The Effect of Slip on Sanitary Pump Performance

August 26, 2014

This post will touch on a topic we’ve discussed a few times in the past- the low slip design the Waukesha Universal series of sanitary positive displacement pumps. Previous posts have talked about how the external circumferential piston design of the Waukesha Universal series makes it nearly perfectly positive and efficient at viscosities over about 250 cps, but why is this important? Outside of efficiency, the low slip design of the Universal series pump allows us to accomplish four useful things- it allows us to pump low viscosity fluids in low NIPA systems, it allows us to pump from vacuum vessels, self-prime, and meter fluids. This post will look at each of these four benefits and why they are important to system performance.

Waukesha Univeral Pumps can be Run at Slow Speeds, Minimizing Shear

Waukesha’s Unique ECP Rotor is More Efficient than Lobe or Bi-wing Designs, Resulting in Less Slip

To begin, the low slip design of the Waukesha Universal series of pumps allows us to efficiently pump low viscosity fluids when net inlet pressure available (NIPA) is also low. When pumping low viscosity fluids in low NIPA systems, slip can greatly reduce pump capacity and increase energy requirements. At low pump speeds this generally isn’t an issue, but if pressure differential across the pump causes excessive slip, little or no flow may result. At higher speeds, internal pump losses may be high enough to limit flow. In a high slip, low efficiency pump design, high velocities of fluids within the chamber can create localized areas of low pressure. If this pressure drops below the vapor pressure of the fluid being pumped, flashing can occur and vapor will fill the pump cavities, destroying the ability to sustain uniform flow.

The next thing the low slip design of Waukesha pumps allows us to do is pump out of a vacuum vessel. Pumping from a vacuum vessel is an example of an extremely low inlet pressure system. Vacuum chambers are typically used to evaporate fluids to hold and process liquids at extremely low temperature. This means we’re going to be operating at or near the fluids vapor pressure in these applications. Because of this, we need to use a pump that is as efficient as possible to allow us to run the pump as slowly as possible. High speeds will create an area of low pressure at the inlet of the pump which can lead to flashing and cavitation, crushing system efficiency and greatly reducing pump life.

As we’ve talked about in previous posts, the clearances in Waukesha pumps are so tight that at higher speeds the pump can even move air. What that means is that the pump can be used to dry prime or actually evacuate the air in the inlet line, reducing pressure, and drawing fluid up the line and into the pump chamber where normal pumping can begin. While not suggested, Waukesha PD pumps can run dry long enough to draw fluid into the pump chamber and begin standard processing. Care should always be taken to ensure the maximum available pressure at the inlet of the pump. Contact a Holland sales engineer for help determining the amount of lift your Waukesha pump can generate.

A low slip Waukesha pump can also be used to meter fluids. If the slip is low, a pump will deliver nearly its theoretical displacement in each revolution. By counting or controlling the revolutions per minute of the pump, we can measure the amount of liquid displaced. In any metering application, it’s important to operate in the metering range of the pump- i.e. the range at which the relationship between change in speed and displacement is linear. System conditions should also be kept constant. We want to maintain constant suction head and minimize the pressure differential across the pump. By coupling this with the efficient design of a Waukesha Universal pump, we can reduce the effect of slip and meter effectively.

To conclude, the low slip design of Waukesha Sanitary PD pumps has further reaching implications than just increasing system efficiency. The low slip design of the Waukesha Universal series of pumps allows us to pump low viscosity fluids with low inlet pressures, it allows us to pump out of vacuum vessels, it allows us to dry prime, and it allows us to meter. If you have more questions about your next Waukesha pump application, contact a Holland Sales Engineer today.