Using 2" HDPE pipe to bring water down a 350 m vertical distance
Using 2" HDPE pipe to bring water down a 350 m vertical distance
(OP)
I need to bring water from a creek at about 2400 metres elevation for 1000 metres down a steep slope to a drill sump located at about 2050 metres elevation.
I'm planning to use a 2 inch HDPE open pipe for this and am trying to determine how much water this will provide, and if I need to worry about the pipe bursting.
My understanding is that as long as I have an open pipe at the bottom, water pressures and bursting won't be a problem.
Answers or references to an on-line resource would be appreciated.
I'm planning to use a 2 inch HDPE open pipe for this and am trying to determine how much water this will provide, and if I need to worry about the pipe bursting.
My understanding is that as long as I have an open pipe at the bottom, water pressures and bursting won't be a problem.
Answers or references to an on-line resource would be appreciated.





RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
In general you must at least check the pressure at all low points along the pipeline. Beginning with any given inlet pressure, as the pipe descends, it gains static head. At any point along the pipe, static head in units of psig, and for water = 62.4 * elevation_drop_feet/144. The elevation drop is measured from the water surface above the pipe intake to the elevation of the point in question. That will give your maximum pressure possible at that point. At the lowest point of your pipeline, assuming that the 350 meter drop is to the lowest point, that should be right around 500 psig, around 3500 kPag.
Now, ONLY when the pipe is flowing, you could reduce the above static pressure by the friction loss of the flow at any given time. If your maximum possible flow is, say 5 liters per second, you might have some frictional pressure drop of 1 psi (7 +/- kPag) per 100 meters of pipe length (I imagined that number, so don't use it. And that's 100 meters of pipe length, not elevation). So, you can see that total pressure, static - friction, depends on where the point is. If the low point is close to the inlet, you won't have much friction loss to subtract. If the low point is at the end of a 1000 meter long pipeline, maybe you will have 10 psig, 70 kPag to subtract. Then your pressure at that point would be 490 psig, but ONLY when flowing at 5 l/s. If your flow stopped at the outlet, then pressure would quickly return to 500 psig.
If your pipe is open, it matters how much its open. If someone that wants more velocity comes along and puts a reducer on the outlet and flowrate goes down, pressures can go up.
If your pipe is not a constant slope, local low points can have a lot to say about maximum pressures.
If your pipe can become clogged, full pressure.
If your pipe breaks for some reason and you have to turn off the water, you will have full static pressure upstream of the point you stopped flow.
Open pipe end theory is generally NOT a safe design practice.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Another way of stating the pressure head is water will give you 9.727 kPa/m of elevation change. Your design develops around 9700 kPa of static head. SDR-13.5 HDPE is rated around 770 kPa at around 15C, it gets worse when it gets cold and at 2050 m elevation it will get cold. I've never seen terrain where you could bury pipe on a slope like that so I'm guessing you'll have at least part of the line above ground or buried in a shallow ditch, even running fast a 2-inch pipe full of water will freeze.
David
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Go speak to your local farmers supply co-operative and they will tell you how it is done.
Perhaps a short piece of smaller bore pipe at the start of the run would ensure that the pipe never runs full.
But take heed of the comments above - no valves except at the top, put a strainer at the top to prevent blockages, avoid low points, beware of freezing, etc.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
See e.g. " Hydraulics of Roman Aqueducts:
Steep Chutes, Cascades, and Dropshafts" by H. CHANSON
Mike Halloran
Pembroke Pines, FL, USA
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
I'm not worried about any debris in the pipe that might clog it- we are tapping into the stream at the outlet of a mountain tarn, so it is crystal clear water with no chance or debris (unless some little marmot-like critter gets too close out of curiosity and gets sucked in, I suppose). And, of course, we'll have a screen at the intake.
The pipeline will feed into an open sump at the bottom in a place where the overflow can be harmlessly directed into a big talus/scree slope and disappear into the ground. The sump will be used to provide water to a big core drilling rig.
Finally, the pipe goes downhill in its entirety- no low points are present.
So right now, my take is that an open pipe would work- but don't let it clog!
