Dam Solution?

[JAParticipant @ja]

Posted by SillyOldDuffer on 03/08/2019 18:14:45:

Posted by JA on 03/08/2019 11:57:00:

Posted by SillyOldDuffer on 03/08/2019 11:21:54:

Posted by Bazyle on 03/08/2019 10:49:03:

Maybe not. A syphon can only apply one atmosphere to overcome the friction per 30 ft pipe…

Or maybe that's wrong too!

I think I'm correct in saying that no-one understands how siphons work. …

Dave

Sorry, siphons are simple. A bit of fluid dynamics –

…

Unfortunately not. Have a read of the Wikipedia article on Siphons. While it agrees Bernoulli equations provide a 'fair approximation' of the flow, the section on 'Modern Research doesn't come to a solid conclusion as to how Siphons actually work. At least not the way I read it.

It's been shown that siphons do not depend on atmospheric pressure because they work in a vacuum. Operation looks to be more to do with gravity and molecular cohesion. I wonder if anyone has tried to eliminate gravity by siphoning liquids in outer space?

The latest research – Hughes and Gurung, Queensland University of Technology, 2014 – concluded "It follows from the above analysis that there must be a direct cohesive connection between water molecules flowing in and out of a siphon. This is true at all atmospheric pressures in which the pressure in the apex of the siphon is above the vapour pressure of water, an exception being ionic liquids." Why are ionic liquids exceptional? Of course being Australian they were upside down when they tried the experiment…

Dave

Edited By SillyOldDuffer on 03/08/2019 18:17:16

Dave

I have just had a look at Wikipedia. I treat Wikipedia as a thoughly untrustworthy source of information. It has a vast amount about what is generally a very simple hydraulic device, a lot of it seems to be grounded in mumbo-jumbo. Away from problems due to pressure approaching a vacuum at the top of the siphon Bernoulli’s equation applies. When the height of the siphon is around 30ft the water starts to boil and the simple Bernoulli analysis fails. The modern studies referred to in Wikipedia concern these low pressure conditions.

JA

[duncan webster 1Participant @duncanwebster1]

………

It seems that siphons are very complex and no-one really understands how they work.

You can do experiments to prove they don't need cohesion (yes water does have tensile strength but you can siphon a broken stream of water.. ) or atmospheric pressure (you can siphon in a vacuum). It seems that there are multiple mechanisms that mean on takes over if another isn't sufficient.

en.wikipedia.org/wiki/Siphon

Neil

This is just trying to make life difficult. Syphons work mainly by gravity, but you need air pressure on the top reservoir to start them off by sucking on the bottom end of the tube. Yes you can have a broken column of water, but then you again need air pressure on the free surface in the top reservoir. Even with a continuous stream of water, if you try to work in too much of a vacuum the 'suction' height will be reduced, because the water boiling point is reduced.

To take a slight detour, all the weirs on the Mayenne River in France have hydro electric turbines perched on top which work by syphoning water over the weir. Neat, the weirs were already there to make the river navigable

Mayenne generators

[john fletcher 1Participant @johnfletcher1]

Re Hydraulic rams there was a good article in ME a few years ago, on making a ram using plastic pipe and fittings. I think it is a large volume of water moving a small volume, John

[Neil WyattModerator @neilwyatt]

Rather than decrying Wikipedia as 'inaccurate why not take a look at some of the plentiful references in the article?

Past research has shown it to be no less accurate than the Encyclopaedia Britannica.

Neil

 

P.S. Looks to me that the pumps are in the floating yellow bits (sorry Original thought was wrong…)

 

Edited By Neil Wyatt on 03/08/2019 20:26:32

[S.D.L.Participant @s-d-l]

Posted by Neil Wyatt on 03/08/2019 20:24:59:

Rather than decrying Wikipedia as 'inaccurate why not take a look at some of the plentiful references in the article?

Past research has shown it to be no less accurate than the Encyclopaedia Britannica.

Neil

P.S. Looks to me that the pumps are in the floating yellow bits (sorry Original thought was wrong…)

Edited By Neil Wyatt on 03/08/2019 20:26:32

They are

Come from the High volume pump sets they have been sending.

The are 50 + sets distributed around the fire brigades that do about 420 m3/hr

High Volume pump pods

Saw somewhere they have 16 sets in place now

Steve

[Martin of WickParticipant @martinofwick]

|On a similar thought can anyone explain how a hydraulic ram works ?

