Syphon break.

[ccscott49]

In reference to the other thread, This syphon break thingy, I'm well aware what a syphon break is for, but in this instance, if it is placed before the pump, it does nothing, unless the impellor is knackered, (as the pump acts as a one way valve, as long as the impellor is OK) then it may flood the engine, if it wasn't there. If it is placed between pump and engine, it does nothing again until the impellor goes, if its placed between the engine and the exhaust, it is the same scenario, it doesn't stop water coming back up the exhaust, the water trap/silencer does that, plus the bend in the exhaust, or am I missing something here? Can somebody put me right here, thank you!

 

[vyv_cox]

As I understand, the siphon break between engine and exhaust manifold is guarding against gas contraction on cooling. In installations where the engine is close to or below the water line, on stopping the engine it is possible that contraction of the exhaust gases could draw water up the exhaust from the trap and into the engine. Once started the water could continue to siphon over into the engine.

It sounds unlikely to me but people here seem to have experienced this. My previous Bukh installation had no siphon break and never had this problem, but when I had the Yanmar installed, the shop that did it insisted on fitting the siphon break.

I agree that putting one between pump and engine makes no sense. Putting one between water inlet and pump could be harmful, as it will put a column of air on the suction side of the pump. Although it is unlikely to prevent priming of the pump altogether it could easily cause overheating of the impeller before prime was established.

 

[ccscott49]

But, the syphon break is in the line between the engine and the injection elbow, it would do nothing, only if it was between the outside and the engine, it would have exhaust gases going through it and a mixture of water, no syphoning action can take place if there is any air/gases present, a flapper valve I could see, but not a anti syphon valve, that has 1" or less connections on it. The ones I have fitted, all went between the pump and the engine, I always wondered what the hell they were for! Apart from the knackered impellor scenario. I still don't understand their purpose apart from that.

A component that fits in the water supply line to or from the engine to injection elbow, is a radically different beast to a valve that fits in the exhaust lline. The more I think about it the more it makes sense.

<P ID="edit"><FONT SIZE=-1>Edited by ccscott49 on 05/08/2002 10:23 (server time).</FONT></P>

 

[philip_stevens]

Colin,
The Vetus catalogue shows the "Air Vent" as they call it, fitted in the water injection line between the engine outlet and the exhaust wet injection inlet.

There's a fair old write-up about it, but would not like to quote the whole piece in case I upset some people, by being long-winded. But I will quote the first paragraph.

"If the injection point of the colling water line is situated at a height less than 15cm (6") above the waterline, then - when the engine is stopped - there is a risk that, as a result of syphoning, the cooling water may enter the engine."

There is a lot more....

I did fit one on my last boat, but have not fitted one on this boat. I also fitted a waterlock and a gooseneck - mainly to reduce exhaust noise, as, when I bought the boat, it had a straight-through exhaust system.

regards,
Philip

 

[Mirelle]

I've got one. Between engine and exhaust elbow, But, let's face it, they plug up with caked salt in no time because they are handling hot salt water, so I doubt if it would work when you need it to do so.

A friend's family boat had a Sabb; they used to close the exhaust seacock in the counter. They left the engine in gear to stop the prop spinning. One fine day, singlkehanding, he heard the engine start to turn so he dropped the decompressor - engine started with the drag on the prop and blew the exhaust off the seacock! So maybe a syphon break is better, but I still doubt if it will work when called on to do so.

 

[ccscott49]

I still say it is impossible (IMHO) for water to be"drawn" into the engine, through the seacock, unless the impellor in the pump is knackered, due to syphon action or depression in the exhaust manifold, which is open to the atmosphere??. They can say whatever they like to sell these things, but I am going to need a little more convincing, sorry. Toilets I can see, they dont have a non return valve in them (IE: Pump) Just as a test, try and "suck" water through a jabsco pump!

