How is the selection calculation for control valves done?
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When I was referring to the manual, it mentioned that there should be a normal flow rate, as well as a maximum and minimum flow rate. I would like to ask: what exactly is this normal flow rate? Specifically, for example, if I need to select a vent valve to be placed downstream of the compressor, with a vent pipe connected after the valve, I should choose a regulating butterfly valve. Since the maximum air supply capacity of the compressor is only 90 kg/s, it is necessary to achieve the maximum flow rate of 90 kg/s required for operation at an opening degree of around 60%. The pressure before the valve is maintained at 2.0 MPa through the valve, while the temperature is 523 K. I would like to ask whether the 90 kg/s I need should be considered a normal flow rate or the maximum flow rate? Logically, when the valve is fully open, the flow rate should not increase; only the pressure before the valve will decrease. First, I calculated the Kv value based on the operating conditions I desired. Then, according to the manual, when the percentage characteristic is at 60%, m takes a value of 3.9; thus, Kvmax was calculated to be 3.9 Kv. Now, how should I verify the opening degree? First, what is the actual opening degree at maximum flow rate (is it around 60%)? Second, what flow rate can pass through at an opening degree of about 5% (this determines the choice of the bypass auxiliary vent valve)? Question: During the opening verification process, the flow rate used is the volumetric flow rate. Should this volumetric flow rate be converted to a value at standard conditions, or should it remain the flow rate under the actual temperature and pressure conditions? Is there anyone who can provide an example of the calculation process for selection, specifically for gas control valves? I’d like to use it as a reference. Thank you all in advance!
90 kg/s is considered a normal flow rate
First, I believe that this flow rate is normal for this vent butterfly valve!
Second, as you mentioned, when the valve is opened, the flow rate should not increase; only the pressure before the valve will decrease. I think for that occasion the pressure should remain stable while the flow rate will increase
Third, verify the opening degree: once the valve setting is determined, the CV value is also fixed; therefore, the CV values at various opening degrees are theoretically fixed as well. Although it is difficult to calculate them, it is possible to do so using the flow characteristic curve. This represents the inherent Cv value of the valve at its specified opening degree. Now that you know the flow rate and the pressures before and after the valve, you can calculate a theoretical Cv using the relevant formula. By comparing these two CV values, you can determine whether the valve’s opening degree in this operating condition is below 60 or above it
Fourth, similarly, you can use the inherent Cv of valve 5 at that opening degree, and by incorporating the pressure difference before and after, you can calculate this flow rate
Fifth, whether it is for verification or other types of flow calculations, everything must be in line with the formulas; since density parameters are used when calculating CV, and density is related to temperature and pressure, it is important to ensure consistency
I did a rough calculation of my own: based on the volume you mentioned and the pressure difference before and after the valve, the opening degree is around 61.4%. This was applicable to our valves, but butterfly valves are more or less the same; with an opening degree of 5%, approximately 11,000 cubic meters per second of flow can pass through, which is equivalent to 4 kg/s
Thank you, expert. However, in our case, the compressor supplies air at the front; its outlet pressure is regulated by valves to keep it operating within the optimal pressure ratio range. 90 kg/s is the maximum capacity of the unit, and opening the valves further will not increase the mass flow rate
Generally, the manufacturers provide the calculations; since these are based on theoretical company calculations and some coefficients are not adjusted, the resulting calculation values can vary significantly.
Well, our manager has asked us to first conduct a preliminary selection on our own, then pass it on to the manufacturers for detailed work, and finally draft the tender requirements