It is based on using the surge pressure or transient caused by the kinetic energy release delivered by the instantaneous stop of a moving body of water. Also known as water hammer to anyone with old plumbing who has fitted an instant cut off tap. The 'clunk' that you may hear or feel through the house on slamming the tap shut is a pressure transient that can be many times the ambient pressure in the system. If you are experiencing this frequently, I suggest you get it sorted before you find your compression fittings starting to leak. (or not find until too late as in my case)

T hydraulic pump functions as water under some sort of pressure flows out of the waste valve of the pump, the fluid drag then shuts the 'waste' valve triggering a positive transient that opens a non return valve (nrv) on the delivery pipe and moves a small quantity of water up the delivery pipe.

Milliseconds later, the reflected negative transient causes the delivery nrv to shut and the waste valve to open allowing flow through the 'power' section of the pump again. The process repeats ad infinitum.

At each pulse, only a small volume is moved, but the process can continue endlessly as long as the flow and pump internals are maintained so over time domestically useful volumes can be moved.

'Stonishingly low tech, but very effective, however in the complex and demanding world we live in they are of limited application these days.

If you have a stream at the bottom of your garden, and wanted to move water up hill to storage at the top of your garden at practically zero cost, this is the device for you.

[Michael GilliganParticipant @michaelgilligan61133]

For the most up-to-date local weather reporting, traffic news, etc. : **LINK**

https://www.buxtonweather.co.uk/

MichaelG.

[duncan webster 1Participant @duncanwebster1]

Posted by Martin of Wick on 03/08/2019 21:25:57:

|On a similar thought can anyone explain how a hydraulic ram works ?

It is based on using the surge pressure or transient caused by the kinetic energy release delivered by the instantaneous stop of a moving body of water. Also known as water hammer to anyone with old plumbing who has fitted an instant cut off tap. The 'clunk' that you may hear or feel through the house on slamming the tap shut is a pressure transient that can be many times the ambient pressure in the system. If you are experiencing this frequently, I suggest you get it sorted before you find your compression fittings starting to leak. (or not find until too late as in my case)

……..

The way to sort it out is to fit one of these near the offending tap, or in my case dishwasher. At least I hope it sorts it out, I'm picking it up tomorrow and hopefully fitting it Monday. It works like a damper, the dynamic head of the water which would have cause a pressure pulse when the solenoid valve slams shut is dissipated in a spring/piston arrangement pegler

[John OlsenParticipant @johnolsen79199]

I can't see why anyone should have the slightest difficulty in understanding how a siphon works. Atmospheric pressure can support a column of water about thirty feet high, which is why a pump can only lift water from that far below itself. Really the pump just removes the air from the pipe, and the atmospheric pressure pushes the water up the pipe.

So OK, we want to do without the pump and start a siphon. For the siphon to work, the low end must be below the level of the water in the reservoir. The highest point must not be too high above the reservoir, eg about 30 feet for water, or 29.6 inches if you are siphoning mercury. You can start the siphon by filling the entire length of the tube with the working fluid. With a hose, this can be done by immersing the whole length, then blocking one end and transferring that end to the low point, then unblocking it.

If you were to measure the absolute pressures along the tube, you would find that at the inlet, the pressure was about 14.7 psi. At the highest point it would be less, a minimum of zero psi. So there is a pressure gradient causing the water to flow towards that point. From the high point to the outlet, it is downhill, so the well known tendency of water to flow downhill accounts for the flow there.

Overall, you can calculate the flow on the basis of the difference in height between the two ends and the resistance of the total length of tube. A siphon will not work if there is a vacuum above the fluid in the top reservoir. It will also not work if the whole system is in free fall. You can still use capillary forces in these situations, but that is a different can of worms.

Do not try to siphon petrol by sucking on the hose. You are likely to get lungful of petrol vapour, which can cause your lungs to go into spasm and can be fatal quite quickly. One guy was found lying dead beside the car with the siphon still running onto the ground. The same would apply to any other volatile liquid.

John

[peak4Participant @peak4]

Now whether this would still be the case with a siphon tube a couple of feet in diameter, I have no idea.

[Barnaby WildeParticipant @barnabywilde70941]

Years ago we drained part of a swamp to recover the land for grazing. We had the experts in & the pump system was duly specified & delivered to an accessible part of the site.

