Looking for advice on siphon design

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Not sure if this is the right place but I need help on siphon design. The siphon must operate at altitude (9000ft ie about 3000m) and with water at 30 degrees C (90F) or more. In calculating the maximum lift the altitude and the temperature are critical due to potential for cavitation but few books or papers I can find state much on this. The only paper I can find is a paper on (of all things) fish farm design which has a good chapter on siphon design but I would like some independent sources or experience brought into the problem.

Main problem is about draining a mountain crater lake that threatens to create a lahar which could cause much damage. In 1953 such a lahar killed

150 odd people. Lake has pH of about 1 just to complicate things further.

My interest is as an engineer in another field who loves mountains and is interested in this problem.

Bob J.

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Just a few thoughts.

You will have to look at the local barometric pressure to determine how high a water column you can support. At sea level it is around 32 feet. At

9000 ft, the atmospheric pressure is about 10.5 psi, giving you about 24 feet maximum vertical lift on a good day. If the edge of the crater is more than that above the water, a siphon isn't going to work. Backing off a quarter of a foot to 23.75 ft maximum height above the lake water level will avoid cavitations.

You may find that you need to drill horizontally through the mountain at some level above the crater water level, stick a pipe through it, and then connect drop legs to the crater and down the mountain. If this is a big pipe, consider some sort of flow control. The water will develop a lot of momentum on the way down the mountain.

If you really want some fun, pipe the water all the way down the mountain, and put a small turbine on it to produce enough electricity to power a pump to get the water from the crater to within 20 feet of the peak of the siphon. You might just have a few electrons left over to power a small village. You will still need a small gas generator to start things off, but after that it should be duck soup. You'll just need to fill the drop leg once to prime the system. Do it slowly, as the water hammer will be fantastic at the bottom of a mountain.

There might be a few other ways to pull this off. You don't by any chance have a pump do you?

If you pull water off, are you likely to "burp" the lake, causing it to release carbon dioxide and sulfurous gasses? That killed a bunch of people a few years back.

If you drop the crater drop leg down far enough, could you pull off water with a very high mineral content? Could this be economically beneficial to the area? What are you going to do with the water you siphon off?

Michael

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Dear Bob Jordan:

What is the lowest altitude the water needs to be lifted to be able to flow down and out of the "cone"? You might be able to keep CPVC together long enough to do this job. You may have to heat bond the joints however.

On a silly note, a few metric tons of baking soda might allow the stuff to foam up and out...

David A. Smith

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One more minor detail. I assumed regular old water was coming down the pipe. If the water in the lake is loaded with something, its density may be higher, so the maximum height of the siphon will be lower.

Materials of construction will be an interesting hurdle.

Michael

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Dear Herman Family:

He could drop a chunk of CPVC in and see what happens. If it is sulfuric acid (which is likely in volcanism?) then: URL:

for lots of acids, just not all acids.

David A. Smith

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I assume you are talking about Crater Lake at the top of Ruepehu in the central north island of New Zealand. My understanding is that the Department of Conservation is just going to let nature take its course

- under pressure from the local natives for whom the mountain is a symbolic icon. If they really wanted to drain the thing, bulldozers could take care of it in a few hours.

The warning systems in place *should* alert people in time (though I'd be wary about skiing there this winter). And hopefully there will be some useful data and even videos of the thing come out of it.

Not all problems have Engineering solutions, you sometimes have to deal with social and culutural issues as well!

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There will also be piping pressure drop losses which depend on the piping configuration and the flow rate. This problem doesn't look all that simple from a practical standpoint. It could take a bit of design effort to get something that works most or all of the time.

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The higher density may well influence the vapor pressure. If there are dissolved solids (e.g., salt), the lower vapor pressure will partially offset the higher density, and the water column that you can support will be higher than expected. As I said in a previous posting, this problem is probably a bit more complicated than meets the eye.

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You might poke around and see if you can find anything from U.S. Bureau of Reclamation. I seem to recall that at least one of their projects (Colorado/Big Thompson?) involves a siphon and is located at a higher altitude.

Also, I believe the California Water Project has something akin to a siphon arrangement for getting water in very large quantities over some mountain ranges. The water is pumped up one side but the outfall is used to generate some of the electricity used by the pumps.

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And don't forget at 30 C vapour pressure louses you about 18 inches of potential lift.

-- Jonathan

Barnes's theorem; for every foolproof device there is a fool greater than the proof.

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You really ought to add a hydraulic ram to this thing. Not so much because you need one, but when's the last time an engineer got to design one?

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As I said last night I would follow up on more details of this.

I am a little further advanced than I think I indicated but as I have found out in the last 24 hours there are a lot more things to consider.

