How to Keep Active Water Supply When Replacing Main!?

[xjken]

I can't figure this out...

Here's the situation:
I have a building that MUST keep its water main active with the normal pressure and volume, while the main is replaced.
The building is long and narrow and there is a 6" water loop that goes through the building lengthwise. In other words, the building is fed from a 6" pipe going into the north and south side of the building. The pressure at the inlet to the building is 115 psig.
The building is 5 stories. There is approx. 680 toilets and lavs and 40 showers.

Heres the problem:
The main can be shut down from one side and still feed the whole building from the other side. However, when several toilets are flushed at once, all the flush valves let loose and all toilets begin flushing. I assume its due to lack of pressure to keep the valve shut.

An idea came up to provide a temporary connection from a fire hydrant. We would shut one side down, connect the temporary line into the main, then begin working on that side.

My task is to figure out if the fire hydrant would provide enough pressure and volume to match the existing conditions, or would a pump and/or tank need to go in this temporary line?

Any ideas or help would be great!
thanks.

 

[bimr]

Why don't you replace the main on the side giving you the low pressure first, then replace the main on the side giving you the high pressure second.

Don't understand why this is an issue. Bring a new main up to the building (alongside the existing main). Then hot tap into the the existing main.

 

[xjken]

its hard to descibe.
The main outside of the building is not being replaced, just the one inside. It runs in a trench through the length of the building.
We have the new main installed above the existing one in this trench. There is a shutoff right outside of the building and there is a valve about 8' down the line inside the building where the prv is. We plan on closing off the outside and inside valves, then cutting that section of pipe out and install the temporary connection to the inside main. Now we can open up that valve and the main inside the building will be supplemented with this temp connection. Now we can do the work to connect to the new main above.

 

[semo]

Why are you replacing the main in the first place. Leaking, undersized? If it is undersized, just lay an additional line along side it and hot tap it in then use both lines.

Another scenario could be to hot tap the new line at the ends, then install cut-in (hydro-stop) valves at the tapping tees to isolate the old line.