Selection of control valve diameter

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Based on the process operating parameters, select an appropriate size for the control valve. Once we have chosen the type and characteristics of the control valve, we can then determine its size. Flow capacity is an important factor in determining the diameter of control valves. From obtaining data related to the process to calculating the flow capacity and ultimately determining the valve diameter, the following steps are required: 1. Determination of flow rate – The maximum flow rate Qmax and minimum flow rate Qmin are calculated based on the existing production capacity, equipment load, and conditions of the medium. When calculating the C value, the maximum flow rate should be taken into account; if an excessive margin is considered for the maximum flow rate, this results in a larger diameter for the control valve ； This not only results in economic waste, but more problematically it causes the control valve to operate at a low opening degree frequently, reducing its adjustability and deteriorating its control performance; in severe cases, it can even lead to oscillations, thereby **reducing the valve’s service life**. When selecting the maximum flow rate, it should be determined reasonably based on changes in the load of the process and the production capacity of the equipment. In applications where high control quality is required, the maximum flow rate should be selected based on the existing process conditions. However, it is important not to overemphasize control quality to the point where, when the load changes or when the existing production equipment is upgraded or expanded and its production capacity increases slightly, the control valve can no longer meet the requirements and must be replaced. In other words, it is necessary to take into account both the current situation and the need to expand production capacity within a certain range in the future, in order to determine the maximum calculated flow rate appropriately. 2. Determination of the pressure difference: The S value is selected based on the flow characteristics of the control valve chosen and the characteristics of the system, and then the pressure difference to be calculated is determined. For a control valve to function as a regulator, there must be a certain pressure difference before and after the valve. The larger the proportion of this pressure difference across the valve relative to the total system pressure difference, the less distorted the flow regulation characteristics will be, ensuring optimal regulatory performance. However, the greater the pressure difference before and after the valve, that is, the greater the pressure loss across the valve, the more power is required. Therefore, it is necessary to take into account both the regulating performance and power consumption, and calculate the pressure difference appropriately. The total pressure drop in a system refers to the kinetic energy losses associated with flow within the system, including those due to control valves, as well as pressure losses caused by local resistances such as elbows, fittings, throttling devices, process equipment, and manual valves. The calculated pressure difference for selecting a control valve is determined primarily based on the total pressure drop across the system, which includes the process pipelines and equipment, as well as any variations in that pressure drop. The steps are as follows:  Select two constant-pressure points in the system, and use the two devices that are closest to the control valve, with relatively stable pressures, as the scope for calculating the values related to the control valve.  Calculate the total sum ΣΔPF of the pressure losses caused by various local resistances within the system (other than control valves), by computing these values for each maximum flow rate and then adding them together.  Select the S value. The S value should be the ratio of the pressure difference ΔPV across the control valve when it is fully open to the total pressure loss in the system (at maximum flow rate), that is, S = ΔPV / (ΔPV + ΣΔPF). It is generally not desirable for the S value to be less than 0.3; typically, S is set between 0.3 and 0.5. For high-pressure systems, to save power consumption, it is allowed to be reduced to S=0.15. For gas media, due to the lower resistance losses, the component of the pressure difference across the control valve is relatively large; generally, the S value is greater than 0.5. However, in low-pressure and vacuum systems, since a smaller pressure loss is permitted, a value between 0.3 and 0.5 is still appropriate.  Calculate the control valve pressure difference ΔPV, using the calculated ΣΔPF and the selected S value. ΔPV is calculated using the following formula: ΔPV = S*ΣΔPF/(1-S). Considering that the static pressure in system equipment often fluctuates, which affects the pressure difference across the valves and further reduces the value of S; for example, in a boiler feedwater control system, fluctuations in boiler pressure affect the pressure difference across the control valves. At this point, the calculated pressure difference should also include an additional 5-10% of the static pressure P in the system equipment; that is, ΔPV = S*ΣΔPF/(1-S) + (0.05–0.1)P. It is important to note that when determining the calculated pressure difference, efforts should be made to avoid cavitation and noise. 3. Calculation of flow capacity: Select an appropriate formula or chart, and using the determined flow rate and pressure drop, determine the Cmax and Cmin values for the maximum and minimum flow rates. 4. Selection of the circulation capacity C value: Based on the obtained Cmax value, from the standard series of product models available, select the C value from the level that is greater than Cmax and closest to it. 5. Verification of control valve opening degree: It is generally required that the opening degree be around 90% at the maximum calculated flow rate, and not less than 10% at the minimum calculated flow rate. The Cmax value is calculated based on flow rate and pressure difference, and the diameter of the control valve is selected from the standard series of control valves provided by the manufacturer. Since the C value of the selected control valve is greater than the Cmax value, it is necessary to verify the opening degree of the control valve during operation. Generally, at the maximum flow rate, the opening degree of the control valve should be around 90%. If this opening degree is too low, it indicates that the control valve chosen is too large; it operates at a low opening degree frequently, which results in reduced regulation performance and economic waste. The minimum opening should be no less than 10%; otherwise, the valve spool and seat will suffer severe erosion by the fluid, leading to a deterioration in their performance or even failure. Due to different flow characteristics, the relationship between the valve’s relative opening degree and the relative flow rate also varies; moreover, there are differences between ideal and actual performance characteristics. Therefore, the opening degree should be determined based on the valve’s specific characteristics. The mathematical expression for the flow characteristic of a control valve is: Q/Qmax = f(l/L), where Q/Qmax represents the relative flow – that is, the ratio of the flow rate at a certain opening degree of the control valve to the flow rate when it is fully open. l/L: The ratio of the displacement of the valve spool at a certain opening degree to its displacement at full opening, for a relative displacement control valve. 6. Verification of the adjustable ratio of the control valve: It is generally required that the actual adjustable ratio be no less than 10. The adjustable ratio of a control valve is the ratio of the maximum flow rate to the minimum flow rate that can be controlled by the valve; it is also referred to as the adjustable range. If denoted by R, then R = Qmax/Qmin. It should be noted that the minimum flow rate is different from the leakage rate. The minimum flow rate refers to the lower limit of the adjustable flow rate, and it is generally 2-4% of the maximum flow rate ； The leakage rate is the amount that leaks when the valve is fully closed, and it is only 0.1–0.01% of the maximum flow rate. In other words, the ideal adjustability ratio equals the ratio of the maximum flow capacity to the minimum flow capacity; it reflects the degree of regulation capability of the control valve and is determined by the structural design. In actual operation, control valves are always connected in series with the piping system or in parallel with bypass valves. As the resistance of the piping system changes or as the degree to which the bypass valve is opened varies, the adjustable range of the control valve also changes accordingly; this adjustable range is referred to as the actual adjustable range. Since the C value of the valve has been set to be greater than the calculated Cmax value when selecting the valve diameter, and especially the limitations on the maximum and minimum opening angles during use, the adjustable ratio decreases; generally, the R value is only around 10. In addition, it is also affected by the distortion of the working flow characteristics, which reduces the adjustable ratio. When the control valve selected cannot meet the requirements for regulating both the maximum and minimum flow rates in the process, in addition to increasing the system pressure, two control valves can be used for split-range control to satisfy the requirement for a tunable ratio. 7. The determination of the valve seat diameter and nominal diameter is made based on the C value, after verifying that it is appropriate.