syphon back

[Guest]

I've installed a yanmar generator set on my power boat, but occasionally i lose all the water from the salt water cooling side of the circuit. The base of the strainer is 15 cm above w/l, the genny jabsco pump is 30 cm above waterline,and the water injection point is 50 cm above w/l. The water syhons back through the heat xchanger then jabsco and levels out at sea level in the inlet pipe below the filter. Once topped up again it may run without draining itself several times before it dissapears again. I'm only familiar with keel cooling hence have i made an obvious mistake?
ps after the water injection point the gas/water goes to a vetus muffler, swan-neck and then out 10 cm above the w/l.

 

[ccscott49]

If the raw water is coming out of the exhaust when the genny is running, whats the problem?

 

[Guest]

thanks for reply, yes the raw water does come out the exhaust and it works ok. But the strainer design is such that the outlet is half way up its side ( vetus 1900 ) it will always only fill up to this point unless manually primed. A 50 % filter screen immersion is not what i paid for...... i wanted a large surface area to allow minimal chance of blockage whilst i'm off the boat, or out of alarm earshot.
Is it standard for an engine to back flush itself like this if above the waterline?

 

[ccscott49]

I dont see any way, apart from a one way valve of stopping it draining. You could try a one way valve, but I dont think thats the answer. If the water level is below the top of the strainer, it will never fill up to the top, it should be just below the water level or just 1/2" above. I have had another thought, the water should not drain back through the jabsco pump either, it should be sealed, the oulet for the strainer, should be at the top, to allow it to fill up all the way, there is something strange going on here, I would have to have a look. But cant, sorry, on drilling rig!
I will think somemore on this!
<P ID="edit"><FONT SIZE=-1>Edited by ccscott49 on Mon Nov 26 22:36:41 2001 (server time).</FONT></P>

 

[Guest]

I'll probably end up with a non return, but I don't think its the best solution.
The filter installation guide from vetus said the filter could be above the w/l but admittedly the diagram then showed the engine below it.
Thanks for your help...... still scratching my head though!

 

[ccscott49]

Wheres the inlet to this strainer?

 

[ean_p]

isn't this what always happens.....because you have a syphon break in the circuit the water can and will drain back down to sea level...and yes it will drain through the Jabsco albeit slowly...without the syphon break the water may continue to fill the exhaust and flood in the exhaust manifold....i think.....

 

[ParaHandy]

Vetus filter seal

Might be very silly, but, check that the "O" ring seal in the strainer (cap) went back in.

 

[Guest]

If it helps, I have fitted a few vetus strainers ( I think they are the best) but I always make sure that the strainer is above the pump level just a couple of cm is enough. That way any syphoning back will only drain leaving the pipe between pump and strainer full. If you lose some out of the strainer it should refill as soon as engine is reved and you should see it bubbling throu' the top glass. The other poster was correct in saying about the seal on the top of strainer, IT MUST be airtight. Try re-siting the strainer, the pump will still lift the water so long as its not more than 10 cms higher. One question is "are the pump and strainer matched" or is strainer to big for pump.Hope this helps a little

 

[Twister_Ken]

Why not close the inlet valve when you've finished with the genny? What stops water coming in will also stop it coming out. Safer, too.

 

[david_bagshaw]

you still get all the useable area, as the lower part clogs, then the free upper area will be used.

What you describe as to loss of water can be prevented a little by having a bit of a loop between the strainer & pump, so when the engine starts there is a ready supply of water to the pump, then the air in the next bit will get sucked through quickly, without burning the impeller.

We have a similar set up on our smaller generator, a 7.5kva Onan, & as a previous post has suggested the strainer and pump need to be about the same level ish, with the strainer above the waterline for safety & convenience of cleaning.

Engine makers use the same principal with the oil pipe to a turbo charger, it has a dip in it to keep oil near the turbo so oil quickly gets to the bearings, as the oil system fills & pressure rises, even if at first it is air.

David
www.yachtman.co.uk

<P ID="edit"><FONT SIZE=-1>Edited by david_bagshaw on Tue Nov 27 11:56:53 2001 (server time).</FONT></P>

 

[Guest]

just got back from work, thanks everyone for replies. In answer to question, the strainer inlet is on the bottom and the outlet on the side. The o ring is good and in place. I've tried disconnecting the shower pipe whilst contemplating the problem in the bath! After exhaustive trials re-creating the set up i can't get the water to stay in it. The strainer is very difficult to move from its present location so i'm edging towards a non return valve.
Thanks again..

 

[charles_reed]

Vane pumps are notoriously poor in suction - and as yours is above the WL it won't self-prime.

Your simplest answer is to turn off the seacock when you stop the generator.

I had this recurring problem (on a Yanmar) which was only resolved by replacing the pump (the back face allowed air past).

The maximum that the 2GM pump will lift is 600mm, that's according to the workshop manual.

 

[ccscott49]

You could put a loop in the pipe from the seacock to the strainer, with the top of the loop above the waterline, as in a toilet inlet, with a syphon breaker, small hole, at the top of the loop, this would keep the strianer full.

 

[charles_reed]

Guarantee no water

Poor thing will just be sucking air when it does get going though it will possibly prevent the wter emptying from the strainer.

 

[ccscott49]

Re: Guarantee no water

Charles, I was talking a little hole, Just to break suction pressure as in toilet anti syphon, does your toilet suck air, or dont you have an anti syphon?