Syphon Break

[lifeofreilly57]

I'm redoing the engine compartment while I have the engine out and have good access. As part of this I'm replacing the hoses and syphon break to the raw water to the engine and generator.
I see both stainless and plastic syphon breaks available but they usually say for plumbing applications, I haven't seen any that say their specifically for raw water engine applications.
Any information or thoughts on the subject would be appreciated.

[deblen]

I replaced the corroded alum. Volvo Penta siphon break in my boat with a brass home type-designed for domestic hotwater heaters,etc- some 8 yrs ago.
Works fine for me.
Cheers/Len
Similar unit https://www.amazon.ca/Cash-Acme-1738...00MNLNF6&psc=1


Pee tube-another option https://www.moyermarineforum.com/for...ead.php?t=7263

[Uncle Bob]

Or you could replace the commercial break with a simple "u" bend and fit a small bleed line from the top to either the cockpit drain or directly overboard, no valves to fail, simple and provides a visual confirmation that the pump is operating correctly, just like the telltale on an outboard.

[lifeofreilly57]

Originally Posted by Uncle Bob

Or you could replace the commercial break with a simple "u" bend and fit a small bleed line from the top to either the cockpit drain or directly overboard, no valves to fail, simple and provides a visual confirmation that the pump is operating correctly, just like the telltale on an outboard.

The only problem there is that this is on the suction side of the raw water feed for the engine and generator, so that wouldn't work in this application.
MOst of the syphon brakes I've looked at say they're for toilets and other non-engine related applications. I'm starting to think they've all gone that way for liability reasons. Tough to overheat a head.

[kenbo]

I don't see the need for a vacuum break on the raw water feed side of the cooling loop as long as there is a vacuum break above the WL somewhere in the loop.

What Uncle Bob suggested is very effective and nearly foolproof. Working on the positive pressure side of the loop makes fitting a vacuum break much easier.

[Kelkara]

Groco make them in bronze and say they are for engine and exhaust applications ... but they won't work on the suction side either. No standard siphon break will ... they break a siphon by letting air into the pipe under suction.

[geoleo]

Even if a suction occurs when engine is turned off-- just how can it pass thru the vanes on the engine raw water pump? ----and even if it could it would just flow thru the heat exchanger then thru the exhaust manifold and the water cooled elbow and out the exhaust hose back into the water outside. I just dont get this action????

[Stu Jackson]

Originally Posted by lifeofreilly57

I'm redoing the engine compartment while I have the engine out and have good access. As part of this I'm replacing the hoses and syphon break to the raw water to the engine and generator.
I see both stainless and plastic syphon breaks available but they usually say for plumbing applications, I haven't seen any that say their specifically for raw water engine applications.
Any information or thoughts on the subject would be appreciated.

I believe this is a common misunderstanding of the application of vented loops. They are to be installed on the outlet (pressure side) of pumps, not on the inlet (suction side).

[jamhass]

What Stu and Uncle Bob said, in spades!

[skipperpete]

Originally Posted by geoleo

Even if a suction occurs when engine is turned off-- just how can it pass thru the vanes on the engine raw water pump? ----and even if it could it would just flow thru the heat exchanger then thru the exhaust manifold and the water cooled elbow and out the exhaust hose back into the water outside. I just dont get this action????

Geoleo, it’s a bit like Russian roulette, mostly the impeller stops in a position that covers both ports in the pump but every now and then they don’t. Then there’s the worn endplate or missing blade that permits the establishment of a siphon, ( it can be a very small inflow)…. And finally, the engine kickback at shutdown , about 5 out of 10 times when I take off a cover plate a couple of the blades are pointing in different directions or one blade is folded flat on itself, engines don’t instantly stop rotating when you activate the stop, one cylinder will always be ¾ of the way up a compression stroke and she’ll rock back a bit.
One of the fatal flaws with new engine installations is the belief that the raw water impeller is a positive seal, and on a new pump it probably is but if the vent loop is badly installed and not functioning , sooner or later an engine flooding is an unwelcome and bewildering surprise for the owner. It’s so common that engineers even have a name for catastrophic early failure of newly installed or rebuilt diesel engines.
Look at it this way, a siphon break could cost you $15-20,000 dollars if you get it wrong.

[skipperpete]

One more thing….. putting the vent loop too early in the raw water circuit causes another unique problem, too much water in the circuit before the mixer elbow ( or less so on the suction side of the pump) can overfill the Aqualock at shutdown or create a water velocity fast enough to splash back into the exhaust manifold in the absence of manifold pressure, one exhaust valve is always open so cylinder flooding can occur in the worst case scenario but more commonly, raw water splash is often seen as destructive corrosion at the joint between the mixer elbow and the ( usually ) aluminium heat exchanger/ exhaust manifold. The hose for the vent loop circuit need to be the same size as the raw water pump outlet so only use a siphon break that maintains that diameter.

[stormalong]

You can use plastic or a corrosion resistant metal. I would not use brass. Brass will fail eventually. This is salt water after all. The water is cold sea water, not hot so plastic is fine. Mine is Marelon and original to my now 38 yer old boat.

The vacuum breaker is the usual problem. They can get clogged with salt. A weep tube in place of the vacuum breaker, IOW a small hose that goes well above the heeled water line and drains to a cockpit drain or overboard solves this.

And yes, you can get a siphon if you are missing a vane on your raw water impeller and may not even know it. The pump can function well enough with a missing vane that you would not notice.

[NevilleCat]

Nanni makes one...
https://images.app.goo.gl/j3NJcA2Cebs5kikA9

[Loredo]

Vetus also makes them:
https://webshop.vetus.com/en/product...tems/air-vents