Features and selection of various multi-stage pressure reduction control valves

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Features and Selection of Various Multi-Stage Pressure Reducing Control Valves A systematic introduction is provided to the common structural types and features of multi-stage pressure reducing control valves. The common methods used by users to calculate the CV value of control valves based on the specific requirements under compressible medium conditions are summarized, providing a reference for users to understand the characteristics of multi-stage pressure-reducing control valves and make appropriate selections. I. Introduction In modern industrial production processes, control valves are devices used in control systems to adjust the flow rate of fluids in pipelines. They serve as the final control elements in piping systems, playing an important role in distributing fluid media and regulating fluid flow rates. In recent years, with the continuous advancement of industrial technology, special operating conditions such as high temperature and high pressure encountered in actual production have placed higher demands on control valves. In particular, control valves used in applications with high pressure differences often suffer from erosion at their internal throttling elements due to high flow rates. Additionally, problems such as cavitation, noise, and vibration caused by cavitation phenomena arise, posing significant risks to safe operation. Therefore, some manufacturers at home and abroad have separately developed multi-stage pressure-reducing structure control valves specifically for use under high pressure difference conditions. This article introduces the structure, working principle, and characteristics of various common multi-stage pressure reduction control valves separately. Furthermore, for compressible conditions, common methods for users to calculate the CV value of control valves based on the required flow rate are summarized. It provides a reference for users to understand the characteristics of multi-stage pressure reduction control valves and make appropriate selections. II. Common types and characteristics of multi-stage pressure reducing control valves The cavitation phenomenon that occurs in control valves is fundamentally caused by an excessive pressure difference before and after the valve. It is generally believed that when Δp > 2.5 MPa, the pressure of the fluid medium drops sharply as it enters the throttling section inside the valve; the pressure reaches its lowest value at the point where the flow cross-sectional area is smallest. When this pressure falls below the saturated vapor pressure of the fluid at the current temperature, part of the liquid vaporizes, forming numerous tiny bubbles. As the fluid flows past the throttle opening and the pressure rises again, these bubbles burst back into a liquid state, exerting force on components such as the valve body and valve spool, and causing problems such as noise and vibration. In recent years, some valve manufacturers at home and abroad have developed various types of cavitation-resistant multi-stage pressure-reducing control valves designed for use in harsh operating conditions. Common multi-stage pressure reduction control valves are divided into three categories. Although they differ in structure, they share the same working principle: by modifying their design, they divide the total pressure difference into multiple stages, ensuring that each stage’s pressure drop Δp1 is less than the critical pressure difference at which cavitation occurs, thereby effectively preventing hazards such as cavitation. 1. Cascade control valve: The cascade multi-stage pressure reduction structure is shown in Figure 1. This structure divides the original single throttling area into multiple separate throttling areas connected in series, thereby converting a large pressure difference into several smaller ones. As a result, the pressure reduction range for each step remains above the saturation vapor pressure, preventing cavitation from occurring. Figure 1: Cascade control valve. Cascade control valves are commonly used in applications involving liquid media. Their characteristics include: 1) They enable the reduction of the continuous pressure difference during opening and closing processes. The operation of each throttle stage occurs after that of the previous stage, which allows the high pressure acting on the valve during these processes to be reduced step by step, thereby sharing the pressure load from the first throttle stage. 2) It has low flow resistance, making it suitable for applications with low fluid cleanliness or even solid-liquid two-phase flows. 3) Cascade-type spools are generally treated with tungsten carbide spraying for hardening, offering good resistance to erosion and cavitation. 4) The manufacturing process is relatively simple compared to other multi-stage pressure reduction control valves; it is easy to process, and the manufacturing cost is also low. 5) Cascade control valves generally have a limited number of pressure reduction stages, usually 3 to 4, and cannot be used in situations with excessively high pressure differences. 