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Old 04-10-2011, 01:24 AM
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Top Fuel oil tank.....BOOM...long

I wasn't sure where to put this so move it if need be.

I build a lot of race car fuel and oil tanks. Most are made from .060 or .050 3003 aluminum. This one was .060. I use 4043 rod unless they are anodized. Even some of those get anodized even though there is a darker shade where they are welded. I use a square wave TIG welder. The tanks have rounded edges all around produced by using a form roller called a tank roll. 3/8" radius. The joint is a butt joint with a carefully fitted "0" gap. I spend a lot of time and effort to get very good fit up as most of the tanks will warp if there is any gap at all. I get really deep penetration but as with most TIG welded tanks there is still a seam visible on the inside. Gas welding is nice but a lost art. I'm going to a 4 day seminar in a couple weeks featuring gas welding aluminum. Anyway the weld is smooth and about 3/16 wide, consistant all around, no stuck electrodes, no holes melted thru, no other uglies. Blowing my horn I haven't had a tank leak after welding for a long time.

Now finally what happened. The tank in question is/was 12 x 12 x 9 inches with a 2 3/4" id vent tube. This is fixed at an angle and not capped. This tank is to capture oil and vent fumes from the motor...crankcase vent. The valve covers vent into the lower chassis tubes then are vented into this tank which vents fumes and captures oil. This type of tank is mandatory on all top fuel cars. So, the car is brand new, run only one warmup a week ago. No problems sounds very good. Last night it was warmed up in prep for a demo at a big local car show. During the warmup while running on alcohol the tank exploded. I mean big time. One side was blown completely out tearing the mount fastener right thru the welded tab. The other side was blown flat...ripped 3 sides totally out right down the weld. The band or center of the tank is stretched bigger than a bowling ball. Of course both sides are stretched in the same manner. This is the first time anyone has seen or heard of this happening but I would have to guess statistics would say it has happened before.

I tried to take pictures but they didn't come out worth posting. The seams split right down the center of the welds except for about 1 inch on the side blown out which ripped near the mount tab. I looked at the welds with a fairly high power magnifying glass and they appear to be stretched and ripped rather than a crystallized cracked area. They are thinned out and there are stress marks into the sheet skin. The bead itself is flattened some as though rolled or planished.

Now why did this happen?? Some information. The motor gets started on gasoline for about 30 sec then the alcohol is turned on. It runs several minutes at fast idle then the nitro is turned on about 1/2 to continue the warm up. Quite a bit of fuel mix gets into the oil pan during the warmup. Sometimes you can see the vapor come out of the catch tank. Since this was brand new, there was no oil in the tank at all. Once oil gets into it, it is very hard to get it all cleaned out. This tank was bone dry after the explosion.

The exact duplicate of the car (and tank) lives in the next stall at the shop. It has had numerous oil venting due to burned pistons and motor catastrophic distructions and the tank is still there in one piece and same shape.

Obviously there was a fume explosion but what set it off???? Maybe metal chips in the chassis tube caused a spark??? Static electricity from the rotating tires?? Something else???

Techinspector let's hear from you.

Thanks.

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Old 04-10-2011, 11:39 PM
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Boom!

Any chance that what happened was actually a "minor" crankcase explosion that sought "the path of least resistance"? The pulse of exploding gases may have blown along the vent tube until it reached the puke tank and compressed the fumes in there and and "boom". Just a thot.....
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Old 04-11-2011, 12:33 AM
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I got a little more information on this today from the team back at the shop.

The 'event' happened about 5 minutes into the alcohol part of the warmup. The motor runs about 2800 rpm. It 'popped' or miss fired then the tank blew up. They continued the warmup and switched to the nitro for another 10 minutes and shut the motor off. At the show they ran another complete warmup without the tank. Here the #8 cylinder was blowing much more smoke than the other cylinders.

These motors smoke a little as they start and warm up then they dry up as we say. So we have the first clue something is amiss.

So tonight back at the shop we started to look things over. The crew just wanted to write it off but I being the dad of 3 generations of crewmembers on car insisted they get to the root cause of this as there really isn't anything that could cause a spark to set the fuel vapors off under normal conditions. I've only been in top fuel 40 years so I felt I had a point. I told them to take the valve covers off and run the valve adjustment which they should have already done but had not.

5 minutes later crewchief son says "hey we have a damaged pushrod on #8 intake". Looking at the pushrod it was blue/black on the top end and the adjuster on the rocker arm is missing the ball and the end of the adjuster smashed badly as well as the rocker arm.

Damaging an intake valve or any part of it can easily cause a blower explosion roughly equavalent to a grenade with many more pounds of shrapnel. Very bad situation.

Well the pushrods are made of high quality hardened tool steel. D-2 I think as is the adjuster and it's ball end. Tool steel sparks very nicely when ground, or 2 sharp edges struck hard together or abrasive rubbed against it. All 3 likely happened very quickly here.

So when the adjuster broke it caused a number of sparks and lit off the fuel vapors inside the motor. I think the flame traveled down the chassis tube to the tank where it encountered more vapors and lit them off too, thus blowing the tank.

I looked much more closlely at the tank welds tonight too. Except for the first 2 inches which are 100 percent or very close but no weld bead, the rest were full pen with a bead. I always purge these tanks with argon too. An old aircraft maint guy told me to preheat to about 150 deg F at the start too so the weld is not on cold metal and it tends to drive off moisture. I do this and measure using my infrared heat gun. It does work quite well if you measure at very close range. Obviously a lot of pressure was inside this tank when it blew up.

So I think we have a reasonable cause. Spark ignition of the fuel vapor in the motor by the damaged pushrod and rocker arm.

Very good comment Dave75210. Failure analyst is a very good profession if you choose.

Last edited by bentwings; 04-11-2011 at 12:39 AM.
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Old 04-11-2011, 01:05 AM
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Boom

I get lots of practice analyzing my own failures......
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