Page 200 - Centrifugal Pumps Design and Application

High Speed Pumps 175

History and Description of an Unconventional Pump Type

Developmental work on the pump type central to the discussions in this
chapter was initiated in Germany prior to World War II to meet urgent
wartime requirements and after 1947 continued in Britain. Need for a
simple, lightweight, and easily manufactured pump suited to produce
high heads at low flow rates existed in connection with aircraft and
rocket propulsion systems. An unorthodox high-speed centrifugal pump
concept resulted and was described by Barske in papers published in
1955 and 1960.
This pump is described as an open impeller type and is exemplified as
highly unorthodox by Barske himself who states: "To a skilled designer
the pump which forms the subject of this paper will, at first glance, ap-
pear most unfavorable and may well be regarded as an offense against
present views of hydrodynamics." Reasons exist, however, to break with
conventional design practice to meet objectives which would otherwise
be difficult to achieve. Intentionally flaunting the rules, in fact, provides
a pump design that can equal or exceed the performance of conventional
pumps in the head-flow design range for which it is intended and for
which it is best suited.
Typical Barske-type pump construction is illustrated by the sketch in
Figure 11-1. The salient features of the design start with a simple open
impeller, which rotates within a case bored concentrically with the impel-
ler centerline. A single emission throat with a conical diffuser section is
oriented tangentially to the case bore. Conical difftisers provide high re-
covery efficiency because of their minimal wetted area. A cone angle of
10° is commonly used, providing good recovery potential and reasonable
cone length requirements.
Radical departure from conventional design practice exists in the ex-
ceptionally tall blade geometry used, with the impeller tip height, b 2, set
equal to or moderately greater than the emission throat diameter, dj.
Blade angle, 8, is unimportant except that the flow area in the impeller
eye must at least equal the area of the suction passage. Further obvious
deviation from normal practice is the use of plain radial blades, with no
attempt made to match inlet flow streamlines.
Performance trends of the Barske pump are generally as indicated in
Figure 11 -2A. The head at zero flow, or shutoff, is about equal to the
design head, with a head peak a few percent higher than design in the
neighborhood of half design flow. This curve shape is referred to as an
unstable curve and is often viewed as undesirable, as described in Chap-
ter 1. A stable curve is one in which the head rises continuously as flow
is reduced from design to shutoff. Head drops rapidly for flows above
design, and zero head or cutoff normally occurs around 130% of design