Anti-sipon on Volvo MD30A

[JMM]

Hi - We have a 1987 vintage Volvo MD30A on our boat ( new to us last year) - that has what I consider a non-standard anti-siphon installed by the PO. The anti-siphon loop is installed NOT in the exhaust, but on the raw water intake. Looking at the exhaust manifold mixing elbow connection into to the exhaust there is no (obvious) place to tee in a hose to install a vented loop in the exhaust, which is probably why the PO installed what he did.

He also installed a manual valve at the top of the loop -- so we have to manually open and close it to allow suction (vent) -- or prevent spray water all over. This I plan on correcting with a proper vent ( like Scots) or a hose.

So here are my questions:

1) Does anyone have experience with an MD30A here and installing an anti-siphon loop in the exhaust? Seems some significant modifications are required.
2) Do we need to ? Does the ant-siphon on the raw water intake (pre - impeller) protect enough to break the siphon and prevent water from entering the engine?

I will not be on the boat until March - so I cannot supply pics - but I can probably find a volvo parts diagram that shows the existing manifold setup.

Thanks, Jim

 

[Alex Dare]

vented loop

The vented loop is not normally installed in the exhaust line but in the raw water line after it exits the heat exchanger but before it feeds into the Exhaust elbow.
It should have a check valve in the top of it rather than a simple shutoff valve.

 

[Don12364]

Thinking it through it would appear that your PO might have had trouble with water getting into the engine.
You will know from other modifications and alterations to the boat whether he was a knowledgeable person or not.
Normally an anti siphon is only required if the water injection bend is below (or could go below) the waterline.
You suggest there is no place to 'tee' one in. Anti siphons are not normally 'teed' in but have the full flow of cooling water flowing through them.

Having one before (upstream) of the pump is most unusual and, mounted here, would act as a siphon breaker but not work automatically as it will allow air to be sucked into the system and starve the pump and overheat the engine.
Hence the manual valve.

Installing an anti siphon anywhere downstream of the pump should be okay and it is usually in the highest point of the piping. If this is genuinely impossible then your set up may be the only practical solution.

As previously stated your PO was either a good engineer who turned this valve on and off with the cooling water inlet through hull every time the engine was started - and stopped - and all was then well, or he was not and just blindly followed advice from others.

From your description I would hazard he was the former and knew what he was doing.

 

[JMM]

Donalux - Thanks! - I am not sure if the PO ever had any issues with water in the engine, but he did install the current setup, and explained to us how to use it. He was not an engineer, but was a very mechanical airline pilot. He still answers my emails so I can ask him if he ever had any issues with water. The engine is > 23 years old, has about 2000 hours, and presently runs great and uses little oil so if he did have water intrusion issues it has held up well -- it certainly appears to have been well maintained otherwise.

Your explanation of how his setup works and why we have to have a manual valve instead of an automatic one now makes sense to me now. Thank you! Until now everyone that has looked at it for me scratched their head and said I should put an automatic valve in - now I know why I should not and will not.

So while unusual, it does sound like it provides ant-siphon protection - as long as you operate the valve properly. The PO put this valve in a very awkward place to operate -- so if you ever needed to either start the engine or shut it down in a "hurried" , distracted or emergency situation we could run the risk of overheating the engine OR having a siphon action getting water into the engine.

Side question -- PO told us to close the valve BEFORE starting the engine, and open it ( siphon break) after killing it. But -- if you did not close it until after starting the engine, would this actually help prevent water from filling the engine if the engine did not ever start quickly -- which I understand that a typical anti-siphon loop (post heat exchanger) does not protect you from.

Now that I know I need the manual vale, I think I need to attach a hose to the top of the brass "loop" where he has it, and run it to a convenient and easy to operate place to operate it.

I will post a stock picture of the engine (w/o the wet exhaust) -- so you can see what I mean about manifold setup. Maybe overkill to have both, but better safe than sorry if I can figure out a way to add one without too much expense.

Jim

 

[JMM]

Here is a stock picture of the engine ( w/o the wet exhaust ):

View attachment MD30_Pic.bmp

The red circle shows the solid pipe connecting the output of the heat exchanged to the exhaust mixing elbow. How would you add an anti-siphon loop to this? Most diagrams I have seen show that the point of injection into the exhaust should be 4" below the exhaust.

Jim

 

[Don12364]

I guess I would start with a hacksaw - after which it should all be straightforward with rubber hose and jubilee clips.

But - first make sure an AS really is needed. i.e. is the mixing elbow really below LWL?
If anxious try buying a spare pipe, if available, to use before cutting the original.
Also, if it all turns to dust then pipes can be reconnected fairly easily and repainted.

However beware; Vented loop valves are not very reliable anyway. The valve in mine seals itself off regularly about every 6 months. I see this because I have put a 2" length of 1"diameter clear perspex tube in the raw water line so I can see if water is circulating. This remains full when the anti siphon valve is gummed. So, in theory, my system could fill exhaust manifold and any cylinders with non perfectly sealed valves.
This doesn't happen so I guess my mixing elbow is above water level and/or there is a tiny air leak in the exhaust so a siphon doesn't develop.
Then, when it is not gummed up, it sometimes drips seawater whilst the engine is running and this has caused the paint on the rocker box to blister and has corroded the electrical connector at the back of the engine (Yanmar 3JH2E).
Boat is 1996 and no untoward water problems seen inside the engine no date.

BTW your PO was a champ. Pilots are very disciplined about the essential procedures before doing anything - like remembering to lower the wheels every time they are .......
Sailors might not be so good!

 

[JMM]

Donalex -- The mixing elbow is definitely below the HWL .. have to check but pretty sure it is close to the LWL. -- I guess the easiest way to check precisely is with a clear hose attached to an open seacock?

I think my first task should be to make the operation of the existing manual valve more convenient -- add a hose and remote mount the valve where we can operate it w/o having to open the cockpit locker and reach way into and under the rear seat. I can probably mount it as part of or next to the engine instrument panel in the cockpit.

If I determine we really need the extra insurance of the other loop - I take it I just cut a section out of that pipe and add the loop there? I am a little confused here as the diagrams I see ( i.e. p379 of Calder's book) show that the injection point should be at least 4" below the exhaust exit ( I guess to make sure it can't flow back into the exhaust manifold?). Since the stock Volvo mixing elbow here does not do this I guess I should not worry about it -- perhaps internally in the elbow the inlet is piped downward to guide the water flow properly.

Our boat is a center cockpit with a side exhaust, so it is easy to see if we have water flowing too. A good reminder if someone forgot to close the manual valve too.

PO was great with the mechanical stuff .. the electrical -- no so good .. at least on some of "his" wiring projects -- he was also very big on using wire nuts!
Thanks again.