I obtained from the Internet a design book which is the most detailed set of equations that I have been able to lay my hands on. It is an FAO document on designing fish farms and has a chapter on hydraulic procedures that may be relevant to that. Section 3.7 of Chapter 8 is entitled Design formulas for Siphons. The full document is located at:

and it is by J. Kövári of Food and Agriculture Organization of the United Nations, Rome, Italy

It includes calculations for atmospheric pressure at various heights which I have parameterised, vapour pressure of water vs temperature, pipe losses, bend losses, suction heads, downstream heads and minimum flow rates for various pipe sizes. I have built this into an Excel spreadsheet and it seems to give meaningful numbers.

However, some other reading on the subject suggests that cavitation may be a limiting design criterion and I am keen to get independent confirmation that the figures for the siphons created will work.

From the comments from this forum I see I have to add two parameters to the model - liquid density which will reduce the atmospheric pressure head compared to pure water, and mole fraction of the liquid which will have a slight lowering effect on the vapour pressure. The latter can probably be ignored as it will only make the design a little more conservative but the former is important. It is easy to add this.

There are three aspects of the design which are given without comment and which cause me concern.

1. I think the calculation of the allowable pressure head before velocity terms are added is rather conservative? They use 90% of the atmospheric pressure head less the vapour pressure head less 1m. Thus at sea-level and with 20degreeC water the allowable pressure head is 90% * 10.3m - 0.24m - 1m which is just over 8m. Is this a good rule to ensure no problems with cavitation etc.

1. They have a mysterious Minimum flow as a function of pipe diameter.

1m/sec at 120mm, 1.5 at 200mm, 1.7 at 400mm etc up to 2.6m/sec at 1200mm. I cannot find the logic behind this. Plotting the function shows it to be very strangely shaped with a rapid rise, then a slow rise then a sort of asymptotic rise again. I wonder if this is the rate that will tend to flush out entrapped air at the top of the pipe?

1. They insist on a minimum depth for the intake. There are various formulae for this and my guess is that it is to prevent air being drawn into the intake by (in my layman's terms) a whirlpool. This is not too much of a worry though as it does not affect the design and builds in more flexibility.

So I would be interested in the groups comments on the first two points above, and any other on the overall design concept if anybody is interested in going there.

Now the lake situation.

The Ruapehu Crater lake (See the google images using those three words) is at an altitude of 2530m, is some 400m by 500m in size, and has a catchment of about 1km2. An average of 5000m3/day flows into the lake. The water has pH about 1,is muddy brown in colour and ranges from 20 to 40 degrees C in temperature.

At its outlet is a 7m deep tephra (volcanic ash) dam over a more solid base. If the lake got to the top of the dam and started to erode it, the whole top

7m of the lake could take off down the valley in about 15 to 45 minutes, picking up on its way 3 to 5 times its volume in "mud" i.e. about 2000m3/sec. It would be fun to watch.

The lake is not in an area that is easy to reach. A good walker could get there in just over an hour if the skifield chairlifts were running, and two hours otherwise. It is in a National Park (I think the second one created in the world), it is a World Heritage site, and is of considerable importance to all New Zealanders particularly our Maori people. Bulldozers are out.

To date siphons have (apparently) been considered but rejected for vague reasons that they "don't work at that altitude" or are "too hard to maintain so far from base". These are the points that I am considering.

Would a siphon work there and would it work well?

My initial calculations say yes and I have passed my spreadsheet to the people concerned. They want the calculations checked by someone with expertise. Read - not an amateur scientist electrical engineer in his mid

50's.

It seems to me that with 60m of 200mm pipe buried in a shallow hand made trench to minimise visual impact and maximise working head one could slow the rate of rise of the lake level and allow the water to infiltrate the tephra and break it down. A lahar from 3-4 metres below the top of the dam would be a much smaller event than the 7m one above and would have much less effect downstream.

The effect of the water downstream is already accounted for. This river does not pass through any power schemes etc. All its neighbours do though. It does not need piping down the hill - spilling down the valley is fine.

Meanwhile all aspects of safety downstream are being dealt with well. And yes people ski there (including myself), people hike there (including myself), and people climb there. It is a wonderful place. And yes I will ski there this coming winter!

At the last eruption (1995 and 1996) we could see the plume from Hamilton where I live 180km away - that¹s about 110miles.

So that is the situation. I wonder if anybody has comments to make on this scenario, in particular to the questions I raise. You have already offered a lot of very useful pointers. Some need more thought from me. Starting, bleeding, keeping it going, density, materials, etc. etc.

Thanks for your efforts so far - it is appreciated.

