Impeller trimming refers to the reduction of the impeller diameter, and thus a reduction of the circumferential speed at the impeller outlet of a centrifugal pump. This is done to match the operating point to specifications.
Trimming the impeller results in a change in the length and outlet angle of the vane as well as the width of the impeller at the outlet. The effect of this measure thus depends on the type of impeller.
Single-vane and diagonal impellers can only be trimmed within narrow limits. They are designed to ensure the free passage required for the fluid handled is sufficient.
The shape of a centrifugal pump's H/Q curve when operating at a constant rotational speed does not allow a simultaneous reduction of the flow rate (Q) and the head (H) by throttling (see Closed-loop control). Reducing the outside impeller diameter represents a relatively simple and hydraulically effective method for permanently reducing both flow rate and head without changing the rotational speed. This reduction can be performed on machining equipment such as a lathe.
If the diameter reduction is limited so that a mutual overlap of the vanes still remains, the relationship between the flow rate (Q), the head (H) and the impeller diameter (D) of the full-diameter (x) impeller and the corresponding values of the trimmed impeller (y) can be approximately expressed as follows:
The pairs of values Q and H with the indices x and y are situated on a straight line passing through the origin of the Q/H coordinate system.
See Fig. 1 Impeller trimming
While the diameter of low specific speed impellers (up to approx. ns = 25 rpm) can be trimmed considerably without impairing the pump's efficiency, impellers of higher specific speeds suffer a notable reduction in efficiency. A similar technique involves reducing only the impeller vane diameter.