Anti-siphon loop for engine

Steve_N said:
Beta are slightly more cautious and go with 25cm in their 'Engine Installation Guide'. They also make the recommendation that, in a yacht, the anti-siphon valve should be as close to the centreline as possible so that there's no possibility of the valve dipping below the waterline when heeled.
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There is a lot of useful info in that guide Posting a link to it might be good.

Beta Marine Engine installation guide
 
Well anyone who can make water syphon 25cm up hill is doing well!

To be fair, you do need a margin for things like wash slamming into your moored boat, and £20 on plumbing to be sure of that is cheaper than the exhaust filling with water.
There are some cheap anti-syphon valves and bends sold for washing machines etc.
 
Regarding the decision to be made as to whether go with valve to pee pipe...it would have been inconvenient for me to run a pee pipe overboard so I went for a valve, but to reduce the liklihood of the valve failing to open when it gets clogged up with dried salt I put it at the end of a tall vertical run of pipe leading up from the U shaped T piece. This way when the engine is running a column of air is trapped in the pipe and the seawater doesn't get near the valve. When the engine stops the valve admits air to break the siphon and allow the seawater to drain out of the inverted loop.
Note that it is always essential that the vented part of the loop is well above the water line as even a vented pipe will still allow the engine to fill with water if the route is downhill/below the waterline all the way.
 
Just noticed pirates cave chandlery on the Medway have an anti-siphone valve that looks remarkably similar to a well known make for 19 squid - he's mine !
 
macd said:
On the contrary, water can easily be made to siphon 25 feet uphill. That's why we're discussing anti-siphons.
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I'm intrigued by the notion that water can be induced to syphon uphill at all, since I had always thought that a syphon worked by having the outlet lower than the inlet. The high point of the loop can't be more than 32 feet above the outlet, though - is that what you meant?
 
RichardS said:
Isn't it 32 feet above the inlet?
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I don't think so, because you need the outlet side water column to provide the suctions at the top, and if that side is more than 32' long the water will just evaporate at 32' above exit. Maybe. Perhaps. Has anyone tried this? Thinks ... my mast if more than 32' tall ...
 
JumbleDuck said:
I don't think so, because you need the outlet side water column to provide the suctions at the top, and if that side is more than 32' long the water will just evaporate at 32' above exit. Maybe. Perhaps. Has anyone tried this? Thinks ... my mast if more than 32' tall ...
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Allrighty... siphons:

Lets start with the basics: 2 buckets and a hose out the top of 1 and into the other, the hose is full of water. If you dont get al funky with big heights and vacuums then the water will siphon from the higher water level to the lower one.
Now then, separately you put one end of a 50' hose in a bucket of water and suck... if the hose is straight up then air pressure on the water in the bucket will push the water 32' up and no more. Suck as hard as you want, thats it. you will get a vacuum at the top so water there will boil, ruining your vacuum.
Now, if the top of the hose is bent down and into another bucket (high up) then the air pressure on the water in that bucket will push the water up the hose, if the top of the loop is less than 32' above the top bucket it will empty pronto into the lower one. If, on the other hand, the top of the loop is more than 32' above the water in the upper bucket then things get a lil bit funky. You still get a siphon, but its not liquid water that is doing the work, its vapour. Since the column of vapour above the water above the lower bucket is taller than for the upper bucket then, when evaporation (ruining the vacuum) has evened out you actually get condensation occurring at the top of the lower column. It gets even more funky since this will cause a very rapid heat transfer from the upper level to the lower, offsetting some of the pressure differential.
 