 

[dickh]

Do you have August PBO? There is an article on P39 about this problem on Volvo MD2030 engine and another bit on p 72 when they 'boat check' a Beneteau Oceanis Clipper 311.
Pat Manley says you must fit a "water break" rather than an "anti-syphon valve" and must be fitted after the water pump(otherwise the pump won't suck water).
I quote:- "The water injection point into the exhaust should be at least 6inches above the worst case waterline to operate without one. If possible, use a syphon break with a bleed pipe running overboard as high on the side as possible to avoid sucking water back in when heeled, rather than an anti-syphon valve which is fail dangerous. The syphon break must be fitted after the water pump - otherwise the pump won't suck water. In my view Volvo should sell the 2030 with a high rise injection bend as standard! The injection point is far too low for most boats, especially ehen theres only a gentle fall of the exhaust pipe. "
This was all regarding a volvo 2030 where the owner had warmed the engine prior to an oil change, stopped the engine to visit the local chandlery, returned to start it again for a final warm up and all he heard was a dull CLUNK! - the engine was full of water!! The engine(cyl head) sits about 3-6" above the water line.
The suggestion from the local engineer who repaired the engine was to fit a syphon braek on the water inlet to prevent water syphonong past the the impellor before feeding the heat exchanger.
I know you say you its impossible to suck water past the impellor but if the impellor/pump is knackered surely this is a possibility? The pump when working might push sufficient water thro' the engine for one not to notice it was knackered but still allow water to syphon past when stationary.
There were other points in the letter regarding the height of the exhaust pipe above the engine etc.

dickh
I'd rather be sailing...

 

[dickh]

Following on from my earlier post, I fitted a Vetus anti-syphon valve on my boat last season as the original was totally knackered and was advised to fit this as the 'break' was for motor boats. In fact the two Vetus fittings are identical with the addition of the internal spring loaded valve - following this article I will remove the valve and fit a pipe overboard.

dickh
I'd rather be sailing...

 

[ccscott49]

You confirm what I said, if the pump/impellor is knackered, then it would stop syphoning, but a knackered impellor won't pump! Not worth a shit anyway. The syphon break is only there, for that eventuality. IMHO You aslo confirm, it is not there to prevent water coming back up the exhaust and flooding the engine, which is what the original enquiry was, thankyou.
Colin.

 

[david_bagshaw]

we have 3 of them, and all are fitted between the cooling water output from the engine, ie at the outlet from the heat exchanger, and the injection point in the exhaust.

their design in life is to protect the engine from syphon , when the engine is stopped, and if there is wave pressure/ water pressure on the exhaust system,starting a cooling water syphon or a direct syphon because the injection point is below or too close to the water line.

hopefully vetus wont mind me copying their diag, see http://www.euroboating.net

 

[JerryHawkins]

Try this...

Get a hose pipe long enough to go from over the side in the sea water to a point near (at the same height) as the water injection point on your exhaust elbow. You may need some help with this, but make sure the pipe is full of water (then block both ends). Keep one end in the sea and take the other to the point beside your exhaust. Unblock the pipe - what happens? If the end inside the boat is below the water line water will pour out of the pipe into your boat, if it is above the water level it wont. If it is only just above the water level, it may go below when the boat is heeled under sail at which time water would pour in again. Now, if water is pouring in, get the person outside to raise the end of the pipe above the water - air is sucked in and 'breaks the siphon'. This is all the siphon-break does - allows air into the system when the engine stops i.e. when the pressure in the pipe drops due to the water pump stopping.

Your engine cooling system is just the same. You end up with a pipe full of water which, if one open end is below the water line inside your boat water will flow even with the engine stopped. With no engine running, there is no exhaust pressure to force the cooling water up and out of your exhaust skin fitting. Unless you have a hole in your exhaust, your boat won't sink but your engine could fill up with water through the exhaust.

You have two options at this point. 1. Close the cooling water intake sea cock or 2. Fit an anti-siphon valve.

Personally I always do "1" and have "2" fitted in case I forget "1" !!!

Cheers,

Jerry

 

[ccscott49]

Re: Try this...

Try sucking water through a jabsco pump, then tell me again how a syphon works! If the pump is in good condition, no water is going to be syphoned through it! I am well aware of the physics involved. Thank you.

 

[bedouin]

Re: Try this...

But it doesn't need to go back through the impeller!

The siphon being discussed is through the exhaust pipe into the cylinders, NOT back through sea-water cooling system (where it would do no harm even if it could siphon - which it can't).

