On September 9, 2010, a 30-inch steel pipe carrying natural gas burst in San Bruno, CA, producing a wall of fire more than a thousand feet high, killing eight people, injuring numerous others, destroying 37 houses, and damaging 18 more. Friday, the National Transportation Safety Board reported that numerous welding defects were found in the pipe that had burst.
Let's note first that pipes used for strength in mission-critical applications ought to be seamless - a solid cylinder of metal from which the insides have been removed. (How that is accomplished is fascinating: You basically roll the tube under high pressure and the insides open up and you push them out. But that's another post.) PG&E (owners of the gas pipe) records indicated that the pipe was seamless, but it was found to be seamed -- and not very well. The pipe had been installed in 1956. Luckily, we know a lot more about welding than we did then.
There are two places you might weld a pipe. The first is a girth weld, where you join two pieces of pipe by welding along the circumference. This type of weld is necessary for seamed and seamless tubing. Seamed tubing, however, has a weld all the way along the length of the tube. Seamed tubing is formed by wrapping a flat piece of metal into a cylinder and welding the two sides where they meet. The process was developed in the early 1800s.
Regardless of whether you're talking steel or aerospace quality titanium-aluminum alloys, the welding process works the same way. We start with two pieces of metal. In a seamed tube, the metals are of the same type, but I've drawn them two different colors because that will make the process clearer.
The welding process starts by cleaning the pieces to the welded very thoroughly. Just like painting a house or laying floor, the details of preparation determine how successful the final product will be.
The metals are then heated. Arc welding converts electrical energy into heat. We're talking about huge amounts of electricity focused into a very small area - tens to hundreds of volts. The metals are placed on a work surface that is grounded. The pointy welding tip is raised to a high voltage and, when the tip gets close enough to the ground, it arcs.
The arc works on the same principle as spark plugs: electrons build up on the tip of the electrode. Charges of the same type want to be as far as possible from each other, but there's only so much space on the tip for the charges to sit. When the electrons reach a critical concentration, they are so repulsed by each other that they jump through the air gap to get away from each other. Once in the metal that you're trying to join, the electrons are free to move around and maximize their distance from each other.
The electrons deposited in the metal have a lot of energy, which means that they go everywhere. The atomic cores (the nuclei and the electrons that are still hanging around thsoe nuclei) are large, and the high-energy electrons bang into them. These collisions produce heat, proportional to the product of the current squared times the resistance of the wire. The small currents in household wiring don't produce much heat. If you plug a high-current appliance into a thin extension cord, the cord gets hot. There's a reason the cords for things like electric dryers are really big. Pass a large current through the type of wires that are in a two-prong extension cord meant for table lamps and you will melt the wires.
That's what happens in welding. So much energy is deposited in the metal workpiece that the two metals you're trying to join, along with the filler metal, all melt into a liquid pool. The atoms mix in the liquid phase and. When the piece hardens, you have something that, while not as strong a a solid piece, has enough strength to resist whatever pressures might be pushing on it. (Edit note: vSee comments below. The weld is actually stronger than the material.) My drawing is not to scale - obviously more than the top four or five layers of atoms are involved.
Macroscopically, a good arc weld looks like the one shown below. It looks like it ought to be fairly straightforward, but I can tell you from experience that it is not as easy as it looks. Now imagine such a weld all the way along the length of a 30-inch diameter pipe that runs for miles. The pipe that was excavated in the San Bruno case had welding flaws in the length welds, as well as in the girth welds.
The NTSB report cites “various defects”, including porous welds (the material didn't solidfy correctly), and some welds in which the fusion between the materials was incomplete. Page 53 of the report shows a weld problem really clearly. The report includes a picture of the burst pipe, which I have reproduced below. Note that the technical term for pieces of pipe that came from a larger piece of pipe is "pups".
The dark coating on the outside is asphalt that was used to protect from corrosion. The report indicated that some pieces of the pipe were not coated, but corrosion didn't have anything to do with the failure.
The pipe that ruptured had 3/8" thick walls and was rated to hold 375 psi of pressure. The NTSB preliminary report estimates that the pressure increased to 386 psi at the time of rupture, likely due to a power interruption that led to a valve opening, which caused a pressure increase in this pipe.
On the one had, it is pretty impressive that this pipe did its job for so long. On the other, it's an all-too-obvious reminder of the decaying infrastructure in the country. The I-35 bridge that collapsed in Minneapolis in 2007 had been deemed unsafe in 2006, but repairs were delayed due to budgetary and engineering problems.
We lump this all together under the term "deferred infrastructure repair", which (as far as I can tell) means that we know we need to fix something, but we're going to leave it as long as we can before we do so.
Imagine the cost to dig up, say, all gas pipes laid prior to 1965 to inspect and/or replace them. Bridges can cost hundreds of millions of dollars. Given the impossibility of fixing even just the spots most in need of repair, I guess the best thing we can do is to ensure that the control systems have enough redundancy that human error can be caught before the situation becomes dangerous -- or fatal.