High-pressure control valve

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High-pressure control valves generally refer to valves with a PN value of ≥16 MPa. The main issue encountered when using such valves is not the high pressure itself, but rather the extremely large pressure difference, which causes severe cavitation and erosion of the valve core and seat, resulting in a very short service life for the valve; typically, such valves can only be used for 1–2 months. To address the issue of short lifespan of high-pressure valves, manufacturers both domestically and internationally have done a great deal of work, conducting extensive research on materials and design, resulting in a variety of products. Structurally, they are mainly divided into multi-stage high-pressure valves, labyrinth high-pressure valves, single-seat sleeve-loaded high-pressure valves, etc. The valve body structures include straight-through type, cage (sleeve) type, and corner type, but ultimately, they all serve one purpose: to extend the service life of high-pressure valves. Here, the author will not introduce each of these products one by one; instead, focusing on improving reliability and service life, he will introduce a product that features a simple structure, easy installation, compact size, and a long service life, thereby addressing the fundamental issue of short service life in high-pressure valves. 1) Reasons for the short lifespan of high-pressure valves: It is well known that high-zhuang valves have a short service life. Generally speaking, there are two main reasons for this short lifespan. (1) Erosion: At the throttle opening, the medium flows at high speed and possesses significant kinetic energy; this energy can quickly carve out streamlined grooves on the surfaces of the valve core and valve seat, which is what is known as erosion. Especially when operating at low openings, the throttling gap is small, the throttling velocity reaches its maximum value, and thus the erosion damage also reaches its peak. The severe erosion will significantly reduce the service life of the valve; this is why high-pressure valves should be avoided from operating at low openings. (2) Cavitation: In the high-speed flow of the medium at the throttle, its velocity can increase sharply. According to the principle of energy conservation, the pressure energy must decrease sharply as well. When the pressure falls below the saturated vapor pressure (Pv), the liquid begins to release gas, resulting in a two-phase flow of liquid and gas – this is what is known as flashing ； When the fluid passes through the throttle orifice, its flow velocity begins to decrease gradually, and the pressure starts to rise step by step. Once the pressure rises above the saturated vapor pressure, the bubbles burst and return to a liquid state. At the moment of rupture, a powerful pressure shock wave is generated, which causes the material on the surfaces of the valve core and valve seat to be damaged, forming honeycomb-like holes, and this leads to vibration and noise – this phenomenon is known as cavitation. The pressure difference condition that causes flashing is: △P = FLfile:///C:/Documents%20and%20Settings/Administrator/Desktop/Control_V valves_Technical_Training_Lectures/Doc/data.WebNoteBooks/20060701205229/zs2.gif (P1 – PV), where FL is the pressure recovery coefficient, P1 is the pressure before the valve, and Pv is the saturated vapor pressure at the inlet temperature. The process of cavitation formation is shown in Figure 4-28: file:///C:/Documents%20and%20Settings/Administrator/Desktop/Control Valve Technology Training Lectures/Doc/data.WebNoteBooks/20060701205229/4-36.gif Figure 4-28 Process of cavitation formation. (3) The relative importance of cavitation and erosion: When operating at low flow rates, erosion is the main issue ； At high opening degrees, cavitation is the main problem. Therefore, users should try to avoid operating at low opening degrees when using it ； High-pressure valves manufactured by producers must have effective anti-cavitation measures; otherwise, the valves will suffer from cavitation damage very quickly. 2) Anti-cavitation high-pressure valve and anti-cavitation measures (1) Structure of the anti-cavitation high-pressure valve The structure of the new type of high-pressure valve developed through these measures is shown in Figure 4-29; its main feature is the distribution of the pressure difference across multiple throttling stages. ●One throttling – G-connector reduction in diameter (at point A, equivalent to orifice throttling, accounting for about 10% of the total pressure drop). ●Secondary throttling – F single-seat throttle (main throttling, structure of the single-seat valve at point B; accounts for approximately 30% of the total pressure drop). ●Three-stage throttling – E-sleeve throttling (at C, sleeve structure, accounting for approximately 60% of the total pressure drop). ●The structure of this high-pressure valve is equivalent to: orifice plate + single-seat valve + sleeve valve. file:///C:/Documents%20and%20Settings/Administrator/Desktop/Control Valve Technology Training Lectures/Doc/data.WebNoteBooks/20060701205229/4-37.gif Figure 4-29 (2) Anti-cavitation measures: ① In the past, using cemented carbide was not a viable option, as its high hardness made it brittle; instead, materials that offer better combined properties of hardness and toughness, as well as resistance to cavitation and erosion, were used ； ②In the 1970s, orifice plates were installed downstream of the valve to reduce the pressure drop across it; drawing on this idea, we introduced resistance within the valve to achieve good results ； ③Multi-stage high-pressure valves increase their service life by having multiple valve cores share the pressure difference; drawing on this concept, we have implemented anti-cavitation improvements based on the multi-stage principle ； ④Shear flow causes severe localized erosion, and we use distributed throttling to overcome it ； ⑤The issue of occasional loosening of the guide bushing has been completely resolved ； ⑥Increase stiffness to prevent valve stem breakage ； ⑦For large diameters and high pressure differences, a powerful piston actuator should be selected. 3) Precautions for proper use: (1) Preventing operation at low opening degrees is crucial; operating at such low levels will significantly shorten the lifespan of the device. In such cases, the DN or dg value should be reduced immediately ； (2) For large-diameter valves, it is necessary to carefully calculate the unbalanced forces and select the appropriate actuator and working spring range to prevent the valve from not closing properly ； (3) For valves with a small diameter and a particularly large pressure difference, it is advisable to have a throttle element available for quick replacement when needed ； (4) In 1995, the Ministry of Chemical Industry reported an accident at a chemical plant caused by a broken valve core. The valve was of the flow-blocking type (with outlet on the side and bottom), and the valve core head closed automatically; high pressure led to the explosion of other equipment, resulting in severe casualties. Therefore, from a purely practical standpoint, it is better to choose the flow-out type (inlet at the bottom, outlet on the side). 4) Performance: This structure was patented in 1992, and over these seven or eight years of use, it has proven to be highly successful. Its service life can reach 2–3 years (except when the valve is operated at a low opening degree due to incorrect calculations), thus eliminating the long-standing problem of short lifespans for high-pressure valves. It is used by dozens of companies in China for level control in ammonia synthesis plants and in P4 valves in urea production facilities (such as Yihua, Chenzhou Chemical Group, Dongnitrogen, Luxi Fertilizer Factory, Heilongjiang Haolianghe Fertilizer Factory, etc.).