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Hydraulic Aspects of Pump Selection · Motor Selection
If there are no specific reasons for doing otherwise, a pump should be selected so that the operating point lies near its best efficiency point Qopt (= flow rate at which efficiency is highest, BEP). The limits Qmin and Qmax (for example due to vibration behaviour, noise emission as well as radial and axial forces) are given in the product literature or can be determined by inquiry [1]. To conclude the selection, the NPSH conditions must be checked as described in section 3.5. A multistage pump is chosen using the same general procedure; its selection chart shows the number of stages in addition to the pump size (Fig. 19). For pumps operating in series (one after the other) the developed heads H1, H2, etc. of the individual characteristic curves must be added (after subtracting any head losses which occur between them) to obtain the total characteristic H = f(Q). For pumps operating in parallel, the individual characteristics H1, H2, etc. = f(Q) are first reduced by the head losses occurring up to the common node (head loss HL calculation according to section 3.2.1.2) and plotted versus Q. Then the flow rates Q of the reduced characteristics are added to produce the effective characteristic curve of a “virtual” pump. This characteristic interacts with the system curve Hsys for the rest of the system through the common node.
��� Drive power relative to pump input power under rated conditions in %
the individual characteristic curve (for example see Fig. 18).
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Fig. 20: Drive power as a function of rated pump input power at the operationg point Example as per ISO 9905, 5199 and 9908 (Class I, II and III) 3.3.2 Mechanical Aspects
3.3.3 Motor Selection
When selecting a pump the mechanical aspects require attention in addition to the hydraulics. Several examples are:
3.3.3.1 Determining Motor Power
– the effects of the maximum discharge pressure and temperature of the fluid pumped on the operating limits, – the choice of the best shaft sealing method and cooling requirements, – the vibration and noise emissions, – the choice of the materials of construction to avoid corrosion and wear while keeping in mind their strength and temperature limits. These and other similar requirements are often specific to certain industries and even to individual customers and must be addressed using the product literature [1] or by consulting the design department.
Operation of a centrifugal pump is subject to deviations from rated speed and fluctuations in the flow volume handled, and, consequently, changes in the operating point (see section 3.4.1). In particular if steep power curves are involved (see Figs. 5 and 6), this may result in a higher required pump input power P than originally specified. For practical purposes, a safety allowance is therefore added when the appropriate motor size is selected. Safety allowances may be specified by the purchaser, or laid down in technical codes, see Fig. 20. The safety allowances stipulated by individual associations are shown in the relevant type series literature [1] or the customer’s specification. When energy-saving control methods are used (e. g., speed control systems), the maximum
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