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
this line is very steep, pipe material is very smooth and there is a chance you may not have full pipe flow. at the bottom, assuming the pipe discharges under water you will have full flow. Farther up the slope you may not have full flow. Capacity will be limited by the orifice at the inlet. The result is you will likely have air in (part of) the line, could have transient surges and assuming the pipe is laying on the ground, it will need to be firmly staked. you will also have thermal expansion. All can produce high stresses on the pipe that you should be aware of.
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
But the big question that no one has tried to answer yet is: how much flow can I expect to get? My little iPhone Pipe Sizer app tells me that at 10 feet per second velocity, I should be able to get 100 gal/min using a 2" pipe. So is 10 fps a reasonable velocity to use- seems low to me for such a big head.
So there must be better ways to calculate pipe flow that take into account the big change in elevation. Is there a good on-line reference that would explain how to do this?
Thanks
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Q = Ca [2gh]^0.5
I will let you estimate the value for the orifice coefficient (Ca)
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
With no outlet valve controlling backpressure, flow at any point will conform to whatever the energy grade line says it is. You may have open channel flow at the subcritical, or at the supercritical level in some sections and pressure flow in another section, etc. Flow exactly at the critical level, usually doesn't remain critical for long.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Have you applied the Manning formula for open channel to get the flow rate (145 gpm)? and if so which value have you entered for slope S?
V = K/n *[R^(2/3)* S^(1/2)]
Where
V = water velocity [ft/s]
K = 1.486
R = hydraulic radius = Cross sectional area/wetted perimeter [ft]
n =manning factor for roughness
S = slope[ft/ft]
If you have used another formula could you tell us which one?
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
I try to expand my previous post.
Using the Manning formula reported above I've found v = 14.67 ft/s (comparable to your value) only if I enter S = 1 for the slope, that is considering a vertical pipe (which anyway corresponds to approx 119 gpm in a 2" pipe). But with a vertical difference of 350 m over a 1000 m pipe length I think I have to enter S = 0.35, which leads to v = 8.68 ft/s (which corresponds to approx 68.6 gpm).
Am I missing something?
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
The value for s will control the flow in each segment of pipe. Flow may be higher in one segment than in another. One segment may flow full, another partially full. The final flowrate will be the minimum of any one of them, maybe even what can enter at the inlet, using inlet head and Cd for the inlet condition.
Once the minimum flow is determined, that's not to say the flowrate might not change, as that minimum flow could affect the flow previous segments, which changes the inlet conditions to the segment with the previous minimum flow.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
I therefore used the Darcy-Weisbach equation with the conditions I gave above (March 8, 15:53) plus I assumed a roughness of 0.005mm. The 145gpm I calculated was USgpm. I agree 15 ft/s gives 119 Imperial gpm.
The slope I have used is a net drop of 350m over a length of 1000m. How this actually occurs could be important - see below.
I suspect that the difference between your calculation and mine is the smoothness we have attributed to the pipe. What value of n did you use for Manning?
@BigInch - The flowrate on a mass basis cannot be different in different sections of the pipe if there is no provision for accumulation. The pipe is assumed to be running full and the "flexible steel bar analogy" we spoke of recently applies. ( thread378-266224: Velocity and Pressure Drop )
Concern over actual slope of pipe
Although I did not make a specific assumption over the actual slope of the pipe, it would have to be fairly even over the length of the pipe to achieve the flowrate I calculated and I do concede that any large deviation from the average would have to be considered carefully. The two extremes to consider are
1. 650m of horizontal pipe followed by a 350m vertical drop
2. a 350m vertical drop followed by 650m of horizontal pipe
In case 1. the slack flow would cause very low pressures at the end of the horizontal section, the water would vaporise increasing the friction and decreasing the flowrate (but the mass flow would still be constant over the length of the pipe). The pipe may even collapse in on itself. In case 2. the pressure at the junction of the vertical and horizontal legs would be high and bursting would have to be considered. At the calculated flowrate the frictional pressure drop is 0.35 metre of water column per running metre, making the pressure at the junction 650 x 0.35 = 227.5 metre WC or 323 psig.
