat\" arrangement with a sleeve throttle element featuring a small-hole jet structure, the problem of the guide bushing coming loose can be completely resolved. Meanwhile, this small-hole jet throttle structure reduces the unilateral localized damage caused by flow resistance to the valve core seat, thereby lowering the throttling noise. Since the orifice jetting method bears most of the valve throttling pressure drop (about 60%), it is possible to shift the cavitation damage from the valve seat to the orifice, thereby protecting the sealing surface and extending the valve’s service life to 2–3 times that of ordinary high-pressure difference valves. 　　III. Anti-cavitation measures (1) The use of cemented carbide in the past was not a viable option; instead, materials that offer better combined properties of hardness and toughness, as well as resistance to cavitation and erosion, should be employed. 　　(2) In the 1970s, the technique of installing orifice plates downstream of the valve was commonly used to reduce the pressure drop across the valve; by applying this same principle, resistance can be created within the valve to achieve a better pressure reduction effect. 　　(3) Prevent operation at low opening angles and reduce the velocity of the scouring fluid. 　　(4) Choose a valve design with minimal fluid flow disruption, such as a straight-stroke angular valve, to reduce the impact of particles. 　　(5) Turbulent flow can cause severe local erosion, which should be overcome by using methods to disperse the turbulence. 　　(6) A multi-stage throttling structure is adopted to divide the pressure drop across the valve into several smaller drops, thereby extending the valve’s service life. 　　(7) Increase the valve stem diameter to improve its stiffness and prevent breakage. 　　(8) For large diameters and high pressure differences, a powerful piston-type actuator should be used. 　　IV. Precautions for Use 　　(1) Preventing operation at low openings is crucial, as operating at such low openings will significantly shorten the lifespan. When facing such situations during selection, the DN or dg value should be reduced; (2) For valves with large diameters, it is necessary to carefully calculate the unbalanced forces and select an appropriate actuator and spring range to prevent the valve from not closing properly; (3) For valves with small diameters but extremely high pressure differences, it is advisable to have throttling elements on hand so that they can be installed promptly when needed; (4) In 1995, the Ministry of Chemical Industry reported a case in which a chemical plant experienced equipment explosion due to a valve core breaking, which caused the flow-blocking type high-pressure valve (with inlet on the side and outlet at the bottom) to close automatically, leading to an increase in pressure and severe casualties. Therefore, from a safety-first perspective, a flow-out type (inlet at the bottom, outlet on the side) should be chosen.