Page 221 - Centrifugal Pumps Design and Application

High Speed Pumps 195

Figure 11-10. Curve shape variation with impeller eye size.

proved pilot recovery at low flow rates. Referring back to the concentric
bowl characteristics shown in Figure 11-2, it is seen that shutoffhead is
2
about the same as design head. The ideal velocity head is u /2g, and some
portion of this head may be converted to static head by either diffusion or
pitot recovery. As flow is reduced, diffusion recovery potential de-
creases while pitot recovery potential simultaneously increases. The
shutoff head coefficient would be expected to be only ^ = .5 without
pitot recovery. A stable curve shape evidently results from improved pi-
tot recovery provided by high solidity impellers.
As would be expected, increased impeller solidity results in an in-
creased head coefficient. In "Partial Emission Formulae," the impeller
blade inner diameter, DI, was said to have small effect on pump perfor-
mance, so was neglected in an illustrative exercise. But the impeller eye
choice does in fact affect curve shape and efficiency with a volute collec-
tor, so presenting a designer's choice. A rising curve characteristic is
achieved by setting Dj appreciably larger than the inducer diameter.
Choice of small DI, or deeper blade penetration into the eye, provides
higher design head and moderately higher efficiency, but results in a rela-
tively flat curve shape.