If the actual slope does not vary too much from the average the small variations in static pressure increase per running metre of length will be absorbed and the flowrate would be as calculated above. If the actual slope were constant over the entire length the pressure gradient along the pipe would be zero because the frictional head loss would exactly match the static head increase. If the pipe dropped below this imaginary constant slope line the pressure in the pipe would rise, but even if it were 50m below the constant slope line the pressure in the pipe would be only 5 bar. A deviation above the average slope would be more problematic because it may cause vaporisation. So the strategy should be to remain slightly below the average line all the way.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
If its one uniform slope, pipe flowing full, Manning works and its a somewhat trivial problem.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
I wouldn't expect to see much available head at the inlet in that kind of terrain where you get 350 meters to play with, unless there just happens to be a very conveniently placed and unusually deep reservoir up there, but he said its a "creek". Probably 2 meters would be an unusually deep inlet head.
I think it will be limited by intake flow and it won't flow full, as the slope surely must be greater than supercritical, well at least not full flow until the pipe gets stopped up. Then it will be full of pressure, but with little flow.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
In fact the entrance effect is (at worst) only 1/3rd of the start up problem. The other loss that has to be overcome by the available head at the entrance is the acceleration of the water into the pipe. For a square entrance the loss would be 0.5 velocity heads and the acceleration is of course 1 velocity head. In order to establish a flow velocity of 15 ft/s a head of 5.5 ft would be required to overcome these two losses. This situation could be improved somewhat by using a radiused inlet, and with a wide mouthed radiused entrance we could reduce the losses to 0.04 velocity heads (Crane TP410 pg A-29). The acceleration losses cannot be improved upon, and there may be losses in an inlet strainer or grid as well.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Direct the inlet into the creek flow and keeping it off the bottom of the creek will both help that as well.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
@Katmar,
Thanks for your explanation. Anyway I still can't get a grip on the use of Darcy-Weisbach equation.
Below the Darcy-Weisbach equation rearranged for velocity V
V = SQRT (hf *2g*D)/(L*f)
Where:
V = velocity
hf = head loss
g = gravity acceleration
D= pipe internal diameter
L = pipe length
f = friction factor.
In this case it is possible to assume elevation head is fully converted into friction loss so hf = 350 m
But friction factor depends both on epsilon/D (being epsilon the pipe absolute roughness) and on Re. But Re depends on fluid velocity V. How could we sort this out?
By the way I have entered 0.012 (no units) for manning's n value (for PE pipe with smooth inner wall).
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
My attitude towards the Manning equation seems to be the same as yours towards Darcy-Weisbach, so I have no feel for what n=0.012 means in prctice. But a Google search suggests that you could use a value down to n=0.009 which would bring our results closer together.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Thanks for swift reply. Meanwhile I have given a shot to Hazen Williams (which doesn't ask for iteration) and results seem to match.
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Absolutely agree the "full pipe" assumption is mandatory for the equations above to be applicable. With this assumption the condition of subcritical flow is safe and the problem requires a simplified approach.
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Or freezes down to the suction level.
Would the loss of the pipe (its water or its physical failure ) be life threatening? Be ready with a backup until the thing can get rebuilt.
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Thanks for the ballpark estimates of flow rate using a numerical approach- always important.
Getting the top of the pipe nicely under water shouldn't be a problem- there is about a one metre deep pond available that covers an area of about 30 square metres- I'll just anchor the pipe in that.
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
Stay tuned.
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
When you have bought and laid all that pipe let us know the actual length and ID as well please.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
We also decided to use a lower intake point- which increases the waterline needed to 1500 metres, but reduces the elevation difference (head) to about 250 metres. The extra length is needed because of how the creek flows relative to where the drill sump is located.
But the big change is to install three pressure relief sumps along the way- which will ensure that any one length of pipe never drops more than a hundred vertical metres, and therefore will never have more than about 143 psi pressure. The HDPE is rated at 150 psi- so should work.
On the subject of professionalism: if I was totally calling the shots on this installation, I would insist on a proper hydraulic assessment by a qualified engineer- who would be expected to issue plans and a signed and sealed report that fully shows the calculations and assumptions he/she used to derive any outcomes, such as flow, etc. Normally, this is not done in situations like this because there is much less head involved and so a waterline is just installed as part of the drilling routine. But this is an unusual situation- if the waterline fails, the drilling would have to be suspended- and so it's important to get it right- something that only a professional and a formal analysis and report can do.
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
RE: Using 2" HDPE pipe to bring water down a 350 m vertical distance
But they won't be running water through the pipe until next week, so I can't yet report on how much water the system is producing.