Advantages of Screw Pumps over Gear Pumps in High Viscosity Applications

1. Screw Pumps can Pump at Lower Temperatures

Gear pumps used as transfer and loading/unloading pumps have often difficulties pumping high viscosity and shear sensitive products without excessive wear and premature failures. One reason is that gear pumps require higher pumping temperatures to work properly whereas the typical Leistritz screw pump is designed with smooth meshing of the screws to pump at reduced temperatures (235 to 250F) for gentler handling of the product and significant reduction of heating costs.

2. No Gearbox is Required for a Screw Pump, Reducing Maintenance Costs

The typical Leistritz screw pump is operating with direct motor drive at synchronous speeds not requiring a gearbox for speed reduction. The gear pump however operates normally at much lower speed due to its internal design and drag losses from viscous fluids. The result is more components, complexity, and maintenance. In addition, it adds to the size of the pump unit often excluding a vertical installation.

3. No Internal Thrust Plates Required in a Screw Pump, Reducing Maintenance Costs

The axial thrust load of a typical Leistritz screw pump is hydraulically compensated by an internal balancing arrangement eliminating the need for thrust bearings. A gear pump however requires an internal thrust plate which lubricated by the pumped fluid to balance the axial load. That in turn can lead to added wear with frequent thrust plate replacements and added maintenance costs.

4. A Screw Pump Offers Larger Suction Capability

Due to low internal velocities and larger pumping chambers the typical Leistritz screw pump has larger suction capability than a comparable gear pump. That means better ability and shorter times to empty tanks and suction lines without vibrations, vapor locking and pulsations. Products with gas and vapor entrainment can be handled without excessive wear and tear.

5. Cold Slugs can Cause Damage to Gear Pumps

During startup slugs or plugs of cold product can be formed in the flow line resulting in pressure spikes which can cause damage to the internal gears of the gear pump. Gear teeth can brake and damage the pump faster than its built on relief valve or the overload protection can react. The typical Leistritz screw pump comes with a pressure balanced full flow relief valve also equipped with a manual handwheel which allows the operator to pass flow, by heating the pump up and protecting it from overload.

6. Shaft Seals have a Longer Life Span in Screw Pumps

Gear pumps are often designed to have the shaft seal in the discharge pressure area leading to more stress and failure possibilities for the seal. As most of modern pumps rely on mechanical instead of a pack box seal there is no need to pressurize the seal. For longer seal life and reduced chance of emissions from the seal area the typical Leistritz screw pump has the shaft seal in the low pressure or suction area. This leads to longer seal life, reduced emissions and load on the shaft.

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