Dougal Tolan said:
Now, if the top of the hose is bent down and into another bucket (high up) then the air pressure on the water in that bucket will push the water up the hose, if the top of the loop is less than 32' above the top bucket it will empty pronto into the lower one.
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Hmm. Intriguing. It seems to me that the pipe size must matter as well, because once the fluid starts moving there will be pressure effects due to friction which could have a significant effect. Supposing you filled your 50' pipe with water and, with both ends sealed, put one end in a bucket of water, led the pipe up 10' and down 40' to another bucket. Clearly the 40' long side would have a 8' gap at the top filled with vapour at the saturation pressure for its temperature, and that would be a lot lower than needed to pull up the water in the 10' leg, so the syphon would start when the ends were uncorked. But would the vapour gap close up, or would water flow down through it. I'm guessing that that would depend on the resistance to flow; if you only slightly unsealed the high end you'd only get a trickle through.

If, on the other hand, the top of the loop is more than 32' above the water in the upper bucket then things get a lil bit funky. You still get a siphon, but its not liquid water that is doing the work, its vapour. Since the column of vapour above the water above the lower bucket is taller than for the upper bucket then, when evaporation (ruining the vacuum) has evened out you actually get condensation occurring at the top of the lower column.
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I'm finding this a wee bit harder to accept (which isn't to say you're wrong). If the pipe goes up 40' and down 50', the "up" side will have 8' of vapour at the top and the "down" side will have 18' of vapour. Since that's at a constant pressure (as before, saturation pressure for the water) then you effectively have two identical water columns 32' long with the same pressure at the top and bottom (ignoring minutely tiny atmospheric head differences at the bottom). I can't see why in that case there would be any imbalance to the pattern of evaporation and condensation. Wouldn't both columns just stay in equilibrium with the vapour space?

It gets even more funky since this will cause a very rapid heat transfer from the upper level to the lower, offsetting some of the pressure differential.
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Hmm. Still can't see why one column would evaporate more than the other.
 
Dougal Tolan said:
Allrighty... siphons:

Lets start with the basics: 2 buckets and a hose out the top of 1 and into the other, the hose is full of water. If you dont get al funky with big heights and vacuums then the water will siphon from the higher water level to the lower one.
Now then, separately you put one end of a 50' hose in a bucket of water and suck... if the hose is straight up then air pressure on the water in the bucket will push the water 32' up and no more. Suck as hard as you want, thats it. you will get a vacuum at the top so water there will boil, ruining your vacuum.
Now, if the top of the hose is bent down and into another bucket (high up) then the air pressure on the water in that bucket will push the water up the hose, if the top of the loop is less than 32' above the top bucket it will empty pronto into the lower one. If, on the other hand, the top of the loop is more than 32' above the water in the upper bucket then things get a lil bit funky. You still get a siphon, but its not liquid water that is doing the work, its vapour. Since the column of vapour above the water above the lower bucket is taller than for the upper bucket then, when evaporation (ruining the vacuum) has evened out you actually get condensation occurring at the top of the lower column. It gets even more funky since this will cause a very rapid heat transfer from the upper level to the lower, offsetting some of the pressure differential.
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Ands that why we have multi stage pumps very simply speaking. Also my good old boys rough and ready Oil field calc, 27
" water equals 1psi.
Stu
 
JumbleDuck said:
I don't think so, because you need the outlet side water column to provide the suctions at the top, and if that side is more than 32' long the water will just evaporate at 32' above exit. Maybe. Perhaps. Has anyone tried this? Thinks ... my mast if more than 32' tall ...
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The 32 feet consideration is all about air pressure and whether the weight of the column of water exerts more or less pressure than atmospheric. In normal circumstances this is only relevant to the higher/source water level.

Once you are below the 32 feet limitation all that matters is the weight of water in the two sides of the U-column. Simply put, the heavier side will always pull the lighter side over.

Richard
 
RichardS said:
The 32 feet consideration is all about air pressure and whether the weight of the column of water exerts more or less pressure than atmospheric. In normal circumstances this is only relevant to the higher/source water level.

Once you are below the 32 feet limitation all that matters is the weight of water in the two sides of the U-column. Simply put, the heavier side will always pull the lighter side over.

Richard
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Hmmm, interesting concept, that, water pulling......
 
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