Centrifugal pumps as inevitable part of any desalination plant are significant consumers of energy. Due to high capacities of desalination plants, efficient operation of all their components, including centrifugal pumps is very important. it is often found that centrifugal pumps do not operate in their optimal points for various reasons. One of the methods to improve efficiency of centrifugal pump is proposed here. Impeller trimming is a common practice performed by pump manufacturers and users when it is necessary to adjust the centrifugal pump head and flow to the actual needs. The trimmed pump is not strictly similar to the initial pump because only a few parameters are modified, while all the others remain unchanged. In spite of this, the assumption of valid similarity was made here and examined through a series of experiments. A centrifugal pump impeller of low specific speed (n(sp) = 19,745 r.p.m.m(0.75) s(-0.5)) was trimmed and tested successively. For this particular pump, the influence of disregarded similarity can be estimated to +/- 3.94% for the pump head and to +/-5.24% for the power, both with a 95% statistical certainty. (C) 2009 Elsevier B.V. All rights reserved.