Screened sewage rising main 1

[JohnWeal]

Hello all,

We have the following defined flows for sewage screened to 6mm (perforated plate)

Daily average flows: 7000 m3/h
Peak hourly to full treatment: 19,000 m3/h
Minimum: 2100 m3/h

The min / max velocities allowable by the client are 0.75 and 1.8 m/s respectively.

Based on the max flow this equates to a 2m diameter pipeline.
For minimum flows the velocity in the 2m pipeline is 0.19 m/s. Clearly well below the minimum required of 0.75m/s.

For average daily flows the velocity is 0.62 m/s. The average daily flows still not enough to move 5e solids along at the minimum velocity required.

Therefore proposing 2 pipelines at 1.5m each to give same cross sectional area. For average flows up one pipe is 1.1 m/s which is acceptable.

For minimum flows up one pipe is 0.33 m/s. Not really acceptable but if average daily flows produce a 1.1 m/s velocity then should be enough to move things along without scepticity.

So on that last point, if maximum flows are not realised for some time, I assume the pipelines would need to be switched over each 24 hours to flush?

Has anyone had any similar flow design scenarios similar to what I have described and any construction/ operation/ maintenance advice would be much appreciated.

Regards
John

 

[bimr]

This scenario is a common problem with the operation of sewage systems. If the pipeline is designed for future flows and the startup flows are much less, the proposal for two pipelines is a solution. Sometimes the two pipeline recommendation includes a smaller and larger pipeline.

 

[JohnWeal]

Thanks Bimr.
I guess the trench is much wider too!!

What is the minimum dimension between the pipelines? I guess enough to join by push-fit (ductile iron) lowering in a welded 1500mm pipeline is no easy task!!

 

[LittleInch]

Well it depends how precise you need to follow the prescribed flow velocities.

With about a 9:1 turndown in flow, but only about 2:1 turndown in velocity, you are looking at something like three pipes with 20%, 30% 50% of flow to be able to meet all the requirement.

Of course different size pipes bring their own problems as a simple manifold won't work as the fluid takes the easy way and will go preferentially down the bigger pipe.

I don't have any specific knowledge on sewage lines, but I think the only way to see this through is to really understand daily flows, times and get the best and most economic fit. Good luck with what seems at the moment like a tall order to get the optimum solution.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JohnWeal]

Many thanks Littleinch. Was thinking exactly the same an hour ago.

Going to 3 rather than two. Balancing flows and introducing control valves is expensive I think unnecessary. Especially on sewage.

What might be ideal for flow may not be easy for installation.

One advantage definitely for a twin rising main is that one can always be isolated for whatever reason. There are a couple of river crossings en route too so like to keep the velocity up to stop crap hanging in the inverted siphon. So was going to put isolation valves and access points in those locations any how.

Ductile iron with sulphide resisting cement lining is typical pipe in the U.K. for sewage transmission lines. Will just need to check the headless and thus overall pressure including surge / test. It is not normal for flanged pipejpints so if the push-fit ductile iron won’t take the pressure then it would have to be steel. I’m pretty sure the heads are not that great though.

We have about 8 miles of pipe route.

Regards
John

 

[LittleInch]

If you don't have some sort of flow control there's no way the flow will divide up in the way you want once you start running in parallel.

Now it could be you dedicate different pump sets to different pipelines but then it starts to get expensive.

However with that sort of turndown that's where pipelines suffer from the inability to change diameter....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

Is not this issue why sewers are designed with slope and only partially filled? The water and solids run along the bottom of the pipe, and velocity stays relatively constant while fill level of the pipe changes.

 

[LittleInch]

The term "rising main" in the title means it's a pumped pressurised pipe, not a gravity sewer.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

There was a recent post on the use of RTP piping for hydrocarbon tainted brackish water. Could the same be used in this application? - vaguely recall they have these in larger pipe sizes too.

https://www.eng-tips.com/viewthread.cfm?qid=445256

 

[JohnWeal]

Yes, definitely pumped!!

Georgeverghese, RTP maybe ok as the pups are pumping 6mm screened sewage which will have grit too but at least bo impact from large debris.

Atvthe moment what seems favourable hydraulically is two DN1400 pipelines rather than a single DN2000.

 

[SandCounter]

In a different application, pumping sand or clay, I've found settling slurries deposit in the pipe, decreasing the hydraulic diameter until it corresponds to velocity limit of stationary deposit. At an increased flow, the higher velocity picks up the deposited bed load and increases the hydraulic diameter until the velocity decreases to the deposit limit.

What you have to watch, however, is that at very low flows, you have a very thin open cross-section at the top of the pipe and you will need to ensure there is nothing you expect to pump greater in size than the minimum open dimension.

Page 18 of this document has a handy nomograph for determining settling velocities in various pipe diameters.


I used to count sand. Now I don't count at all.

 

[bimr]

What is the minimum dimension between the pipelines?

For parallel large diameter pipelines, concern should be given to pipe-trench interaction and trench requirements.

When buried pipes are installed in parallel, principles of analysis for single pipes still apply, however soil cover must be greater than minimum. In other word, the design of parallel buried pipes requires an additional analysis for heavy surface loads (given that a common trench is used). In fact for buried parallel pipes soil slip between the pipes becomes the bottleneck for the distance calculation. Talking about 1 m or more separation then the concern becomes faded but it all depends on your pipelines diameters and soil density.

If the pipelines are in separate trenches, then embedment stability becomes a concern when a trench is excavated parallel to an existing buried flexible pipe. You should address what would happen to a buried flexible pipe when some or all the side support is removed in a parallel excavation. Ans also the existing trench stability itself is an issue to concern. At less than minimum side cover, X, side support is lost and the soil on the pipe must be supported by the pipe. If ring stiffness is inadequate, the pipe collapses.

For a detailed discussion, please refer to "Buried Pipe Design" by A. P. Moser, Steven Folkman chapter 3

See also page 34 of the Welded Steel Pipe:

Steel Tank

The min / max velocities allowable by the client are 0.75 and 1.8 m/s respectively.

The two pipeline solution will be workable. With sewage, you just have to make do with what you have. You don't mention the upward slope. If the slope is steep, a higher velocity is necessary to force the air bubbles downstream.

You also do not mention whether or not the pipeline will stop. If so, a higher velocity of at least 1 m/s (3.5 ft/s) is necessary to resuspend solids that have settled out.

"A lower minimum velocity of 0.5 m/s (1.6 ft/s) can be tolerated if a twice-daily velocity of 1 m/s (3.5 ft/s) is attained." Pump Station by Garr Jones. This is something that you can program into the control system.