What is Impeller Trim?
The performance of a centrifugal pump can be modified by changing the diameter of the impeller. The impeller trim is the finished ouside diameter of the impeller.
The majority of centrifugal pumps are available with a range of impeller trim sizes. The maximum trim is the largest diameter impeller that can be installed inside a specific pump. The minimum trim diameter is the minimum diameter impeller that will still produce acceptable operation.
By increasing or decreasing the diameter of an impeller, the production of a pump can be increased or decreased. Impellers with a larger diameter produce greater head and flow than impellers with a smaller diameter.
There are a few factors that limit the maximum impeller size that can be installed in a pump:
- Physical space limitations: The impeller cannot be larger than the space available within the pump casing.
- Mechanical limitations: The pump shaft and bearings are designed based on certain maximum power and thrust loads. The impeller cannot be so large that it exceeds the power and thrust handling capabilities of other mechanical components.
Depending on the production method used to produce a specific centrifugal pump, the number of available impeller trims may be limited to a few pre-defined increments selected to coincide with specific motor ratings, or the impeller trim may be customized to match the specific requirements of a project.
The impact of a change in impeller diameter can be calculated by applying the affinity laws.