Under what flow rate are the pressure before the valve and the pressure after the valve specified by default in the control valve specification sheet?

This post was last edited by HEJIYUER on 2021-7-25 at 22:30. Under normal circumstances, the process calculates the pipeline pressure drop based on the normal flow rate (the parameters under 100% load of the factory), and the pressure assigned to each valve is determined based on this \"normal flow rate\". The above applies to continuously flowing pipelines, but for venting conditions, the pressure drop at the \"maximum flow rate\" is generally given. If the process is detailed, enter the pressure parameters in the table below the corresponding flow rates. A more advanced process takes into account the pressure changes within the operating range; for example, it provides the pressure changes at 0.6, 1, and 1.1 times the normal flow rate, offering three sets of data. This allows the instrument to estimate the opening degree of the selected valve within its operational flexibility range. If the process only provides a normal flow rate, it means that load variations are not taken into account; the strategy for the instrument is to set the opening degree at 60-70%. If the valve is fully open, the pipeline flow rate can be increased by roughly another 20%.

Thank you, Expert H. In the data sheets provided by valve manufacturers, when there is only one set of pressure values before and after the valve, the manufacturer’s calculation reports use this set of pressure differences to determine the opening degrees for the maximum, normal, and minimum flow rates. In that case, is it only the setting for normal flow rate that is accurate, while the setting values for maximum and minimum flow rates can only be used as a reference?

If there’s a set of pressures, then treat it as one set; otherwise, there’s no way to create something on your own.

A pipeline flow rate corresponds to a distribution of pipeline pressure drops, which is essentially the pressure drop data for a valve. In terms of the process, the pipe pressure drop is generally calculated using the \"design flow rate\", which corresponds to 100% load, with the remaining pressure drop being assigned to the control valve. Therefore, your claim is plausible. In the situation you described, the actual opening under \"normal flow\" is close to the calculated opening, while the actual opening at maximum flow is greater than the calculated opening ； The actual opening degree at the minimum flow rate is lower than the calculated value; this is because the flow rate in the pipeline represents a \"fixed demand\", and any changes in pipeline flow rate that result in pressure drops must be compensated for by the control valve, as it is the only component in the pipeline that can change the area/resistance coefficient. The magnitude of the deviation from the above opening degree depends on the valve resistance ratio (pressure drop across the control valve / total resistance drop in the pipeline); the smaller this ratio, the greater the deviation. (A tip: Be particularly careful when estimating the opening degree of butterfly valves.) The diagram below comes from Shell’s DEP; it shows a typical pump circuit, and the distance between the two black lines represents the pressure drop across the control valve at different flow rates.

Thank you, teacher. I’ve learned a great deal from your lessons. I’m very interested in your explanation of Shell DEPs as depicted in the diagrams; I wonder which section of those documents deals with Shell DEPs, so that I can read it in detail

{:1_90:}Thank you so much{:1_90:}{:1_90:}

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