Centerfugal Pump

You have to reduce/cut 6.mm from out side of impeler by grinding m/c. And what abut impeler wide?

Then it would be a different pump. Provided the performance is correct, why worry about the impeller diameter <rhetorical question>?

A 9-inch impeller with narrower openings could perform similarly to an 8-inch impeller with wider openings; although the 9-inch generally would be capable of higher head. If you are comparing two pumps, they might perform similarly at the given conditions of 440 gpm and 230 ft head. Check also their efficiencies. And prices. Also note where each pump's BEP (best efficiency point) is in relation to the duty.

If it turns out that actual piping resistances are less than estimated, either pump will deliver greater flow at the lesser head. Thus it is possible that one of the pumps could cross over to an overloaded condition, while the other might not. I think this issue would be more likely with the 9-inch impeller.

In other words, the pumps might perform equally at design conditions, but not equally if actual conditions vary.

It is customary for the pump manufacturers to offer series of pumps of different capacities and head using a standard size casing with different impeller diameters. If the same manufacturer is offering you 9" dia impeller, then it fit in the same casing. If you are buying from other source, check whether casing is designed to accommodate it. If no, impeller OD is to be machined in lathe to get 8" OD.

Higher impeller diameter will result higher head (much more than 230"). If you have no problem with higher head then OK, other wise impeller OD is to be machined in lathe to get 8" OD. Other method is to use VFD to reduce speed to get required head, but it is costlier.

I can narrate a case study in this regard. Our cooling water pumps were procured with specifications as per our need, 52500 lpm and 37 mtr head such that two pumps run parralal and third remains stand by. With these specifications, impeller dia was 500mm. After actual instalation it was found that neither flow nor head is achieved as per specs, which were confirmed by manufacturer. After lot of discussion and R&D, impeller dia was increased to 640mm in three steps. At that level we could achieve the required flow and head but at that point of operation pump efficiency was down then specified.

I want to specify that by increasing impeller dia you may loose some of the efficiency of the pump.