pump

Your question implies that you are concerned about the "added" cost of that reducer. Is that is the real basis behind your question? If so your question is a valid one. It is one of the many "Why" questions associated with piping.

I will not try to give you a technical answer for this but I will try to give you an understanding of the situation.

First you do not change the pump nozzle to match the line size just to save the cost of a reducer. To change the size if the nozzle part of the pump case would be very much more expensive than the reducer.

So why the different size in the first place. Try to look at it this way. It is a matter of "Supply and Demand". The "Demand" relates to the pump. It is designed to move a certain amount of a specific fluid having specific design conditions. The pump is a specific size but it has that motor (or Turbine) driver providing the power that moves the fluid along.

To make the pump work properly you must guarantee the proper conditions at the inlet to the pump. This is called NPSH (Net Positive Suction Head). The suction line is included in the physical system that is the "Supply" part of this matter. The suction line is normally only a gravity driven conduit that must pass the same amount of fluid as the motor driven pump. To do this you must have a larger suction line than the pump nozzle. If you have a larger suction line then you have a "Supply" that meets the "Demand".

Like I said, this is not a highly technical answer but I hope it helps.

So go ahead and add the reducer in the pump suction. But remember, in pump suction lines, install the eccentric reducer with the flat side on top.