Minimum Gradient of Foul Rising Main

[BrianIreland]

I am trying to find the minimum slope (gradient) for a rising main (sewage). I presume that slope "up" or "down" is irrelevant as I am worried about both the location of air valves and scour (washout) valves.

While I'm at it, any maximum gradient?

Also, are scour valves required at all low points or are rising mains quite often constructed omitting scours at "not really low points"?

I appreciate the last question isn't very scientific but I'd like to know if there's grey between the black and white.

Thanks in advance,
Brian.

 

[rconner]

You may want to to check archives and the many similar, some lengthy discussions and references mentioned in several prior threads on various forums, e.g.

http://www.eng-tips.com/viewthread.cfm?qid=125263&page=12

,

http://www.eng-tips.com/viewthread.cfm?qid=108273&page=9

,

http://www.eng-tips.com/viewthread.cfm?qid=108898&page=1

and

http://www.eng-tips.com/viewthread.cfm?qid=24447&page=8

.

 

[cvg]

thread161-24447
thread161-79751

try these threads which have discussed "sewer force mains"

 

[rconner]

Brian,
I have tried a couple times to post a message hopefully to point you toward some references concerning the "scour" part of you inquiry, but every time the draft replies somehow apparently disappeared in some black holes without being sent! I am thus trying again!
You might also check the archives of this site with regard to this key word, as well as perhaps keywords "blowoffs", "blow-offs", "drains" etc. (that may be more common terminology in some areas) as I think I remember reading some such threads in the past).
I know also ASCE MOP #36/WPCF MOP #8 1977 contains the verbiage, "On all low spots where gritty material is likely to accumulate and restrict flow through the pipe, a blow-off should be installed if practical." Another ASCE publication, "Pressure Pipeline Design for Water and Wastewater" (I think latest edition 1992), also briefly discusses such structures, referring inplaces to installation at "critical" low points and also with one quote under the heading "Transmission Pipeline Blow-Offs" (large enough for possible future man-entry), "Frequently blow-offs are not only located at critical low points in the pipeline profile but next to a manway pipeline access inlet." While the structure/syntax of this sentence may not be too good, I believe the technical intent of this guidance is well-meaning (perhaps so as to be especially able to evacuate such lines most effectively for man entry, and subsequently maybe even also provide a means to subsequently get better/more dependable circulation of fresh air into a pipeline in these areas?)
Based on the specific verbiage in these references I suspect folks probably don't have blow-off structures at every inadvertently or slightly by grade "low" point in every working pipeline. There may now be other ASCE and WEF references.
As far as movement (or not movement as the case can be) of various sorts of solids such as "grits" in otherwise fluid pipelines in response to flow shear forces etc., I think there have even been some more specialized technical papers in various forums on this subject, if same would be of interest. An example is "Grit Buildups in Large Sewers - Projections versus Real World", by Mark Holstad of URS et al that I think was presented at a 2001 WEF conference.
I have noticed some designers are even now designing "pigging" structures into some contemporary sewer and even water pipelines, and I guess this practice could enter into discussions/dynamics of such issues as well.