 

[ccscott49]

Re: Try this...

Sorry, No it isn't, the water cannot syphon through the exhaust pipe! it is open to the atmosphere! What people are talking about, is a partial depression in the exhaust system, when the engione is shut down, causing a syphon action to start in the raw water feed to the engine, or why would you put a syphon break in the water pipe, between engine and exhaust elbow. The original thread was about exhaust flooding, but it got sidetracked, because there is no such thing as an exhaust syphon break. The question I asked was why do we have a syphon break, when the only time it would be needed, was if the impellor failed.

 

[dickh]

From what I remember...

The initial post did not actually say wether the engine was flooded via the exhaust or from syphoning from the inlet water. I agree there is not such a thing as an exhaust syphon break, certainly most engine installations recommend a syphon break/valve in the raw water feed and if this could prevent a potential disaster, then I'm all for it.
Anyway, I'm off on holiday tomorrow, to France, unfortunately not with the boat...
Will return in 10 days, bye...

dickh
I'd rather be sailing...

 

[ccscott49]

Re: From what I remember...

Bye, Dick, have a goodun'
Colin.

 

[vyv_cox]

Re: From what I remember...

I was out on business yesterday and bought a PBO at the airport. I read the article about siphon breaks, referred to up the thread somewhere. Pat Manley most definitely says that a siphon break should be situated between the raw water pump and the engine, to avoid water being drawn into the engine when the boat is heeled. I'm entirely with you Colin - this is a non-existent situation. Water cannot come past the Jabsco pump, I would suggest that even if it was badly worn it would still be sufficient block to flow that no siphon could be established.

Another point made in the article is also rather contentious, in my view. He says that if the engine does not start after 30 seconds or so, the raw water inlet should be closed to prevent water from filling the exhaust, only being opened when the engine is running. I think this is very dubious advice, firstly because there will inevitably be a delay between engine starting and turning on the seacock, during which the pump will run completely dry, but also because I don't know whether these rubber impellers have the capability to draw to near vacuum. It seems there is a likelihood that the vanes could be sucked the wrong way round, and then to be forced "backwards" around the pump. Sounds like a recipe for failed impellers to me. Any experience?

 

[ccscott49]

Re: From what I remember...

I agree with you about the jabsco, but I can see the point about the engine being filled with water if you crank it over for five minutes or more, the impellor won't be completely dry and will run for a LITTLE while with no water, I hadn't considered the vanes being turned, but can see your point, if the impellor is in good nick, this should not worry it too much, as you can fit the impellor either way round, (in some pumps, before I get a slaggin') but an old impellor might well give up the ghost right then! I change mine every year anyway and my engines start, immediately, plus my water injection manifolds are up in the air and it would take a lot of water to fill my exhaust sytem to manifold level. I don't need syphon traps, either, I was just trying to understand the reasons. Thanks for your input and everybody elses.

 

[bedouin]

Re: From what I remember...

That is not the first time I have heard that advice - and I do tend to follow it. Probably mostly out of fear of the cost if the cooling water did manage to get back into the cylinder. However I am glad to say that it is a rare occurrence with my engine that it doesn't start in the first couple of pulls.

On my installation the limited self-priming capabilities of the water pump shows that it is not very efficient at pumping air, so in my case I don't think there is a risk of the impellor being sucked the wrong way round and (accidental) experience shows that the impellor will stand being run with the seacock closed for a couple of minutes without suffering any apparent ill effects.

On the other side of the argument, I know that at idle my engine pumps about .5 - 1 gallon a minute into the exhaust. That water has to go somewhere, and I am not sure that there is sufficient pressure in the exhaust to push it out in the usual fashion without the engine running.

 

[JerryHawkins]

Sorry if you thought...

I was trying to teach you 'how to suck eggs'. The long-winded explanation was for those who, by their comments through the threads, didn't seem to realise what was going on!

The impeller in my Yanmar 3GM30 pump does not seem like it would produce a seal between inlet and outlet when stopped, i.e. water could siphon if allowed! I must admit that I haven't actually tried to suck water through it though.

Regards,

Jerry