2. Multi-layer sleeve control valve: The multi-layer sleeve multi-stage pressure reduction structure is shown in Figure 2, and it is commonly used in industries such as power plants or the chemical industry. Figure 2 Multi-layer sleeve control valve. The typical structural feature of a multi-layer sleeve control valve is that the throttling element in the valve core consists of several layers of sleeves with holes drilled in them; there is a certain gap between each layer of sleeves, which allows the fluid to be buffered as it flows through these sleeves, thereby keeping the fluid velocity within a specified range. Its features are as follows: 1) The number of pressure reduction stages in the multi-stage sleeve-type control valve can be designed to be large, giving it a stronger pressure reduction capacity compared to cascade types, making it suitable for applications with high pressure differences. 2) The multi-layer sleeve structure can meet high pressure drop requirements while ensuring a large flow rate during operation. 3) It has good cavitation resistance; when used with liquid media, the fluid flows from the outermost sleeve to the innermost one. The pressure of the liquid medium decreases gradually as it moves through the sleeves, which helps to reduce the occurrence of cavitation. Ultimately, the fluid is ejected through small holes in the innermost sleeve into the central valve chamber, where the bubbles burst, thus avoiding any direct damage to the metal surface of the valve. 4) It has excellent noise and vibration resistance; when used with gaseous media, the flow occurs from the inside to the outside of the sleeve. The diameter and gaps of the outer sleeve are larger compared to those on the inner side, allowing the gaseous medium to expand as pressure decreases step by step, which effectively reduces the harm caused by noise and vibrations. 5) The sleeve processing process is relatively complex and costly. But it is easy to install and maintain, and easy to replace. 3. Labyrinth control valve: The multi-stage pressure reduction structure with labyrinth disc plates is shown in Figure 3; its core throttling element is composed of multiple metal discs equipped with labyrinthine grooves, stacked on top of each other. As the fluid flows through the labyrinthine channels, it undergoes numerous collisions and turns, thereby losing energy; simultaneously, as the pressure decreases step by step, the flow velocity is also controlled. Figure 3: Labyrinth control valve. These valves are generally used in industries such as nuclear energy and power plants in situations requiring high temperature and pressure drops; the working medium is usually superheated steam, but they can also be used with liquid media. Its features are as follows. 1) The number of turning stages in the labyrinth flow path corresponds to the number of pressure reduction stages in a labyrinth-type control valve; this number can generally range from a dozen to over twenty stages. Therefore, the labyrinth-type multi-stage pressure reduction structure offers the strongest pressure reduction capability among common multi-stage pressure reduction control valves. Some foreign products can achieve a pressure reduction of up to 40 MPa. 2) Excellent resistance to cavitation erosion, as well as sound suppression and vibration reduction capabilities; the multi-stage curved labyrinthine flow channels can effectively control the fluid flow rate, preventing issues such as cavitation, noise, and vibration. 3) By combining different types of labyrinth discs, labyrinth control valves can achieve various flow characteristic adjustment curves. 4) The manufacturing precision required for labyrinth-type disks is very high; they are generally clad with Stellite alloy, which ensures a long service life ； Installation and maintenance are relatively simple, and the discs are easy to replace. 5) Labyrinth flow channels require a high level of cleanliness of the fluid medium; otherwise, they are prone to blockage. III. Calculation of the CV value of multi-stage pressure reduction control valves. The flow coefficient (CV) is generally used to indicate a valve’s capacity to allow fluid to pass through it. To select an appropriate control valve, it is necessary to calculate the required CV value based on the operating conditions, and then choose the suitable valve model according to that rated flow coefficient. Under compressible conditions, the fluid experiences a decrease in pressure during throttling, its volume expands and its density decreases; the flow behavior within the valve is much more complex compared to that in incompressible fluids. Therefore, for multi-stage pressure-reducing control valves that are generally used in compressible conditions, the method for calculating their flow coefficient is also rather special. The typical methods for calculating the CV value under compressible conditions include the compression coefficient method and the expansion coefficient method. 