Pumping Rate Discharge Transition to Gravity Flow

[stupickles]

I am designing a sewer project with 3 pumping stations that all tie together but are 3000LF + apart from one another.
My the most upstream pumping station is designed for the EDUs that gravity to it.

Should the middle pumping station be designed for the EDUs that gravity to it directly plus the upper pump station pumping rate even though the forcemain discharge point to the middle pump station is 2000+ LF away? At some point, gravity takes over?

This expands to the lowest pump station also. The direct EDU flows plus the pumping rate for the middle pump station makes for extremely large pumps.

If I calculate the TOTAL EDUs (i.e. population) for the lower pump station, the size is a third compared to using the pumping rates.

 

[LittleInch]

I can't work out your system from the words. Can you sketch the system, scan and post and add suitable levels and flows to make it easier to comment. Also don't assume we all know or share your abbreviations. Thanks.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[stupickles]

Sorry. I attached a quick hand drawn flowchart. I am looking for a theory answer only .
Basically my question is, should pump station #2 be sized for the 200 homes which equates to 60GPM plus the pumping rate of pump station #3? The discharge location for pump station #3 is over 3000 feet from the next pump station. Would the gallons per minute (GPM) be the same entering at pump station #2?

 

[LittleInch]

It certainly looks like you need PS 2 to be PS 3 plus your 200 homes to me assuming that the levels are such that PS#3 can't flow by gravity to the man hole beyond PS#2. In that configuration PS3 is just a big input ( a super sized home if you like) into the PS 2 inlet flow.

If you can flow past PS2 by gravity then you might be able to have individual pumps, but we would need much more info on the profile and invert levels of the pumps, pipes and man holes.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[bimr]

No, gravity does not take over.

You have to design the middle pumping station for the EDUs that gravity to it directly plus the upper pump station pumping rate.

Note that you should be using some type of peaking factor for the design peak hourly flow. For the sewer flow from: 200 homes * 350 gallons per day per home * 4 (daily average flow to peak hourly flow (peaking factor)) = 226 gallons per minute out of the sewers.

Because your project is relatively small, you need a slightly larger peaking factor (approximately 4) as shown in the attached table:

http://ilga.gov/commission/jcar/admincode/035/03500370ZZ9996dR.html

The peak flow from a lift station is different than the peak hourly flow from the sewers entering the lift station. You have to consider the pump cycle times and lift station retention. If you use VFD's the peak flow from the lift station may be lower than that for a lift station with fixed speed pumps.