Regarding the CV and KV of control valves
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This post was last edited by Tuna Yiye on 2017-10-22 at 18:17. The parameters below are part of the data sheet for the control valve provided by the design institute; they differ from the Cv=1.156Kv value obtained from Baidu. How are the CV and KV values of a control valve calculated? Images cannot be uploaded; the basic parameters are as follows: Process medium: circulating chilled water. Operating temperature: 15°C. Pressure before the valve: 0.35 MPa. Pressure after the valve: 0.3 MPa. Flow rate (m3/h): maximum 6.6, normal 5.5, minimum 1.1. Operating density: 1000 kg/m3. Valve diameter: DN40. Calculated Kv value: 1.81–10.89. Selected Cv value: 28. Selected opening degree: 30%–76%
Is it relevant that the design institute has chosen a normal opening degree of 30%–76%?
The formula is quite complex, so it’s possible to download a software to assist with the calculations. There are two types of CV values: the rated CV and the selected CV. Generally speaking, a linear control within the range of 30-76% is considered to be good
Choosing a CV value that is much higher than the calculated CV value is necessary; what you searched for on Baidu was the relationship between calculating CV and calculating KV. The value chosen for CV is sometimes referred to as the rated CV, and it’s generally advisable to select a value that is one level higher
In fact, there’s no need to doubt this calculation result, as it is usually provided by the valve manufacturers. What you should doubt more is whether the process parameters provided are accurate; how can one adapt if they are not? Then, which valve type is the best to choose!
For control valves of different brands, the calculated values of their CV and KV values vary. Under the same operating conditions, the calculation results obtained using the software from Wu Zhong and Chuan Yi differ from each other. Some of the data provided in the control valve tables issued by design institutes can only be used as a reference.
I think you’ve mixed up two concepts: one is calculating the CV or KV values, and the other is selecting the CV value. Generally, choosing a CV value means using a value that is two more than the calculated CV value