If you don’t understand, ask; regarding control valves, the flow coefficient and opening degree of control valves
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A control valve regulates flow and pressure based on changes in the valve’s opening degree; whether linearly or in proportion, there is a specific opening degree that corresponds to a particular flow and pressure level. Then why is it still necessary to calculate the flow coefficient? As long as a control valve with the appropriate diameter is selected and the valve is fully open so that the medium can pass through completely, the lower the pressure difference, the better ； With the valve fully closed, the flow of the medium is blocked, and the pressure difference corresponds to the pressure before the valve ； As long as there is a relationship curve between flow rate and opening degree, or a corresponding curve between pressure and opening degree, why is a flow coefficient still needed? I know what the flow coefficient means; it indicates the ability of a control valve to allow fluid to pass through when it is fully open. What I don’t understand is the significance of this parameter. Yet it is necessary when selecting valves, along with information on pressure loss. Isn’t regulating flow simply a matter of adjusting the valve’s opening degree? Of course, the lower the pressure loss when the valve is fully open, the better. I can’t understand the relationship between them; I would be extremely grateful if experts could give me some guidance! ! !
This post was last edited by kkbbaa on 2019-11-26 at 19:17. I don’t understand it – if changing the opening degree can create a pressure difference, then why is there still a pressure difference when it’s fully open? I have a background in chemistry and am currently studying the principles of chemical engineering. Seeking answers online. . I’ve been struggling for a long time here trying to choose a valve.
This post was last edited by kkbbaa on 2019-11-26 at 19:37. I asked the engineering team about pressure loss, but they didn’t consider it at all; they said that the lower the value the better when the valve is fully open, and that the flow rate can be adjusted depending on the degree of opening, with the maximum flow rate ranging from 0 to 100%. As for pressure loss, they told me to estimate it using the valve’s resistance loss coefficient. But if that’s the case, wouldn’t the pressure loss for all valves of the same type be identical? What’s the use of that? How am I supposed to choose a valve in this situation? I actually think the same way, and I’m reaching out to the seniors here for advice; I really can’t figure out this relationship.
I don’t understand this, but I’ll just silently help hold it up for you
Although I’m not an expert in valves, having heard about this issue, I think it’s more related to process regulation; the flow coefficient is closely tied to the diameter of the control valve. For a DN100 valve, whether the aperture should be 80 or 50 depends on the requirements of the process. Whether the aperture is too large or too small has a significant impact on whether the control loop can be successfully put into automatic operation in the future. These are my thoughts; I hope everyone will offer criticism and suggestions.
Well, was it posted in the wrong section? Should I go to the Piping and Valves section?
This post was last edited by Hariky on 2019-11-27 at 10:28. Essentially, valves regulate the flow area, thereby affecting the flow capacity. As you said, the flow coefficient is a parameter used to measure a valve’s flow capacity; this parameter effectively accounts for the properties of the medium and the pressure difference before and after the valve, and it represents the ability of the valve’s flow channel to allow fluid to pass through. There is a concept related to valves, namely the intrinsic flow characteristic, which is the curve showing the flow capacity as a function of the valve opening degree. The actual curve and the theoretical curve for this type of curve are different; for example, even for curves with the same percentage characteristics, the actual curves vary among different manufacturers and different valve components. This means that the same flow capacity corresponds to different opening degrees. This is why the opening degree cannot directly measure the flow capacity of a valve. One more thing to note is that a lower pressure drop on the valve is not necessarily better; after all, the valve’s function is to regulate the flow resistance of the entire pipeline. Therefore, valves must carry a certain weight; typically, the pressure drop across the valves needs to account for more than 30% of the total pressure drop in the entire pipeline in order to achieve effective control.
It is necessary to calculate the resistance of the entire piping system; at the normal operating flow rate, the resistance of the control valve accounts for around 30% of the total resistance in the piping system. Either a too high or too low proportion is not favorable for proper regulation
Thank you for your reply, but I think that since there is resistance when the control valve is closed, it should be possible for the medium to pass through completely when it is fully open. To adjust, reducing the opening of the control valve creates resistance, thereby achieving the purpose of throttling and pressure reduction. Perhaps there’s something wrong with my understanding; I hope you can point it out. Thank you very much! ! !
This post was last edited by kkbbaa on 2019-11-27 at 11:40. I’m not very knowledgeable; I can understand the first two paragraphs, but I’m confused about the last one. Since the flow resistance in a pipeline can be adjusted by closing the valve, why is there still a pressure drop when the valve is fully open? If adjustment is done by closing the valve, isn’t that the same as having a pressure drop already? When regulating pressure, there are definitely specific requirements regarding the pressure drop; the flow rate changes according to those pressure requirements. When regulating flow, there is only one requirement regarding the flow rate, while the pressure changes based on the flow rate requirements, right? Thank you for your reply, big bro. I really can’t understand it; maybe I’ve been thinking too one-sidedly.
Additionally, if the design requirement is that the pressure drop should be as low as possible when the valve is fully open, then can I choose a control valve with the highest Kv value that meets these conditions?