Calculation and selection of control valves

Generally, you only need to provide the process parameters (required physical property parameters, normal/max flow rate, and pressure differences before and after), and then give them to the supplier who will carry out the selection and calculation for you. The Cv values vary among different manufacturers, so there is no need to calculate them yourself.

After determining the manufacturer, there is a value in the sample corresponding to the size of the control valve you’ve selected. That value can be considered the rated Cv value. So when performing the calculations, you should first determine the diameter and then calculate the Cv value; for equal percentage control valves, a Cv value of around 90% of the rated value is generally sufficient. Check whether your calculated value is appropriate; if not, adjust the valve diameter accordingly.

We are submitting it for calculation by the instrumentation team

It should be like this: first, calculate the CV value using the given process conditions, and then select one with the desired CV value. Choosing the rated CV means selecting the valve body diameter and the seat diameter ； The so-called rated CV refers to the combination of these two factors; the selected rated CV requires that the opening degree of the control valve fall within the corresponding technical specifications. If this is not the case, then the diameter of the valve seat or the bore size of the valve body must be adjusted.

What was said on the fifth floor is correct: the manufacturer will provide the CV value corresponding to a specific diameter. Different manufacturers use different calculation methods and testing conditions, and varying correction formulas also lead to different CV values. However, generally there is no significant difference in orders of magnitude. You need to calculate the CV value using formulas appropriate for your own process conditions – whether the medium is gaseous, liquid, or steam, as well as considering the relationship between the pressure before and after the valve. This is the CV value you calculate; then you should refer to the manufacturer’s data sheets to find the rated CV value, and compare them. By examining the relevant curves, you can determine the valve opening degree to ensure it meets your requirements. If it doesn’t, it may be necessary to reduce the valve diameter or discuss with the pipeline supplier whether the flow rate is appropriate. The CV value still needs to be calculated by oneself, as this gives more confidence~

What was said on the fifth floor is correct – generally, when selecting a control valve, there is no need to carry out calculations oneself. The user only needs to provide the process parameters, and then the supplier will use its own specialized software to perform the calculations based on its products. In fact, even if you spend a lot of effort on calculations, the results may not be accurate. For the same operating conditions, different brands of products can have varying CV values; although this difference isn’t very large, it still exists. The values you calculate can only serve as a reference, not the final values

Once the diameter and valve seat type have been determined, the corresponding CV value can be found in the selection guide. Then, based on the maximum and minimum values provided by the process, the adjustment range is calculated to determine whether the chosen diameter is suitable.

Friend, may I ask why the Cv value is generally around 90% of the rated value? The Cv value calculated in this way is obtained under operating conditions, while the rated Cv value is calculated under standard conditions; there should be no difference in magnitude between the two. For example, if your operating conditions are 4 barg and a flow rate of 40 m/h, then the rated Cv value corresponds to the flow rate at 1 PSI and 60 degrees Celsius. There is no relationship in magnitude between the calculated Cv value and the rated value.

Reply to 9# zbz_happy: I may not have explained myself clearly. What I mean is that the rated Cv value is generally the value given in the manufacturer’s specifications. Given that there is a variation of around +20% to -10% in the rated Cv value, it is generally recommended to use the value of the proportional valve at an opening degree of 90-95% as the maximum Cv value. This is what the manufacturer recommends. But generally, when making calculations, we determine the maximum Cv value based on a series of values corresponding to the maximum flow rate, and then select a rated Cv value that is close to this value (for equal percentage valves, I usually refer to 90%); of course, the diameter of such valves should be smaller than that of the pipeline. Then compare to determine whether the diameter you selected in your initial calculation is appropriate; if not, adjust the diameter and recalculate. Finalize the appropriate bore size, and then determine the rated Cv value. These are all my own interpretations; discussions are welcome~