Pressure testing fuel tank for leaks?

[Woodsong]

So the boat I am buying has a fuel leak somewhere. *Seller suspects the port tank. *Boat is a 1983 MMC/Monk 36. *Tanks are black iron. *Access to inspect the tanks is tough due to bulkhead between tanks and engine. *However, from parts I can see the tanks look decent. *On top of that, we have found absolutely no visible signs of diesel traveling down the hull from the tanks to the bilge. *Bilge seems to be a mixture of some fuel and water. *I know the water pump has a small leak so pretty sure that is the source of the water. *Engine has a diesel leak when underway via one of the injectors and injector pump but the bilge under the engine is separate from rest of bilge so "should" be isolated from rest of the bilge. *I have found no leaks from any fittings or lines, etc. *Basically- I find no leaks anywhere beyond the injector leak at the engine that is easy to fix. *Obviously my first step will be to repair the injector leak and the water pump, dry/clean up the bilge and then see what happens. *But- it's frustrating to not know the source of it yet! *Has anyone ever pressure tested their tanks to confirm if they are leaking or not? *I would think if it was a pinhole leak the 50 gallons of diesel in it would be spilled out of the tank by now unless it is just slowly seeping out through corroded/porous metal at the bottom of the tank? *Short of pressure testing the tank I don't know how else I can isolate the cause and determine if it is the tanks or something else???


Any ideas?

 

[RickB]

"Any ideas?"

If you don't have access to visually inspect the tank exterior you are fighting an uphill battle, as you probably already realise.

Be careful "pressure testing" an old square tank in an unknown condition. Since you cannot see all the sides and corners, pressurizing it with air and soap bubble testing it is out, unfortunately.

Your best bet now is to completely fill it and do a hydrostatic test. To do this you need to:
*****
Shut off the tank outlet valve.

Remove the connection*to the fuel return line*on the tank to be tested.

Route a line (clear plastic tubing will work nicely)*from that*connection to*the flying bridge or to a hand pump so that*you*can*either*pour or pump a small quantity (a quart maybe) of fuel to the tank.
*
Extend the tank vent line with clear plastic tubing (the smaller diameter the better)and route it to the flying bridge.*Secure it to a mast*above eye level. Double it back into a bucket to catch any "oops" incidents - there will be one.

Fill the tank completely full with fuel, all the way up to the deck level. Rock the boat to remove as much air as possible before closing the fill cap, be careful of burps.

Secure the tank fill cap tightly so as to prevent any seepage.

Pump (from any level)*or pour fuel into the return line*extension from a height*equal to or above the level of the*extended vent tubing until you see fuel in the vent tube at about eye level.

Give things a*minute or two to settle then mark the level with a piece of blue tape.

Leave*it alone for a couple of hours and see what happens to the level.

If it rises it's because the fuel is warming up. If*it goes down, you have a leak somewhere. If it goes down*a bit and stops, it is either temperature or air absorption. If it continues to fall there is definitely a leak, the size*of which is indicated by the rate*at which it drops. Your most likely leak is in*one of the tubing connections you just made. Check them first. Beyond knowing for sure you have a*tank leak, what you do next*depends on how bad it is and*how much work you are willing to take on.*

If*you connect things the way I just described, you will apply about .355 psi per foot of fuel. That means if the top of the fuel level in the tube on the mast is 15 feet above the bottom of the tank, the tank bottom*is subject to about*5.3 psi. I wouldn't go much above that at the top of the tank or you might find a bigger leak than you care to deal with. Be careful and good luck.



****

 

[skipperdude]

Rick,
That is amasing. Who figures stuff like this out.
Perfect. I printed the post you made. It will go in my How to book
What did you do graduate from MIT.

SD

-- Edited by skipperdude on Thursday 29th of July 2010 02:31:43 PM

 

[RickB]

skipperdude wrote:Who figures stuff like this out.

Old guys like Archimedes, Boyle, Pascal, Torricelli, Huygens ...

*

 

[skipperdude]

Yeah,
*But I don't think they owned trawlers.
So I'm giving you the credit weather you like it or not.

SD

 

[Woodsong]

Rick,
thanks a bunch for your suggestion- I agree about the ingeniousness of it! Only issue I have is that it necessitates filling the tank. Since I have not yet closed on it, I am not 100% keen on the ideal filling the tanks for essentially the seller. is there another way to test it that does not require filling the tank with fuel? On top of that, I would be concerned that if it is a fuel leak that filling the tank could result in a lot of spilt diesel in the bilge which would be a real mess to clean.
That is where I thought of lightly pressurizing the tank with a gauge on it since I can't see all around it and check via soapy water. If tank is drained, all connections sealed off, pressure gauge installed at inlet where air is pumped, and then slowly pump it by hand to like 5 lbs pressure and watch how it acts an afternoon or a day, would it not tell you if it had a leak via drop in pressure?

