Hsv

has proven to be useful especially for high-specific speed pumps and turbines. The loading on the blades of a variable-pitch propeller, and therefore the minimum suction-side static pressure, is directly connected to the head developed. As this head increases with increasing blade setting angle, the loading increases, the minimum pressure drops, and the NPSHR goes up. Hence an identifiable limiting value of s is found to exist for such a machine. s-limits are presented for a range of specific speeds in Figure 25.

Performance-NPSH data are sometimes given in terms of s; for example, Rutschi13 presents the effect of pump hydraulic efficiency on NPSH in these terms, a consistently higher value of s being required for pumps with lower efficiency (Figure 26).

Suction-Specific Speed For pumps in which the eye diameter of the impeller is smaller than that at exit, the pressures on the suction-sides of the blades that are associated with most of the head addition are in a region of higher pressure than in the vicinity of the eye. Thus Wislicenus29 was able to decouple the eye from outer diameter of the pump (where most of the head is generated in a radial-flow machine) and show that only the eye geometry determines performance-NPSH and not the magnitude of the head created by the impeller. He demonstrated that for most centrifugal pumps (except the propellers just described), there is a small range of suction specific speed—from which the performance-NPSH can be determined. As in defined in Section 2.1, this parameter is defined as follows:

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

Get My Free Ebook


Post a comment