Sounds easy doesn't it The keyword here is "to an accessible part of the site", we couldn't get the delivery vehicle with it's lovely HIAB anywhere near where the pump was required.

I remember cursing the office chair labourers & especially the clean shoes site surveyor as we dragged & lugged everything into place.

Not many engineering types will know this but sweat is actually your skin crying in distress !

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I follow and understand the experiment in the video that peak4 has posted, however they have started with the system primed. If you stick one end of an empty tube into a container of water and then have the other end outside the container and below the end that is in the water, nothing will happen. Now assuming that the tube is large enough not it allow capillary action to take place and you evacuate the air from the low end of the tube (i.e. suck the air out) then the syphoning will proceed to take place and to my mind atmospheric pressure is the only thing that will drive the water up the tube and over the top of the container. Something that they could not achieve in the video. A vacuum has no pulling power, even in a vacuum cleaner you have to have an air flow for it to do its job.

Regards Nick.

[SillyOldDufferModerator @sillyoldduffer]

Posted by JA on 03/08/2019 11:57:00:

Posted by SillyOldDuffer on 03/08/2019 11:21:54:

Posted by Bazyle on 03/08/2019 10:49:03:

Maybe not. A syphon can only apply one atmosphere to overcome the friction per 30 ft pipe…

Or maybe that's wrong too!

I think I'm correct in saying that no-one understands how siphons work….

Dave

Sorry, siphons are simple. A bit of fluid dynamics –

Looking at the BBC news site the dam is about 45 ft. Let us have a siphon pipe, 1 foot diameter, full of water from and to the bottom of either side of the dam. With the dam full the water pressure difference between the leak surface and bottom of the dam is

height of water x density of water = 45 x 62.4 [lb/ft^3] = 2808 [lbf/ft^2] = 19.2 [lbf/in^2]

This drive the flow of water whose velocity can be calculated using Bernoulli's equation (really conservation of energy). The velocity in the pipe is

square root of (2 x pressure difference / water density) = (32.2 x 2 x 2808 / 62.4)^0.5 = 54 ft/s

[the 32.2 is required because imperial units gets into a bits of a mess when handling mass and force]

The water flow rate can then be calculated for the full reservoir

water velocity x pipe cross sectional area = 54 x 3.1415/2 x 1^2 = 85 ft^3/s = 530 gallons/s

…

Many thanks for the maths, I accept JA's logic entirely, but I don't think it explains how siphons actually work.

Isn't this failure to explain rather than describe generally true of all formula? Maths allow natural phenomena to be quantified and predicted, which is extremely valuable. But formula don't explain anything, they're just models. It's true formula improve understanding by eliminating explanations that don't match the numbers, but – bottom line – they don't explain physical causes. (A nice example of elimination by maths in JA's post : the sums don't feature atmospheric pressure, strongly suggesting atmospheric pressure is not what drives siphons. Gravity does feature in the formula and thus remains a player. )

Ohms Law is similar. V=IR does not explain electricity. V=IR works without the concept of electrons let alone the fact that a deeper look proves electron behave like waves (not particles) and particles (not waves), which cannot both be true in ordinary life. Just as well – it means engineers can exploit electricity even though the scientists don't fully understand it.

Are liquids in a siphon pushed from the high pressure end or pulled from the low? My feeling favours 'pull' but I wouldn't bet the farm on it!!!. Provided the pressure on the output is lower than the pressure on the input and the molecules are bound well enough to their neighbours to haul them through, I can imagine it working. Note a chain being pulled over a lip by the weight of the hanging end has no pressure on the static end – there is no push at all. Also, provided the tube is full and output is lower than input it is not necessary to suck on the end to start a siphon.

Curiosity about such things is valuable. Huge engineering benefits have come from science penetrating deeper in to the workings of the natural world. Chasing electrons has given us a much better model of the atom, resulting in laser beams, semi-conductors, advanced communications and chemical engineering, and much, much else. I doubt understanding siphons will open the door to Nirvana but who knows?

Possibly a parallel between siphons and global warming. Lack of conclusive scientific proof and a full understanding of the mechanism does not mean the results cannot be predicted…

Dave

[Alan VosParticipant @alanvos39612]

I see it this way. A siphon runs on gravity, not air pressure. Creating a partial vacuum is a convenient way of priming the system and getting the process started, but not the only way.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Alan Vos on 04/08/2019 09:58:17:

I see it this way. A siphon runs on gravity, not air pressure. Creating a partial vacuum is a convenient way of priming the system and getting the process started, but not the only way.