 

[RickB]

Woodsong wrote:would it not tell you if it had a leak via drop in pressure?

It would do an even better job telling you how the temperature rose and fell.*

Seriously though, if temperature remained constant, you could use a U-tube manometer made of that same small plastic tubing with a column of mercury or water. Five psi is just over 10 inches of mercury or about 11.5 feet of water.

If you went that route you should leave it over a period of days and chart the tank temperature and barometric pressure as well as the column height to determine if the level is actually dropping over a large number of observations.

A few years ago I would have considered sealing the tank and then relesing a healthy dose of freon into it and sniffing all around it with a refrigerant leak detector. That would be illegal now of course even though it would be highly effective.


-- Edited by RickB on Thursday 29th of July 2010 05:08:10 PM

 

[Arctic Traveller]

Perhaps a far simpler method would be to get a flourscent die / leak detection kit possibly at an auto parts store.* You put in a few onces of the dye, run the engine a bit to circulate the stuff, then examine the whole area with a black light.* The more powerfull the light, the more the leak will stand out.* Most kits come with some type of light, and a pair of special glassees that enhance the die.* If there is a leak, you WILL see it.* If it's a really small pin hole, it might take longer for the dye to leak out, but once it's mixed properly, and some leaks out, you can be 100 % sure there is a leak if you see the dye.* Works with motor oil, transmission oil, refigerant, and coolant too, but be sure to use the correct type for what ever fluid leak your looking for.* Cat has a kit with a really powerfull light, and sells the fluids, but it's industrial strength and very expensive.* Consumer grade kits are far more reasonably priced.

Another method I have used on an injection pump was to completely sprinkle it with talcom powder.* Close observation will then pinpoint the source of the leak, and it's easy to clean the stuff up too, just get out the shopvac.** ...................Arctic Traveller

 

[RickB]

"Perhaps a far simpler method would be to get a flourscent die / leak detection kit .."

No pun intended but that is a brilliant idea!

Of course the tank will have to be filled and pressurized lightly or any leaks above the current low level will not show up unless the leak is on the return piping.

 

[Woodsong]

The dye idea is actually intriguing. Right now the tanks are down to around 1/4 full so nearing empty. I have watched the bilge for 10 days now- no increase in levels in the bilge and no evidence anywhere of active leak. However, I can see where some of the bulkheads next to the tank appear to have been saturated at one point. My guess is either the seller is just wrong, it's not leaking, or most likely, it does have a pinhole leak but it is at a level higher than the current fuel level in the tank. I am guessing that is the case. So- before I go pumping 150 gallons of diesel into the tank, I really need to confirm what is going on. The 10 gallons of fluid in the bilge turned out to be 99% water as I discovered Friday when I hand pumped it all out- slight amount of diesel on the top but rest just plain ol' funky bilge water. Only other idea I had was to pump 5-10 gallons of fuel in at a time and sit and watch the tanks/bilge for leaks. This would result in possibly some diesel leaking out but would help diagnose the level at which it is leaking.

 

[Peter B]

I'd be inclined to just tell the seller/owner to identify and fix the leak, then give you a call when they are sure its done. You should not have to do all that messy stuff to someone else's boat to help him sell it.

 

[Woodsong]

Peter,
Ordinarily I would agree with you 1000% about just having the seller repair. However, in this instance, the deal is priced incredibly well and as such these little things I will be handling but the price he is selling at more than makes up for any work or headache I incur by having to address the tanks.

 

[Keith]

I usually don't like having the owner fixing things. He's likely to crap it together in the least expensive way that will get*him through the sale.

 

[Larry M]

When we sold the last boat we put money in escrow for the buyer.* We got estimates for the work and the buyer signed off on them.** We gave the buyer the option and they elected to take care of the repairs.* It allowed the closing to happen on time and with no ill will.

Larry/Lena
Hobo* KK42
La Paz, BCS, MX

 

[sunchaser]

It is easy to conceive a leak in a 1983 Monk with iron tanks. A weep of a tablespoon a day is enough to smell up the vessel. I doubt pressure testing*or tank measurement would pick up "incipient" leaks. If any water resides in or on*an iron tank, 30 years or sooner shows the problems for sure. Me, I'd assume the tanks need fixing/replacement - price out tank replacement from a reputable yard and build it into your offer with a season likely lost too.