Convert pipe size to multiple smaller pipes 1

[jffr602]

I am bidding a project that will require a low water truck crossing( Peak data= 500CFS and 7F/S V) I calculate a 108" ID pipe will handle the flows but I need to use multiple smaller pipes (say 36") in order to keep the crossing accessible to trucks. The length will of pipe will be 30' and I will likely be using steel casing. I will have this fully engineered if we get the job but I am trying to deide how much money to put into this item.

 

[rconner]

In this case I think it may take a lot of them! [See e.g. per the footnote to the table "Equivalent Number of Pipelines" at

http://www.acipco.com/adip/products/Sect17.pdf

, i.e. it roughly takes about as many smaller pipes as the ratio of larger to smaller diameters raised to the 2.63 power.]

 

[rconner]

[The rough rule I threw out is for all full flowing pipelines, which I realize this might not necessarily be.]

 

[jffr602]

I too came up with 10 pipes and thought I would add a factor of 25% for friction but started to doubt that would be enough

 

[ColonelSanders83]

I occurs to me that the comparison criteria you should use here may be pressure drop (or head loss) rather than flow area.

Maybe calculate the pressure drop across the large diameter line, set that as your max allowable, and then back calculate how many small lines are needed to stay under that value (I would also toss on a 5 to 10% margin for good measure).

Just my two cents worth, I am by no means an expert.

 

[BigInch]

Its not proportional to velocity because the object is to have at least an equivalent head loss to prevent any additonal buildup of head upstream that might cause run over the top of the roadway.

The rough answer is <about> .. yes ... twentysix (26) pipes of 36" diameter, flowing full, so an increase in the number is needed again, if not flowing full. PS that's 36" I.D.

Basically why a lot of culverts are big diameters.


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[cvg]

without knowing the slope or tailwater conditions, it could be 10 or 30 pipes. you need more information to solve this puzzle

 

[BigInch]

I just assumed the same minimal slope in both diameters. For cost estimates, its safer to use 30 until the detail engineering is done.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[racookpe1978]

At a certain point (maybe 15 pipes ??) it will become more economical to "raise" the truck grade (by, perhaps, 12 to 15" so that the number of pipes (greater length of culvert, greater expense, greater construction time and delays, simple available space in the ditch) becomes less.

So, analyze 30x pipes at 36" ?

10x pipes (in the middle) at 48" plus 5x each side at 36" ?

4x pipes in the middle at 72", plus 2x each side at 48" plus 2x more at the far ends at 36" = this would let the "middle of the sloped (?) ditch to be deepest at the center, with additional capacity as more flow is needed as the water gets wider in the "vee" shaped ditch. But the total height of the ditch + culvert is still substantially less than the original 108" pipe.

 

[dicksewerrat]

Look at some box culverts.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com