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Shawner (Civil/Environmental) (OP)
14 Sep 11 23:14
What velocity would I look for to achieve a proper scouring in a water main?  2.5 ft/s?  5 ft/s?

Any links?

Thanks!
Helpful Member!  bimr (Civil/Environmental)
14 Sep 11 23:29
The velocity required to suspend and flush out the deposits depends on particle size and specific gravity. Although most small animals are of low specific gravity (about 1), inorganic deposits may have a specific gravity of up to 3. Table 4.4 provides the volumetric flow rates required to transport loose particles of 0.2 mm diameter.

http://www.who.int/water_sanitation_health/dwq/en/piped4.pdf

A typically flushing velocity of 3.0 to 3.5 ft/ sec will normally suffice.  
coloeng (Civil/Environmental)
15 Sep 11 10:16
Of course with small water lines it is fairly easy to get the velocities necessary to scour them.  As your mains get larger, the large volume of water required to achieve the scouring velocities may be difficult to manage.

For example an 8-inch main at 3.5 fps will give you a flowrate of 548 gpm, which is fairly easy to get rid of.  For an 18-inch main, 3.5 fps will give you a flowrate of 2775 gpm, which becomes more difficult to dispose of.

It is not always practical to get scouring velocities is larger pipes and you may have to live with the fact that there may be some sediment in the pipe.  This is why it is very important that the pipes are kept as clean and free of debris as possible during construction or repair.
Shawner (Civil/Environmental) (OP)
15 Sep 11 17:41
I drew up a quick spreadsheet on excel for the different values (attached), does this seem correct?

 
Helpful Member!  zabrab (Civil/Environmental)
22 Sep 11 20:48

Minimum flushing velocity per AWWA C651 is 2.5 fps so 3.5 fps is a good target for public mains. There is a table in the standard that indicates flows needed for 2.5 fps and the size/number of taps needed at a pressure of 40 psi.

For private fire mains, flushing velocities may potentially need to be 10 fps or higher (about 1,560 gpm vs 400 gpm for 2.5 fps).

No direct links, but you can do a search for "AWWA C651 Flushing" and find specifications or guidance documents referencing C651 for the stated flushing requirements. For private fire lines, NFPA 24 is available for viewing if you register (free) at the NFPA site.

 
ch81pc (Civil/Environmental)
6 Jan 12 19:27
Without over engineering - 3 ft or 1 meter per second is the rule of thumb generally used.

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