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soapy water test

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prkaye

Well Known Member
I began pressure testing my pre-built fuel tank today. I used Van's fuel tank test kit, manometer method. I pressurized the tank as per Vans instructions (just over 5' separation on the manometer). I left it to sit for a while. Over about 6 hours the tank lost about 3 or 4 inches (on the manometer) of pressure. I sprayed soapy water and found the fuel cap to be leaking. I so I put some plastic food-wrap around the inside of teh fuel cap, and then put tonnes of duct tape all over the cap. I re-pressurized, but still getting a leak through the cap (through all the duct tape).
I proceeded to check for other leaks by spraying soapy water over all the rivet lines and other fittings/joints. There was no other obvious bubbling.
I know one builder who bought a special plug for the fuel cap opening (expandable rubber plug), and he used that to hold the pressure and then let the pressurized tank sit for a few days to check for slow leaks.

1) Just wondering what others have done? Soapy-water test only, or seal it up the fuel cap somehow and let it sit for a few days to watch for pressure changes?

2) is it normal to notice some bulging of the skins between the ribs on the bottom of the tank, when it is pressurized?
 
1. Remove the "O" ring and coat it with fuel lube. The ring tends to dry out over time and the lube will help soften it and allow a better seal. Don't try to tighten the nut on the bottom of the cap. This will likely only result in breaking the latch pin.
2. If the skins are bulging, you're probably using too much pressure. Also, keep in mind that you'll see movement in the manometer due to changes in atmospheric pressure.
 
prkaye said:
I began pressure testing my pre-built fuel tank today. I used Van's fuel tank test kit, manometer method. I pressurized the tank as per Vans instructions (just over 5' separation on the manometer). I left it to sit for a while. Over about 6 hours the tank lost about 3 or 4 inches (on the manometer) of pressure. I sprayed soapy water and found the fuel cap to be leaking. I so I put some plastic food-wrap around the inside of teh fuel cap, and then put tonnes of duct tape all over the cap. I re-pressurized, but still getting a leak through the cap (through all the duct tape).
I proceeded to check for other leaks by spraying soapy water over all the rivet lines and other fittings/joints. There was no other obvious bubbling.
I know one builder who bought a special plug for the fuel cap opening (expandable rubber plug), and he used that to hold the pressure and then let the pressurized tank sit for a few days to check for slow leaks.

1) Just wondering what others have done? Soapy-water test only, or seal it up the fuel cap somehow and let it sit for a few days to watch for pressure changes?

2) is it normal to notice some bulging of the skins between the ribs on the bottom of the tank, when it is pressurized?
Click to expand...
1)I used EZ-Turn lube on the cap o-ring to seal that leak.
2) 5 ft of water column is about 2.5 psi and that seems a bit high to me and would definitely cause a bit of bulging.

-mike
 
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thanks guys, I'll get some ez lube.
Vans instructions say "the maximum pressure in the tank is 1.61psi. Add a 50% safety factor and you get 2.4PSI (5.5 feet on the manometer).
 
Soapy water will work fine.

I used Snoop (industral leak detecting solution, MSDS) on my tanks. I am not sure I would go buy it if I did not have it on had as Soapy water will also work.

Be careful not to over pressurize the tank. It will buldge from the presure.

Coat everything in the fuel cap with "Fuel Lube" including the center O-Ring and shaft. Best to get the cap to seal now with fuel lube as it will leak fuel out into the low pressure air above the wing when flying and can allow water in when parked on the ramp over night.
 
prkaye said:
thanks guys, I'll get some ez lube.
Vans instructions say "the maximum pressure in the tank is 1.61psi. Add a 50% safety factor and you get 2.4PSI (5.5 feet on the manometer).
Click to expand...
It's not a pressure vessel and probably shouldn't have arbitrary factors of 'safety' added to the leak test pressure.

