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Author Topic: Tank failure?  (Read 360 times)
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Duncan McBride
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« on: July 02, 2020, 07:23:47 PM »

I was flying a free flight model with a G-160, brand new.  I was test flying and all the charges were gas charges from a 20 oz. paintball tank that was nearly empty.  On about the fifteenth flight or so, I filled the motor tank at the back of my van, where everything was in the shade, and walked out to the middle of the field to launch.  I was holding the plane upright at the level of my shoulder as I was walking when the tank burst apart.  It was like a rifle shot, making my ear ring on that side.  The top of the tank with the cap and filler valve was blown somewhere out in the field and never found.  The bottom part of the tank blew through the fuselage and into my thigh tearing my pants and raising a nasty bruise.  I've looked at the tank and it appears the top is screwed into the bottom with some kind of thread locking glue.  The joint failed.   Has this ever happened to anyone here or have you heard of it? This is worrisome.
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billdennis747
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« Reply #1 on: July 03, 2020, 12:03:23 PM »

That does sound bad. Are you saying the top is screwed in and glued - not just glued? If screwed, how did it blow out? Did it destroy the thread on its way out? I gave up on Telcos first time round because I didn't trust the chargers - stories of them exploding apart in pockets.
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Duncan McBride
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« Reply #2 on: July 05, 2020, 08:08:00 AM »

Here is a shot of the tank that failed.  You can see the threads.  The darker grey streak appears to be the sealant/adhesive.  The threads are pretty shallow but it is a thin-walled tank and I can't tell if they were damaged or not.
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Duncan McBride
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« Reply #3 on: July 05, 2020, 08:11:46 AM »

Here it is next to another tank the same size.  I think they are 5cc tanks. They came with the 2 Gasparin g160s I bought from Old Engines. Jiri confirmed that a sealant is used on the threads and the tanks are tested to higher than operating pressure.  I will send him these pictures as well, and offer to send the tank back if he wants to inspect it.
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raggedflyer
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« Reply #4 on: July 05, 2020, 11:12:46 AM »

In my opinion the mechanical joint between male and female threads should retain the parts together at the highest operating pressure plus a safety factor also taking in consideration the worst combination of operating temperature. The sealant should be to eliminate leakage and may add some additional joint strength but should not necessarily be relied upon because the strength will vary with temperature and is probably affected by thermal cycling.

The cut thread in the vessel looks more like a scratch thread and perhaps not full depth and has stripped on the left side.

Be interesting to learn if these pressure vessels are 100% over pressure tested or only a representative sample tested.
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pedr01
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« Reply #5 on: July 05, 2020, 12:13:40 PM »

Just throwing a curveball out there. Could it be that your paintball tank is High Pressure Air and not CO2?

Pressure in a CO2 canister is typically 850psi. If there is more CO2 in the cylinder, it will then be converted into a liquid form.

Whereas pressure in a high pressure air canister is typically 3000/4500psi. Big difference.
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billdennis747
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« Reply #6 on: July 05, 2020, 12:28:34 PM »

Crikey! But then wouldn't the motor run much faster (until it blows up)?
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Duncan McBride
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« Reply #7 on: July 05, 2020, 04:24:25 PM »

No, the tank was filled at a sporting goods store with CO2 about two years ago.  Used on and off ever since with several motors, no problems.  Doing some internet browsing I found that 850 pound number as well, the partial pressure of gaseous CO2.  The article went on to say how dramatically the pressure rises with temperature.  I sent the pictures to Jiri, and I hope he wants to have the tank returned - it would be good to find out whether he thinks there was some issue with the tank or if temperatures above 90F with exposure to direct sunlight is beyond the capability of the tank.  I'll let you know what I find out.
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FLYACE1946
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« Reply #8 on: July 05, 2020, 07:47:33 PM »

Some people put the co2 tank in an ice chest. I thing at 90 degrees that is a GREAT IDEA. This would be the tank that is used to charge the system.
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Nigel Monk
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« Reply #9 on: August 18, 2020, 05:16:19 PM »

The charging tank should be pressure tested every 5 years (typically) and should have the renewal date stamped in or a non-removable label attached. A reputable re-filler will check and charge an exchange fee if a customer's has expired. This is common practice for welding-gas cylinders.

