Following on from Impeller trimming Part 1 and 2, this article will continue to outline things that can happen when the impeller vane diameter is reduced. In Part 3, we will look at noise changes that can occur with impeller trimming.
When writing a pump specification, many practicing engineers limit the impeller diameter to 85 per cent of its maximum diameter. Such a limitation is actually a misunderstanding of a design concept known as quiet pump operation.
This misunderstanding may force the selection of a larger pump for the application. The idea here is not that the impeller diameter should be 85 per cent of the maximum published diameter, but 85 per cent of cutwater diameter (0.85 cutwater ratio).
To fully understand the quiet pump operation design concept refer to Figure 1.
In designing a pump casing, a design engineer first determines the volute scroll (A) necessary to handle the desired volume of water.
This volute scroll terminates at the volute cutwater (B) at the base of the discharge nozzle (C). The volute scroll is drawn around a base circle (D), which is sufficiently large enough to allow insertion of the impeller.
The distance from the shaft centerline to the volute cutwater is called the cutwater radius and twice this distance is the cutwater diameter.
Hydraulic noise becomes a factor when the periphery of the impeller passes too close to the cutwater. When designing a pump, the distance between the impeller and the cutwater is a compromise between pump efficiency and pump noise.
Typically, cutwater ratios (D/F) of 0.9 and above produce higher noise, and cutwater ratios of 0.8 and below produce significantly lower pump noise.
Cutwater ratio of 0.85 is commonly specified by practicing engineers, thereby realising a minimum reduction in efficiency with a mean reduction in noise level.
From the above, it may be understood that a specification should more properly read “impeller diameter not to exceed 85 per cent of the volute cutwater diameter”, rather than “impeller diameter shall not exceed 85 per cent of the maximum impeller diameter capable of being installed in the pump casing”.
Specifying the later statement is safer since the impeller diameter would be even smaller than the desired maximum.
Article courtesy of Kelair Pumps Australia “When Pump Knowledge Matters”. Phone: 1300 789 466 or visit www.kelairpumps.com.au.