Q - What is the best way to cut, or trim, an impeller diameter?
a. Cut both the impeller vanes and shrouds to the same diameter. b. Cut the vanes only, and leave the shrouds uncut.
A - In most cases it is best to cut both the impeller vanes and the shrouds to the same diameter for these reasons:
The uncut portion of the shrouds causes additional and unwanted friction loss resulting in additional brake horsepower and reduced pump efficiency.
With uncut shrouds it is more difficult to reach inside the impeller discharge passageway to clean the casting and to profile the discharge tip of the vanes.
The added weight of the uncut shrouds may increase the shaft deflection specially in overhang horizontal pumps.
In rare instances it may be preferable to cut the vanes only and leave the shrouds uncut, such as:
When the impeller requires a large cut in diameter and is operating at low flow condition. The combination of large volute clearance (B-gap) and low flow operation can induce discharge flow recirculation. In this case, leaving the shrouds at a bigger diameter than the vane would help mitigate this problem.
When a pump is anticipated to operate at a higher head at some future time where it may be more economical and faster to simply weld up and extend the vane diameter (to obtain the increase in head) rather than to buy a replacement impeller at a bigger diameter. However this approach is advisable only in big, high capacity and low head (high specific speed), low speed pumps such as those in large de-watering, sewage control, flood-control, irrigation, and similar light to medium service, where the impeller is not subjected to high pressure and high peripheral speed, and when the liquid is not highly corrosive or abrasive.