Inverted siphon ? 5

[PELS]

We are designing a 10" diameter gravity sewer that has to cross a major thoroughfare. Unfortunately, an existing 20" diameter water transmission main lies in the direct path of the gravity sewer. We have no flexibility in the grade of the sewer as the downstream end is fixed. One proposal was to use an inverted siphon to dip under the water main. We have no experience with this method.

Is this a common solution to this type of conflict?

Are there any pitfalls to be aware of when using a siphon?

Are there other methods to get by the water main without having to relocate it?

Your thoughts and suggestions are appreciated.

 

[Bbird]

Saw one constructed before. It was like your case for overcoming obstruction. Can't remember the details now but I suppose it requires periodical check/maintenance to ensure the system doesn't get blocked at either end. The system I put in was near the outfall and the flow was always continuous but I didn't hear anything about it since.

 

[jheidt2543]

The only inverted siphons (depressed sewer is the better term, since there is no real siphon action) I did was as class exercise in school. I did bid on the construction of one once in the past too. They are an accepted design, but not used very often, basically as a last resort, as in your case.

My references are "Water Supply and Pollution Control" by Clark and Viessman, 1970, page 186 and "Design and Construction of Sanitary and Storm Sewers", WPCF Manual of Practice No. 9 (ASCE MOP No. 37), 1970.

"The minimum flow in a siphon must be great enough to prevent deposition of suspended solids. Normally, velocities less than 3 fps are unsatisfactory. When a siphon is required to handled flows which vary considerabley during any 24-hr period, it is customary to provide two or more pipes. A small pipe is provided to handle the low flows; intermediate flows may be carried by the smallest pipe and a larger pipe. Maximum flows may require the use of three or more pipes. By subdividing the flow in this manner, adequate cleansing velocities are assured for all flow magnitudes. The entrance transition structure is designed to properly channel the flow into the various pipes."

 

[PECPESC]

Can U not add a manhole just upsteam of the 20" water and change the grade as U cross the thoroughfare? ie:steepen the slope on the upstream side & flatten the downstream side ?

 

[Focht3]

You could also use a small lift station if the flows are low. Not as elegant or cost effective, but another possible answer -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[cvg]

your first question needs to be answered. Is this a common solution...? No, the more common solutions are to either relocate the pressure line, or go deep with the gravity line. A siphon (in my humble opinion) would be the last option.

Generally, the only time I have seen siphons used is when you have a conflict between two gravity lines. Either you siphon, or go deep with the new line.

 

[JedClampett]

I'm more structurally oriented, so I just know enough to be dangerous. I've seen them used some. They are used when one towns sewage line runs through another town and the town they're in makes them do it miss something. Also when a sewage line crosses an recessed expressway. My involvement begins when the upstream or downstream structure deteriorates due to hydrogen sulfide gas. This is a result of smelly gases being released in the turbulent conditions of the entry and exit. The gas turns to sulfuric acid through some bacterial magic and eats up the concrete. If there are residences nearby there might be complaints of odors.
Also, as the above posts mention you'll need to increase velocities. This means you'll need a smaller pipe than the 10 inch one you are designing. It will be a problem to keep such a small pipe clear. All the ones I've done repairs on were much larger so that the siphon pipe still had considerable size.
They're tough to design and maintain. I'd call someone with specific experience to take this on.

 

[nealt]

Given the potential problems associated with an inverted siphon and the long term costs of monitoring, cleaning,etc., why not lower or raise the water line to accommodate the sewer at its desired profile?

 

[chicopee]

WHY CAN'T YOU JUST GO UNDER THE 20" LINE AND THEN BRING IT BACK UP TO ELEVATION TO MAINTAIN YOUR ORIGINAL GRADIENT?

 

[91markus]

As someone has already eluded above, siphons require CONSTANT flow and above average velocities in order to work properly and problem free. Siphons were around long before pump stations were and work on sound engineering principles.

If its a 10" line, I would assume that you would be subject to the usual diurnal flow patterns and that isn't necessarily a good thing with a siphon. Namely, during low flow periods, debris could settle out in the "belly" of the line and therefore over time, it could "choke" off the cross section until it no longer functions (therefore periodic flushing/maintenance is critical). If this line has periods of zero flow, I would highly recommend against NOT using a siphon for the reasons previously mentioned.

The benefit with siphons are that they usually less $$$ to construct and you don't have constant energy expenditures as you would with a typical pumpstation. From a maintenance perspective, I don't think either a siphon or pumpstation are better than the other.

My 2 cents.

Regards,
MB

 

[RDK]

Why not put a dip in the 20” line. All it will take is 4 45-degree bends and some pipe.

It’s a water distribution line and will be pressurized. Also it will not have any solids to deal with.

If timing is an is you can use restraining bolts on the offset so that the line can be put back into operation in a very short time without waiting for the concrete on the thrust blocks to set up.



Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[Focht3]

We are designing a 10" diameter gravity sewer...

It's a sewer line, not a water line - per the first sentence of the original post -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[RDK]

The water transmission line is the one that I am suggesting that be relocated.




Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[Bbird]

Inverted siphon is a term used mainly for sewerage design.

It is just a "U" tube section within a gravity drain.

If one fills one side with discharge the hydrostatic pressure will force the flow through the "U" tube section.

Nobody would want to put a "U" tube section in a gravity drain to risk the blockage by the suspended solid from the discharge unless there is a gun pointing to his head (metaphorically speaking).

 

[wolfhnd]

Put one line under and one over. The top line works as a kind of overflow safe guard. Put a valve on the upstream side and on the overflow line. Since the water line is already there put a back-flow proof connection to it on the upstream end of the overflow and all you have to do is open the water line to flush the lower line.

 

[wolfhnd]

Oh I forgot to say you had to close both valves before you open the water main connection . Open them after it is cleared lol.

 

[Focht3]

Actually, I think [blue]RDK[/blue]'s answer is the best: move the water line that's in the way, and avoid the inverted siphon entirely. The water line could go above or below - it doesn't matter since the water line is pressurized.

Guess I was too busy focusing on the inverted siphon to see the simplest answer. Good call, Rick!



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[Bbird]

In term of cost and interruption can one justifies moving a 20" water main to suit a 10" sewer?

Siphons are maintained by periodically flushing the system. What is the objection to it? Flushing water is already available from the adjacent 20" main.

 

[Focht3]

[blue]Bbird[/blue] asked,

In term of cost and interruption can one justifies moving a 20" water main to suit a 10" sewer?

It depends on two considerations: what is the life cycle cost of maintaining the siphon, and what does the city think about the maintenance issues involved with the inverted siphon?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[PELS]

Thanks everyone for the thoughtful suggestions and comments.

The 20" watermain was recently constructed, tested, bacteria-cleansed and accepted by the local water district. We did not think the district would want to temporarily take it out of use, relocate it, re-test it and re-chlorinate it to accomodate the new sewer line......but that is exactly what happened. Four 22.5 degree bends were placed in a distance of 12 feet and the 20" water line was raised 1.4 feet above the 10" sewer alignment. The district did not want the maintenance problems associated with a syphon. This particular district also has authority over sewer lines as well so the decision was easy for them.