Reversing Direction of Flow in Water Main 1

[mflam]

We are planning a water project in an urban setting. We will be taking a section of a large water main out of service. The temporary water service will result in the direction of flow in some streets being reversed. What are the risks? How can this be assessed in advance? What can be done to reduce the risk?

 

[77JQX]

I wouldn't worry about reversal. I would worry that increased velocities around the closed pipe will stir up sediment and result in dirty water complaints. I would model the before and after velocities, and consider a cleaning program (unidirectional flushing) prior to taking the large main out of service.

 

[fel3]

mflam…

Assuming a strongly looped and well sized distribution system with relatively low head losses, it is very possible that the flow in the pipes you are concerned about already reverses during emergency conditions (i.e. fire flows) and maybe even during normal operation.

Water systems are typically modeled for extreme conditions to establish an envelope of operation. But, what happens in the middle of the envelope? It is likely that flows sometimes reverse in the loops as demands continuously vary.

For modeling purposes, demands are usually assumed to be consistent by type, location, and time, but in reality demands continuously vary. While assumed consistency suffices for global modeling, it is not actually the case on a connection by connection basis or a neighborhood by neighborhood basis. For example, I can guarantee that your water use pattern and rate is as unique as your next-door neighbor's, and this applies system-wide. A neighborhood with lots of school-age children and working adults will probably show a much stronger morning peak than a neighborhood full of retired people. These little nuances can add up to temporary reversal of flow in a looped pipe from the normal direction. Only the dead ends are guaranteed to have one direction of flow.

Fred

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[bimr]

Most water mains are sized for fire flows, not for the normal service water flow. The normal service water velocity is probably around 2-3 ft/sec which over time allows sediment to deposit in the water mains.

Check the pipe velocities of the current layout and compare the velocities to the proposed layout. If the velocities are similar, you don't have much to worry about. You will typically flush out the water mains after you make the connections and chlorinate.