A process requires increased flow. As the pressure is?

Due to changes in a pumping process, sometimes you need to resize a centrifugal pump to meet the new conditions.

Consider a recirculating washing system, in which fluid is drawn from a reservoir, circulating through a pipe, with geometric elevation, passes through one or more spray nozzles and returns to the reservoir. Initial calculations indicated a pump with flow 200 m3/h, total head and (AMT) from 27 mcl, already considered the operating pressure of the spray. The system starts operating satisfactorily. Suppose, later, due to a change of the product to be washed, necessary to increase the current flow 250 m3/h.

It is possible that the first "reaction" of the designer is: “OK, we just need more flow, or pumped liquid, the spray and the rest of the system will not be changed. Then the new pump should flow 250 m3/h, and the same pressure (same or AMT) the old (27 mcl)”.

This initial impression would result in the acquisition of a new pump under-sized for system. Why?

Just as a bomb, the entire hydraulic system has a characteristic curve (x curved flow pressure or flow head height x). In practical terms: the greater the required flow in the system, the greater the "answer" shaped hydraulic resistance, that is, the greater the "back pressure" that the pump should win.

If we raise the pressure drop values ​​of our system, for various flow rates, we get a curve as the graph 1. The initial value corresponding to zero flow represents the geometric height system.

Our current pump would have been sized to operate 200 m3/h x 27 mcl. His characteristic curve would be like graph 2.

The combination of curves showing the operating point of the washer assembly - Pump, or as Chart 3

It is important to remember that when you put a bomb in a hydraulic system, the operating point "natural" set will always be the point of intersection of the two characteristic curves. In extreme cases, it may occur that the curves are not. This would reveal a coarse sizing error.

Defining the new pump only by the flow increase, We incurred in the error shown in Figure 4.

The new pump, Flow chosen only by the criterion, keeping the original AMT, would keep operating point provided (250 m3/h x 27 mcl) out of the washer curve. The "natural" adjustment would occur at the meeting point of the curves, that is, 220 m3/h x 29,5 mcl, therefore insufficient, to meet the new condition.

A previous analysis of washer curve would have shown that, for a flow 250 m3/h the system "calls" AMT = 34 mcl. The correct choice would be a pump curve which pass through this point.

Remember also that the increase in flow can lead to the need to resize the suction line, avoiding cavitation or breaking of the liquid column.

To a survey, with good accuracy, the characteristic curve of a system refers to data provided by the manufacturers of the various components (valves, filters, spray nozzles, etc), and the pressure drop tables in pipes and connections. There are also software for these calculations.

Eng.Mauro Baldini





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