There's two parts to the temperature issue, environmental temperature and exposure to direct sunlight. CE marked cylinders (UL in the USA I think) will have been tested to the equivalent of temperatures well above normal ambient extremes (65oC for the ISO standard). However, direct sunlight can heat an object almost without limit - at least up to the point where the heat loss matches the heat gain rate. That depends on absorption and emission coefficients for the surface. Heating the surface will heat the contents.

A 'standard AM1.5 atmosphere' assumes 1000Watts/m2 irradiance which is a very sunny day in the UK. If the charging tank is 5cm diameter x 25cm long (European fizzy drink maker cylinder) the exposed surface is effectively the rectangular envelope so 125cm2 or 1/80th m2 so could absorb 12.5Watts, but only if it were a perfect absorber which the painted surface is nothing like so I'm relatively confident the charging tanks are 'safe'. Shade and/or a beer cooler is even better.  Cheesy

I'm less confident about the shiny aluminium (?) flight tank. I have access to the British Standards library through work so I had a root around looking for the relevant standard. There isn't one, unless it is very strangely titled. There are standards for aluminium and steel seamless refillable gas cylinders. These contain calculations for minimum wall thickness but for both materials the absolute minimum is 1.5mm. Also, putting a threaded joint in the sides kinda stops it being seamless! I didn't look up a standard for joints in pressurised thin walled tubing, might have a go at that tomorrow. There is also a standard for single-use 'compact cylinders' less than 120ml, i.e. the 'sparklet' bulbs some of us are still using up old stock from (I'm sure they leak over the decades...). The test regime for these is heat them until the internal pressure is 1.5 times max working pressure and hold for 1 minute, then check for leaks.

Finally, there is an ASTM document F2856-12 (2020) "Standard Practice for Transfilling and Safe Handling of Small CO2 Cylinders for Use in Paintball". I don't have access to that one but I guess it does what it says on the wrapper. It's not an international standard and may have the status of best practice for the paintball industry organisers. It doesn't appear to relate to manufacture and probably not to testing from the title, but if anyone stateside has access, please let us know.

Duncan, it shouldn't be too difficult to calculate the approx failure pressure for the wall to cap joint and/or the end fitting joint for the flight tank. I can measure the pipe fitting but not the cylinder joint. I would need to know the thread pitch and number of threads engaged to calculate area of material that sheared. Lots of assumptions about manufacturing method/s and taking an average type of aluminium but it might be interesting. If you still have the failed cylinder, could you estimate those values? I'm sure there are lots of better engineers than me on HPA that can correct me where my calculations go wrong but I was basically going to use cross sectional area to find force then sheared area to find material strength, or rather the other way around...
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Nigel
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Duncan McBride
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« Reply #10 on: August 19, 2020, 09:03:35 AM »

The charging tank is a 20 ounce cylinder bought new in 2019.  I have an adapter that fits the Gasparin filler.  All that has been working well.  The cylinder has a burst disc.  I have always kept it in the shade - I don't know why I didn't think the motor tanks did not need the same precaution.

Jiri did send me two new tanks.  He said the high temperature coupled with exposure to direct sunlight was certainly a factor.  He also said it was advisable to start the motor as soon as possible after filling, and not to let the tank warm up.  I will measure the cylinder as best I can and post here. 

In the meantime I will  avoid temps above 90F, keep everything out of the direct sunlight, launch asap after filling, and make sure neither the cap or the cylinder is pointed at anything dear.  Here in Florida the hot weather lasts another four or five months, so it will be a while before I get back to flying CO2.

Thanks everyone for your help and suggestions.
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