Flow Away in Force Main Calculations

[jartgo]

I'm sure this question has been discussed before but I didn't find exactly what I was looking for so I figured I'd just post again. Feel free to direct me to a previous post.

Consider a force main with multiple high points. Also assume that one particular high point to low point is 40 feet (elevation difference). A hazen-williams calculation along that same section of pipe for the design flow, shows a friction loss of 25 feet. Am I able to subtract the balance (15 feet of head) from the next low-point to high-point TDH calculation?

I intend to work out the hyraulics of it later this week, just thought I'd see if anyone had a layman's response.
Thanks!

 

[bimr]

You use the difference between the staring elevation and the endpoint elevation plus the headloss in the pipe. Your pump must also have enough dicharge head to be able to pump the water over a high spot in the middle of the force main (if the endpoint has lower elevation than the high spot).

 

[jartgo]

bimr,
What you are describing is essentially ignoring any benefit from gravity flow in the force main, correct?

 

[micalbrch]

Check

http://www.engineeringtoolbox.com.

I don't have the time at the moment to tell where exactly you find the description of your problem plus formula but I know it is there.

 

[bimr]

No, you will get the energy recovery.

However, you pump must have the necessary power to initially push the water over the summit. Once opeating, the hp will drop as you recover the energy. If you do not add the extra power to the pump, you may not be able to push the water over the summit.

 

[sow]

You will get some presure back, but bear in mind that the more of a hump you have the more potential for -ve pressure you have + you need to ensure the "gravity" section is primed.

You need to draw the hydraulic profile and the pipe profile to make any sense of it.

 

[rconner]

Just a comment, for whatever it is worth, regarding force mains with "multiple high points". While I understand the drivers for same (e.g. with regard to specific terrain/avaialble ROW's and also minimizing numbers of lift stations etc.), I believe multiple references mention that it is generally best when sewer forcemains are designed with generally just ascending profiles. [While it is obviously a different application, see incidentally how a project with small plastic/compositite pipelines and apparently multiple "Nessie"/vertical curves in alignment is currently dealing one gentleman fits at

http://eng-tips.com/viewthread.cfm?qid=275705&page=1

].

 

[bimr]

There generally is no feasible method to avoid elevation differences when pipelines are laid across distances.

The problem that may occur in pipelines is entrapment of air or air pockets. The remedy for this is to ensure that the pumped fluid has adequate velocity that will force the entrapped air out of the pipeline.

 

[rconner]

Ah yes, “not feasible” have perhaps been two of the strongest (and arguably most stifling?) words in the English language. I wonder how many prospective innovators have ever heard, and been influenced one way or another, by such words? [In the face of such bar one really wonders how the less schooled ancients ever built the gravity flow (it would appear by nature mostly just “descending” in nature), aqueducts up to hundred of kilometers long in the Old and New World! ;.)]

 

[bimr]

feasible from the Latin word facere: capable of being done or carried out

Where depressions deeper than 50m had to be crossed, the Romans also used pressurized pipelines called inverted siphons. Modern hydraulic engineers use the same techniques to enable sewers and water pipes to cross depressions.

 

[rconner]

[…though it appears quite early on a few others decided instead to keep the descent/slope/grade in effect constant by putting the conduit on piers or piles etc. across the “depression”. Among other things, perhaps this facilitated easier access or long-term maintenance (e.g. cleaning or operations etc.), that at least where “feasible” was thus perhaps at least somewhat easier than with “inverted siphons” – see the fascinating examples of apparently quite long-term and successful service e.g. at

http://news.bbc.co.uk/2/hi/uk_news/wales/north_east/3271665.stm

,

http://itotd.com/articles/574/pontcysyllte-aqueduct/

,

http://whc.unesco.org/en/list/1303

, and

http://en.wikipedia.org/wiki/Pontcysyllte_Aqueduct

etc. Maybe a secret to long-term success was the “ox blood” originally used in the mortar for the supports mentioned in the latter site –or maybe just derived from the moniker aqua or "water" and ducere "to lead".? Maybe my two years of Latin were good for something. Everyone have a good weekend! ;>) ]

 

[bimr]

I don't recall reading about any Roman force mains.

 

[rconner]

fyi

http://www.eng-tips.com/viewthread.cfm?qid=270920&page=1

 

[bimr]

Are you posting that as a lesson's learned?

 

[rconner]

Sorry for the last link I grabbed by mistake, that is probably irrelevant. I think I instead meant to provide links to a couple other FM problem inquiries I had seen not long ago with also "intermediate high points", including

http://eng-tips.com/viewthread.cfm?qid=229700

(and some mentioning false head, or static head) .