Answer
The GPM rating notes the pump's optimal flow rate. While just about every pump has a listed GPM rating, that doesn't mean that it only puts out that amount of GPM. You still need to know how high and how far you're pumping the water, the piping diameter, your required GPM and any possible pressure requirements needed in the application. Once all that is known and calculated to what is called your TDH or Total Dynamic Head, you can then use the pump curve to determine the best option for the application. For example, let's say you need 10GPM and your pressure requirements are 40psi and with your lift, distance and piping diameter all figured in, you come out with a TDH of 225 feet. If you look at the pump curve for this pump, you would see that it would come to about 9GPM. Not what you need, but close. If, on the other hand, your TDH came to about 150', that pump would be giving you about 13GPM. And in looking at the curve, you will also notice that the same pump can actually provide anywhere between 0 and 15GPM. It all depends on the pumping distance, lift, piping diameter, and your requirements.
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