Page 32 - Centrifugal Pumps 2E

Specific Speed and Modeling Laws 19

ter and the achieved performance. The larger the impeller cut, the larger
the discrepancy as shown in Figure 2-6.

Example

What impeller trim is required on a 7-in. impeller to reduce head
from 135 ft to 90 ft?

Step L
From affinity laws:

D 2 = 5.72 in.

Calculated percent of original diameter = 5.72/7 = .82

Step 2,
Establish correction factor:
From Figure 2-6 calculated diameter . 82 = Actual required diame-
ter ,84.
Trim diameter = 7 x .84 = 5.88 in.

Impeller trims less than 80% of original diameter should be avoided
since they result in a considerable drop in efficiency and might create
unstable pump performance. Also, for pumps of high specific speed
(2,500-4,000), impeller trim should be limited to 90% of original diame-
ter. This is due to possible hydraulic problems associated with inadequate
vane overlap.

Model Law

Another index related to specific speed is the pump modeling law. The
"model law" is not very well known and usually applies to very large
pumps used in hydroelectric applications. It states that two geometrically
similar pumps working against the same head will have similar flow con-
ditions (same velocities in every pump section) if they run at speeds in-
versely proportional to their size, and in that case their capacity will vary