SAN BRUNO GAS EXPLOSION--WHAT HAPPENED?--Carl Schwab

A blast believed to be caused by a natural gas explosion destroyed a San Bruno, Calif. neighborhood Thursday September 9, 2010. With a thunderous roar heard for miles, a natural gas line explosion ripped through the neighborhood Thursday evening, ending in a geyser of fire that killed at least one person and injured more than 20 others, and igniting a blaze that destroyed 53 homes and damaged 120 other, authorities said.

I am reading what I can find about the circumstances around the 30", 3/8" wall thickness, gas main in St, Bruno. The investigation has released little info but one can conjecture from the observables like crater size and area of damage. At first look it is equivalent to 5000 lb bomb penetrating to 20' then detonating.

The pressure in the 30" main was several hundred psig but not published by PG&E yet. Knowing that, one can compute the gas/air volume, at 1 to 20 ratio, to produce the equivalent blast energy based on blast damage. The cratering suggests the explosion was under the main. If it were a line rupture just 3 feet down you get a heluva of torch effect but no explosion.

PG&E and the Federal Safety Group say that a report with findings and recommendations will be issued 12 to 18 months from now and aren't volunteering much more. The crater from photos is 80'- 100' long and 30'-40' wide and 10'-15' deep. The length of the 30" gas line main was physically blown up in the air and landed more then 100' away. The ends of the 30" main appear to have severed cleanly in weld seams. The line was fabricated and installed in 1948 and was buried only 3' below the surface. Originally located in a "right-of-way" of some minimum dimension (like 150' minimum), over the years has been encroached upon to the point houses have been built directly over the 30" main.  THIS SHOULDN'T HAVE HAPPENED. Because of the age (1948) the turn-off valves on the main were manual. Apparently it took some 30 minutes to find and turn off the turn-off valves at St. Bruno. Newer work requires the valves be electric and remotely controlled for quick operation.

Early info release says the length of 30" main clear has axial weld seams and appears to include a "bend" fabricated at the time of installation (1948).

The writer's interest in understanding gas explosions stems from the belief that as a nation, we should maximize the use of NG for home and business heating as well as generating electrical power. Compared to oils and coal, NG produces about 45% less CO2 into the atmosphere. Also the supply of NG is huge and 98% comes from within our own borders. NG is efficiently moved via pipelines hence understanding what happened, and how to avoid it in the future is important. Also maintaining the NG gas line infrastructure is an INVESTMENT, and if the Obama administration IS looking for "shovel ready" projects this refurbishment should qualify.