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Thoma Cavitation Parameter S All the terms in Eqs. 15 to 17 may be made dimensionless by dividing each by the pump head H. The resulting parameter
σ = h(sv) / H
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 σ is found to exist for such a machine. σ-limits are presented for a range of specific speeds in Figure 25.
Performance-NPSH data are sometimes given in terms of σ; for example, Rutschi 13 presents the effect of pump hydraulic efficiency on NPSH in these terms, a consistently higher value of σ being required for pumps with lower efficiency (Figure 26).
