Selecting a Pump Model, Horsepower, & Impeller Size

This part of the Pump Tutorial Series.  The 1st page is at  Pumps: Selecting-a pump step-by-step.

Selecting a Pump Model, Horsepower, & Impeller Size

The best and easiest way:  contact a pump dealer and give him/her the pressure you need (feet head or  PSI), and the volume (GPM) you want to pump, they can figure out which pump you need.  This is the best way to select a pump, as the dealers know their pump lines much better than you. They may suggest a pump you don’t even know about.

The Do-It-Yourself way

The rest of this page will take you through the process of selecting the right pump model for your irrigation system.  It is assumed you read the previous pages and already have decided what type of pump you need and you know, or will soon know the pressure and flow your pump will need to produce.

Generally it is not a good idea to buy an “off-the-shelf” boxed pump at a store for use on an irrigation system. Most of those pumps do not provide sufficient performance information on the packaging for you to be able to tell if the pump is sufficient for your system.  Off-the-shelf pumps are fine for fountains, draining a pool, running a sprinkler on the end of a hose, or other uses that don’t require precision. The wasted power used by the wrong size pump on an irrigation system can easily cost you far more than the savings you get from buying a discount pump. If you guess wrong and the pump isn’t big enough, you will become one of those poor people trying to unload a brand new pump for half what you paid for it at an online auction!

Pump Curves

A pump curve is a simple graph which shows the performance characteristics of a particular pump.  Some pump manufacturers use charts rather than curves, the idea is the same, curves just tend to be more flexible to use.  Pump curves are created by the pump manufacturer based on test results of the various pump models the manufacturer produces. The pump manufacturer should be able to provide you with performance curves for the pumps you are considering. Remember, there is always an inverse relationship between pressure and flow. Higher pressures mean lower flows. Lower pressures result in higher flows. Here’s a sample pump curve for study:

OK, that was a big help, right? No? OK, so how in the world do you use the thing?  Here’s a step-by-step guide to the pump curve above. The pump curve above is for a centrifugal type pump, but the curves for other pump types are used the same way.

Each pump curve typically reflects a single model of pump made by the manufacturer. At the top right of the chart you will notice it gives the pump speed, in the chart above this is 3500 RPM. It is said that higher speed pumps wear out faster, but to be honest, we have never noticed a difference.  We pretty much ignore the speed and try to select the pump with the best performance for our needs.  There are other opinions on this, so be aware it is one of those areas of disagreement in the industry.

For each model there are two variables which effect the pump performance. The first is the horsepower of the motor attached to the pump. Remember, what we commonly refer to as a pump is actually combination of two items: a pump and motor. The pump is the part that moves the water, the motor is the part that moves the pump! Most pumps can be attached to several different sizes of motor. Bigger motors mean more volume and pressure.

The second variable is the size of the impeller. As you remember the impeller spins inside the case and this is what moves the water. Larger impellers move the water faster creating more centrifugal force.  This results in higher pressures.

Look at the left side of the curve and you will see a label HEAD – FT and numbers starting with 0 and increasing as you move up the chart. This is the pressure that the pump is capable of producing, measured in feet of head (not PSI!)  The bottom of the curve is labeled US GPM. This is the flow that the pump produces.

Finding the proper pump is just a matter of selecting a model and size that will produce both the head and GPM that you need. To do this you select the horsepower and impeller size that will give you the desired performance.

Using the Curves

Notice the red color curved lines (the top one in the pump curve above is labeled “6.00 IN. DIA.”). These represent the various impeller sizes. Now notice the green color straight lines which intersect the impeller curves (the top one in the pump curve above is labeled 5 HP). These lines represent the motor horsepower ratings available for this pump. Together the impeller curves and horsepower lines represent the best performance the pump is capable of if that horsepower or impeller size is selected. Some pump curves do not have horsepower lines, and some pump curves combine the horsepower and impeller lines into one single line. This is usually because the pump only is available with one motor, so you don’t get to select the horsepower. The pump may also only come with one size of impeller, so you will only see a single line on the entire pump curve!

To use the curves you select the pressure you want on the left and then move horizontally across the chart to the vertical line that corresponds with the flow (GPM) that you want. You then select an impeller size curve and horsepower line that are above this point to determine the impeller size and horsepower you will need for your pump.

The following examples are based on the pump curve above.

Example #1: You want a pump that produces 125 feet of head while pumping 100 GPM. Start at 125 ft. hd. on the left of the pump curve. Now move straight across the curve to the right until you reach the line that goes down to 100 GPM on the bottom of the curve. From the point where the two lines intersect move up the chart to see what horsepower pump will be needed. In this case a 5 HP will be needed as the next horsepower line above our intersection point is the 5 HP line. Likewise, the impeller curve must also be higher in the chart than our line intersection, so a 6.00 IN. DIA. impeller will work.

Example #2: For 70 ft. hd. and 80 GPM. Make a mark at the point where the horizontal 70 ft. hd. line intersects the vertical 80 GPM line. This point is just above the 2 HP line, so a 2 HP pump will NOT work. We will need to use the 3 HP motor. The next higher impeller size is 5.00 IN. DIA. so we could use that impeller (but we might not, see “custom impellers “below).

Custom impellers

If you order it, they will build it!  Pumps can be ordered with custom impeller sizes. In fact, this is the most common way pumps are sold.  This often does not cost much more than a “off the rack “pump, but it does take a little longer to get the pump since they need to custom build it!  Almost all of the pumps pros use have custom impeller sizes. To determine the custom impeller size you simply draw a new impeller curve through your ft. hd./GPM intersection point so that the curve is parallel to the other impeller curves.  Then you determine the impeller size by comparing your new curve with the other impeller curves.

Example:  If your new curve is midway between the 5.00 inch curve and the 6.00 inch curve then you would need a 5.50 inch impeller!

The good news is that you don’t really need to do this, when you order the pump the manufacturer will ask you what your feet head and GPM requirements are and they will calculate the best impeller size for you! Then they will grind one down to the correct size for you. Now that has to make you feel pretty important. Plus, you get bragging rights; “I had this pump custom built for my yard, it’s one of a kind!”  The negative side of this is you generally can’t return a custom built pump.  So get that pressure and flow requirement right.

Pump Efficiency

On many pump curves you will see an additional set of ellipses labeled “efficiency ” or you may see percentages labeled on the other curves. For clarity I left these off of the example pump curve. These ellipses indicate the efficiency of the pump. To use them you simply look for the smallest ellipse that your line intersection point is inside. This is the efficiency at which the pump will operate. The higher the efficiency the better! A high efficiency pump uses less energy ($$$) to operate than a low efficiency pump. If possible, it is best to avoid any pump that has an efficiency of 55% or less. 55% efficiency is the industry standard used to estimate the performance of a pump when the actual efficiency is unknown.

Read the Manual

All pumps have oddities and special and unique requirements. Before installing a new pump always read the Owner’s Manual.

Go to the next page –>   Pump Controls