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Old 28-04-2017, 16:16   #1
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Air Lock in raw water system downstream of pump

Our sailboat has a very annoying (and sometimes dangerous) issue where the raw water system will airlock downstream of the water pump">raw water pump. I know it is downstream of the pump because once the system is airlocked, nothing can be done to prime the system on the upstream side. The problem is prevented by closing the thruhull while sailing but this is unacceptable as a long term solution.
I've checked out this thread: air lock in raw water pump , but most people there are talking about airlocks on the upstream side of the pump.
My air lock happens when I sail in rough enough seas to allow air bubbles to accumulate in the system from the thru hull. Once the engine is started the air bubbles are pushed past the pump where it seems they can't get pushed past the anti siphon valve.
Here is my setup: 1986 Volvo MD30 with heat exchanger, engine oil cooler and gear cooler. The raw water runs from the pump, to the oil cooler, to the anti siphon valve then to the heat exchanger after which a portion flows to the gear cooler and back before it exits out the riser.
The only way I can prime the system is by removing the hose on the downstream side of the anti siphon valve and sucking on the hose with the engine running. I usually can't suck hard enough by mouth so a shopvac is required.
There must be something wrong but I can't imagine what. I have no leaks, the pump and impeller are new with the correct size cam and the heat exchangers are free of debris.
Could there be something wrong with the anti siphon valve? I have a small hose attached to the air vent that redirects any leakage into the bilge. I can blow and suck fine on the end of that hose so the valve is not clogged.
Help?
Thanks
Gary
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Old 28-04-2017, 16:27   #2
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Re: Air Lock in raw water system downstream of pump

Quote:
Originally Posted by gpeacock View Post
Our sailboat has a very annoying (and sometimes dangerous) issue where the raw water system will airlock downstream of the raw water pump. I know it is downstream of the pump because once the system is airlocked, nothing can be done to prime the system on the upstream side. The problem is prevented by closing the thruhull while sailing but this is unacceptable as a long term solution.
I've checked out this thread: air lock in raw water pump , but most people there are talking about airlocks on the upstream side of the pump.
My air lock happens when I sail in rough enough seas to allow air bubbles to accumulate in the system from the thru hull. Once the engine is started the air bubbles are pushed past the pump where it seems they can't get pushed past the anti siphon valve.
Here is my setup: 1986 Volvo MD30 with heat exchanger, engine oil cooler and gear cooler. The raw water runs from the pump, to the oil cooler, to the anti siphon valve then to the heat exchanger after which a portion flows to the gear cooler and back before it exits out the riser.
The only way I can prime the system is by removing the hose on the downstream side of the anti siphon valve and sucking on the hose with the engine running. I usually can't suck hard enough by mouth so a shopvac is required.
There must be something wrong but I can't imagine what. I have no leaks, the pump and impeller are new with the correct size cam and the heat exchangers are free of debris.
Could there be something wrong with the anti siphon valve? I have a small hose attached to the air vent that redirects any leakage into the bilge. I can blow and suck fine on the end of that hose so the valve is not clogged.
Help?
Thanks
Gary
Anti siphon valve ?? between pump and heat exchanger. My understanding is that the circuit should be intake, strainer, pump, oil cooler if fitted, heat exchanger, vented loop mounted as high as practical then returning to the exhaust elbow. My guess would be a problem with the anti siphon valve, even so far as to say that it shouldn't be there.
Anyway, good luck.
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Old 28-04-2017, 16:34   #3
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Re: Air Lock in raw water system downstream of pump

Might have my terms mixed up... it is a vented loop. It has to go from the oil cooler to the loop then to the heat exchanger because the pipe from the heat exchanger to the elbow is metal. The plumbing is stock from Volvo.
I could very well be wrong but I dont think the order matters to the pump. Cooling priority is a different story.
Thanks for the reply
Gary
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Old 28-04-2017, 17:07   #4
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Re: Air Lock in raw water system downstream of pump

Air locks don't happen downstream of positive displacement pumps (sort of what an impeller pump is....sort of).

I'd bet that it's something with the pump like a worn cover plate or cam or even an air leak on the suction side.
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Old 28-04-2017, 18:27   #5
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Re: Air Lock in raw water system downstream of pump

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Originally Posted by Sailmonkey View Post
Air locks don't happen downstream of positive displacement pumps (sort of what an impeller pump is....sort of).

I'd bet that it's something with the pump like a worn cover plate or cam or even an air leak on the suction side.
Almost certainly this is the correct answer.