-mike
 
Blowhard

Duct tape is just too inherently porous to serve as a leakproof barrier in this pressurized application. After cleaning the local area with MEK, I used clear packing tape over the fuel caps with success. Yes, the tank will slightly bulge with pressure. I prefer Van's balloon method because the balloon will burst before the fuel tank can. It does not take much pressure. I have to wonder if sometime, somewhere......some hapless builder overpressurized an RV fuel tank causing it to split. I believe we used only 4 pounds of pressure on the T-45 cockpit and that was enough to hear creaks and groans throughout the cockpit station. We once had a F-18 wing split wide open when someone used the wrong cart to pressurize it. The wing subsequently became a training aid. Here is what happened to a KC-135 when someone forgot to open the outflow valves prior to pressurization. The rear hatch blew out over 70 yards! http://www.hanford.gov/rl/?page=525&parent=506 :eek:

llkc135db8.jpg
 
Almost any kind of grease will work on the O-ring. I use Lubriplate.
Click to expand...
Is it important that the grease be of a type that won't interact with fuel (disolve, or chemicaly react) in any way?
I have some Lithium grease at home, but not sure if I should use this...
 
Hi Phil


You can get aluminum tape at Home Dumpo. It has a thin layer of aluminum on one side. This is what I used over the caps and it worked great.

Cheers
 
Phil-
I can't tell from how you described what you are doing, but are the water levels in your tubes different by 5 feet? The first line or two of your original post seems to imply that you did. If so, I think you are WAY over-pressurizing your tank. From how I read the instructions, I THINK what Van's is saying with needing 5 feet is that you need at least 5 vertical feet between the tank outlet and the floor in order to build an accurate manometer. From that point on, the difference in the water level between each side of the tubing should not be more than a couple of inches if you're looking for 1-2 psi. I tested my tank and got nothing that looked remotely close to bulging the tank. I used both the balloon and manometer method.

Again, I might of misread your post, but just in case I haven't I'd hate to see you blow open your tank.

Also, a good trick for sealing the cap is to put the cap into a rubber glove then into the tank. A little ezturn helps with this as well.

Good luck.
 
To put 1 psi on your tank, you need 27.68" of difference between the water level in each tube. 60" is way too much pressure for testing a tank. You may blow out a seam or damage your skins/baffle permanent! Remember that 1 psi is exherting 1 pound of force to a square inch of surface area. Imagine how many square inches of surface area there is on the inside skin of your tank. :eek:

This reading will change significantly with temperature and atmospheric pressure. You must chart this and end up with similar temps and pressures after a day or so to see if you have a small leak. Larger leaks will show up quickly.

I used fuelube to seal my cap o ring and it worked great.
 
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Don't worry about making the fuel caps airtight if you're doing the balloon and soapy water test because the test will be completed in less than 5 minutes and the only bubbles you'll see are around the duct tape over the cap.

Steve
 
Vans says 2.4PSI

Vans instructions definately suggested 2.4 PSI, and the instructions definately say the water levels should *differ* by 5.5 feet. I'm using a bicycle pump to pressurize. I've de-pressurized the tank, and I'm pretty sure nothing was damaged. Still no obvious leaks with the soapy water and I didn't hear any cracking or poping while I pressurized.
 
You can get aluminum tape at Home Dumpo.
Click to expand...

if this is the stuff they sell for taping-up ducting, then I have some of it at home. My only fear is that removing this stuff would be difficult.

As someone else suggested, ultimately the fuel cap will have to be airtight before flight, correct? During flight the low-pressure over the wing would suck out the fuel otherwise, wouldn't it?
 
prkaye said:
Vans instructions definately suggested 2.4 PSI, and the instructions definately say the water levels should *differ* by 5.5 feet. I'm using a bicycle pump to pressurize. I've de-pressurized the tank, and I'm pretty sure nothing was damaged. Still no obvious leaks with the soapy water and I didn't hear any cracking or poping while I pressurized.
Click to expand...

