Selection, calculation of control valves, adjustable ratio, etc
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I would like to ask my colleagues for advice on the calculation methods for selecting control valves. I’m quite confused about how to approach this issue, and I hope you can help me out. Specifically, when selecting a control valve, how should factors such as diameter, control ratio, opening degree (20%~75% is optimal), and flow rate be taken into account together? Are there any good methods or simple calculations available, and how can the CV value be determined in the end?
It is covered in the \"Petroleum and Chemical Industry Automatic Control Design Manual\".
When selecting the diameter of the control valve, the following factors are taken into consideration: 1. It is possible to do so at the maximum flow rate (with an opening degree of less than 90%), and then at 2. At normal and minimum flow rates, there is a relatively reasonable opening degree (20%–75%). For example, the calculated Cv values for minimum/normal/maximum operating conditions are 1, 2, and 3 respectively. When using a standard single-seat valve, the process involves: for Option 1: 2X(2.5–3) = 5–6; for Option 2: 3X(2) = 6. By considering both options, a valve with an appropriate diameter is selected, and its rated Cv value is determined; thereafter, it is checked whether the opening degree is reasonable. If the opening degree of the operating point is appropriate, there is no need to be overly demanding regarding the adjustable ratio.
I would like to ask: where does the optimal opening range of 20~75% come from? According to the chemical engineering design standard HG/T-20507-2014, for linear valves, an opening degree of 10~80% is optimal, while for equal% valves, an opening degree of 20~85% is optimal. Per this standard, the Cv value is calculated based on normal and maximum flow rates; the selected Cv value is obtained by rounding off the calculated value. In the case of linear characteristics, the required Cv value should be ≥1.63 for normal flow rates and ≥1.1 for maximum flow rates, with the highest of these two values being chosen ； When waiting for the % characteristic, select cv/ and calculate the maximum value of cv between 2 (for normal flow) and 1.3 (for maximum flow). According to chemical engineering design standards, there are two criteria for determining the diameter of control valves: First, when the valve is fully open, it should be able to handle at least 1.25 times the normal flow rate – this serves as a safety factor to prevent the valve from operating in the fully open or fully closed position. Second, the characteristics of the valve and economic considerations play a role in the selection process. It is hoped that at normal flow rates, the wide opening range will be 70% on a linear scale and around 80% on an equal percentage scale.
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