Rotary Lobe vs. Non-Clog Centrifgual Pumps

[Nordic119]

Hi
We are a sub doing the process mechanical work for an MNR project. The prime is insisting that we spec rotary lobe pumps for the recycle & WAS pumps since in their eyes it is a solids applicatation, even though the highest ML contect we expect is 1.5%. The ML design temp we are using is 10-degrees C.

We are saying that Rotary Lobes would induce a higher shear to the ML, shearing the flocs recycled to the anoxic basins & sludge holding tank. We are saying 1.5% isnt really a solids application since the liquid is still pretty much Newtonian.

Can anybody give me any references/rules of thumbs for densities or viscosites of ww liquids having a similair solids content. Also, are there any published rules of thumbs stating typical maximum viscosity values for centrifugal pumps?

Thanks in advance.

 

[bimr]

Rotary lobe pumps are not necessary for your application.

Here is a reference:

Design of Municipal Wastewater Treatment Plants published by WEF

The first classification of fluids is dilute sludges. Typically wastewater activated sludge at concentration less than 2% falls under this classification. Centrifugal pumps operate successfully with this concentration. Thixotropic behavior is not a problem with dilute sludges.

Thickened sludges have higher concentrations are typically produced though a mechanical thickening process. These sludges can be characterized as having a significantly higher viscosity and are not reliably pumped with centrifugal pumps.

As sludge is dewatered, thixotropic behavior begins to predominate.