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Gear pump sizing tips and advice

Though gear pumps may seem simple enough, it actually takes five pump curves to correctly size them for your application. Understanding each of these curves will help you select the right pump for the job.

Most gear pumps handle viscous liquids, so it makes sense that viscosity plays a serious role in these curves.  The liquid filling the space between the gear teeth usually provides enough pressure to do the pushing. The pump runs faster with thin liquids – and must be slowed down as the liquid’s viscosity’s increases to allow that space between the gear teeth to fill.

So the first curve, Curve 1, (see below) equates pump size, viscosity and the maximum RPM the pump can be run. The curve is predicated on atmospheric pressure at the pump suction. Changes here affect the speed and require a conversation with your pump manufacturer.

Gear Pump Curve 1

Curve 2
(see below) reveals the capacity – GPM – produced by the pump at normal operating speeds.

Gear Pump Curve 2

Curve 3
(see below) helps measure volumetric efficiency. It’s significant because gear pumps have clearances and as pressure increases, slip – or lost GPM – can occur. Higher viscosities reduce the amount of slip. This curve reveals how much you’ll want to boost your pump speed from Curve 2 to get the GPM you need.

Gear Pump Curve 3

Curve 4
(see below) lets you know the horsepower necessary to operate the gear pump at the appropriate speed and pressure.

Gear Pump Curve 4

Curve 5
(see below) provides another horsepower requirement that can be combined with the horsepower from Curve 4 to counter the effect of the viscosity. Notice that with the last two curves, if you are running your gear pump at higher speed and viscosities, the viscous horsepower can equal or exceed the input horsepower.

Gear Pump Curve 5

Many outdoor gear pumps run with liquids that thicken up in the cold. Because of this, it can be tempting “over horsepower” your pump at installation. But think twice – this can cause the gear pump to produce pressures much greater than the pump’s pressure containment capacity. This is why a relief valve system is absolutely necessary.