Reducing Impeller Diameter

[rota1]

Hi Guys!
I have to reduce an impeller diameter. Should I cut both vanes and shroud or only the vanes. My thinking is only the vanes are to be reduced, maintaining impeller to cut water original clearance (Gap B).A strange school of thought has surfaced viz; that reducing the shroud eliminates unbalance that may be present due to heavier mass in the impeller.

 

[BigInch]

http://www.mcnallyinstitute.com/

offers this advice,

Although both the vanes and shrouds are often cut, in end suction volute type centrifugal pumps, it is not a good idea to do this in double suction designs. With these types of pumps you can reduce the vane diameters, but the shrouds should remain untouched.

Structural strength is a consideration when deciding how much to reduce the vane diameter in double-ended pumps because you could leave too much unsupported shroud. Some manufacturers recommend an oblique cut that will improve the vane exit flow and add some strength to the shrouds.
Machining a radius where the trimmed vane meets the shroud is another good idea to add strength to the assembly. Square corners are never a good idea.
Under-filing the exit vanes is usually a good idea.

Under-filing will increase the pump capacity, especially for large circulating pumps. One look at the above diagram will make this obvious.
The exit angle of the fluid will change, resulting in a higher head at design flow, but no change in shut off head.
Because of reductions in the wake of the fluid exiting the vanes. The efficiency of the pump should improve slightly. The smaller the size of the pump the larger the effect.
The technique of under-filing is critical. Sharp corners, where the vane joins the shroud, can initiate cracks and eventual impeller failure.
At least 0.0125 inches (3 mm) of vane tip thickness must remain after the under-filing.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[25362]

It has been published that it is possible to increase pump heads at reduced flow rates (toward shut-off), by either removing part of the shrouds, or by opening -specifically located- slots on the shrouds.

 

[Artisi]

You haven't said what type of pump /impeller you are talking about and how much reduction you are considering, ie from 21" to 19.5".

You have used the plural - vanes - so I assume it is an enclosed impeller but -

Is it a process / water pump or a slurry pump etc
Is it and end suction overhung impeller with pump-out vanes on the rear shroud.
Is it a single suction between bearings design.
or is it a double suction between bearing design.

All factors for consideration.

Naresuan University
Phitsanulok
Thailand