sewer pump wet well sizing
sewer pump wet well sizing
(OP)
Hi everyone,
We're in the planning stages for a project in the middle east, mixed residential and commercial (~60% of the flow is residential). The downstream sewer flow has to be pumped across a bridge.
My guess is that you would size the pump for the max day flow (approx 2 x average flow), and size the wet well to store the peak flow in excess of the max day flow. When I sketch a simplified diurnal curve (simplified to triangles), it looks like the necessary storage would work out to about 6 hrs x (2 x average flow)/2 = 6 hrs x average flow. (The diurnal curve only applies to the residential portion, I know, but it would be more conservative to treat it all as residential, which has a higher peak factor. Or maybe it should be considered, since they peak at different times, and would therefore reduce the peak hour flow significantly?)
Another option would be to have two pumps that each handle average flow (and would therefore both switch on for max day flow), plus an additional emergency pump, and store the excess peak flow. I think there's also usually an allowance for emergency storage.
How should the pump and wet well be sized? I don't have experience with sewer pumps and wet wells, so would like to know how those of you who do have experience would do it.
Thanks,
entp
We're in the planning stages for a project in the middle east, mixed residential and commercial (~60% of the flow is residential). The downstream sewer flow has to be pumped across a bridge.
My guess is that you would size the pump for the max day flow (approx 2 x average flow), and size the wet well to store the peak flow in excess of the max day flow. When I sketch a simplified diurnal curve (simplified to triangles), it looks like the necessary storage would work out to about 6 hrs x (2 x average flow)/2 = 6 hrs x average flow. (The diurnal curve only applies to the residential portion, I know, but it would be more conservative to treat it all as residential, which has a higher peak factor. Or maybe it should be considered, since they peak at different times, and would therefore reduce the peak hour flow significantly?)
Another option would be to have two pumps that each handle average flow (and would therefore both switch on for max day flow), plus an additional emergency pump, and store the excess peak flow. I think there's also usually an allowance for emergency storage.
How should the pump and wet well be sized? I don't have experience with sewer pumps and wet wells, so would like to know how those of you who do have experience would do it.
Thanks,
entp





RE: sewer pump wet well sizing
Steve Wagner
RE: sewer pump wet well sizing
As a general overview process, you first determine both the average and peak flows (in GPM) and you will use them to calculate pump cycling frequency.
Through an iterative process you try different wet well diameters (usually 4' 6' or 8', etc.) to arrive at a pumping frequency that is commensurate with pump manufacturer specs. The depth of the wet well is a function of your physical constraints, i.e., ground elevation, incoming pipe elevation, downstream conditions, storage depth in the wet well and the dimensions of the pump you are specifying.
Then using Hazen Williams, you analyze the hydraulics, trying different outfall pipe (force main) diameters (4, 6, 8 or 10-inch, etc.) and taking into account length of outfall pipe, TDH, minor friction losses, hydraulic slope and velocity (2-5 FPS). Then you utilize manufacturer's specs and pump performance curves to zero in on a model of pump that satisfies TDH, peak flow capacity (GPM) and pump efficiency.
That's it in a nut shell.
RE: sewer pump wet well sizing
We're not designing it in detail yet, we just want to know what flow to design it for. It sounds like you guys are saying that the pumps should be designed to pump the peak flow (~4 x average flow), and not the max day flow (~2 x average flow) that I had guessed.
So you're always pumping it as fast or faster than it comes in, and are *never* letting it lag. ie. It is never pumping and filling up the wet well at the same time, unless there is an equipment failure. Is that correct?
(I was previously thinking that you would store the excess flow while it peaks beyond the pump capacity).
Thanks,
entp
RE: sewer pump wet well sizing
Steve.
RE: sewer pump wet well sizing
RE: sewer pump wet well sizing
RE: sewer pump wet well sizing
Teddy