3 m/s is probably the maximum recommended velocity. A maximum mean velocity of 3 m/s at the design depth of flow will not damage the 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.
bimr, I should clarify.
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.
I don't have the M&E book handy but I presume that have a recommended maximum velocity of 3m/s or so as you say bimr. What I am looking for is any recommendations as to a pipe type/design that would allow us to use much higher velocities (e.g 10 m/s). I saw a reference in another thread to velocities of up to 40 fps being used in water and causing no damage to pipe.
You can probably pump 40 fps through a pipe, but it would not an economical method of pumping. The headloss will be too extreme. The economical water pumping velocity is 1 to 1.5 m/s.
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.
Our drainge design software shows that with the slopes we have (approx. 1:5) and with a roughness values of 0.15 mm that we will have velocities in the pipe of approx. 10m/s under gravity conditions. I have spken to a PE pipe mfr. who says 'it can be done' but am looking for something more definitive.
Reviewed the maximum velocity again and it does show a larger velocity with larger pipes.
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.
Of course it can be done, but it seems a bit foolish to recommend this to your client. This is not a potable water line, so comparing it to one is apples to oranges. I have seen gravity storm sewers carrying sediment erode away very fast with much lower velocities than this. Replacement of the sewer was not a cheap option. It is very likely that continued O&M costs for the steep pipe will far exceed the perhaps added capital cost to install it at a flatter slope with drop manholes.
Interesting discussion (I think you have some good advice to sift through). Your pipe manufacturer has really put themselves out on a limb with their response (of course nuclear fusion also “can be done” to produce energy - hehe!!!) The first reference I can remember to “40 fps” is however from “Design and Construction of Sanitary and Storm Sewers” (WPCF Manual No. 9 or ASCE MOP #37, 1982 and maybe even before). I believe the specific quote reads as follows:
“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
. 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.
Use Manning's n value of .010 and see what your Q will be. Will this line ever get flat or turn a corner? You will blow out the side of a corner with the grits associated with sewage.
