Maximum Velocity in Gravity Sewer
Maximum Velocity in Gravity Sewer
(OP)
Has anybody come across a gravity sewer with very high slopes and velocities (approximatley 10m/s). Can it be done?
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Maximum Velocity in Gravity Sewer
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RE: Maximum Velocity in Gravity Sewer
High velocities in small pipe sewers and the corresponding low depth of flow may allow large objects to become lodged in sewers such that the next rush of wastewater may not dislodge the objects.
RE: Maximum Velocity in Gravity Sewer
Proposed pipeline is a 450mm OD PE100 pipe with an ID of approx. 397mm. I assume that erosion/abrasion would be the biggest concern but is there any data/literature/recommendations i could use.
RE: Maximum Velocity in Gravity Sewer
RE: Maximum Velocity in Gravity Sewer
RE: Maximum Velocity in Gravity Sewer
The Civil Tools software calculates a maximum velocity of 7 m/s in a "vertical" pipe, so what you are asking for does not seem to be possible.
RE: Maximum Velocity in Gravity Sewer
RE: Maximum Velocity in Gravity Sewer
Here are a couple of references. The piping handbook states that you can use a velocity of 7.6 m/s with FRP pipe. Steel would not be acceptable. Of course, the amount of sediment is not specified in that reference and that will have a major bearing on the potential for erosion.
As the PVC manual states, you should consider energy dissipation, erosion, and anchorages.
RE: Maximum Velocity in Gravity Sewer
RE: Maximum Velocity in Gravity Sewer
"For clear water in hard-surfaced conduits, the limiting velocity is very high. Velocities in excess of 40 fps (12 m/sec) have been found harmless to concrete channels." (It is not stated whether those were actually somehow measured velocity, or back-calculated from someone's slope and formulae assumptions). From what little information that has been supplied here, I'm not really sure that applies to your service. I will note however that that phrase is followed immediately in that manual by:
"Erosion of inverts may result from much lower velocities when sand or other gritty material is carried." [It then goes on to say that in continuous high velocity flow conditions where grit erosion is expected to be a problem the limiting velocity often is taken to be about 10 fps (3 m/sec)]
While I guess your "drainage" situation might well justify greater velocity particularly if same is infrequent, it may however behoove one to be a little cautious. Many months ago I happened to hear a comment from a large utility contractor I know who had just installed a quite large polymeric drainage pipe in new construction. After the pipe was installed but before they left the job they had a couple quite significant rainfall events that resulted in large flows. He told me that after these couple events, but even before they left the job, he saw evidence of some wear on the bottom of the specified pipe. [I guess therefore maybe the claims of abrasion resistance of softer pipes to harder particulates could conceivably depend on the size of the rocks, and also whether they kind of bounce or instead sort of grate along the bottom of the pipes in the specific application!]
Many manuals and standards incidentally also suggest or require anchorage on steep slopes that can be a little trickier to dependably apply with plastic pipes. Also, buried plastic pipes normally require good bedding with specific/select material, and steep slopes can sometimes result in not only erosion of pipe on the inside, but also of bedding on the outside.
I think the quite knowledgeable Mr. Bimr may incidentally in his 9:35 response have been referring to a sort of "terminal velocity" of flow that is apparently reported to occur in substantially vertical drain stacks (in perhaps quite non-obvious fashion) e.g. as discussed at http://xnet.rrc.mb.ca/rcharney/interior.htm . While obviously at least not directly applicable to your situation, note that this particular reference claims this terminal velocity may be only about 10 fps in 3" stacks, and this reference happens to show a tabulated "capacity" of 12" stacks at I figure is based on not far from this velocity in even that larger pipe.
RE: Maximum Velocity in Gravity Sewer
Virtified tiles and glazed bricks 4.5-5.5
CI sewers 3.5-4.5
Stone ware sewers 3-4
Cement concrete sewers 2.5-3.0
Ordinary brick-lined sewers 1.5-2.5
Earthen channels 0.6-1.2
You should try to construct drop MHs and energy dissipators to break the Vs in case the more than above.
RE: Maximum Velocity in Gravity Sewer
Richard A. Cornelius, P.E.
WWW.amlinereast.com