Centrifugal Pump Casing and Shaft Design

As indicated, "The shaft is designed for the maximum diameter impeller."
The natural second question could be, " If so, why should a bigger shaft to be used for a lesser requirement?"
The energy (because of excess mass of the shaft) required by the bigger diameter of the shaft is minimal compared to the total hydraulic load.
But the gain on the overall is huge.
In one of the modern plants I came across recently, the following are the population.
Total centrifugal pumps: 200 (around)
Overhung (Back pull-out pumps): 160
Pumps from the same make: 140
These 140 pumps have following interchangeable major parts:
Bearing Bracket, Bearing Sets, Volute Case: Only 5 sizes.
Recommended stock level for Shafts: 1
Recommended stock level for Impeller: 1
Recommended stock level for Bearing: 3 sets
What a big saving on inventory !

Shafts are designed for not only the maximum diameter impeller, but for the running speed. If you take a pump designed for 50Hz and use it in an 60Hz application, the gearing on the shaft would be different to achieve the same output.

No question of change in Frequency.

Question is for one perticular case either with 60 hz or 50 Hz.

The pump curves as published by the manufacturer for any given family of curves would all have the same shaft diameter. This shaft would be adequate for the complete family of curves as shown on that page.

The same is true for the casing. However, if the pump is a centrifugal pump, as I assume it is, you must be very careful about the inlet or suction pressure. The centrifugal pump will add its discharge pressure to the suction pressure it is seeing. The result could very well be more pressure than the pump casing is designed to take. I had a situation where the pump, an end suction centrifugal water pump, was installed with the suction pressure erroneously listed as 65 psi when in fact it was 165 psi. This, added to the discharge pressure of about 60 psi means that the pump casing was subjected to a pressure of 225 psi. The casing was only rated for 175 psi. The solution was to replace the pumps with ones that were cast of ductile iron to get the rating required.

Typically the shaft of a pump, gear box, etc is sized to match the shaft of the largest motor that will drive it. Motor shafts are standardized per horse power.

Travis

The manufacturer of the impeller will be the expert on what shaft is acceptable for any given impeller. If the impeller is available, they will have already decided on the shaft diameter. You should not have to make that decision. Good luck.