Example 3-15 Reducing Impeller Diameter at Fixed RPM

If you have a non-cavitating (sufficient NPSH) operating 9-inch impeller producing 125 GPM at 85 feet total head pumping kerosene of SpGr = 0.8 at 1750 rpm using 6.2 BHP (not motor nameplate), what diameter impeller should be used to make a permanent change to 85 GPM at 60 feet head, at the same speed  

When viscous liquids are handled in centrifugal pumps, the brake horsepower is increased, the head is reduced, and the capacity is reduced as compared to the performance with water. The corrections may be negligible for viscosities in the same order of magnitude as water, but become significant above 10 centistokes (10 centipoise for SpGr = 1.0) for heavy materials. While the calculation methods are acceptably good, for exact performance charts test must be run using the pump in the service. 

When the performance of a pump handling w ater is known, the following reladons are used lo determine the performance with viscous liquids [17]  

The values are averaged from tests of conventional single-stage pumps, 2-inch to 8-inch, with capacity at best efficiency point of less thar 100 GPM on water performance. 

Tests were on smaller pumps, 1-inch and below. 

---