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Issue with existing 6" force main being too large

Issue with existing 6" force main being too large

(OP)
I am working on a small grinder pump tank design for a sales office. Daily design flow is only 150 gpd (6 employees x 25 gpd). The issue that I have is that we have an existing 6” force main that discharges into a gravity sewer and it appears that it will be challenging to get 2 ft/s scouring velocity in this pipe. The municipality requires the use of 2” service pipe from the pump tank to the force main. However, there is only one house that is currently connected to the existing 6” force main with no other pumps connected to it.

We don’t have much information about this existing pump and the owner would not let us open up the tank and see what pump model number was used. We also could not locate any drawings for the existing 6” force main that was installed. The town engineer who designed the system is deceased and his wife has gotten rid of the drawings/calculations that he kept. However, we do believe that the existing pump is operating at a discharge rate of 30 gpm since that is the pump discharge that the municipality requires for residential buildings. If we use 30 gpm pump discharge rate for our proposed system, then we will have no issue with scour velocity in our proposed 2” pipe (it will be 3.06 ft/s). However, our issue is with the existing 6” force main. If our pump is the only one that is pumping, then we cannot obtain scouring velocity of 2 ft/s in the 6” force main unless we increase the size of 2” service pipe and/or use excessively large pump discharge rate of somewhere in the range of 176 gpm.

One engineer who ran into this issue before, where the force main was too large, said that the issue was resolved by periodic scouring in the line. I have contacted the state and was told that they have no minimum scour velocity that they will accept if it is below 2 ft/s. The state has suggested designating 6" force main as a high priority line that will require periodic inspection and cleaning. However, we don't think that there are flushing ports in this line and are unsure if they can be easily installed after the fact.

Our TDH is very low (less than 15 ft) and if we upsize the service line and use 176 gpm pump discharge rate, it seems like overkill to pump out 150 gallons just to get that 2 ft/s scour velocity. Public works director did mention that the residence connected to this force main had issues with control panel and pump but no issues with the force main itself.
What are our options here? Has anyone here ran into this issue before and if so how did you resolve it and did the solution work?

RE: Issue with existing 6" force main being too large

The 2 ft/s scouring velocity that you are using is for a continuously operating forcemain. If the forcemain stops, you will need approximately 3.5 ft/s scouring velocity to resuspend the solids in the forcemain.

You do not mention the length of the forcemain. Is it possible to install a pressure sewer system instead of using the forcemain?

In situations such as this, it is common to install two forcemains. One is smaller for initial operations.

RE: Issue with existing 6" force main being too large

(OP)
The length of the proposed 2" force main will be around 152 feet. The length of the existing 6" force main is around 775 feet from the point where the 2" tie-in to the manhole. I've ran analysis in WaterCAD and it just isn't feasible to get good enough scouring velocity. With just one pump on I get about 0.3 to 0.4 ft/s scouring velocity in the 6" force main. When they are pumping simultaneously I get about 0.8 ft/s.

It is not possible to install another force main since the town does not want to pay for it. The state review agency has suggested installing flushing ports and designating this as a high priority line that will have to be periodically inspected and cleaned. I could not locate a permit number for this existing force main so it looks like it was never permitted, which would explain the issues that I'm having.

Is anyone aware of a website with some generic details of flushing ports/drain valves for an existing force main?

RE: Issue with existing 6" force main being too large

Is there any chance you could simply insert a 2" line into the force main and run along inside up to the connection to the gravity sewer?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Issue with existing 6" force main being too large

For oonly 775 I would flush that 6" line to get it as clean as possible and push a smaller line into it. EssentiallyuUse the 6" as a casing pipe. Then if you ever need a larger force main you could just pull the smaller pipe.

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