Engine siphon-break before or after heat exchanger?

[Iceblink]

I have a Perkins 4.108 and am in process of renewing all hoses etc. The engine has been set up with the siphon-break/vented loop before the heat exchanger rather than after it (the latter being the more ususal arrangement from what I can gather) Can anyone see any advantage either way? It doesn't seem to me that it would make much difference but thought I'd check as it's a fairly important part of the system..

Just to be clear my set-up goes like this: seacock - strainer - raw water pump - siphon break - heat exchanger - mixing elbow - exhaust

Also the vented loop/siphon break appears to be at or slightly below water level with a narrow diameter hose running up from the top of the loop and plumbed into the galley sink waste.. Is this a good idea?

 

[BabaYaga]

Also the vented loop/siphon break appears to be at or slightly below water level with a narrow diameter hose running up from the top of the loop and plumbed into the galley sink waste.. Is this a good idea?

No, IMHO.
The siphon break (top of loop) must be some distance above the water level in order to prevent water from entering the exhaust system. If not, the water level in the narrow hose will just adjust to the outside level and water might continue flowing in the larger hose below.

 

[rob2]

Just to be clear, the opening of the syphon break must be above water level so that the static pressure at that point will allow the water in the loop to settle back to water level, creating the break required to stop any syphon. Even though it rises to be become the highest point of the closed loop system at present, it is actually only opening to a drain hose which could be close to the waterline when heeled, so may promote a syphon.

My own preference is to have the loop as high as possible in the engine bay, which will usually give you around a foot above WL, then the hose is taken through into the cockpit immediately above a cockpit drain so that you can glance down and confirm there is water flow whilst the engine is running. Older boats without a self-draining cockpit need the hose to be taken to a skin fitting somewhere easy to see, but which won't give passers-by on a pontoon a shower or fill the dinghy...

It really doesn't matter where the loop is in the system so long as it has done it's cooling work before you bleed off a little of the flow, so either side of the heat exchanger is good.

Rob.

 

[pvb]

For their siphon breaks, Vetus recommend that the top should be at least 40cm above the heeled waterline, and the tube from the top must go downwards.

 

[Steve_N]

Can anyone see any advantage either way?

I think that functionally they give the same result.
My outgoing VP MD21B has the syphon break between heat exchanger and exhaust manifold whilst Beta's recommendation for the new engine is to fit it between the combined heat exchanger/manifold and the exhaust elbow.

 

[pvb]

I think that functionally they give the same result.
My outgoing VP MD21B has the syphon break between heat exchanger and exhaust manifold whilst Beta's recommendation for the new engine is to fit it between the combined heat exchanger/manifold and the exhaust elbow.

I'd have thought that putting it straight after the raw water pump would be better long-term, as it would only have cold water going through it, so less chance perhaps of deposits being formed.

 

[sailorman]

When i re engined ( 4108 with out vent ) i fitted a Nanni 4150 & fitted a vac-break between the raw water pump & the HE, it "piddles" into the cockpit drain so i can monitor the water flow, if the raw water inlet does get blocked ( 3 times last season with fine weed) the vent runs dry & makes a noticeable noise which grabs ones attention fast.
I have had no issues with this set-up in 1300 hrs

 

[BabaYaga]

Actually, if the top of the loop doesn't reach over the outside water level, it is not even a siphon – just a pipe connecting the sea to the injection point on the exhaust bend. The fact that the top of the loop is vented makes no difference.

 

[prv]

I have a Perkins 4.108 and am in process of renewing all hoses etc. The engine has been set up with the siphon-break/vented loop before the heat exchanger rather than after it (the latter being the more ususal arrangement from what I can gather) Can anyone see any advantage either way?

Nope, either should be fine.

My D1-30 has the heat exchanger discharging almost directly into the elbow - there's a bent hose about an inch long between them, covered over with engine paint. I think some other engines have the equivalent connection made in metal within the exchanger/exhaust manifold. In either case the anti-siphon obviously has to be before the heat-exchanger.

Pete

 

[Steve_N]

I'd have thought that putting it straight after the raw water pump would be better long-term, as it would only have cold water going through it, so less chance perhaps of deposits being formed.

That's a fair point and I could easily fit it just before the heat exchanger/manifold.

 

[Iceblink]

Thanks for the replies. So the concensus seems to be that it doesn't make a difference before or after the heat exchanger (before might be better as it will be on the cold side) but that my current 'vented loop' set up may nothing of the sort if the loop itself is not significantly above outside water level and only the narrow diameter tube is extended up above the water..
With a product like this http://www.asap-supplies.com/marine...ose-line-siphon-loop-and-breaker-valve-410033 what is the advantage of its feature that only allows water to flow in one direction? Surely the concern is that water will carry on flowing in the same direction as usual and swamp the engine/boat?

 

[VicS]

Thanks for the replies. So the concensus seems to be that it doesn't make a difference before or after the heat exchanger (before might be better as it will be on the cold side) but that my current 'vented loop' set up may nothing of the sort if the loop itself is not significantly above outside water level and only the narrow diameter tube is extended up above the water..
With a product like this http://www.asap-supplies.com/marine...ose-line-siphon-loop-and-breaker-valve-410033 what is the advantage of its feature that only allows water to flow in one direction? Surely the concern is that water will carry on flowing in the same direction as usual and swamp the engine/boat?

I think it does not mean what it appears to say.

A more detailed explanation from elsewhere on ASAP's website reads:

Anti-Siphon Devices and Antisiphon Valves

An anti-siphon unit is an extremely important part of any plumbing system. An anti-siphon device is recommended for use in installations such as 'below waterline' applications of engines with wet exhausts, toilet systems and pumping systems to name but a few.

Under normal operating conditions, the anti-siphon valve is closed as the water passes in the required direction, pushed passed by the pump. Under fault conditions the weight of the column of water in the hose can create a vacuum causing more water to be drawn into the system and starting a flow of water. Under these conditions the anti-siphon valve will be drawn off its seat and allow air into the pipe, breaking the water column and stopping the siphoning effect. Non-valved anti-siphons work in a similar way, but under normal conditions a small quantity of water flows out of the vent pipe. Air is drawn in should a vacuum be created.​

The valve is the bit screwed to the top of the loop.

 

[BabaYaga]

In my view the explanation linked to in post #11 is just wrong and the one in post #12 a bit odd.
Direction of flow in the loop is not something that the anti-siphon can respond to.
Also, it is not under "fault condition" that the anti-siphon valve is put into action, but every time the engine/water pump shuts down.

 

[Iceblink]

I think it does not mean what it appears to say.

A more detailed explanation from elsewhere on ASAP's website reads:

Yes that sounds more like it..:encouragement:

 

[VicS]

In my view the explanation linked to in post #11 is just wrong and the one in post #12 a bit odd.
Direction of flow in the loop is not something that the anti-siphon can respond to.
Also, it is not under "fault condition" that the anti-siphon valve is put into action, but every time the engine/water pump shuts down.

Would not disagree with you there.

but its not exactly rocket science.................

 

[lw395]

If the syphon break is before the heat exchanger, the salt water in the exchanger will run into the exhaust when the engine stops.
Probably no issue in a normal exhaust design.
When the engine re-starts, the exchanger will be dry. This may increase deposition of salt and so forth.

 

[BabaYaga]

If the syphon break is before the heat exchanger, the salt water in the exchanger will run into the exhaust when the engine stops.
Probably no issue in a normal exhaust design.
When the engine re-starts, the exchanger will be dry. This may increase deposition of salt and so forth.

There is also the issue of corrosion. An anode, like in Beta engines, might be left hanging in the air.