Sewer Lift Station: steep slope inlet pipe or inlet free fall?

[CrimsonEngineer]

For a sewer lift station with duplex centifugal pumps in the wetwell I am unsure of the best option for the gravity inlet pipe. I only have about 20 LF of pipe between a screen plant and the wetwell. I want to avoid turbulance in the wetwell created by the gravity inlet. I can either have the inlet pipe at a mild slope to decrease inlet velocity but have a 4-ft freefall or have a steep slope (25%+) with the IE at the pump off elevation. In one scenario I have a free-fall which may entrain air and in the other scenario I will have a high inlet velocity(12 fps+) which could create turbulance.

My thought is to avoid free fall and go with the steep slope and install a baffle on the outlet to reduce turbulance/swirling. Any other ideas or thoughts? (The wetwell is going to be 10-ft dia., inlet directed to center of the two pumps).

 

[jgailla]

In all the lift stations I've designed the invert elevation determined the depth of the wetwell. The invert should be at the pump on level, minimizing the drop. Is your wetwell existing, so that you cannot have a shallower wetwell? If you cannot change the wetwell design, I would put an open pipe vertically below the invert and use a baffle to direct the flow into the open pipe. I've used a large reducer for a bowl at the top of the vertical pipe to catch more of the flow. I wouldn't be too worried about a 4 foot drop except for the odor issue. You will likely have erosion issues with a 25% slope.

 

[Artisi]

Any free-fall is likely to entrain air into the sump- avoid free-fall at all costs.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[piffer]

Reliner / Duran Inc. makes "drop bowls" which help alleviate air entrainment. Do a quick internet search.

 

[CrimsonEngineer]

Thanks all for your replies.

Jgailla - This is a new wetwell so we can design the inlet and wetwell as needed. The need to provide emergency storage is the driving factor for the depth of the wetwell in this case and the proximity and elevation of the upstream screen plant dictates my elevation drop.

The "drop bowls" look like a good option to reduce air entrainment but is there any advantage to design a drop inlet rather than a lower steep pipe inlet (other than some cost saving on pipe excavation)?

It seems the steep pipe may be better to reduce air entrainment and the inlet velocity might be similar in both cases because of the drop.

Does anybody have experience with any problems because of steep inlet pipes into the wetwell. Would pre-swirling be an issue in a 10' dia wetwell with the pipe aligned with the center of pumps?

 

[KiwiMace]

You can also branch the inlet pipe into the wet well. One branch goes high, the other to your normal flow driven height.
Run the pipe at typical slope and step down at a 45deg close to the wet well.

 

[KiwiMace]

This is what I meant above. A Tumbling Bay.

 

[bimr]

You would probably get better advice in the waste forum.

The 12 f/s velocity is way too high. The 4 ft drop is probably not a problem but if you are concerned about it, install a drop manhole before the lift station.

Get yourself a copy of Pumping Station Design by Robert L. Sanks before you do any design.

This subject is covered in depth in this comprehensive book.

http://www.amazon.com/Pumping-Station-Design-Second-Robert/dp/0750694831

This book should be on the shelf of every sanitary engineer.

 

[bimr]

Here is what you are looking at.

 

[stanier]

ANSI/HI 9.8 standard is a document worth reading. Also the US Army Corps of Engineers have done a lot of work on pump station design. This is freely available on the Internet.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

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