Convert a centrifugal pump's water performance to viscous-liquid head, flow, efficiency, and power per ANSI/HI 9.6.7
Understand why viscosity degrades centrifugal performance and how the ANSI/HI 9.6.7 parameter-B method works
Everything keys off one dimensionless parameter B, computed from the pump's water BEP point and the liquid viscosity:
Power follows as Pvis = Qvis·Hvis·SG / (3960·ηvis) in USC (367 in SI).
The ANSI/HI 9.6.7 correlation is validated for:
Outside these bounds — high specific speed, non-Newtonian slurries, B ≥ 40, or above 300 cP — treat the result as indicative and consult the pump vendor; very viscous service often points to a positive-displacement pump.
Pump curves are published on cold water (about 1 cSt). When the actual liquid is appreciably more viscous, the head, flow, and efficiency all drop while the absorbed power rises. The ANSI/HI 9.6.7 method screens this with a single parameter B: if B is at or below 1.0 the viscous effect is negligible and no correction is needed; once B exceeds 1.0 you apply the correction factors. As a rule of thumb, liquids above roughly 4–10 cSt at the pump's flow and speed begin to warrant a check.
B is a dimensionless correlating parameter that collapses viscosity, BEP head per stage, BEP flow, and pump speed into one number: B = 26.6·ν0.50·HBEPw0.0625 / (QBEPw0.375·N0.25) in US units (16.5 in SI). The correction factors are all functions of B. B ≤ 1.0 means no correction; 1 < B < 40 uses the equations; B ≥ 40 is highly uncertain and a detailed loss analysis is warranted per §9.6.7.5.2.
A viscous liquid increases disk friction on the impeller shrouds and skin-friction losses in the impeller and casing passages. Those losses convert shaft power into heat instead of head, so the developed head and the best-efficiency flow both fall and the pump efficiency drops — which in turn raises the brake power needed for the same duty. The viscous power scales as Qvis·Hvis·SG/(3960·ηvis) in US units.