CONTINUOUSWAVE

A compressed air tank exploded under the deck of a Boston Whaler boat yesterday. The news article explains that compressed air was being used to pressure test the fuel tank. I wonder at what air pressure does the factory normally test the fuel system at? https://goo.gl/zW8V4k

The local NBC news affiliate also covered the story via helicopter, from the video the boat seems to be one of the larger Outrages with triple mercury outboards: http://www.wesh.com/article/boat-explod ... er/8260016

The television report of "breaking news" gets almost everything wrong about this incident and has just about zero information value--more or less typical of the usual no-information but sensationalized television news approach.

The other print-journalist story is much more accurate. I highly recommend that link as a source of information. See

https://goo.gl/zW8V4k

There is no mystery about what happened. A large-volume aluminum fuel tank on a boat was over-pressurized with compressed air, probably during a leak test.

The force that can be exerted by a slight pressurization of a fuel tank with compressed air can be significant. For example, if the boat has a rectangular fuel tank with dimensions of 4-feet x 6-feet x 2-feet, that means there is a surface area of three sets of sides:

2 x 4-foot x 6-foot = 48-foot^2
2 x 2-foot x 6-foot = 24-foot^2
2 x 2-foot x 4-foot = 16-foot^2
Total area = 88-foot^2

In terms of inch^2 that is an area of 88-foot^2 x 144-inch^2/1-foot^2 = 12672-inch^2

If the internal pressure of the tank is raised to 10-PSI, this exerts a force on the tank of 126,720-lbs. You can see how a tank might rupture (violently) with that kind of force applied to it.

While the term "explosion" is used, I don't get the impression there was an explosion as a result of very rapid combustion. I think the tank just ruptured when too much force was exerted on it, and apparently the rupture was sudden and perhaps violent.

When I had a tank built for my 15 center console, it was labeled as having been pressure tested to 2 psi.

jihm,

Saying a tank (pressure vessel) is under a force of 126,720 lbsf is a bit mis-leading. It's not mathematically wrong, but only part of the engineering/materials concerns. (Saying a tank/pressure vessel under load of 10 psi is much more appropriate.) -- This is like me saying I am under a force of ~43,500 lbsf at this exact moment, and anyone reading this is also under about ~43,000 lbsf (unless you are in space or deep under water). Or saying your generic aluminum 80ft^3 scuba tank is under ~1,500,000 lbsf while it's on your back before you get in the water... These numbers aren't wrong, but a small part of the equation in determining what is going on materially and structurally.

Taking the area of the fuel tank internal surface and multiplying by the force per unit area applied is a good indication of the total force acting on the material. If all of that force is released in a short period of time, the amount of WORK produced can be significant. I don't think most boaters realize how much force is involved to create that much pressure over a large area like the internal surface of a fuel tank.

It would be interesting to know more about where the fuel tank material (aluminum welded plate, I assume) yielded. Did it yield at a weld? Did a fitting yield?

The leak checks on the tanks are tested at either 2.5 or 3.5 psi for about a half hour. The welds on the tank held, the sheet metal sheared. This happened on a 350 outrage.