an Engineering & Service Bulletin

Technical Topics

J.H. WRIGHT & ASSOCIATES

GEAR PUMP CURVES, A PRIMER

Most engineers and plant operators are pretty familiar with centrifugal pump curves, but how many understand the curves used to size gear pumps? Strangely enough, we need more curves on the simple little gear pump (five in all) than we do for the centrifugal. Let's look at them one at a time.

Since gear pumps are primarily used to handle viscous products, it's not surprising that viscosity plays a serious role in these curves.

First, let's consider the problem of getting the product into the pump­­remembering that the liquid has to fill the spaces between the gear teeth with generally only atmospheric pressure to do the pushing. With thin liquids the pump can run faster, but as the viscosity increases the pump must be slowed down in order for the teeth to be filled. If you have ever heard a gear pump operated under cavitating conditions, you know this noise is much like a centrifugal with the same problem.

Therefore, the first curve (Curve 1) equates pump size, viscosity and the maximum RPM the pump can be run. Notice that this curve is predicated upon atmospheric pressure at the pump suction. Changes from this condition one way or another affect the speed and usually require a discussion with the pump manufacturer. This curve, by the way, is pretty much the result of specific testing. The next curve (Curve 2) simply tells you the capacity (GPM) produced by a certain size pump at normal operating speeds­­enough said.

Curve 3 is important because gear pumps have clearances and as the pressure increases­­slip (lost GPM) occurs. This is actually a measure of volumetric efficiency. Note that higher viscosities decrease the amount of slip. This curve tells you how much you are going to have to increase the pump speed from Curve 2 to get exactly the GPM you require.

Curve 4 gives you the horsepower to run the pump at the speed and pressure you choose. Curve 5 is an additional horsepower requirement to be added to the horsepower from Curve 4 for the effect of viscosity. Some gear pump manufacturers combine these curves in various fashions, but the answers are the same.

Looking again at Curves 4 and 5, notice that when you are operating these pumps at higher speeds and viscosities the viscous horsepower can equal or even exceed the input horsepower. Since many of these pumps are run on liquids that thicken up radically under cold conditions, it is not surprising that problems can occur on outdoor applications.

This often leads to "over-horsepowering" gear pumps on installation. The downside to this practice is that the pump then can produce pressures far in excess of the pump's pressure containment capacity.

For this reason alone, no gear pump should ever be installed without a relief valve system of some sort.

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