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Choosing between rotary lobe and ECP

19 September 2016

A rotary piston (ECP) pump.
A rotary piston (ECP) pump.

Cliff Warne explains the differences between two similar pump types – rotary lobe and ECP pump technologies. 

All areas of process engineering include applications where the transfer of fluids requires sensitive pumping action so as not the damage or change the structure of the media being handled. Typical applications include fluids that contain solids, display either high or low levels of viscosity, have high or low temperature levels, are non-lubricating or could be made from ingredients that can be aggressive and cause damage to the pump.

There are a variety of pump types that can address some, if not all, these challenges. This article focusses on two similar pump types – rotary lobe and ECP (External Circumferential Piston) pumps.

Rotary lobe pumps and ECP pumps are most frequently employed in food processing because they can handle more delicate solids without damaging the product. The size of the particles contained within the fluid being pumped can be much larger in lobe pumps than in other types of positive displacement pumps (PDP) because the large rotor cavities.

There is very little difference between a rotary lobe and an external circumferential piston pumps as they both essentially use the same operating principle involving interlocking lobe shaped rotors to move a fixed volume around the pump chamber pushing the liquid out of the discharge as the rotors mesh together. However, a closer examination of the rotors shows them to be of a different shape and herein lies the difference. ECP rotors have much larger heads (known as ‘pistons’) than their rotary lobe counterparts, following the profile of the pump body. They also utilise a ‘rotor hub’ on the pump body. Both of these features result in a much longer sealing path between the rotor and the pump body. This reduces the slippage within the pump which can be associated with pump wear when handling abrasive liquids. The leading edge of the rotor provides a gentle scooping action of the pumped media moving solids away from outer edge. 

While offering many of the benefits of the rotary lobe pump the ECP pump employs arc-shaped rotary pistons, or rotor wings, that travel in annular-shaped cylinders machined in the pump body. The resulting long sealing path reduces slippage and produces a smooth product flow without destructive pulses or pressure peaks, and without the need for valves or complex parts. 

Because the rotors produce a scooping action, they do not squeeze and compact the medium being pumped between the rotor and the pump body. This pump type combines a very gentle, pulse-free pumping action with the high suction capacity necessary for allowing thick mixes to be drawn into the pump without any separation of the ingredients.

The longer sealing path of the ECP pump makes manufacturing harder for the pump company as the rotor and body are more complex in their design. However, the longer sealing path considerably improves the level of containment within the liquid cavity, reduces the pressure at the point of contact and results in a smoother flow with virtually no pulses or pressure peaks.

ECP pumps can pump the most delicate of products including pet foods and yogurts containing fruit pieces, they can cope with extremely viscous fluids, such as re-work products and dough mixes that are beyond the scope of most other pump types. The long sealing path of ECP pumps also ensures virtually no back slippage of liquid making their dosing accuracy as high as 2-3%.  The reduced pressure and velocity between the outer edge of the rotor and the chamber wall also prolongs the life of the pump and minimises heat within the pump head.

With the ECP pump there is no contact between the rotors and the pump casing. This becomes important when pumping abrasive liquids and dilatant slurry. When the pump is stopped any remaining fluid falls to the bottom of the cavity and begins to solidify. When the pump is restarted, the slurry is scooped up by the ECP rotor and is moved into the collapsing cavity where fluids from the two rotors meet on the discharge side of the pump. There, the slurry material is again mixed and re-suspended without stress or damage to media or the pump.

Summary
While for some applications and duties there are areas where both pump types could be employed, careful consideration has to be given to the nature of the media being pumped and the expectations required of the pumps before a final decision should be made. For applications where product degradation, viscosity, abrasion, pressure and life-cycle costs are of higher concern, I would advise the use of an ECP pump. However, for medium duty lobe applications, where viscosity, pressure and abrasion are of lower concern, lobe pumps provide a perfectly adequate solution.
Cliff Warne is UK sales manager at AxFlow.


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