Wet Well Design: Negative Head

[Trackfiend]

I am trying to design a sewer line for a new public restroom. The only option is to install a wet well and force main to pump out the effluent (due to existing features a gravity line is not possible). Here are all of the knowns:

Proposed Wet Well - TOG = +16.0', Inv = +10.0'
Existing Manhole - TOG = +10.0', Inv = +0.7'
F.M. Discharge Inv = +3.0'
Length of F.M. = 161'
Dia. of F.M. = 4"

In trying to keep the cost of wet well down, the invert of sump is +10.00' (TOG = +18.00'). The nearest sewer manhole that can be discharged into has a invert of +0.7(TOG = +12.00'). The discharge line in the existing manhole will have an invert of +3.0'. It should also be mentioned that the F.M. needs to go over a floodwall with a TOC elevation of +21.00'. Running through the Hazen-William's equation with inputs for all head losses through the pipe, I still get a negative head of -7'. How do I approach this problem? Is the static head the difference between the invert of the sump and the invert of the disharge line into the manhole or the top of the floodwall? How do I pick a pump to figure for negative head?

I've attached my excel sheet to highlight the approach I've used. Please refer to the sheets "Wetwell 2-pumps", and "Foster's". Thanks for any input into this matter.

 

[Trackfiend]

To make it simple, is the static head measured to the highest point in the dishcharge line or to the water level in that manhole where the F.M. is discharging to?

If I do have negative head, then do I extrapolate the system head curve on the pump chart?

 

[bimr]

The static head is the difference between the high point of the discharge force main and the low water level in the wet well.

If your force main has to go over the flood wall at TOC 21', the static head is 21' - 6' (6' is the assumed low level in wet well). That would give you a static head of 5' (21-6 = 15').

 

[dicksewerrat]

How much flow are you expecting from this restroom. A 4 inch FM seems a little on the large size. I'd look at a about a 1 1/2 to 2 inch grinder pump and F.M. the same size.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Trackfiend]

My apologies, the TOG for the wet well and existing manhole should be +18.0' and +12.0', respectively.

The client didn't give any info on expected flow but there are 16 fixtures in the restroom, 6 toilets, 1 urinal, and 9 faucets. From there I estimated 325 gal/day average use. Using a peaking factor of 4.5, the peak flow rate is 23,400 gal/day or 16.25 gal/min.

bmir - I assuming you mean +10.0' is the low level of the water in the wet well? That would give an initial static head of 11.0'. I was taught that the static head difference is the low level of water in the wet well and the low level of water in the discharge tank. I understand that the pump needs to generate enough head to overcome the high point in the line, but in order to size the pump and highlight the operating point on a given pump curve, shouldn't I be using the overall static head? This would be the difference between the wetwell and wall (21 - 10 = 11) and then the difference between the wetwell and manhole 10 - 3 = 7', so 11' - 7' would be 4' of overall static head. Please correct me if I am wrong.

 

[bimr]

The static head is the difference between the high point of the discharge force main and the low water level in the wet well. Static head is the highest pressure difference that can be expected to pump against.

If you size the pump for the low level in the discharge manhole, you may not have enough power to turn on a pump when the discharge manhole is at the high level. (This assumes that it is possible to have a high level in the discharge manhole.)

Static head is not measured from the low level in the discharge manhole. It would be the high level in the discharge tank or the highest elevation of the force main, whichever is higher.

You say that the sewer invert into the wet well is 10'. Your operating water level in the wet well should be below the level of the 10' invert. I assumed that you operate with 4' of difference between the high point 10' and the low point 6'. So the low level in the wet well is 6'.

The high point in the force main appears to be the point where you discharge over the flood wall, which you stated was 21'.

Therefore the static head is 21'-6' = 15'.

 

[bimr]

Most wastewater standards require a minimum force main size of 3" diameter with a grinder pump and 4" diamter without a grinder pump.

I am not following your flow calculations, but the worst case scenario would be 1 gpm per fixture unit. This is about the flow that you have calculated.

 

[Trackfiend]

I believe that I've got my static head and discharge head mixed up. Thanks for the clarification. Much obliged.

 

[redbridge]

I agree with dicksewerat. I have designed several projects with 1.5" and 2.0" force mains.