Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

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 diameter
,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 diameter.
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 conditions
(same velocities in every pump section) if they run at speeds inversely
proportional to their size, and in that case their capacity will vary