.

Sounds good to me, Alan

MichaelG.

[Kiwi BlokeParticipant @kiwibloke62605]

Well, if no-one really understands how syphons really work, allow me my 50 cents' worth. I've skimmed the Wikipedia entry, so am now an instant expert and world authority…

The 'flying droplet' syphon is interesting. Elsewhere in the article, it's said that a syphon will work (assuming that it has been started, presumably) in a vacuum. But would the 'flying droplet' type work in vacuo? It seems to me that the discharge tube tries to empty, by gravity, just like water running out of a simk, thus reducing the pressure in the 'air chamber'. This 'sucks' up liquid in the feed tube. But this suction should only work if air pressure pushes the liquid up the feed tube, into the 'air chamber'. So, for this type of syphon, it seems to me that air pressure is needed to 'get over the hill', and gravity takes care of the flow down the discharge tube. Of course, the cohesive property of liquids helps the bulk flow in the discharge tube from breaking up and allowing air up the tube, to break the syphon. So, a combination of gravity and air pressure are required.

Does this mechanism seem a reasonable explanation for 'conventional' syphons too? …Well, those not in vacuo, at least.

[Kiwi BlokeParticipant @kiwibloke62605]

However, on thinking about it a bit more, the laws of thermodynamics and energy conservation have to be satisfied, and I've conveniently ignored the energy accounting. I think I'll go to bed…

[not done it yetParticipant @notdoneityet]

C’mon guys. Syphon is caused by gravity alone. The weight of the column of water below the level in the source will follow the rules of gravity which Newton sorted out for us some 300 years ago.  Atmospheric pressure will only affect the level in the pipe in as far as it will fill to the same level as the surroundings, nothing more! Only that reduced pressure (in a head of less than 9.81m) will be causing the atmospheric pressure to drive the water up the header, until the forces are in equilibrium.  Forces will be in equilibrium when a vacuum space appears above the liquid in the header. Or the level of the source and receiver are equal.

The famous 9.81m is only applicable for standard atmospheric pressure here on Earth, for water of unit density. It would be more or less, dependent on where the syphon was set up – Earth, Moon, Mars for instance. That static height is fixed by atmospheric pressure, gravitational fields and density of the liquid involved.

Think mercury barometer. The instantaneous height of the mercury column depends on the prevailing atmospheric pressure. The general height also depends on the density of the liquid – Mercury is 760mm, and a water barometer would be ~10 m (errors come in due to the difference in gas vapour pressures above the liquid.

One would not even manage to make a sensible barometer with a volatile iquid like ether as it would actually boil at room temperature under any sort of high vacuum.

Gravity and mass create the weight of everything (the force which, for syphoning is an unbalanced force, so Newton’s Laws of linear motion will apply and the unsupported water clumn will accelerate according to those Laws).

Other things to think about when syphoning, is to consider the diameters and fluid mechanics of flow, along with a pipe that will not collapse under the conditions applied 9often happens when syphoning beer or wine from one bucket to another using a plastic pipe!).

One underlying principle is that liquids are incompressible (measured with equipment available to us laypersons). That makes them unexpandable by the same reasoning.

End of tutorial. Sort out the rest yourselves!

Edited By not done it yet on 04/08/2019 12:01:41

[SillyOldDufferModerator @sillyoldduffer]

Can't resist pedantically pointing out a repeated typo in NDIY's post. 9.81 is the acceleration due to gravity in metres per second, not the maximum lift achievable by a pump using only atmospheric pressure, which is about 10 or 11 metres.

Apart from that, can't disagree with NDIY except to ask:

1. Can a very thin liquid, like a heavier than air gas, be siphoned? My guess is yes.
2. Can a very sticky liquid like glue be siphoned? My guess is no, because the friction on the sides of the pipe may be enough to overcome friction, much as wet paint stays on ceilings.
3. Which spelling do you prefer. I like Syphon but my allegedly British spell-checker insists otherwise.

Meanwhile, 12pm at Whaley Bridge, they've still not drained enough water to make the dam safe and more rain is on the way. Poor people.