Anything short of a complete plug in the discharge side is going to let the pump prime if it isn't worn out. When it won't prime on is own, it's because the pump housing and/or cover plate is just plane worn out. Seen it many times. Buy a new pump, don't waste time trying to fix it.
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Old 28-04-2017, 18:34   #6
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Re: Air Lock in raw water system downstream of pump

I'm with sailmonkey and boatbod. Air won't block pressurized water. I think you've got a weak pump. Replace the impeller and look for wear in the pump body, face plate and cam.
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Old 28-04-2017, 18:40   #7
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Re: Air Lock in raw water system downstream of pump

Quote:
Originally Posted by Sailmonkey View Post
Air locks don't happen downstream of positive displacement pumps (sort of what an impeller pump is....sort of).

I'd bet that it's something with the pump like a worn cover plate or cam or even an air leak on the suction side.
When I have the issue I pull off the hose coming from the downsteam side of the vented loop and use a shopvac to suck on it. No water comes through the system until the engine is on (hence the pump is running) That says to me the pump is fine... Am I wrong?
I can also raise the strainer about 3 feet above the pump and supply a continuous supply of water with no air on the inlet side but it will not prime until I pull the vented loop discharge hose and suck.
I have 3 water pumps in good shape (I expect the previous owner had the same issue) and they all do the same thing.

It's my understanding that these pumps should be able to easily handle starting with an empty strainer bowl when they prime. Could a cover plate that leaks no water be bad enough to stop flow? The cam is brand new.

Thanks again for putting some thoughts into this. I might try pressurizing the whole system to check for air leaks. Just need to figure out the best way to make that happen
Gary
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Old 28-04-2017, 20:09   #8
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Re: Air Lock in raw water system downstream of pump

Pull the cover plate and look at it. If you see wear on the inside replace it. The side of the impeller has to seal on the face plate. A worn face plate won't leak water. It just won't pump. There is sometimes a wear plate at the back of the pump. The same problem can happen with the wear plate.

On a lot of pumps you can flip the cover plate over and use the other side. Be sure to write down any numbers before you flip it. The numbers will be worn off in time.
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Old 28-04-2017, 20:26   #9
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Re: Air Lock in raw water system downstream of pump

Everyone here is convinced its the pump. Well, guess that gives me somewhere to start.
Tomorrow I will hook up one of the pumps to my drill and see what we can pump when its free of the engine.
While Im doing that. The pump is a Johnson F7B-9 and Im using the 6mm cam. Does anyone know where I can find expected flow rates vs RPM?
I'll report my findings back
Thanks
Gary
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Old 29-04-2017, 08:32   #10
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Re: Air Lock in raw water system downstream of pump

temporarily take the anti-siphon valve out of the system, then see what you've got.
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Old 29-04-2017, 10:14   #11
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Re: Air Lock in raw water system downstream of pump

Love my Superseal. My guess is the pump.
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Old 29-04-2017, 10:53   #12
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Re: Air Lock in raw water system downstream of pump

Technical data sheet for the MD30a engine shows the following water flows for the sea water pump:

2000 rpm. 33 l/min
2500 rpm. 41 l/min
3000 rpm. 47 l/min
3500 rpm. 54 l/min
3800 rpm. 58 l/min

Hope this helps.

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Old 29-04-2017, 14:27   #13
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Re: Air Lock in raw water system downstream of pump

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Originally Posted by DougR View Post
Technical data sheet for the MD30a engine shows the following water flows for the sea water pump:

2000 rpm. 33 l/min
2500 rpm. 41 l/min
3000 rpm. 47 l/min
3500 rpm. 54 l/min
3800 rpm. 58 l/min

Hope this helps.

DougR
Hey Doug, thats great info... Do you have a link to that datasheet? Im sure there is lots of other good stuff that I could use in there as well.
Thanks
Gary
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Old 29-04-2017, 14:31   #14
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Re: Air Lock in raw water system downstream of pump

Might well be a blocked exhaust, exhaust back pressure pressurizing raw water system, look for internally melted exhaust hose coming of riser or the riser itself blocked with coke and corrosion near the water injection point. Good luck with it. Simon
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Old 29-04-2017, 14:41   #15
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Re: Air Lock in raw water system downstream of pump

So.
I set up a pump mount out side of the engine and ran a bunch of tests today. Learned a bunch and yes, everyone was correct... It was the pump.
The cover plate had a very slight grove on it where the impeller rubs against it. Almost undetectable with my finger but when I put some grinding paste on the plate it was easy to see. This caused all the problems. With that plate the pump would not prime even with the water intake above the pump. Once ground flat it would easily prime the full height of the hose I was using (about 10 feet).
Just for fun I threw the vented loop on the pump discharge and it primed/pumped with or without air trapped in that pipe.
A rough estimate of drill rpm shows water flow pretty close to the specs that Doug posted so I think the problem is solved.
So the lesson here: flat sand your pump cover plate when you pull your impeller for the season. It was very surprising how small the grove needed to be to prevent priming of the pump.
Thanks for everyone's help
Gary
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