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Syphon or Siphon - but the dictionary is wrong anyway. 6

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PS - I have not designed siphons with dynamic pressures below vapour pressure. The maximum crest height of a siphon is atmospheric pressure (32 ft) - vapour pressure.


I have designed siphons that prime by evacuating air out of the downstream leg - is the water then sucked over the crest by tension or is it pushed by atmospheric pressure ?

If you had no atmosphere but still had gravity a siphon would not work !

 
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If there is acceleration, its acceleration, if there is pressure, its pressure, if there are both, its both. If pressure is due to gravity [ρ]*g*h, or [∑]1/2 mv^2 of the gas molecules is irrevelant, its the sum = total head (as energy) of an external fluid that supplies the motive force to the fluid within the siphon.

I've commented to the article in the newspaper as follows,
====================================================
Dr. Hughes isn't entirely correct, in fact still about as erroneous as the original definition. The definition in the dictionary was correct for its time, when travel outside the Earth's gravational field was not seriously contemplated. While still correct on Earth, there is no doubt that it needs to be updated for modern times by replacing its reference to atmospheric pressure with the sumation of the molecular kinetic energy, 1/2 mv^2 + pressure, p*g* h, of the external gas supplying force to a siphon. These days, it is not limited to either Earth's gravity or atmospheric pressure. Gravity could be replaced by acceleration of a spaceship and atmospheric pressure by a connection to any external pressure source, such as the atmosphere in a space ship.

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"Being GREEN isn't easy" ..Kermit

 
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Sorry to come in a so long thread, but really can't see why a siphon could not, in principle, work in the absence of atmospheric pressure.
Granted that any real liquid will vaporize at zero absolute pressure, making the siphon behavior impossible, but we can easily imagine, without breaking any fundamental law, of an ideal fluid that, just like many solids, would not vaporize in vacuum, or at least would at a very low rate (like solids that sublimate).
Gravity, on the contrary, is intrinsically and in principle, required (in a stationary system), just because is the loss in potential energy of the fluid that makes it possible to overcome friction losses associated with fluid flow.
So I'm more with the position that siphons are gravity fed (in a stationary system) than they are fed by atmospheric pressure.

prex
: Online engineering calculations
: Magnetic brakes and launchers for fun rides
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Of course they can work in the absense of external pressure. External pressure is only one component of potential energy, or head, acceleration being the other. A siphon would work if you have either one of them, or both. A siphon simply converts some potential energy to the kinetic energy of motion of the fluid within. Where they won't work is where there is zero potential energy, no 1/2 mv^2 or acceleration.

What I totally fail to understand is why that debate continues. Can anybody explain why they do not choose to accept Bernoulli's theory and back it up with a spreadsheet calculation of an example why it does't work?

I think it would be more productive use of remaining time to discuss if a siphon could continue, after being started, due to liquid tension effects, if net pressure and accelarations drivers suddenly became nil.

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"Being GREEN isn't easy" ..Kermit

 
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Well, if you have a glass full of water next to an empty glass, and have a cotton string string draped into both, then the water will rise up the string by capillary action (surface tension) and drop into the empty glass due to gravity plus capillary action. The level in the two glasses will equilibrate. In this example, clearly, no air pressure is involved. However, I don't think it would be correct to call it a siphon.

If you have a chain in a bucket that rises to a pulley and then drops into a lower bucket would that be called siphon?

Another interesting example: in a pool of fresh water you submerge a container of salt water (higher density than fresh). If a tube is draped over the edge of the bucket to a lower level nothing will happen if the tube is filled with fresh water. "Prime" it with salt water and the salt water will flow out of the bucket. There is no air pressure involved and no interfacial surface tension. Since all liquids, by definition, have surface tension, you can do the same experiment with two gasses with differing density.

The siphon principle is not about what makes fluids flow down-hill. It is about what makes it go up first. In most practical siphons it is air pressure pushing the liquid.
 
