Pump Piping

I can't answer the first part but I can tell you what I was told about using the reducer. When I was learning the business, in the fifties, we were designing and building refineries all over Europe. I opted to do pipe supports because it gave me some interesting structures to design. The structures that I liked least were the ones where we anchored the big lines to protect the pump bodies from the forces. I asked, "Why don't you buy bigger, stronger, pumps so we don't have to spend so much on these anchors, or use smaller pipes to reduce the pipe loads?". I was told that the pipes were selected to move the product at a preferred rate, but the bigger pumps would move it too quickly. I asked, "Why don't you order the same size pumps but with bigger nozzles and stronger bodies to suit?. I was told, "We can't do that, it would mean more cost from our mechanical equipment budget" "But it would save even more from the structures budget". "We buy the pumps from our budget, we are not going to spend more than we have to, we are judged on the mechanical budget alone!".

And so, I gather from the OP, it still goes on today.

Perfect. I'd just like to add that if he is seeing a reducer in the pump inlet, this should be because someone have installed a pipe larger than the pump requires for convenience, like, all the pipes until there are from size |X|, so keep using the same to avoid buying another pipe set and different fittings... Or to help reduce the load, but this may not be true.

Look at this from the pump manufacturer's viewpoint. It doesn't cost that much to make the suction connection one size larger. But it sure does cut down on complaints from customers who don't understand NPSH. Believe me, there are a lot of them.

Ed Weldon

See CR4 Threads: pipe sizes and pump size, Selecting Pipe Size & Pressure drop in piping for specification of Pump disch. pressure.

There are many ways of explaining this need, involving NPSN required, NPSN available, Cavitation, pipe friction etc. That is how it is been and being explained in textbooks. Let me try explaining this in a simple way as to get an idea even by non-engineers.

Pumps are used to transfer liquid form one place to another. Pump receives water/liquid from a well or from a tank and pushes to an overhear tank or into a high-pressure vessel like boiler.

Say our required pumping rate is 10 gallons per minute. Suction pipe should supply this 10 gallons per minute continually. If suction pipe is small in its bore or it is too long, liquid may not be ensured to the pump at the required rate. If a pump is capable handling but the supply system/suction line is unable to supply, then this pump starves. This situation is known as Cavitation, which has undesirable effects. Ultimately the pump would supply only the amount (a flow less than 10 gallons per minute), what suction line is capable of supplying.

Well, on reaching the pump the liquid is forced out off it unlike the suction pipe wherein the liquid has to reach the pump by it self (by gravity/static head). 10 gallons per minute can be forced through 2" pipe or 1" pipe. Leading 1" pipe is obviously lot cheaper than 2" pipeline. Of course 1" pipe would impose more frictional resistance (in handling same amount) than 2" pipe and in turn poses an increased load on the motor. So selecting the discharge pipe (as well suction pipe size) is a trade-in between initial investment on bigger pipe and running/maintenance cost by going to smaller pipe line.

In line with all this, bear in mind that if the suction pipe size is larger than the pump connection, there should be an eccentric reducer, top side flat, followed by a length = 6 x diameter straight pipe entering the pump.