Limitations of the Trim Process


Trimming of impellers is a commonly accepted process of reducing flow and head of a pump. There are some limitations one needs to consider:
1.	The power coefficient m is largely affected by the specific speed, the impeller vane curvature and application.
2.	The impeller diameter should never be reduced below a diameter where the vane to vane overlap no longer exists (See Sketch).
3.	General purpose pumps: both the impeller vanes and the shrouds are being trimmed (Cases A and B).
4.	Boiler feedwater pumps: typically only the impeller vanes are trimmed. The shrouds are left at full diameter. This is done to minimize pressure fluctuations in the spaces between the impeller shrouds and the the stationary casing walls (Cases A and B).
5.	Flat performance curves may require trimming the vanes under an angle to avoid curve instability sometimes referred to as "drooping curve" (Case C).
6.	The calculation method can be applied for hydraulic specific speeds n_s < 2500 ( n_q < 50 metric). For higher specific speed large changes in diameter should be determined in several steps with testing in between trimming.
7.	Mixed flow or Francis impeller should be trimmed using the method shown in case D (See also item 6 above for impeller with higher specific).
8.	If in doubt contact the manufacturer for recommended trim diameters. They typically have test results available for a much more accurate calculation of the required impeller diameter and trim configuration.
9.	Certain impeller designs cannot be trimmed. They are of the single vane design for sewage applications (KSB's E impellers). The methods described can also not be applied to vortex type impellers (KSB F impellers).