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That's only because it is much easier to provide the necessary potential energy differential by changing the pressure, rather than changing acceleration. However, if you fitted a siphon hose to the side of a gas tank and began spinning the apparatus about the vertical tank axis, until flow would initiate within the tube as the result of centripital acceleration forcing the gasoline to flow up towards the gooseneck, cross over and flow downwards thereafter, at which time you could stop rotation and flow would continue without further aid. No change in atmospheric or siphon outet pressure would have been necessary for initiation.

**********************
"Being GREEN isn't easy" ..Kermit

 
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Is there any real dispute that once a siphon is primed it is gravity pulling the water down the downstream leg that creates a negative pressure which gives a pressure differential allowing air pressure to push water over the hump.

In "gravity" pipes and siphons water flows from a higher to lower elevation by gravity - the feature of a siphon, (as it is traditionally understood) is that water first goes uphill, and unless anyone really believes that this is achieved by tension in the water, it clearly goes uphill due to the difference in pressure between atmospheric pressure at the inlet and the less than atmospheric pressure at the crest.

 
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Bris - if the siphon is fully primed, then it is acceleration due to gravity or any other source (of acceleration) that does it and it has very little to do with atmospheric pressure. In fact, I have never seen a siphon initiate simply because of atmospheric pressure differences. In normal situtations, such as on earth and not spinning rapidly - it requires either a) artificial suction on one end, b) pressure on one end or c) full priming to begin a siphon which will then continue simply due to gravitational acceleration.
 
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The only "dispute" I have is that you're seeming to say that the only thing that can give rise to potential energy is gravity. That would ignore the other terms in Bernoulli's theorm and seemingly prohibit siphons in weightless conditions, which obviously can exist where a gas pressure, not caused by gravity (as is atmospheric pressure), provides the potential energy difference that continues the flow. Other than that, nothing else I can put my finger on.

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"Being GREEN isn't easy" ..Kermit

 
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One step past Bernoulli, you could heat an expansive liquid in a closed container, even with no driving gas pressure, until fluid was forced up a siphon in a direction opposite a net acceleration and then let heat and acceleration continue the flow over the gooseneck, or remove the acceleration if you like, and allow the flow to continue as long as the fluid continued to expand due to heat alone. In that case only heat would provide the required potential energy by increasing the molecular kinetics of the liquid, which was responsible for its expansion to begin flow.

**********************
"Being GREEN isn't easy" ..Kermit

 
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It is certainly possible to move a liquid through a pipe by applying a gas pressure to one end of the pipe, as was done traditionally with acid eggs. And of course this can be done whether we have gravity (or any other form of acceleration) involved or not - only the magnitude of the required pressure differential would change. But I would not call that a syphon.

For my money, to be termed a syphon the system must take advantage of the natural tendency of liquids to flow from a source at a higher level down to a lower level under the influence of gravity, but with the added requirement of having to flow via a third level that is higher than the source level.

Further, I would say that if there is no external atmospheric pressure that prevents the liquid from boiling at the apex of the syphon you do not have a true syphon system. The direction of the force of gravity (or other acceleration) would cause the liquid to flow away from the apex, and it is only the external atmospheric pressure that prevents boiling. As soon as boiling occurs the liquid heights in the two legs equilibrate and flow stops.

Many different flow systems, from ancient trees to modern space vehicles, have been described in this thread and while they may indeed cause liquid to flow from one point to another I would not call them syphons unless these two requirements are met.

Katmar Software
Engineering & Risk Analysis Software
 
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Katmar, "at a higher level down to a lower level under the influence of gravity, but with the added requirement of having to flow via a third level that is higher than the source level."

Why limit yourself to "gravity"; when it is the effect on the liquid that could be caused by any applied acceleration which gives the potential energy needed to make it work. Flow moving through a waypoint "higher than the source", yes, but higher in relation to what? In space there is no higher or lower level. Define it with respect to the direction of acceleration and a siphon can exist using the remainder of your definiiton.

Further, what's your fixation with pressure caused by atmospheric pressure and is it relavent to whether a siphon flows or not.

