Is there still a final verification of the pressure drop based on the actual instrument specification sheet when selecting pumps for the process?
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There are two aspects of pump design that I’m not quite clear about; I would appreciate some guidance from those with more experience: 1. Is the pump design process carried out in stages? In the first stage, the pressure drops caused by orifices, control valves, etc., are estimated to determine the head required; in the second stage, detailed calculations are done once the ISO charts are available; and in the third stage, the actual pressure drops are calculated after the control valves and orifices have been purchased. What if, after a rough calculation and purchase of the pump at the initial stage, it turns out that the pressure drop across the orifice plate is very high, significantly different from what was estimated, and the required head is not achieved? Is it a problem with the pump’s calculation program, or an issue with the empirical estimates? 2. After the pump selection is completed, before on-site installation, is it still necessary to calculate the pump’s installation height Hg to determine whether it meets the requirements? Previously, it was only about performing calculations to select the pump, specifically calculating NPSHa; it was sufficient for this value to be just 1 meter higher than NPSHr. I’m not sure if this is correct
Answering your first question. There are only two stages. 1. During the plant design phase, the process requires an initial selection of pipe sizes (by drawing PID diagrams) and pump parameters (by preparing pump data sheets). Therefore, it is necessary to calculate the initial head based on the plant layout; an economic flow velocity of 2–3 M/S can be chosen. The height difference is determined by the equipment layout, while the dynamic pressure drop consists of \"fixed resistance elements\" and \"movable resistance elements\"; the former refers to pipes, fittings, and flow meters such as orifice plates ； The latter refers to the control valve. “The calculated pressure drop for the \"fixed resistance element\" should be increased by a margin of around 15%. As for the pressure drop across the control valve in the pump circuit, it is generally set at 0.5 to 1 times that of the fixed resistance element; the sum of these two values is used as the design parameter for the pump. For flow meters such as orifice plates, the process specifies a maximum pressure drop value @ MAX FLOW, so you don’t need to worry about the pressure drop exceeding this limit. 2. As mentioned earlier, there is no need to inspect the flow meter; only the actual pressure drop across the control valve needs to be checked, as the control valve can adjust its opening degree to compensate for any initial estimation errors. For example, if the actual pressure drop in the pipeline is 6 meters higher than before, the instrument will recalculate the opening degree of the control valve; if it remains below 80%, there is no need to replace the valve ； If it exceeds 90%, then the flow-limiting orifice plate no longer has the capacity to increase flow, and it is necessary to consider replacing the valve (it is not possible to replace the pump at this point). Except for some specified critical circuits that must be recalculate and checked, the rest can be compared with the pipeline estimates from the “first phase”; if the differences are not significant, that’s fine, as the control valve, being a \"variable resistance element,\" can absorb any changes in the actual pipe resistance.