“Difference between “Calculate Cv value” and “Select Cv value”
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What do “calculate the Cv value” and “select the Cv value” mean respectively? 1. The CV value should indicate that, under the process conditions you specified, when the control valve is opened to 100% of its capacity, it will be able to meet your maximum flow rate requirements. For reasons related to process efficiency and safety, there is always a margin left for opening the control valve; therefore, a specific CV value is chosen, and the corresponding data sheet also shows the normal operating range for the valve opening. This is my personal understanding for reference only. 2. The CV value is calculated based on process data to determine the flow capacity required of the valve; choosing a CV value means selecting one from the values listed for valves, based on that calculated value. The selected CV value should be higher than the calculated CV value required for maximum flow, as per the standards, the valve opening should be between 60% and 80% at maximum flow.
For selecting control valves CV, our approach is generally to choose a value that is about twice the calculated CV value; the opening degree is also kept at 80%, with a maximum of 85%. An opening degree greater than 80% essentially eliminates the regulatory effect, while too small a degree represents a waste of the valve.
The cv value is calculated based on the process conditions and parameters, and it represents the minimum flow requirement. The selection of the CV value is based on the valve manufacturer’s product manual. Valves of a certain diameter have only a few CV values; they are standardized products. Manufacturers cannot produce a product with every desired CV value for users. Based on the calculation results, the manufacturer selects an economical and practical product that meets the process requirements and has an appropriate range of opening degrees.
Based on the process parameters and the calculated CV value, the manufacturer selects another CV value; generally, the maximum opening is around 85%, while the normal opening range is between 60% and 70%.
I don’t agree with what was said on the second floor. The selected CV value is the one corresponding to a constant pressure difference before and after the control valve is fully open; it is also referred to as Cvs. By comparing the calculated CV value with the Cvs at full open, the opening degree of the control valve can be roughly determined. On this point, I think the 4th floor explained it very clearly.
What you mean is that if the calculated Cv value is A, then the manufacturer should choose a value close to (A/0.85) as the Cv value for the control valve they select?
The appropriate range for the opening degree of a control valve should be 20% to 80%. When selecting products back then, we would provide the manufacturer with the pipe diameter as well as the maximum and minimum flow rates and the pressures before and after, so that they could determine the appropriate components. If the adjustment precision and accuracy are insufficient, they need to make improvements.