Seeking advice on the calculation of the Cv value for selecting control valves
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I’m a beginner and this is my first time dealing with control valves. After reading some information, I learned that when selecting a control valve it’s necessary to calculate the Cv value. Now that the pipe diameter has been determined, the manufacturer’s catalog only provides one Cv value corresponding to it. Is it okay to simply choose that value? I asked the process technician (who is also a beginner), and he only told me the flow rate. I mentioned that a control valve requires a specific range of operation, and he said it’s 0–100%, which refers to the normal range of flow rates; he didn’t mention anything about pressure loss. The entire process hardly takes into account the pressure losses associated with pipes and fittings. What should I do in this situation? The control valves supplied by the manufacturer come with only one value for the flow coefficient. Thank you all~~~ I also think that regulation can be achieved as long as there is some degree of flow restriction depending on the degree of opening; then why is it necessary to calculate this Cv value? Please help me understand this, guys~~~ I’m extremely grateful
Calculating the pressure drop in the pipeline based on the controlled flow rates Qmax and Qmin, as well as determining the corresponding valve pressure drop, falls within the scope of process design. (0-100% does not represent flow control; flow control refers to the flexible range of operating flow rates under device operation.) The instrumentation specialists calculate the Cv values at Qmax and Qmin, and then select a valve with an appropriate size based on the manufacturer’s specifications. The valve opening should be between 30% and 75%, with it not exceeding 85%. If the manufacturer does not provide a table corresponding to %-Cv, you can calculate the Cv value for Qmax by multiplying it by (2.5–3), and then use this value to compare it with the manufacturer’s Cv values in order to estimate the valve diameter approximately. If reasonable, it should have a diameter one size smaller than that of the pipe. Below is the scheme for the pump circuit.
Thank you for your reply. The situation is as follows: when I asked about the operating parameters, he told me that the minimum flow rate is 0 m³/h, which corresponds to the valve being fully closed, while the maximum flow rate is 8 m³/h, which corresponds to the valve being fully open. He doesn’t know about the pressure difference either; he says that it should be very small when everything is fully open. I really can’t do anything now either
Without considering the pressure difference, the flow rate is a minimum of 0 m3/h, which corresponds to the fully closed position, and a maximum of 8 m3/h. CV2.5
Hello, how can Cv be calculated without considering the pressure difference?
The flow rate is here: 8 cubic meters per hour, so the pressure isn’t high.
Hello, according to the calculation formula, the lower the pressure loss, the higher the Cv value should be. Why is the value I obtained so low? Using the flow coefficient Kv=60 provided by the manufacturer and our maximum flow rate of 8 m³/h, I calculated the pressure loss, and it turned out to be only 0.8 Kpa, which seems quite low. Could it be that the adjustment range of this valve is too wide? The medium in question is LNG, with a density of 450 g/cm3. Would it be possible to leave your contact information so that I can ask you for some advice?
I previously did a rough measurement using water; now I’ll measure again using what you’ve given me
Since the pressure drop assigned to the control valve at a flow rate of 8 cubic units is very small, it indicates that he doesn’t want the valve to absorb the pressure drop in the pipeline; therefore, a full-bore ball valve would be a good choice. :)
I’m wondering that since reducing the valve opening creates a pressure difference, why does one still need to consider pressure differences when the valve is fully open? When it’s fully open, of course, it’s desired for the medium to flow through completely, with as low pressure loss as possible. If one wants to change the flow rate, simply reduce the valve opening – isn’t that enough? Thank you for your explanation
It depends on which valve you choose. Ball valves with a diameter matching that of the pipeline generally incur no pressure loss. All other valves have pressure loss. Problems with the valve core structure. Ball valves are generally not used as control valves.