Viscosity, Speed and Flow: Sizing a Gear Pump

Pump Notes · engineering notes from Vimpo Pump

A gear pump is simple to size. You need four numbers: the fluid, the temperature, the flow and the pressure.

Flow = displacement × speed

Each pump size has a fixed displacement per revolution. The drive sets the speed. Flow is the product of the two, minus a small internal slip.

• VD-25: 0.0287 litres per revolution at 1,450 rpm gives 2.5 m³/h.
• VD-150: 5.556 litres per revolution at 300 rpm gives 100 m³/h.

The same pump at half speed gives half the flow. A variable frequency drive therefore gives one pump a range of flows.

Viscosity sets the maximum speed

A thick fluid needs time to fill the space between the gear teeth. When the viscosity increases, the maximum permitted speed decreases, because filling of the pump becomes the limit. This is why our capacity tables give one speed range for each viscosity band, and why the VH series runs at low speed, max. 461 rpm, behind a gearmotor.

Pressure and viscosity set the motor power

Pressure changes the flow only a little. It changes the motor power. The shaft power is the sum of hydraulic power, viscous drag and bearing and seal friction. We round it up to the next standard IEC motor step.

• Cold start: select one motor step above the table value.
• Above 10,000 cSt: select one motor step above the table value.
• Above 50,000 cSt: use flooded suction; a suction lift is not possible.

Typical viscosities at pumping temperature

• Paving grade bitumen 50/70: 300 cSt at 150 °C · 90 cSt at 180 °C
• Polymer modified bitumen (SBS): 2,000 cSt at 160 °C · 500 cSt at 180 °C
• Oxidised bitumen 85/25: 4,000 cSt at 170 °C · 800 cSt at 200 °C
• Heavy fuel oil HFO 380: 380 cSt at 50 °C · 90 cSt at 80 °C

These are typical values from our datasheet reference table. Your product data sheet is the final source.

Send us the four numbers. We give you the pump size, the speed and the motor.

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