Centrifugal-Pump Economics

Abstract

In this paper the author presents some developments in the theory of the centrifugal pump with which he has been connected and shows their application to pump practice. In reviewing the technical and theoretical aspects of the problem, he refers to the fact that the performance of the centrifugal pump is shown best graphically by the quantity-head curve. In the extensive centrifugal-pump literature existing there are to be found a number of theoretical analyses of the action of such pumps, all of which are based on somewhat similar assumptions. In each case the striking fact is the wide difference between the so-called theoretical curve and the one actually obtained by a test. To bring about any sort of an agreement it is necessary to assume in some cases large and disproportionate losses. To show this difference the author presents a chart showing the theoretical quantity-head curve and the actual curve obtained by a test of the pump. Around the point of zero discharge there was a discrepancy of about 100 per cent between so-called theory and practice. At or near the shut-off point the head actually obtained from the pump was nearly twice as much. A careful study of these matters resulted in the discovery of a surprising error in the derivation of the fundamental theory for the centrifugal pump.

Another interesting point brought out in the paper is that the use and the benefits claimed for guide vanes are founded on a misconception of theory. The same is true of the so-called volute or spiral casing into which the runner discharges the water.

The author states that the real function of the pump case is that of compelling the water to revolve in a more or less circular path. Thus the natural form of a case should not be volute or spiral, but rather a plain concentric circular case of constant cross-sectional area. Tests of a circular case pump and a good design of the volute type showed a remarkable increase in head development by the former. At its point of maximum efficiency the circular case pump developed an excess head of 50 per cent over a similar volute design.