Dave

[JAParticipant @ja]

Dave

For a siphon to work the fluid must have an appreciable density. All gasses have densities much lower than liquids so gas siphons are not met in practice. I write this knowing that someone in an obscure university lab has probably made one. As for ordinary viscous Newtonian liquids a siphon will work. However for non-Newtonian liquids such as many paints, cow muck etc all bets are off.

Your earlier point about having an understand of what is actually happening is very pertinent. My college text book of fluid mechanics describes what a fluid is in five or so pages. For the next 695 pages a fluid, liquid or gas, is treated as a continuum (that is "no part of which is perceptibly different from the adjacent parts". This model is very successful until one has large free molecular paths such as in a vacuum or hypersonics.

Siphon, or syphon which I also prefer, does not feature in the index of my text book.

Enough said, this is a long way from a dam in Derbyshire.

JA

Edited By JA on 04/08/2019 13:09:10

Edited By JA on 04/08/2019 13:10:32

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 04/08/2019 12:19:45:

Can't resist pedantically pointing out a repeated typo in NDIY's post. 9.81 is the acceleration due to gravity in metres per second,

[ … ]

Meanwhile, 12pm at Whaley Bridge, they've still not drained enough water to make the dam safe and more rain is on the way. Poor people.

Dave

.

[ pedant alert ]

… acceleration due to gravity in metres per second per second

MichaelG.

.

Now: Thanks for remembering what this is all about, Dave and JA 

The A6 remains closed [not that the idiots driving past the signs believe that, until they are turned back by the Police] and they have stopped people nipping back to their houses in Whaley, because of the high level of risk.

… If you know a 'No Rain' dance, please do it.

Edited By Michael Gilligan on 04/08/2019 13:39:12

[old martParticipant @oldmart]

I wonder if anyone has used the sheet metal piling used in sea defences, river and canal banks and cofferdams to repair dams such as this? It is relatively fast to use if the machinery could reach the water side of the dam. I don't know what the length of the piles could be or if they could be driven through the puddled clay core. I could imagine a complete wall of piles across the dam, with a 50 feet section in the centre a foot lower for the spillway with a thick tarpaulin like cover secured from the spillway down to prevent the erosion.

[Neil WyattModerator @neilwyatt]

Posted by JA on 04/08/2019 13:03:55:

Dave

For a siphon to work the fluid must have an appreciable density. All gasses have densities much lower than liquids so gas siphons are not met in practice. I write this knowing that someone in an obscure university lab has probably made one. As for ordinary viscous Newtonian liquids a siphon will work. However for non-Newtonian liquids such as many paints, cow muck etc all bets are off.

Indeed they have, but it's behind a paywall:

opscience.iop.org/article/10.1088/0031-9120/46/4/006/pdf

Neil

[Bruno TaylorParticipant @brunotaylor21701]

Not wishing to detract from this discussion can l make a practical observation. This is an emergency situation and the urgent need is to do something with the equipment you have immediately available and you know works. Hence pumps pumping.

Many years ago l was involved in a similar situation with a flooded quarry caused by an overflowing river, the ground was sand and the erosion of the sand threatened the railway embankment (a little south of LBSC for those that know their geography) Working for the DCE we had first 6 then 12 6"" pumps working before the "Consultant" design scheme of 2 x 12" pumps were even installed. Yes once they were working those pumps started to remedy the situation. But the bank of 6" pumps from the local plant hire companies off the shelf with miles of pipes kept the trains running and commuters happy.

In retrospect l don't think Chinnook's would of helped but it would have been fun trying.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 04/08/2019 21:03:06:

Posted by JA on 04/08/2019 13:03:55:

Dave

For a siphon to work the fluid must have an appreciable density. All gasses have densities much lower than liquids so gas siphons are not met in practice. I write this knowing that someone in an obscure university lab has probably made one. As for ordinary viscous Newtonian liquids a siphon will work. However for non-Newtonian liquids such as many paints, cow muck etc all bets are off.

Indeed they have, but it's behind a paywall:

opscience.iop.org/article/10.1088/0031-9120/46/4/006/pdf

Neil

.

Well, the authors did borrow the use of lab facilities [at Carleton College, Northfield, MN 55057, USA] for the low pressure experiments.

You may find that you can also read/download the article here: **LINK**

https://www.researchgate.net/publication/231125972_Siphonic_concepts_examined_A_carbon_dioxide_gas_siphon_and_siphons_in_vacuum

MichaelG.

[Author]

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