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Density and Specific Gravity- How they affect pump selection

May 8, 2014
Note negatively sloped horsepower lines when calculating pump motor size

Note negatively sloped horsepower lines when calculating pump motor size

While most fast talking sales guys eager to get an order can get any pump sizing application reasonably close, few truly understand the rheologic and fluid properties that go into proper pump selection. It goes without saying that the correct application of a sanitary pump means not only understanding the process, but also understanding the physical properties of the fluid being pumped. This post will offer a quick look at two important properties- density and specific gravity- and how they affect pump sizing and selection.

To begin, the density of a fluid is its mass per unit of volume and is usually expressed as pounds per gallon. A closely related, but slightly different, measure of a fluid’s mass per unit volume can be expressed by its relative density to water, otherwise known as specific gravity. To reiterate, specific gravity of a fluid is the ratio of a fluid’s density to the density of water. As it is a ratio, specific gravity is unitless. A fluid with a specific gravity of less than 1 would be considered “lighter” than water, while a fluid with a specific gravity greater than one would be considered “heavier” than water.

So how do specific gravity and density affect pump sizing and selection? Let us explain. When sizing a centrifugal pump, most pump curves are based on water, not that liquid sugar you’re trying to pump. These pump curves often show a pumps discharge capacity in gallons per minutes vs. a discharge head. The pump curve will also have some negatively sloped, dashed lines that show the horsepower requirements of the pump. And while a centrifugal pump curve is great because it puts everything into terms of flow and head, the curve does not consider how fluid weight impacts the amount of work, or HP, required to perform the duty.

If you recall, back in February, we posted a blog that talked about the difference between pump pressure and pump head. The formula we gave from head was:

H=2.31p/SG

This formula tells us that as fluid specific gravity increases, head actually DECREASES. So if you have a fluid with a specific gravity of 1.05, and you want a 35 PSI gauge rating, you will need to size for less head than you would if you wanted a 35 PSI gauge rating with water.

Getting back to this idea of how much “work” is required, let’s try and understand the impact of liquid weight by converting flow in GPM and head in feet into units of work. The equation for that is as follows:

GPM x Density (lb/gal) x Head (ft) = Work (ft-lb/minute)

Dividing this by 33,000 gives us HP at that particular intersection of flow and head. As we can see, if either the density, flow, or head increase, so will the amount of work required. That means while we’re going to need more horsepower to pump a heavy sugar solution than we will need to pump gasoline (SG=0.71), we will not need a bigger pump.

To conclude, as long as the viscosity of a fluid is comparable to that of water, specific gravity and density will have no effect on pump capacity. Specific gravity and density will, however, directly affect the input power required to pump a particular liquid. Density and SG can also have an effect on the onset of cavitation in a particular pump, but that is beyond the scope of this blog. If you have any questions about how the physical properties of your fluid impact pump selection, contact a Holland Sales Engineer today.

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