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What's the Difference? Pump Head vs Pressure

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It's time to get pumped about reading pump curves, because we're going to explain the difference between head and pressure. Both terms are used often when talking about sizing pumps, but how do they differ?

A typical pump curve plots head (in feet) against flow rate (typically in gallons per minute). So why is head expressed as feet rather than pounds per square inch (psi) of pressure? Because head is fluid independent.

Different liquids have their own specific gravity, which is the ratio of the density of a liquid to the density of water. But flow will be the same for a given feet of head. If we measured it in pressure, we would have a different curve for every liquid with a different specific gravity.

So, how do we get back to pounds of pressure? Head and pressure are different, but they have a simple mathematical relationship. Imagine a tube that's one inch by one inch. On the bottom of that tube is a pressure gauge. Using water as our baseline, we would have to fill the tube with 2.31 feet of water to make one psi. So, if we stick with water, you can multiply one foot of water by 0.434 to get psi.

Thanks to head, however, you can use the same pump curve to figure out pressure for all liquids.

See Pump Systems from the U.S. Department of Energy for information and resources for properly sizing pumps and optimizing their performance.

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