Analisis of downhill pipeline 1

[EloyRD]

Dear community,

I'll appreciate your oppinions in how a downhill gravity pipeline is being analyzed.

The pipeline has changes of internal diameter and slope, altough it always is going downhill. The pipeline starts in a tank and discharges to a pond and has a flow requirement.

1) We analyzed each section that maintained diameter and slope, with a full pipe and no pressure scenario. We found that the flow in each section exceeds the required flow, so we concluded that the pipeline will work as an open channel.

2) We analized each section again supposing uniform flow. Started guessing how full the pipe was, then finding the losses and checking against the available height difference, and iterated until the values had no difference.

3) We found for each section the critical water height, and after comparing with the water height in each section found the flow was in supercritical regime. We don't know how to calculate the Froude Number for the situation.

I'm seriously in doubt in how to analize the discharge of the tank to the pipeline, the discharge of the pipeline to the pond, the changes of slope or diameter in the pipeline and if the uniform flow assumption is . My guess is that these are minor aspects of the pipelyne analisis.

Thanks for reaching here. I'll be expecting your comments.

Kind regards.

 

[zdas04]

Changes in slope probably introduce perturbations that are too small to measure. You really need to look at elevation at the head and foot of each pipe size to get the driving force. I would look at the smallest line to see if it is running full (q=10.2*ID^2.5 with ID in inches and q in gpm, it is an empirical equation and it only makes sense in those units) using the head from the tank to the head of the pipe for an upstream pressure on the pipe in Darcy Weisbach. If Darcy is smaller than the minimum to run full, then calculate the whole thing as open channel flow. If Darcy is greater than the minimum to run full then us the flow rate from the smallest line to check the other lines. If they are all running full then you know that the flow in every segment is the same (you really can't say that with open channel flow) and you can calculate the dP due to friction sequentially. It really is an easier problem is the lines are not running full.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

If the pipe flows as an open channel everywhere, no water touching the top of the pipe, your analysis is correct. You need to look at areas with changes of slope go from steep to flatter slopes. There you may have more water entering than leaving, in which case the flow will tend to back up and may tend to fill the upstream pipe. If it does fill the pipe, you cannot use open channel analysis in that segment and you will need to determine the maximum flow in that segment as you would with a culvert with a submerged outlet, or if the entire segment is flowing full, by (perhaps Manning's) pipe flow head loss equations.

Independent events are seldomly independent.

 

[cvg]

if it is all supercritical, than there will be no backwater effects
this could be solve in minutes using stormcad

 

[EloyRD]

Thanks for the replies.

@zdas04 We have checked each pipe segment that changes size or slope and have found that some work as open chanel and others work as full pipe. I'm not sure if that is possible or undesirable.

@BigInch Will it be correct to asume that the flow backs up to the point where the pressure at the start of the full pipe segment is enough to compensate for the pressuro losses for the required flow?

@cvg Sadly, the pipe now has some segments that work as full pipe. But I'll keep StormCAD in mind for other situations.

Let me know if anyone wants to see the profile. We are checking a pipeline of 30inches that runs for almost 6Km that drops 150m and is required that it handles 2400m3/h.

Thanks for all the sugestions.

EloyRD

 

[BigInch]

I think it might.
Not much slope, very high flow. I didn't run any numbers on it, but it looks like that will be tough, and if it will handle the flow, which I doubt, the fast velocity will need some pretty good erosion resistant material.

Independent events are seldomly independent.

 

[cvg]

stormcad and other hydraulics packages work just fine for both subcritical, supercritical, full or partial flow with any number of changes in diameter, slope, roughness or discharge

 

[zdas04]

What your modeling is showing (some pipes full and others open channel is both possible and probably inevitable. One thing is for sure in this system you can't rely on the continuity equation since the flow is able to back up.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

Stormcad needed for 1 pipe?

Independent events are seldomly independent.

 

[cvg]

6 km pipeline, multiple slopes and multiple diameters, with falling head, thus varying discharges and perhaps tailwater conditions also, I would not consider this to be trivial and perfectly suited for a software application. Besides, my point was I could have done this in an afternoon with Stormcad, no need for all the hand wringing.