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Force Main Relocation Issue

Force Main Relocation Issue

Force Main Relocation Issue

(OP)
We are relocating a force main to higher capacity gravity sewer system. The issue is that the proposed route starting at the PS (Datum 0') would have to pump 7,000 lf to the top of the hill at Datum 55' then would have to flow back down to 5' (2,000 LF) to where it would discharge to the high capacity gravity sewer. The estimated flow rate is approximately 500 gpm with a proposed 10" force main to allow for future pump station expansion capacity.

The potential scenarios are as follows:
1) Install Vacuum Valve at Top Hill
2) Install 6" main from top of hill to discharge point to maintain positive head
3) Discharge into existing 8" gravity sewer running at 2.5% (720 GPM Capacity) that flows to a larger 24" sewer
4) Discharge to gravity sewer at top of hill and upsize gravity sewer.

With scenario 1, I have read into potential issues from the previous threads and do not think its advisable

With scenario 2, reducing to a 6" pipe severely restricts my future expansion ability and also seems counterintuitive after installing 7000 lf of 10" forcemain

With scenario 3, i am concerned about backing up the discharge manhole although the 8" gravity sewer has the capacity at the existing grade.

With scenario 4, I am concerned about the additional cost plus the gravity sewer (although shallow) runs through some high profile properties.

Any other thoughts or discussion on the options would help, thanks

 

RE: Force Main Relocation Issue

In the long run Scenario 4 will be cheapest. I'd go with 4, then 3, then 1. Ultimately it's the utility company's decision.  

RE: Force Main Relocation Issue

Scenario 3 appears to be the most cost effective option. Force mains are less expensive to construct than sewers and have less maintenance.

RE: Force Main Relocation Issue

No. 4 seems the more practical approach.

RE: Force Main Relocation Issue

(OP)
thanks for the replies, option #3 is looking the best so far

Does anyone have any information on how to model a manhole once the force main starts discharging into it? Just want to make sure it does not surcharge when experiencing higher flows

We took into account the lowest sloped 8" sewer downhill, which was listed above at 2.5% (capacity varying from 700-900 GPM, depending upon manning #)

 

RE: Force Main Relocation Issue

If you are concerned about possible surcharging, think about upsizing the sewer by one size. It is relatively inexpensive.

RE: Force Main Relocation Issue

(OP)
it is just that there are several other utilities weaving in and out of the existing gravity sewer

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