Pump Feet of Head and Foot Pounds

Wed, January 01, 2019

Why do pumps use Feet of Head instead of PSI? Centrifugal pumps show flow in gallons per minute (in the US).  That makes sense, right?  So why are we showing the differential pressure created by the pump in Feet of Head? Why doesn’t the pump curve show pressure differential in PSI? In a sense, it does, but with one very important difference: Specific Gravity. Specific Gravity is the ratio of the density of a fluid to some standard that we all agree on.  In the case of liquids for our purposes, the standard is water at 39.2F, where water is most dense. In short, feet of head is the pressure differential and the specific gravity of the liquid being pumped, or: Ft = (2.31 x psi) / Specific Gravity If we are pumping a liquid with a Specific Gravity of 1, we only need to multiply psi by 2.31 to get the equivalent feet of head.  If the Specific Gravity is something other than 1, we need to modify our number based on the formula above. Here’s another way to look at it. The head of a pump is a statement of energy per unit mass, or foot-pounds per pound (again in US).  Simply stated, it tells us that for every pound of water, we have a certain amount of work being performed. Energy  = (ft - lbs) / lb The pounds in the numerator and denominator cancel out, leaving feet. This means that the pump will produce the same amount of pump head in feet regardless of the liquid being pumped, assuming relatively similar viscosities.  This DOESN’T mean that the pump will require the same pressures in psi. For closed loop systems such as heating hot water or chilled water systems, we need to make the corrections for pressure drop based on the actual fluid being pumped. Glycols will require a slightly higher pump head than plain water, although the difference is reduced at higher temperatures. Once this calculation is made however there is no need to change the pump based on the liquid.  A foot is a foot is a foot.  Therefore, the pump manufacturer doesn’t need to create a different pump curve for every fluid and at every temperature.   Next time: Understanding NPSH – The Great Fear of Pump System Designers!