How to calculate flow rate using the CV value?

This post was last edited by feng*aosa on 2019-10-10 18:28. It can’t be calculated. At most, an estimate with a range that is too broad to be of much practical use can be obtained. The flow coefficient Cv value refers to the value, after removing the units, of the flow rate through the valve, expressed in units of 3.785 L/min, for water that is fully open at the valve, with a pressure difference of 1 pound per square inch across it and a temperature of 60°F. The Cv value has no units. Reasons why it is not possible to calculate the flow rate based on the Cv value: 1. The traditional method for determining the Cv value (the one described in textbooks) is designed for older needle-type control valves, and it is not applicable to most valves used today. For example: the pressure difference when a full-bore ball valve is fully open is extremely low; applying the definition of Cv leads to the clearly incorrect conclusion that the flow rate is also extremely low when the valve is fully open ； 2. The specific method for calculating the Cv value is considered proprietary intellectual property of the manufacturer. For example, when using publicly available Cv calculation software on the internet, the calculation results obtained from different software programs can vary significantly (due to differences in the product structure) ； 3. According to the traditional method for selecting the Cv value, after calculating the Cv value, an empirical coefficient is applied based on the state of the medium; this coefficient ranges from about 1.2 to 3 (I have seen cases where a value of 5 was used as well). In other words, even if all the operational data from the design phase are available, and flow rates are calculated using the traditional Cv value method, the error will still be around 2-3 percent or even higher ； 4. Flow rate is related to the valve opening degree, and according to the definition of Cv, the selected Cv value is only related to the valve’s maximum flow rate. In other words, even if the result is calculated backwards, it doesn’t have much to do with the flow rate under actual operating conditions ； When selecting the Cv value, an important parameter is the maximum flow rate; in other words, the reverse calculation process must be based on the results of that reverse calculation, and this is something that only engineers with high expertise can handle. Strictly speaking, it is not impossible to calculate the flow rate based on the Cv value. Rather, there are too many parameter conditions that need to be determined; if all of these conditions are known, there is basically no need for reverse calculation. Of course, on a specific device, engineering technicians can estimate the flow rate based on experience and roughly using the Cv value, but this has nothing to do with calculation.