Regards

Bob Jordan

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What's to parameterize? For a well behaved atmosphere that lapses uniformly with height at 6.3 kelvin per kilometer, a standard expression for pressure vs. height is h = 153.8 * T0 * ( 1 - ( p/p0 )^0.1902 ) where h = height, T0 = temperature at sea level, p = pressure at the given height, p0 is sea level pressure

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Uh-oh this request for qualified opinion is (sad to say) a give away for proposals offered by amateur scientist electrical engineers, like you?

[Means it would be better to leave it to the pros with local insight.]

Good luck

Brian W

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Dear Bob Jordan:

That would be my vote, yes.

I concur on the whirlpool/vortex.

I don't know if you could, but I would do two things:

1) encourage local native growth on the slope to try and bind the soil, 2) reinforce a section of the dam (pH 1, right!) so that the outflow, when it occurs, is limited in rate.

Bulldozers

I doubt this. A reduction in water added would allow the suction end to become effectively exposed. Then you are depending on someone to hike up the hill to restart it.

I read this as "we understand your concern and will sit on it until a lahar occurs, then you can say 'I told you so'." Those people will throw all kinds of money at repairs, but nothing at constant preventative maintenance.

You'll need some pipe, a little more than the immersion depth, to create your vacuum. After that, you are correct.

Good luck!

David A. Smith

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Remember to design for the maximum temperature of the water. Add 10 or 15 C just to be sure, so design for 40 to 50C. Conservative designs now can reduce problems later. Overdesign is a good thing when it will be undermaintained. It does add weight, however. You might want to go much faster than minumum in order to prevent the mud from settling out. Calculate the allowable height for water, divide that number by the specific gravity of the lake. With a high mineral content you may see another meter knocked off of the allowable height.

Make the intake deep enough so that the water level never gets below it. In your case several meters deep wouldn't be a bad idea. One thing to consider here is that your pH, density, and mineral concentration may change at lower depths. Consider some method for keeping the intake well off of the lake bed. You don't need to be eroding the lake at your siphon intake. That could eventually cause an even larger lahar. I'll assume you don't need fish screens with a pH of 1. You won't have to worry about swimmers. Burial will prevent casual curiosity and vandalism.

The outlet should be below the minimal level of the lake. Your driving force will be the difference in height between the water level and the opening of the outlet. That driving force will largely determine your flow rate and also dictate some of the piping design. I'd make sure that the outlet was at least 10 meters below the inlet. Having it drain into some sort of trough or tank might not be a bad idea. It would prevent air from getting in at startup.

If you have enough of a dropleg, you could put a really nice perpetual fountain up, or several. That might not be in the spirit of things, but it is fun to consider. You have so much head, that it's fun to think of what could be done with it. This could become part of the attraction for the area. I'll bet it would be warm around that fountain in the winter.

Since you must prime this, I'd consder putting a valve at the inlet, a valve at the outlet, and a valve at the top of the siphon. To start, close both the inlet and outlet valve, open the top valve, and fill the siphon with water. Then close (and lock) then open the inlet, then the outlet (slowly). The lock will prevent tampering. Burying the whole thing afterwards will finish the deal.

Your pipe track may be visible in the winter because it may melt some of the snow above it.

Michael

Bulldozers

So they need a real engineer, not an electrical engineer :)

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Hi Bob, I have come into this thread quite late but you seem to have a lot of good ideas fed back. One point I have not seen made is the need to prevent a siphon break travelling back up the outlet pipe. As the water accelerates due to gravity, the same volume will only occupy part of the pipe section thus allowing air to travel against the flow and stop the siphon. There are many ways of avoiding this. A vertical fountain would do it as would submerging the pipe end under water. A nozzle would probably work but would create a high discharge velocity that could cause erosion. In my opinion, you really need a working party to fully investigate the problem and to formulate a sound workable design. As it happens, I have always wanted to go to NZ and have some holiday time already booked. No doubt others could be persuaded to join you as well. Good luck

John

Bulldozers

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It should be patently obvious, but don't try using copper/brass valves at pH 1.

So much for stealth! 8-)

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Thanks a heap to all who responded to this thread you have been most helpful.

In particular:

Solutions reducing vapour pressure Density reducing head Starting mechanisms Back flow of air into the outlet Sucking up lake bottom debris Pipe materials

These have been most useful

By the way this lake is located way above the tree line in New Zealand so there is no vegetation, and the stream is diverted from power systems etc and travels some kilometres before reaching civilisation by which time it has had much other water added to dilute things.

And the fun thoughts were fun too.

Sorry if I look to some of you like I am trying to be an expert. It just fascinates me as a scientist/conservationist and outdoors person. The experts have given up on a siphon too quickly I believe. I am writing a report to collect all the ideas and my basic designs for giving to the people concerned. If anybody further interested please contact me directly.

Regards and thanks again

Bob J.

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Dear Bob Jordan:

For myself, thank you. We'd like a status update a few months from now, assuming there is anything of note to report...

David A. Smith

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