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Sanitary Pumps: PD VS Centrifugal. When Do You Use One Over the Other?

October 25, 2013

When trying to make the decision of whether to use a sanitary  Positive Displacement (“PD Pump”) or sanitary Centrifugal pump it may not always be a clear-cut decision. To make the beneficial choice between these pump types it is important to understand that these two types of pumps operate very differently.

Flow rate versus pressure. The centrifugal has changing flow dependent on pressure, where the PD pump has more or less continuous flow regardless of pressure.

An additional important difference between the pump types is the effect viscosity has on the capability of the pump.

Centrifugal pump in let and out let

Centrifugal Pump

The sanitary centrifugal pump loses flow as viscosity goes up, where the PD pump flow essentially increases.  The reason for this is the higher viscosity liquids fill the clearances of the pump introducing a higher volumetric efficiency.

When efficiency versus viscosity, viscosity takes on prominent role in pump mechanical efficiency.  Considering mechanical efficiency and efficiency versus pressure, the pumps do act in a different way.  Changes in pressure have a minor effect on the PD pump but a modest one on the centrifugal.  With centrifugal pumps the efficiency goes down as the viscosity rises due to increased frictional losses inside the pump. Efficiency will increase in a PD pump with increasing viscosity.

A PD pump operates by making fluid move by catching a fixed amount and pushing (relocating) that trapped volume into the discharge pipe.

Sanitary PD pumps come in  a variety of configurations including piston, diaphragm, twin screw, rotary piston/lobe, progressive cavity to name a few.   Sanitary PD pumps, not like centrifugal can in theory construct the same flow at a given speed no matter what the discharge pressure. Hence, positive displacement pumps are known as constant flow machines.

A centrifugal pump uses a rotating impeller to boost the pressure and flow rate of a fluid. Centrifugal pumps are the most common type of pump used to move

PD inlet and outlet

Rotary PD Pump

liquids through a piping system. The fluid goes through the pump impeller along or near to the rotating axis and is sped up by the impeller, flowing centrifugally outward or axially into a diffuser chamber, from where it departs into the downward piping system.

In summation PD pumps normally can produce more pressure than centrifugal pumps. PD pumps operating at a lower speed than the centrifugal will have effect on the seal life, and consequently PD seals tend to last longer than seals on a centrifugal pumps.

Sanitary Centrifugal pumps by contrast tend to do well in high flow conditions.  They are ideal for transfer applications of low viscosity liquids.  Sanitary PD pumps are better suited for high viscosity fluids.  They are also normally preferred for high pressure/low flow rate applications. In some applications, sanitary PD pumps can be used as a metering device as well.  This cannot be achieved using a sanitary centrifugal pump.

Hopefully this information will make it a little easier in determining the proper pump for your sanitary pumping application.  Holland offers a large variety of both sanitary centrifugal and sanitary PD pumps and has over 50 years experience of applying them to sanitary process applications.  Contact us you are not sure what pump you need for you application.

Holland Applied Technologies

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