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Old 14-03-2012, 07:14   #1
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Changing Fittings on a Full Tank ?

I need to swap out some fittings at the bottom of my fuel tanks, and the tanks are full. Somewhere I remember reading about a trick where you attach a shop vac to the filler pipe, plug the vents, and run the shop vac. The vac creates enough vacuum to prevent the fuel from flowing out of the tank bottom when the fitting is removed.

Has anyone ever done this? Will the vacuum motor ignite all the diesel fumes that get sucked into the vacuum and blow up my boat and burn the marina to the ground?

If this approach is a bad idea, does anyone know of a better way short of pumping out the tanks?

Thanks
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Old 14-03-2012, 07:52   #2
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Re: Changing fittings on a full tank?

BAD IDEA!!!!!! Have you ever looked at the motor of a vacuum cleaner while it starts and runs???..........Mucho sparks!!!! do it safe and empty the tanks with a properly protected pump designed for gasoline service.
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Old 14-03-2012, 08:03   #3
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Re: Changing fittings on a full tank?

Stay alive, and empty the tanks.
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Old 14-03-2012, 08:20   #4
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Re: Changing fittings on a full tank?

I'm wondering what fittings you have that are at the bottom of the tank...we've talked about how common this way or may not be on the forum in the past...For me, I've never seen it.
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Old 14-03-2012, 14:36   #5
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Re: Changing fittings on a full tank?

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I'm wondering what fittings you have that are at the bottom of the tank...we've talked about how common this way or may not be on the forum in the past...For me, I've never seen it.
The fitting for the fuel pickups all come off the side of the tanks just above the bottom. They are not on the underside - sorry I wasn't very clear. The key in this instance is that they are below the fuel level and if you open them fuel will spill out. I too recall a conversation a while back about what was and was not ABYC and USCG compliant. When checking it out, I concluded the arrangement on my tanks was compliant, though there is an valid argument that a top fitted pickup tube cannot leak if opened or disconnected.

In my case, I'm adding pressure sensors to serve as fuel level gauges, so they need to be at or very near the bottom of the tank.
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Old 14-03-2012, 15:21   #6
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Re: Changing fittings on a full tank?

By the way, point taken on the whole vacuum approach.
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Old 14-03-2012, 16:14   #7
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Re: Changing fittings on a full tank?

Your basic idea is good, its just the method that's dangerous. You could try a decent sized hand operated diaphragm type bilge pump instead of the vaccuum, it shouldn't take too much work ( at least that's what you'll tell your assistant).
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Old 14-03-2012, 16:24   #8
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Re: Changing fittings on a full tank?

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Your basic idea is good, its just the method that's dangerous. You could try a decent sized hand operated diaphragm type bilge pump instead of the vaccuum, it shouldn't take too much work ( at least that's what you'll tell your assistant).
Regards,
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I would VERY much welcome any ideas on safe ways to swap the fittings without emptying the tanks. I think the vacuum (as in depressed pressure, not the apparatus) approach is very clever and as you say should work and not take much negative pressure (2 PSI would be more than enough). I know I read about someone who did it.
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Old 14-03-2012, 17:57   #9
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Re: Changing Fittings on a Full Tank ?

I have seen a vacuum pump used firsthand when the dealer used one to keep the oil in the hydraulic system when changing a pump. It was a while ago and I didn't pay much attention but it seemed to work well in that application. Maybe ask at a hydraulic shop or garage and see if they have any experience. Good luck.
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Old 14-03-2012, 21:57   #10
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Re: Changing Fittings on a Full Tank ?

... and if anything goes wrong your bilge is full of diesel. Is it really worth the risk?
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Old 14-03-2012, 22:25   #11
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Re: Changing Fittings on a Full Tank ?

Cant you install the sensors from the top of the tank with a tube extending to the bottom...this is typical in the Hart system.
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Old 15-03-2012, 04:44   #12
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Re: Changing fittings on a full tank?

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Originally Posted by twistedtree View Post
.............

In my case, I'm adding pressure sensors to serve as fuel level gauges, so they need to be at or very near the bottom of the tank.
I might be missing something basic here but as these are pressure sensors, can't you add them AFTER the fuel shut off valve. Again I am assuming there is a shut off valve immediately in the line at the tank. If not, then disregard my post .
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Old 15-03-2012, 05:07   #13
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Re: Changing Fittings on a Full Tank ?

I was going to say what's wrong with pumping the tank out - but then I saw your boat description! My guess is that a lot of fuel is involved!

How about selling the fuel to someone on your dock - or even borrowing there empty / half empty tank(s) - even if you don't get rid of all of it might make what's left manageable to decant?

Anyway, just an idea.
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Old 15-03-2012, 05:17   #14
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Re: Changing Fittings on a Full Tank ?

Read up on Boyles and Charles Law. When you reduce the atmospheric pressure (pull a vacumn) on a flammable liquid you alter its entire combustion and ignition characteristics. BAD IDEA.
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Old 15-03-2012, 05:39   #15
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Re: Changing fittings on a full tank?

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I might be missing something basic here but as these are pressure sensors, can't you add them AFTER the fuel shut off valve. Again I am assuming there is a shut off valve immediately in the line at the tank. If not, then disregard my post .
No, you are absolutely correct, and it's a possibility. I'd have to add a second shut off after the existing one so I can still have the same fuel management capabilities without shutting out the pressure sender. Also, given the specifics of how the boat is set up, I would need to have two new fuel lines made up. They are essentially hydraulic lines with hydraulic fittings crimped on the ends and are pretty pricey.

I also wonder how much pressure drop will be created by the fuel circulating through the lines while the engine is running, and whether that will mess up the gauge readings in any appreciable way. I don't have the tools or knowledge to calculate localized pressure drops of liquid flowing in a pipe to really answer the question.

And last, I think having the senders in the main fuel line makes maintenance harder, requiring at least one engine to be shut down, and possibly inducing enough air into the system to require bleeding. And in the worst case of a sender leak, it would take out an engine and possibly the generator too. With the senders on a tap separate from the fuel lines, maintenance is easier and can be done without any impact to normal operation of the boat.

So, it's certainly a possibility, but as you can tell I'm leaning strongly towards the approach that creates the best end-product, even if building it is harder.
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