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Centrifugal vs positive displacement grinder pumps

Selecting the correct pump for your application is important to extend the overall lifespan and success of your pumping installation. However, the choice is not always clear so it’s important to understand how different pumps behave in different scenarios. Here we discuss the differences between centrifugal and positive displacement grinder pumps, and look at what is most suited for pressure sewer systems.

Positive displacement pump

Positive displacement pump

Under pressure

One of the main features of a centrifugal grinder pump is that it is self-limiting in terms of both flow and pressure. This means when sewage volume increases it will automatically reduce its flow while increasing pressure.

Furthermore, when there is a system overload, such as too many pumps on-line at one time or system failure, the pump will reduce its flow to nearly zero (shut off). When conditions improve it will automatically increase its flow to the optimum point. Therefore, a centrifugal grinder pump will always perform within the present conditions.

On the other hand, positive displacement grinder pumps are not self-limiting and are designed to maintain constant flow at all times. When there is an increase of sewage volume, the pump will increase its output pressure to support a steady pumping rate.

In certain applications this is a good feature, however, in the case of pressure sewer systems, over-pressure can occur causing the pump motor to overload and trip its thermal switch or circuit breaker. In some cases, this can also cause pipes to rupture or damage to the pump itself.

Velocity is key

Velocity is important for pressure sewer force mains and laterals as sometimes they have a relatively small diameter pipe and depend on the velocity of the pumpage to keep it clean from debris. This is especially important when flow and velocity is reduced by a low number of stations that are actively pumping.

Under these conditions, positive displacement grinder pumps contribute to the condition due to its low flow, fixed rate design. On the other hand, a centrifugal grinder pump automatically seeks the highest flow it is capable of producing under the conditions, thus increasing the fluid velocity and scouring action it provides.

In fact, one small centrifugal grinder pump can produce the same fluid velocity as five small positive displacement pumps.

Flexible design

Liberty Pumps' Omnivore V-Slice cutter technology

Liberty Pumps’ Omnivore V-Slice cutter technology

What makes the centrifugal grinder pump flexible is its unique grinder mechanism and impeller design in combination with a standard centrifugal pump operating system.

The high speed cutter assembly of the centrifugal grinder pump performs over 3,200 cutting operations per second, producing an extremely fine slurry that is over 96 per cent water.

Since this slurry is almost all water, it is able to pump like water and is therefore able to take advantage of the unique operating characteristics of the centrifugal pump.

There are two characteristics of centrifugal grinder pumps that make them important in the design and operation of pressure sewer systems: the ability to vary flow and pressure automatically, and the reduction of energy consumption as flow is reduced.

The centrifugal grinder pump’s vortex impeller design  resides completely out of the pump volute and allows it to take full advantage of centrifugal action in a high head, low flow environment.

A major advantage of this design is that heat producing friction is reduced when running at shut off head. This allows the centrifugal grinder to operate under zero flow conditions for extended periods of time.

Maintaining the pump

Another characteristic of the centrifugal grinder pump is its low maintenance requirements.

Since the vortex impeller resides completely outside of the volute, it experiences almost no wear and can therefore be expected to remain operational as long as the pump itself.

In contrast, the close tolerances and running clearances of a positive displacement grinder means there is more maintenance and costs, and the pumping components will have a shorter life.

The high speed cutting action of the centrifugal grinder reduces torque over that experienced by the slower action positive displacement pump. The reduction in torque relates to lower wear of cutting components and longer life.

Liberty Pump's Omnivore grinder pump

Liberty Pumps’ Omnivore grinder pump

Which pump is right?

According to Jason Lynch, Director at FITT Resources, centrifugal grinder pumps provide the most efficient outcomes for pressure sewer system applications.

“Centrifugal grinder pumps such as Liberty Pumps’ Omivore grinder pumps are designed to deal with the conditions present in pressure sewer applications.

“They are able to slice through items such as feminine products and cleaning/wet wipes if they are introduced into a sewer system, which a positive displacement pump is unable to do without clogging up.

“The Omivore grinder pump can get the job done, and can last many years longer depending on the application.”

For more information, contact the FITT Resources team or visit