Maybe you have a different version of the instructions than I do but mine specifically warn you about applying more than 1psi of pressure to the tank. They go on to say that the pressure inside the tank is significanly less than 1 psi during flight.

My instructions came with the tank leak test kit that was shipped late 06.
 
Phil-I think I read the instructions the way Brian Chesteen did above. It's been a while, though I do know that I had nowhere near 60". 27-ish sounds about right. I'll go home and check tonight.
prkaye said:
Vans instructions definately suggested 2.4 PSI, and the instructions definately say the water levels should *differ* by 5.5 feet. I'm using a bicycle pump to pressurize. I've de-pressurized the tank, and I'm pretty sure nothing was damaged. Still no obvious leaks with the soapy water and I didn't hear any cracking or poping while I pressurized.
Click to expand...
 
When I get home tonight I'll pull out the instructions, check the date, and transcribe exactly what is written on them...
 
Chapstick works well on the cap o-ring and mechanism. I like cherry, personally. Fuel won't wash it away very readily, cheap and is safe for human and aircraft consumption (in moderate amounts).
 
prkaye said:
if this is the stuff they sell for taping-up ducting, then I have some of it at home. My only fear is that removing this stuff would be difficult.

As someone else suggested, ultimately the fuel cap will have to be airtight before flight, correct? During flight the low-pressure over the wing would suck out the fuel otherwise, wouldn't it?
Click to expand...
That stuff came off really easily. I don't recall any residue (assuming it is the same stuff I have). Any residue will come right off using laquer thinner.
 
Great Idea

rv8bldr said:
Hi Phil


You can get aluminum tape at Home Dumpo. It has a thin layer of aluminum on one side. This is what I used over the caps and it worked great.

Cheers
Click to expand...

Great idea.

About the pressurization statements.

If it cant take 2&1/2 lbs per sq in. I would not fly it let alone do aerobatics.

Look on the of the wing during a steep turn, does the top skins bulge slightly between the structures?

A ltlle bit of bulging shouldn't scare you as long as you don't want to burst test it. I would definitely keep it 2 1/2 or less.

But like my mother in law sometimes tells me...

Quote to me " Your entitled to your own stupid opinion!"
 
3 psi

Seems like I remember my Mooney manual recommending 3 psi leak check after resealing. I'll check my manual tonight.
 
I used the nitril glove methode. With the glove on the fuel inlet, I put in air through the water check valve on the bottom of the tank. I only put in enough air to inflate the glove. Then I put dish soap in a sprayer and sprayed the entire tank, no leaks. Then let the tank set for a few days and the glove stayed inflated. Good enough for me.
 
allbee said:
I used the nitril glove methode. With the glove on the fuel inlet, I put in air through the water check valve on the bottom of the tank.
Click to expand...
Neat idea - but how do you hold the glove in place and not leak?
 
WSBuilder said:
Seems like I remember my Mooney manual recommending 3 psi leak check after resealing. I'll check my manual tonight.
Click to expand...

Having had the nasty job of re-sealing Mooney tanks before, the structure is fairly similar. I would accept 3psi. Looking forward to finding out for sure.
 
Just got home and checked the instructions that came with my tank pressure test kit from Van's.

The instructions are pretty clear that there is no reason to pressurize the tank above 1 psi and warns about the dangers of putting too much pressure in (but doesn't say what "too much" pressure would be.) It goes further to mention that 1 psi on a manometer would mean that the water level in one end of the tube would be 2'3" higher than the level in the other (or that one side has gone up 1'1.5" and the other down 1'1.5").

Van's may have changed the instructions for later versions--mine is two years old--but 1 psi will definitely show the leaks so I'm not sure why there would be any reason to pressure it higher.

Hope this clarifies the procedure a little.
 
