This is from yesterday:
http://www.indystar.com/article/20121112/NEWS/121112004/Conditions-must-be-perfect-for-severe-blast-experts-say?odyssey=tab|mostpopular|text|NEWS02
What does it take to turn a 2,700-square-foot home into a bomb large enough to flatten two other houses, pummel dozens more, rattle dishes for miles and set off earthquake sensors 30 miles away?
According explosives experts: 90 pounds of natural gas, a lack of ventilation and an ignition source.
The Star spoke Monday with four explosive experts after they reviewed photos and news accounts of Saturday evening’s explosion on Indy’s Southside. Each said the blast could very well have come from a natural gas explosion.
City public safety officials, however, cautioned that their investigation into the blast that killed two people and damaged 80 homes, is incomplete and ongoing. While they say it appears gas may have caused the blast, other factors could be in play so they avoided giving a definitive cause.
The experts The Star spoke to said it’s extremely rare for a gas explosion that huge to occur inside a home, but it’s entirely possible.
For that to happen, they say, the conditions inside the home have to be just right.
In order for natural gas to detonate, there needs to be a very specific mixture of air and fuel inside the home.
Each said the blast could very well have come from a natural gas explosion.
City public safety officials, however, cautioned that their investigation into the blast that killed two people and damaged 80 homes, is incomplete and ongoing. While they say it appears gas may have caused the blast, other factors could be in play so they avoided giving a definitive cause.
The experts The Star spoke to said it’s extremely rare for a gas explosion that huge to occur inside a home, but it’s entirely possible.
For that to happen, they say, the conditions inside the home have to be just right.
In order for natural gas to detonate, there needs to be a very specific mixture of air and fuel inside the home.
The experts say that if less than 5 percent gas is floating in the air, there wouldn’t be enough fuel to spark a blast.
If there’s more than 15 percent gas in the air, the fuel mixture also wouldn’t ignite from lack of oxygen.
Nina Scotti, a California fire and explosions expert and former investigator with the Pasadena Fire Department, described the process as being similar to a carbureted small engine with a choke that controls the air and fuel mixture that causes the motor’s pistons and cylinders to fire.
“Think of your lawnmower or your Weed Eater when you can’t get it started,” Scotti said.
You can pull and pull the starter chord, and the machine won’t start because either it doesn’t have enough fuel or you’ve flooded the cylinder with too much.
The most volatile mixture for a massive blast of the magnitude of Saturday’s is around 10 percent gas.
“The perfect storm you might say,” says Paul Worsey, an explosives engineering professor at Missouri University of Science and Technology.
Worsey said to fill a 2,700-square-foot home you’d need around 90 pounds of fuel — about as much as a tank of gas in a car.
“I can get about 300 miles on that,” Worsey said. “If I drive at 50 mph, it will take me six hours.”
The difference, of course, is that inside his engine there are 7,500 tiny explosions happening each minute.
“The problem you have here is it happened in one big one all at one time,” he said.
Of course, inside the home there’d need to be an ignition source to coincide with just the right mix.
“Maybe a thermostat kicking in or something like that,” Worsey said.
But how did that much gas get into the home without anyone noticing the fuel’s foul smell?
W. Michael Shinkle, a Des Moines, Iowa, attorney who specializes in natural-gas related civil suits, says it sometimes is caused by an underground leak coming from a supply line feeding the home.
Shinkle at one time represented a company that manufactured a chemical additive that gives the otherwise odorless gas its noxious smell.
Shinkle said the soil acts as a natural filter that eliminates the smell. He said the leak then could have filled the home from the ground up without anyone noticing. Officials say the home was empty when the blast occurred.
The gas utility is continuing to investigate the explosion. Citizens Energy Group spokeswoman Sarah Holsapple said crew members inspected main gas lines Sunday and found no problems.
She said Monday they were in the process of testing service lines that run into the home where the blast occurred; the results were pending.
Holsapple said a meter reader had been at the house on Oct. 26 to test energy consumption levels, and found nothing out of the ordinary.
The experts say a natural-gas explosion also would have certainly had enough force to pummel nearby homes even several streets away.
That also includes the strange scene inside the neighborhood where homes blocks from the blast had garage doors caved in — even if they were pointed away from the blast.
Worsey said that’s because a blast causes a vacuum to form as superheated air is forced away from the ignition source. Air rushes back into the void caused by the explosion.
He said the air sucking back in very easily could have caved in the opposite-facing doors.
“Garage doors are pretty weak,” he said. “And it doesn’t take much pressure to cave one in.”
Star reporter Jill Disis contributed to this story. Follow Star reporter? Ryan Sabalow on Twitter at twitter.com/RyanSabalow. Call him at (317) 444-6179.
Note: Italics added
