The fluid does not come to the tip of the impeller vane at exactly the precise angle discussed in impeller theory. This deviation reduces the velocity, causing a reduction in the predicted head produced by the pump. The phenomenon is called “slip”.
Accurate predictions of slip are impossible since the causes of slip cannot be predetermined in any practical way. The slip factor is not only affected by the impeller configuration, but also by the interaction of the volute and the entire diffusing system.
This is one of the greatest drawbacks to existing centrifugal pump theory.