Engine Anti-siphon Valve position

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biscuit

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My boat has a freshwater cooled Volvo 2020, installed from new by Volvo agents a few years before I bought it. It has a Vetus anti-siphon valve on the raw water inlet side, after the water pump. There is no anti-siphon valve between the heat exchanger and the injection point on the exhaust. Although there is a short exhaust riser fitted, the vertical distance between the injection point and the water level in the water lock is close to the minimum recommended (300mm) distance. So:

Will the inlet anti-siphon prevent seawater siphoning back through the exhaust system?
Should I move the anti-siphon from the inlet side to the exhaust side?
Should I have an anti-siphon on BOTH inlet and exhaust to be safe?
(There is no inherent difficulty in putting an anti-siphon on the exhaust side.)

I have asked local engine experts these questions, and have had a confusing variety of answers! Even Nigel Calder doesn't make this clear.
Advice gratefully received. Siphon or syphon, either will do (O.E.D.)








http://www.ybw.com/forums/showthread.php?t=301225
 
biscuit said:
My boat has a freshwater cooled Volvo 2020, installed from new by Volvo agents a few years before I bought it. It has a Vetus anti-siphon valve on the raw water inlet side, after the water pump. There is no anti-siphon valve between the heat exchanger and the injection point on the exhaust. Although there is a short exhaust riser fitted, the vertical distance between the injection point and the water level in the water lock is close to the minimum recommended (300mm) distance. So:

Will the inlet anti-siphon prevent seawater siphoning back through the exhaust system?
Should I move the anti-siphon from the inlet side to the exhaust side?
Should I have an anti-siphon on BOTH inlet and exhaust to be safe?
(There is no inherent difficulty in putting an anti-siphon on the exhaust side.)

I have asked local engine experts these questions, and have had a confusing variety of answers! Even Nigel Calder doesn't make this clear.
Advice gratefully received. Siphon or syphon, either will do (O.E.D.)








http://www.ybw.com/forums/showthread.php?t=301225
Click to expand...

My engine is just as yours, it makes no difference where the anti vac is as long as its after the raw water pump
 
Your installation is exactly in accordance with Volvo recommendations. You can't put it in the outlet because there is no hose - the injection point is in the exhaust riser. The purpose of the valve is to break the syphon in the seawater system so anywhere after the pump will work. Just make sure it is cleaned regularly.
 
Anyone got a diagram of this setup?
I don't see how it will stop water coming back to the exhaust manifold and into the engine if the vent is at the other end of the circuit.
 
Billjratt said:
I don't see how it will stop water coming back to the exhaust manifold and into the engine if the vent is at the other end of the circuit.
Click to expand...

So where do you think the water is coming from?

Anti-siphon valves aren't to stop water coming in through the exhaust pipe in the transom and back-filling the engine that way. The solution to that is loops, water traps, and in extremis a big valve or bung.

The only other hole in the hull for water to come from is the intake seacock. You want to form an air gap somewhere in the hose between the seacock and the exhaust bend where the water gets injected into the exhaust - once you have such a gap, the water will not siphon through of its own accord. The anti-siphon valve allows the air gap to form by being high up (so the water is trying to fall down the hose from each side of it) and having a hole so that air can get in to fill the space the water leaves behind.

You could put the anti-siphon valve anywhere between seacock and exhaust bend and it would prevent flooding - but if you put it before the pump then the pump would suck air in through the anti-siphon instead of (or as well as) water in through the seacock. So you have to put it after the pump where there's pressure instead of suction. But anywhere after will do.

Pete
 
Billjratt said:
Anyone got a diagram of this setup?
I don't see how it will stop water coming back to the exhaust manifold and into the engine if the vent is at the other end of the circuit.
Click to expand...

This is from Vetus. The anti-siphon is shown after the heat exchanger but anywhere on the delivery side of the pump would do.
 
Last edited:
Tranona said:
Your installation is exactly in accordance with Volvo recommendations. You can't put it in the outlet because there is no hose - the injection point is in the exhaust riser. The purpose of the valve is to break the syphon in the seawater system so anywhere after the pump will work. Just make sure it is cleaned regularly.
Click to expand...

Mine is the same. If you listen carefully you can hear it vent shortly after the engine stops. It means the water level in the exhaust is at sea level.
 
Ruffles said:
Mine is the same. If you listen carefully you can hear it vent shortly after the engine stops. It means the water level in the exhaust is at sea level.
Click to expand...

The hose to my anti-siphon is reinforced transparent. You can watch the water drop down to sea-level after the engine stops. Same with the flush hose on the heads (but not the discharge! :eek: :D )

Pete
 
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