centrifugal pump selection

To calculate the flow and head requirements of a centrifugal pump, you must first develop a system head curve. To do this, you need to use a table of friction losses in the type of pipe you are using. These tables are created from a formula called the Hazen & Williams formula. They have a table showing the values for each type of pipe. The tables show the head loss per 100' of pipe and they are available in many pump manufacturers' catalogs. Probably also can get them on line somewhere.

Having gotten a copy of the appropriate tables, you will see that they are shown for various flows. Take a sheet of paper and write down the flows in a column on the left side of the paper. Take the total length of the pipe in hundreds and multiply that by the value given for each of the flows you have listed. Put those numbers in the next column. Then you must know the static head (eleveation difference between the start and the end of the pipe). Add that number to the friction loss calculated for each flow listed. The result is the total dynamic head (TDH) of the system at the given flows.

Now you must determine the flow you need. This you will determine for your application. Let's say you need a flow of 150 GPM. You will look at the chart you have developed and pick a flow near to or at 150 GPM and see what TDH you have listed for that flow. For example, let's take 45' of TDH. Your conditions, then, would be 150 GPM @ 45' TDH. That is what you will look for when selecting your pump.

Now, you choose a pump that has that condition point in a good location on its curves. You will take the pump curve and plot the numbers in your chart for the various flows on that pump curve. Then you will connect these points to reveal a curve. Where your system head curve crosses the pump characteristic curve is the delivery point. The pump curve will show various impeller diameters or it may show various speeds or both. The system head curve will cross each of these curves and each crossing is a delivery point for that curve.

Now, there are various other considerations that you may want to discuss. Some of those are the bends and fittings in the line, as those will exert some friction losses on the system; the type of pump you need, such as solids handing pumps, grinder pumps, high head pumps, and any number of other specialties. These are all concerns that you may wish to address.

And, of course, since you have asked your question in this forum, I will tell you how I would advise anyone who would asked me the same question, "Get an engineer."