The NTSB will be looking into the San Bruno natural gas explosion and probably won’t release their findings for a while.
The 30”, high pressure pipe was apparently laid in the 50s. There are now reports that several people smelled gas in the neighborhood in the days or weeks leading to the explosion. PG&E says they can’t find any records of reports made to them. :dubious:
What was the likely sequence of events? Did the escaping gas from a pinhole (or larger) leak collect in a low spot, meet an ignition source (a water heater pilot light, etc.) and the resulting fire raced back to the pipe causing it to explode? Or did the aging pipe blow out first, and then meet the ignition source? (I realize the either sequence could be near instantaneous and so my question may never be answered.)
Here’s a whole bunch of gas pipe explosion reports. Glancing through them, it seems a common thread is that a) someone smelled gas for a period of time before the explosion and b) repair crews who got there before the explosion often found a flaw of significant size in the pipe.
IINA chemist or an engineer, but if I recall, the gas /air mix has to reach a certain ratio before it can explode. Too little OR too much gas in the mix won’t sustain ignition. It’s my guess that a pipe failure (I wouldn’t say “blow out” allowed gas to leak out to such a point where enough collected and met an ignition source to cause an explosion.
This seems to me to be the same thing happening, just at different speeds. The gas escaped the pipe and found an ignition source. Or am I misunderstanding your question?
NinetyWt, I think the OP is asking, did the cumulative gas leaking out of a pinhole in the pipe simply accrue until it hit critical mass in some way and was ignited; or did a leak become a hull breach and the resulting outpouring of gas ignite?
(probably wondering about relative probability. and how much to worry.
My 17-year-old son lives in San Bruno, where I grew up, just a few blocks away from the disaster. He could so easily have been skating there when it happened, he has friends nearby. Fortunately, he is okay. Gave me a hell of a scare, though.
Anybody know what kind of offerings the FSM prefers?
From the descriptions of some of the residents who were in the neighborhood described a whoosh, then very quickly a fireball.
So my guess is that a “pinhole leak” abruptly became a major leak as the pipe failed catastrophically, but within seconds (maybe less than a second), that rushing volume of gas found an ignition source from somewhere in the neighborhood…a hot water heater in a garage is a good bet.
Several years ago an interstate high pressure gas line a half block from us was broken by a construction crew. No fire, thankfully.
There was a day long roar as the gas rushed out. “Whoosh” doesn’t begin to describe the noise.
(It’s rarely a good thing when a helicopter shot of your neighborhood leads the 5 o’clock news.)
Note that all too often people here overstress the issue of the need of the fuel-air mixture being just right. There is a gradient. From too low to too high. Somewhere along there is “just right”. When that gets sparked, the ensuing mixing due to the fire churns things so rapidly that a very large volume is now flammable. In short, once things get going, and it really is easy to do, then it all goes to hell.
They just shipped the section of pipe that blew out to DC for analysis. That should tell if it corroded or if was physically damaged. There was sewer work at the point of the explosion two years before, but the pipe had been inspected several times since.
This even reminded me of the August 19th, 2000 gas pipe rupture/explosion that killed 12 people camping close to the pipe. The NTSB released a report in 2003…
…that points to corrosion as the source of the leak. I also learned a lot about pigs, drip lines, corrosive gas and anerobic bacteria.
The pipe’s interior was full of natural gas and devoid of oxygen. It was under pressure, so atmospheric oxygen would not have entered the pipe at all; instead, gas leaked out, probably through pinholes, and then through a major rupture due to simple mechanical failure of the pipe due to age and corrosion. Combustion inside the pipe could not have taken place at any time during the event.
The videos on the news have been showing a segment of pipe under scrutiny that appeared to have been blown outward from the inside. As asplained above, this wouldn’t have been from combustion, but just from the raw force of system pressure inside the pipeline. According to these guys, system pressure is 200-1500 psi.
Does the possible smell of gas even point to this pipeline? From some of what I’m reading, the artificial odor may be added further down the line, and might not be present in a big transport pipeline like this.
That makes sense. The city or county planners shouldn’t have allowed this, but I guess that depends on how it was developed. I wonder if the area was developed before or after incorporation into a municipality.
There are many this size in our area, running through residential neighborhoods, usually located within a right-of-way; sometimes that right-of-way is only 50 feet or so in width.
Spot on. Likely sequence of events is: 1) corrosion thins pipe wall to a dangerous degree, creating pinhole leaks and reports of gas smell. Pinhole leaks don’t create a flammable cloud - methane is lighter than air so the most likely result is a gas plume.
2) pipe finally lets go under internal pressure, which can be up to 100 atmospheres. This will make a hell of a loud bang all on its own. A whole lot of gas rushes out at once, mixing with air. Some of it will form a flammable mixture and find an ignition source. The shockwave from the pipe rupture is not particularly strong but may be enough to shake any nearby electrical contacts: switches, streetlights etc. making arc sparks.
3) the flammable gas mixture burns in big fireball. It’s extremely difficult to get a free cloud of natural gas to “explode” in the sense of generating any kind of large overpressure or shock wave. Instead what you get is a “flash fire”, possibly lasting a couple of seconds. It’s very similar to the display “fireballs” set off at airshows and other events. A whole lot of heat is released which is bad enough on its own and can set the whole neighbourhood alight. If the flash fire propagates through confined areas and/or around complicated obstacles, the combustion becomes locally turbulent and creates a few psi of overpressure - enough to blow windows in or out, shift walls and lift off a roof.
4) The escaping gas continues to burn like a monster blow torch until the supply can be shut off and the residual gas burns away.
What is vanishingly unlikely however is that the large rupture of the pipeline itself resulted from a flammable gas explosion, particularly an explosion initiated outside the pipe.