READING A PUMP CURVE
Have you ever wondered how to read a pump curve or even what a pump curve is?
The pump curve will tell you the performance of the pump prior to installing it. This is very helpful because there is no guessing involved on whether or not the pump will meet your needs.
First, we want to know the gallons per minute (gpm) and the psi needed to operate the sprinkler system. For this example, let's assume we need 30 gpm at 55 psi.
Secondly, with an irrigation pump, the manufacturer typically rates the pump in Head not psi, so we must convert our 55 psi to Head. In order to do this we take 55 psi and multiply it by 2.31, which equals 127 Head, or is sometimes referred to as 127 Total Dynamic Head (TDH).
Now, we can look at the pump curve to size our pump for 30 gpm @ 127 TDH.
You may notice by looking at the graph, that the bottom is labeled gpm, and the left side is labeled TDH. We pick the gpm and Head we need to operate the system. We follow these two lines up until they meet, usually somewhere in the center of the graph. At this point we draw a dot. This is the place where we want our pump to perform at.
Now, we look at the curve in the middle of the graph. This curve usually starts in the upper left hand corner and ends on the lower right hand corner. This curve represents what the pump will perform at. Hopefully our dot is on the curve or just below the curve, indicating that our pump will meet our needs. If the dot is above the curve, we must select another pump.
That's it. Now, a few items to make note of:
Make sure your dot is always on the curve or under the curve.
Make sure your dot is generally on the left side or center of the curve. Never choose a point on the far right side of the curve. If you're on the far right side, choose another pump.
You may have noticed that there is a direct relationship between gpm and Head. The more gpm you want, the less Head the pump delivers. Always size your pump for gpm 1st, and Head 2nd.