Cement truck pressure vessel explosion (Pennsylvania)

Aluminum 200-gallon water tanks for cement trucks built by an Iowa subsidiary of a large defense contractor. Designed by a lawyer using exclusions in the boiler and pressure vessel code and a D.O.T. opinion to skip doubler plates around port holes. Two failures recounted: (1) Iowa hydrotest explosion that took a worker's legs off when the tanks were new; (2) Pennsylvania field failure during overpressurized weld-repair testing (100 psi shop air instead of 5 psi) that killed the welder, found in four pieces. Used to illustrate (a) doubler plate requirements at stress concentrations, (b) the National Board inspection regime under state law, (c) the cost of replacing engineering with legal interpretation. The Connecticut steering-failure embedded anecdote (§4.p3) is a sub-story illustrating muriatic acid corrosion, not a separate case.

I didn't talk to you about the air tanks on the cement trucks today? Let me tell you the story. They're aluminum, two of them — aluminum tank. Here's the dome on the end, here's the cylinder, and they put a backing bar in here. This is typical so that you can put the weld from the outside, because it's circular. You could do things like — this is a simple butt weld, like we have, with no problem; but in a little dome you're liable to use a sleeve and essentially double the welds, a sleeve on the outside. On this one, it's just a simple tank and cylinder, and there's a story that goes with this type of joint.

Extended teaching case. Aluminum water tank, no doubler plates, regulatory gap (DOT defers to ASME, ASME excludes water tanks, Pennsylvania attorney reads regs and concludes no code applies). Iraq-veteran welder killed during 100 psi over-pressure test repair. Earlier near-miss (leg amputation) had been attributed to a fluke; company responded by building a test cage. Tom got involved through Roger, ex-ASME committee. Hundred thousand similar vehicles on the road; National Board of Pressure Vessel Inspectors says they can't be inspected at that scale.

I had a situation once on cement trucks. They have a 200-gallon tank of water, and they use air pressure from the air brakes — the truck has 55 psi air to run the brakes. In some residential parts of the country it says "air brakes not allowed," because if you've ever heard a truck with air brakes, it's pretty noisy. So you have this 55 psi air, and cement trucks always have people spilling cement, so they carry this 200-gallon tank of water with hoses they can pressurize. About the pressure of your garden hose. They take it, spray off the truck, so they don't get cement caked on it. When they really have a problem they often carry a couple gallons of muriatic acid — HCl, hydrochloric acid — but you'll see corrosion if they don't rinse with water.