Under-filing is the retrospective machining of impeller vanes on centrifugal pumps with low specific speeds for the purpose of slightly increasing the head at the design point. Under-filing the concave side of backward curved impeller vanes results in a slightly steeper vane outlet angle. The H/Q curve is shifted upward, increasing the head by approx. 3 % at the flow rate at BEP (Qopt) depending on the type of impeller and the manner by which it is under-filed. In some cases, under-filing also slightly improves efficiency.
See Fig. 1 Under-filing