I think you would probably have a different view of things, if you got your head off the ground. Can a siphon work in an airplane? Can it work when its flying upside down? Can it work when its upside down while flying a weightless trajectory? Can it work while executing a loop; at the top of the loop, a 9G loop. There you have it, Earth gravity is ony 1G, so 10G would be caused by centripital acceleration and -1G by Earth gravity. Net 9G "up" and reduced atmospheric pressure at 10,000 feet, causing flow to go up the normally "downward" pointing downspout. Now go to a U2 at 100,000 feet. Take the next step. Where do you draw he line? Earth's gravity technically never goes to zero, only approaches zero until you get somewhere near another body that has a gravitational attraction greater than Earth's and the net gravitational vector flips the other way. :-)

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"Being GREEN isn't easy" ..Kermit

 
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I like katmar's definition, however it does not address an inverted siphon, used commonly (well not all that commonly) on sewer mains which go under a crossing obstruction such as a river, freeway, or another pipe. These siphons travel through a third level that is lower than the source. I'm not sure if they are truly siphons at all and am sure we will not decide it on this forum.


 
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now I keep hearing that BP is siphoning off the oil through a long thin tube to the bottom of the gulf. I have seen numerous news reports showing a schematic of the "siphon". Care to comment on if this is truly a "siphon"
 
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If its a bent pipe, I'll bite.

I figure it probably goes up to the surface, then goes higher, crossing over the top of the tanker's hull before turning back down and going below sea level into the tanks, thereby officially qualifying to be a "siphon" or perhaps a "syphon", by anybody's definition, since unfortunately, this is happening on Earth and within its gravational field.

Not trying to give anyone a hard time, only to find a bit of humor in a very bad situation. Apologies in advance.

**********************
"Being GREEN isn't easy" ..Kermit

 
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BigInch - Yes, the gravity could equally be some other acceleration and I mentioned this several times in my "definition". Of course by "higher" I meant in the direction of the acceleration.

You cannot have a syphon if the atmospheric pressure is insufficient to prevent boiling at the apex. This was one of the first points made (by 25362) right at the top of the thread. If this were not so you could have the apex of a water syphon (on earth!) more than 10 meters above the source level, and we all know that is impossible (barring capillary action).

And if you want to operate a syphon under a gravity (whoops, I mean acceleration) of 10G you need a proportionally higher atmospheric pressure.

cvg - I would not call an "underpass" a syphon, but I know what you mean. Also I think that the use of the word "syphon" in regard to BP's activities in the gulf is more akin to the figurative use of the word as in "my bank syphons money out of my account disguised as service fees" than to the literal sense of "some thief syphoned the gas out of my car". Please note that there is no hidden meaning in the fact that I use the words "bank" and "thief" in the same sentence.

Katmar Software
Engineering & Risk Analysis Software
 
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Why must there be atmospheric pressure? Pressure is pressure. Pressure level? Just keep the temperature below the bubble point for any pressure you happen to have at the gooseneck and you won't vaporize. I only thought vaporization was prohibited if you wanted to consider tension force. Besides, you could have partial vaporization, with a vapor bubble forming at the top of the pipe and liquid flow underneath it. Would that not be a siphon? You could have a vapor bubble form across the entire cross section and, if the velocity was fast enough to carry it down below the bubble point line in the downcommer, before the siphon "broke", it would be a 2phase flow siphon. If the bubble reached a high enough pressure to collapse as it was swept down the liquid column, it would go back to one-phase siphon flow.

**********************
"Being GREEN isn't easy" ..Kermit

 
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katmar, inverted siphon is not my word - it is a published and frequently used term. I have not heard any other term used to describe what you call an underpass. It is published in the following dictionary:

Sci-Tech Dictionary
McGraw-Hill Dictionary of Scientific and Technical Terms, 6th edition, published by The McGraw-Hill Companies, Inc.

 
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If its a bent pipe, I'll bite.

:) Author Terry Pratchett (I think) once described all life as being a bent tube. So, a python is a syphon, and it'll certainly bite...though you may disagree:

Is a perstaltic python a syphon?
 
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