gravity flow rates
gravity flow rates
(OP)
I'm back checking the flow rate from a building with a gravity line connecting to a manhole. The flow rate I have developed from the building is giving me a less than 2 f/s velocity in the pipe. Should gravity sewer pipes be sized for the peak flow rates and checked for the daily to meet a 2 f/s velocity?
RE: gravity flow rates
https://www.broward.org/WaterServices/Engineering/...
RE: gravity flow rates
Every jurisdiction I have dealt with in California sizes sewers to flow half full at peak dry weather flow. For examples, the City of Los Angeles: https://eng2.lacity.org/techdocs/sewer-ma/f200.pdf, Section F250. They also want a minimum velocity of 3 fps (Section F253). Some agencies, such as the City of San Diego, allow trunk sewer to flow deeper, but typically no more than 75%. The City of San Diego Sewer Design Guide, Section 1.3.3.3, says, "New sewer mains 15 inches and smaller in diameter shall be sized to carry the projected peak wet weather flow at a depth not greater than half of the inside diameter of the pipe (dn/D not to exceed 0.5). New sewer mains 18 inches and larger shall be sized to carry the projected peak wet weather flow at a depth of flow not greater than 3/4 of the inside diameter of the pipe (dn/D not to exceed 0.75)." (https://www.sandiego.gov/sites/default/files/legac...)
I don't know if Los Angeles set these criteria because it has a lot of combined sewers (it's been more than 30 years since I dealt with any sewers in LA), but I see the same standards in other jurisdictions that don't have many (if any) combined sewers.
The moral of the story is: check with the agency having jurisdiction.
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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
RE: gravity flow rates
It must be recognized that decreased slopes may cause additional sewer maintenance expense and special linings or materials should be considered for corrosion protection.
However, you appear to be dealing with a service sewer. Service sewers are the drains that run from the building to the municipal sewer in the street. Service sewers generally have a minimum slope of 1%. You should check this service sewer requirement with the local AHJ.
RE: gravity flow rates
The answers in previous posts point out some guidelines. Worst case: what happens if the receiving municipal pipeline gets clogged or run full?
RE: gravity flow rates
RE: gravity flow rates
Most municipalities require a minimum size sewer service for a residential development to be 6-Inches in diameter and the minimum size sewer main for commercial and industrial developments to be 8-Inches in diameter.
The reason for over-sizing service sewers is for ease of cleaning.
RE: gravity flow rates