Here is an image of the instructions I have. As you can see it clearly says 2.4 PSI. This was given to me by another builder, so I can't verify its vintage, but at the bottom left it says 7/03, which suggests its a few years old. Perhaps somebody blew a tank with 2.4 PSI and Vans changed thier recommendation? ANyway, I am glad my tank survived 2.4 PSI.

 
wrap the glove around the fitting a couple times and then use a hose clamp. On my first one I started putting air in the tank, didn't know how much and the glove exploded, I guess it's a good thing to protect the tank as well. Didn't take much air.
 
hey Phil,

When I read the instructions I read it as 5 1/2 inches difference. If you remember when you came over I showed you that i looped the tubing down to the floor about 6 feet and then up to about eight feet. I filled the tube with water until it was even on both sides a couple of inches below the outlet. When I pumped the tank up and the water level moved the 5 1/2 inches that was plenty to show me any leaks and I did indeed find air leaking around two rivets (around the drain). After spraying with soapy water I saw these two leaks immediatly. I couldn't imagine pressurizing the tank so the difference was five feet in the tubing, if that is the case then i am glad I read them wrong!

Steve

ps:sitting in a hotel room in SYR waiting out the storm trying to get to S-N-F!!!!
 
Phil,

Ignore the log-in name in my last post, I'm using an old laptop of Mikes and it still recognized his log-in info.

Steve
 
prkaye said:
As someone else suggested, ultimately the fuel cap will have to be airtight before flight, correct? During flight the low-pressure over the wing would suck out the fuel otherwise, wouldn't it?
Click to expand...

Absolutly, I've seen it happen on a Beech Barron during a run-up. The tab on an inboard fuel tank cap, was facing forward and hadn't been pushed down all the way. During the runup the airflow pushed the tab up and the cap came out. Fuel then started spitting out of the tank pretty quickly.
 
Can anyone tell me where to find this "Fuel Tank Pressure Testing" document. I don't seem to have it in my 9A instructions.
 
the instructions I have came with the fuel tank test kit that can be purchased through Vans web store.
 
You get it when you buy the "Fuel Tank Test Kit" - an optional Vans accessory that consists of an AN type cap and a tire valve!

BTW - mine came in December and it describes a 1 psi test and a 2.3' total difference on the water manometer. The instruction sheet is labeled "Fuel Tank Test r2 4/04" Hope this helps.

dave
 
why the change?

So it looks like we've discovered that in 2003 Vans was recommending a 2.4 PSI, and then a year later they changed to recommending a 1 PSI test. The question is, why the change? Some possibilities:

1) The tank designs were changed, making them weaker?
2) Somebody blew a tank with 2.4 PSI?
3) They decided that 2.4 PSI was way overkill, and not worth the risk.

Thinking about sending an email to Vans support to ask about hte change... I did 2.4 PSI and my tank was not damaged, but after this tread I'm reducing to about 1.5 PSI (~3 feet water separation on the manometer?) for future tests.
 
For general interest: I got the following from Vans in responses to my question about the change from the 2.4 PSI test to the 1 PSI test:

"The original calculations were wrong. The newer instructions have
the correct number. The PSI suggested reflects the pressure on
the vent at about 200 MPH. That is the max that the tank should
see in use."
 
how slow a leak is OK?

Last night I started the balloon test on both tanks . In the first 6 hours, both balloons shrunk about the same (used a digital camera to verify...) as the shop temperature droppped.

In the next 12 hours, both balloons got bigger as the temperature increased, but one is not as big as the other one. In fact, I'm not sure it was as big as it was at the start of the test. It may be very slowly deflating. I would guess that there is a very slow leak on that tank, or on the balloon fitting.

I have rubbed soapy water on all the usual places and I can't see any bubbles forming. Maybe I need to spray it on with a spray bottle?

My question is - at what point is slow leak OK? At some leak rate you can't see any bubbles, so how do you find it? I don't want to make this a science project and I'm not going to use mogas (gives me a headache).

I'm unsure of what to do. I may rebuild the balloon attach on the "bad" tank or convert to a water manometer, but I'm not sure I can make that any more robust.

Thanks for any help,
 
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