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Vertical Bends in Sewer Force Main

Vertical Bends in Sewer Force Main

(OP)
I have an existing (in-place) pipe that must be used as a casing for a new force main. The existing pipe is 20 feet below the roadway. The current plans show two vertical bends after coming out of the casing in order to get the force main back up to workable grade, where the pipe laying would continue for the duration of the force main alignment. I guess I feel like it would be better to gradually make up that grade rather than doing so with vertical 90's. However, gradually making up 20 feet of grade would require some distance of deep sewer laying, that I'm not sure other site constraints would allow. What are your thoughts on the inclusion of the two vertical 90's to bring the force main back up?

RE: Vertical Bends in Sewer Force Main

Not a good idea. 45's would be as sharp as you should go.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Vertical Bends in Sewer Force Main

I'd go with a pair of 45s on each end. A couple questions first. What is the diameter, flow rate, and pressure if known. Is it steady or intermittent flow. The steadier, faster, and higher the pressure, the better. Chances of a clog are remote if the solids content is low. Consider putting clean outs in with bolted on flanges in the event you have a problem.

But back to your original question, 45s will be as good as sweeping in a curve.

RE: Vertical Bends in Sewer Force Main

In concept, there is nothing wrong with using 90 degree bends in a force main. However, your post is incomplete.

For example, what is the fluid velocity? Velocity if important if you have vertical pipe segments that may trap air. You may need a velocity of 3.5-4 ft/sec to push an air bubble downstream if the pipe direction is verical.

RE: Vertical Bends in Sewer Force Main

Another question would be what kind of pipe are you using. You might consider directional boring HDPE or yellowmine pipe for your gradual grade you discussed.

If PVC and you do use fitings; restraints are important. Not only on the fittings; but, also on the carrier pipe within your casing and potentially your vertical risers. You don't want the pipe to push apart in the casing and you have no friction to rely on inside the casing.

RE: Vertical Bends in Sewer Force Main

While I'm not sure there are any hard and fast rules here, in general and for many reasons I believe it may be best where room or right-of-way and/or obstructions allow to make pipeline transitions as gently or with as small of bends as possible (even just deflect a succession of pipes instead of using fittings when this can be done). Obviously, it is theoretically possible(i.e. on paper with the stroke of a pen, or now w/ cursor on screen) at least without aforesaid restrictions to make the same off-set with very small angle bends, or instead 90's .
However, the smaller the bend angles are, the less will be the minor (head) losses, and if applicable pumping costs forevermore as a result of same. The lighter, and at least maybe some less costly will be the fittings as the angle reduces. Very small angle bends can also result in relatively small pressure and flow-induced thrust force resultants, whereas thrust forces and effects at larger bends like 90's can be huge. When thrust forces are huge, larger thrust blocks or more restrained joints are required, perhaps further increasing overall construction costs. Finally, if anyone ever wants to run anything other than fluid (or anybody) through a pipeline, it may be easier traversed with gradual slopes than with 90's and vertical risers.

RE: Vertical Bends in Sewer Force Main

(OP)
Velocity is 2.5 fps. Diameter of the force main is 8 inches. I plan to use ductile iron within the casing pipe, and likely continue that as we make the transition to workable grade, in whatever configuration we come up with. Flow will be intermittent as with a standard sewer lift station. I do plan to include a VFD so hopefully that will relieve some of the thrust issues. Of course the restraints will be designed without consideration for the VFD in the case of an across the line start. Thanks for the responses. I was thinking two 45's or a directional bore, and I don't think I've seen a decidedly different suggestion here.

RE: Vertical Bends in Sewer Force Main

You probably realize the 20' depth transition wouldn't all have to be at 45 degrees, the 2-45's could be fairly close together.

RE: Vertical Bends in Sewer Force Main

There are plenty of 90 degree bends in a sewage treatment plant. Same sewage.

What's your problem with using 2 90s? If your concerned about the size of thrust blocks use flanged fittings? The difference in head loss through 45s or 90s is not going to be worth losing sleep over. Do the numbers.

Use 2 90s and get on with life.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/

RE: Vertical Bends in Sewer Force Main

(OP)
Stanier,
Ill withhold the word "useless" and just say that your post wasn't very helpful. Not that i expect all responses to be helpful, but it also seemed to carry a tone that suggested you might be a person who is surrounded by people telling you how smart you are. If I were more concerned with your opinion I might argue that it's not the same sewage. I think instead, I'll get on with life.

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