welding: a matter of life and death.

Cpp_avatar 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.  Welding_Start_Side

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.

Welding_End_SideThat'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.  Arc-weld-bead-200X200Now 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".

SanBruno_RupturedPipe 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.

9 thoughts on “welding: a matter of life and death.”

  1. A couple standard tests for weld quality are a tension test, where a section of weldment is pulled apart; and the bend test, where a section of weldment is bent on the weld. If it fails on the weld, the weld (and if it is in a class, the weldor) is failed and does not pass go. These guys have pictures of both fixtures on site.
    All of this is a long-winded way of saying a proper weld is stronger than the base metal. Obviously, here, we’re dealing with a defective weld, so you’re quite correct to say “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.” However, in the general case, the weld will be stronger than the base metal.
    Loved “The Physics of NASCAR”.

  2. diandra@trivalent-productions.com

    Thank you (*blush*) for the comment about Physics of NASCAR.
    A welding seminar I attended for welding rollcages in racecars emphasized that you choose a weld material that is weaker than the materials you are joining. Am I remembering wrong about that?

  3. Great Post. I’m always interested in hearing about how things are made – or why they fail.
    Regarding the cost of repairing infrastructure – it doesn’t cost as much as you might think from first glance. It creates a lot of jobs, which means that a good chunk of the money comes back to the government in the form of payroll taxes. If the materials are purchased from American companies, then quite a bit of that money comes back in taxes, too. It puts people to work, which means that it’s probably one of the most effective ways to get money into the economy. I haven’t been able to understand why politicians don’t see that we have an infrastructure problem and an unemployment problem, and come up with a solution to both.

  4. Actually you are wrong on one point.. The weld, if done properly, is stronger than the base metal, and hence the joint is stronger. This is determined by the type of welding rod used. For instance a 6010p5 rod has 60k lbs tensile strength, and a 7018 low hydrogen rod has 70k.
    Also, you would be stunned if you knew the condition of pipelines in the ground. Many suffer from SCC (stress corrosion cracking), and external and internal corrosion. This pipe was running at a paltry 375 psi. Triple that is the norm in rural areas.
    Further, just because a pipe is rated at 375 psi does NOT mean it will burst at 380 psi. 375 is the MAOP or maximum allowable operating pressure. When pipelines are hydro tested after construction, test pressures are typically 1.5 times the desired working pressure. This line was likely tested to 563 psi, which is still far from 100% of SMYC or the actual pressure that will cause a rupture.
    As mentioned above, I bet they find microscopic SCC on the surface of the pipe. This does not look like a weld failure, as evidenced by the ripping longitudinally, and not following a spiral or straight seam, or butt weld.
    All in all this is way too complex to cover in a short blog piece.
    You should write something on the fascinating technology involved in testing pipelines that are in service and under pressure, called smart-pigging.

  5. Paul is correct! The weld, if done properly, is stronger than the pipe. Unfortunately pipeline welding codes only require something like 60% weld penetration…. I don’t know the exact number but it is in the right area.
    There are also many other factors that compromise the weld quality like letting the weld cool down between passes, wind, electrode moisture and many others. For example if a weld is cooled down to quickly then the material is like to have coarse grain structure in the heat affected zone. That results in material failure like in this failed weld test coupon. I have a page about welding certifications that gives a good rundown about weld quality and testing.
    Another problem I am seeing is cheap pipe and metals. I personally weld for a living and can’t believe how companies are cutting corners these days. They are using everything except American products and the quality is terrible! Cheap electrodes, bad metals and now they are cutting labor cost by more the half. The new trend is if you don’t speak English and don’t question anything then we will hire you. Real scary!
    If companies really wanted weld quality they would TIG weld the pipes (superior quality welds), use quality materials (American or not) and hire skilled labor!
    In the end I don’t think companies will replace infrastructure unless it is the worst case scenario. It just comes down to paying for a few deaths cost less then it would to fix the problem. Just think of the BP oil spill. Nobody remembers that anymore and they are still making money!

  6. Dave: I talked to a weldor about this, and they said the issue here with safety gear is that you weld with lower-grade filler to reduce the heat affected zone. They don’t (contrary to what I expected) put airframes or car cages into an over to stress relieve them, so they need as small a HAZ as possible.
    Diandra was given some specialized information on an exceptional process and generalized from it; probably the reason it was emphasized in the forum she was in was due to the exceptional nature of that type of welding.
    Diandra, if you’re still reading along, thanks for writing here, there, and everywhere.

  7. diandra@trivalent-productions.com

    Ry: I certainly am reading along! The beauty of blogging in a forum like this is that I learn as much from my readers as I do writing. I write enough “authoritative” stuff in my real life. I appreciate that the folks attending this particular cocktail party are knowledgeable and willing to share without needing to put each other down. Thank you!

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