submersible centrifugal vs dry pit centrifugal pumps 1

[rpclayman]

Designing wastewater treatment plant for non ferrous metal processing wastewater treatment in China. The Chinese typically prefer submersible pumps vs dry pit pumps. The tanks that submersible pumps will set in will also have large mixers running at over 100 rpm with a high rate of tank turnover. On the face of it the submersible seems to be a better choice, low NPSH, lack of cooling problems. My concern is air intake. Thoughts on this appreciated.

 

[Lan123]

Can you share a sketch of your proposed arrangment with us.

 

[rpclayman]

I uploaded diagram to ENGINEERING.com. The name of the file is "nonferrous_metal_EQtank.jpg"

 

[Artisi]

attach the link to a posting, makes life easier and saves us the problem of searching for it.

 

[QualityTime]

Air entering the submersible pump (or dry pit pump for that matter) is a good possiblility if the water levels in the tank vary. The mixer will introduce air into the process when the water levels in the tank drop too low. submersible mixers perform differently with different working water levels.

For a similar situation, see thread407-276105 .

 

[rpclayman]

Artisi(and others)

Please see URL for diagram

 

[Artisi]

nothing there.

 

[Lan123]

Correct. Link is not working.

 

[rpclayman]

Sorry about the busted link. This link works.

 

[Artisi]

Whether you have submersible pumps or dry pit pumps and a mixer in a common sump you run the same chance of air being beaten into the pumped product ending up in the pump.

If entrained air looks to be a problem the solution is to ensure the mixer is always submerged - however,this in itself may not be cure, even submerged mixer can entrain a lot of free air from the surface, the second solution is to shut down the mixer at a pre-determined sump level so as not to entrain air - alternately use both, keep the mixer submerged and shut it down when air is starting to be entrained.

I have seem a change of direction of the mixer eleviate an air entrainment problem.

Mixers and pumps can be a very poor combination, especially in small sumps.

 

[rpclayman]

Artisi

Thanks for this great tip, I thought as much so I am going to go ahead and make necessary design changes based on your input.

 

[Artisi]

One other point you need to consider is the inflow, if it is allowed to free-fall into the sump this can also drive air into the pumped product - try and discharge via a pipe if necessary below the sump working level.

 

[QualityTime]

Your mixer manufacturer will advise you what the minimum working water level in the tank should be to prevent air being entrained in the water. The blade design greatly affects things. You really have to talk to the mixer manufacturer. There is a lot of science behind the mixing


For a similar situation, that was solved, see thread407-276105: Centrifugal Pump Cavitating on tank changeover .

 

[rpclayman]

Artisi and Quality Time

A great lesson in troubleshooting pump noise, air entrainment problems from mixers, NPSH and so forth Based on this education, going to use variable speed centrifugal pumps, keep the EQ tank large and deep and also going to use submersible pumps. Going to independent mixer expert to review mixer selection recommended by sales engineer.

 

[Artisi]

Guess you also need to review mixer effect on the sump and the level at which it is installed / stops / starts etc. If the mixer shuts down a too high a level will it keep the lower level of the sump agitated and the solids in suspension?

 

[QualityTime]

You are welcome...try Chemineer....they have some engineering depth