Would it be wise to design multi-stage non-clog sub. sewage pumps? 1

[CrazyHorse81]

This is a spin off of my last thread in the Pump Engineering Forum "Non-clog low flow high head pump." My original question was to find a pump that could perform at 314' TDH and 170 gpm flow. So this is my follow up question.

Why don't pump engineers/mfg's design a multi impeller non-clog submersible sewage pump? Would it be unfeasible? Is it such a unique situation that only one or two would be sold per year worldwide? Is it in the design of the impeller and its application? And why can't these impellers be more efficient at lower flows like like 80-percent instead of 35-50%.

Maybe I should find a pump engineering training course for my IDP (individual development plan) here at work...

 

[Artisi]

Yes, a pump hydraulics training course would be a good idea, this will give you a better understanding of why multi impeller solids handling pumps are rare and why low flow solids handling small capacity sewage pumps are low in efficiency.

 

[micalbrch]

It will make sense to develop such a pump, if there are no other type of pumps which can do that. "Submersible" is something exclusive for centrifugals. But flow, head and sewage fit for a lot of positive displacement pumps and these pumps - if properly sized - have a rather high overal efficiency. And most of them can lift a suction head, too (and then do not have to be submerged).

 

[Slagathor]

Solids Handling Pumps (don't say non-clog...ANY wastewater pump under 8" can and will be clogged eventually) utilize overhung impellers, blunt vanes, and rounded hubs.

This is to prevent the accumulation of stringy material. Subsequent stages in multi-stage pumps would have an impeller mounted between bearings, with a shaft through the eye. This is considered too risky for solids handling services, from a solid passing perspective. That is why no one designs multi stage solids handling pumps.

The need for high heads (in conjunction with the poor solids handling characteristics of very low Ns/skinny impellers) leads station designers to utilize single stage standard pumps in series. Luckily, this situation does not arise too often. It occurs most often in areas where wastewater must be pumped to a central interceptor or regional WWTP, in a very hilly area..some areas of the Western US for instance....

 

[GaTechTheron]

Depending on the solids, you should look into a progressive cavity.

 

[micalbrch]

Depening upon solids concentration and size (fibres) you should look into a PC pump. That is more precise. Although you have to find a solution for the rotor shaft sealing. It will be submerged, too.

(Raw) sewage pumps are usually centrifugals. They can be submerged and are still the best choice.

 

[FACS]

I might be missing something, but... don't they? Or do they need to?

I have a grinding station in my front yard. Every two houses has one. It grinds up the solid waste so it can be pumped efficiently to the treatment plant.

Charlie

http://www.facsco.com

 

[Artisi]

Facs, that's a solution to the problem and allows you to use low flow high head pumps (limited solids handling capabilities) for raw sewage applications.
However the question was about designing a multistage low flow high head raw sewage pump, and anyway CrazyHorse81 seems to have vanished.

 

[Artisi]

Slagathor, Not true about sewage pumps under 8" eventualy clogging. torque flow impellers, and "Shearpeller" from the old Allis Chalmers pump range - now Goulds, will handle what ever comes thru the inlet without clogging including long stringy "stuff".

 

[CrazyHorse81]

Grinders are only allowed in our designs in special and specific circumstances...not an option in this case. A big problem that we have to deal with is that the O&M is virtually non-existent.

And how did you know that I vanished Artisi? I guess I don't live and die at Eng Tips. I went on a two day trip to visit the lift station that we are replacing. It is a mess...there are three phase pumps running off of vfd motors powered by single phase...one pump is running constantly...one pump is out of order...the wet well is full of rags paper rocks sticks anything you can imagine...there is approximately 1/2 of the wastewater flow going into the primary cell and the remainder is overflowing...

 

[Artisi]

A sump full of rags, paper, rocks, sticks and probably a couple of dead dogs could probably be classified as a special case for grinders or other pre-treatment. Could be as easy as self cleaning trash racks - but not my field of expertise so suggest you widen the net a little for information.