1. Compression coefficient method: The compression coefficient method was proposed by the Soviet Union in the 1950s; it is one of the early formulas used to calculate the flow coefficient under compressible conditions. The compressibility coefficient method takes into account the compressibility of gases by adding a gas compressibility coefficient ε to the general formulas for liquids, thereby correcting those formulas. This method significantly simplifies the calculation model by reducing various types of control valves to identical flow nozzles. It then assumes that the flow of the gas medium within these nozzles is an adiabatic process, and uses the energy balance equation to derive the calculation formula, namely: Equation (1), where γN represents the specific weight of the gas under standard conditions, with the unit being kgf/m3 (1 kgf = 9.8 N) ； Q — Volume flow rate under standard conditions, in m3/h ； T — gas temperature, in K ； p1 —— Pressure before the valve, in kgf/m2 (1 kgf = 9.8 N) ； ∆p — pressure difference before and after the valve, in kgf/m2. The compression coefficient ε can be determined through experiments; generally, for air, the following value can be obtained: (2) In addition to the compression coefficient method, earlier methods included the density before the valve method, the density after the valve method, and the average density method. Early formulas can only be applied in situations where the degree of pressure recovery is low, and they can ensure good calculation accuracy within the non-critical flow regime. However, due to the simplification of the calculation model by the formula, larger errors occur as ∆p/p1 increases to the critical pressure ratio, and the requirements cannot be met in the transition zone and the critical zone. 2. Expansion coefficient method: Since the early calculation formulas did not take into account the impact of the valve’s pressure recovery characteristics on the calculations, in the 1970s some foreign manufacturers developed a series of later-formulae represented by the expansion coefficient method, polynomial method, and sine method. These improved upon the earlier formulas and were able to meet the required calculation accuracy for the range from the non-critical zone to the critical zone. Compared with earlier formulas, the calculation results of later formulas, represented by the expansion coefficient method, are more economical, allowing for the reduction of unnecessary waste. Among them, the coefficient of expansion method is recommended by IEC as the standard formula due to its simplicity in calculation. The coefficient of expansion method is derived by introducing the coefficient of expansion Y into the calculation formula used for liquid conditions as a correction; when Y=1, this method is also applicable to incompressible liquid situations. (3) In the formula, ρN is the density of the gas under standard conditions, with the unit of kg/m3 ； Q — Volume flow rate under standard conditions, in m3/h ； T1 — Gas inlet temperature, in K ； p1 —— pressure before the valve, in kPa ； X — pressure difference ratio, X = Δp/p1 ； Z — Compression coefficient. The expansion coefficient Y refers to the ratio of the flow coefficient of a compressible medium to that of an incompressible medium at the same Reynolds number. It shows the change in density of the fluid as it flows from the valve inlet to the throat, which is the section with the smallest flow area downstream of the orifice, as well as the change in the area of this throat when the pressure difference changes. (4) In the formula, FK is the specific heat ratio coefficient, with FK = K/1.4. Since the calculation formula itself does not include the actual density of the fluid under upstream conditions, the coefficient of expansion method introduces a compressibility coefficient Z to compensate for the deviation between real gases and ideal gases under certain conditions. The expansion coefficient Y is used to correct for the change in gas density from the valve inlet to the throat; Y is related to factors such as the ratio of the throat area to the inlet area, the shape of the channel, the pressure difference ratio X, the Reynolds number, and the specific heat ratio coefficient FK. The expansion coefficient method takes comprehensive account of the many factors that affect the flow of compressible fluids, thereby ensuring high computational accuracy across the entire range of flows. It is suitable for various types of valves and is therefore widely used. IV. Conclusion The multi-stage pressure reduction control valve, when used under high pressure difference conditions, serves as a key device in pipeline systems and plays a crucial role in the control process. This article provides a systematic introduction to the working principles, core structures, characteristics, and applicable scenarios of three common types of multi-stage pressure-reducing control valves, offering users a reference for understanding the basic types and features of such valves. Furthermore, since multi-stage pressure-reducing control valves are often used in compressible flow conditions, this paper also summarizes the typical calculation methods for the flow coefficient under such conditions, enabling users to select the appropriate model of control valve based on the correct calculation methods. In summary, this paper provides some reference for users to understand the characteristics of multi-stage pressure-reducing control valves specifically designed for high-pressure difference applications and to select them appropriately.