So they’re doing some sewer work in the street, and “a worker operating a backhoe accidentally cut into a natural gas line, causing gas to flow into 17 Danny Road,” which subsequently exploded. Not the first such incident, nor will it be the last. But I’ve always wondered, how they heck does that happen? Surely when the gas line is cut, gas will stop flowing into the house. So how does the house, which has to be at least some distance from where they’re working, suddenly fill with gas and explode?
One thing to realize is that a normal gas line filled with natural gas is above the upper explosive limit, because not enough oxygen is present to support combustion. When the line is cut or damaged, however, oxygen can be introduced from the surrounding air, which can contribute to a fire or explosion.
If the house was serviced by gas and the furnace was on, then gas was already flowing into the house. However, instead of having pure natural gas carefully combined with air in the furnace, you might instead get an explosive mixture coming into the house which, as soon as it contacts a spark or heat source, explodes–possibly even inside the gas supply line.
Pressure in the gas line will prevent oxygen from mixing within the line on the side upstream from the break. Downstream of the break, if there’s still pressure, oxygen won’t get in, and if there’s no pressure, there’s not enough gas to explode.
As Whack-a-Mole noted, the gas is probably finding another path into the house - maybe airspace around the gas line or a nearby sewer line. If the house doesn’t have much ventilation, it can be a slow leak that builds up over days until it reaches the right gas/oxygen mixture and a spark sets it off.
Gas leaks in the street do follow pipes into adjacent buildings. The big factor is disturbed earth, not necessarily the pipe itself. The soil around a pipe provides a conduit for the gas to seep through. It doesn’t have to be a gas pipe, either. The gas will follow the path of least resistance, so if there is a water or sewer pipe to follow, so be it.
The more frightening aspect of leaks in the street finding their way into a building is that the odorant, a chemical called mercaptan, is “scrubbed” out of the gas as it percolates through the ground. Straight natural gas (methane, etc) has no natural odor to it, mercaptan is added by the gas company to make the customer aware of leaks. The gas that leaked in through the ground might not smell, making the “boom” the first indication of a problem.
We’ve had more than our share of houses going “boom” in Massachusetts in the past few years. Somerset, Plymouth, Norfolk, now Boston. I love my gas heat and stove, but oil is looking better and better.
Watch the video in the link provided by the OP. In the video at the 1:12 mark it states the worker struck a gas main pulling the meter off of the house. My assumption is the is the break occurred in the wall of the house at a 90 degree elbow and the gas poured inside.
That’s true if the gas is static (i.e. not flowing). However, I would think that if the gas is flowing, air could be drawn in downstream via the venturi effect, like how an air ejector or venturi pump works.
I’m a little skeptical of these explanations. The path-of-least-resistance argument seems implausible. Water, gas and sewer lines to the house are usually surrounded by dirt; there’s no cavity that provides a conduit into the house. And what would push the gas selectively into the basement of the house? Thinking of how a backhoe operates, the pipe would almost necessarily have to be exposed at the point of rupture. So you would have gas collecting at the bottom of the trench. Obviously, the surrounding earth is not airtight, so you’d have some dispersal of gas into it. But that enough gas should go in the direction of the house, through however many yards of pretty compacted soil, and collect there in enough volume to cause an explosion not very long after the initial rupture – I’m not really buying it.
And robby, in the Wikipedia article you linked to, I read the following: “Concentrations lower than the [lower explosive limit] are ‘too lean’ to burn. e.g.: Methane gas has a LEL of 4.4% (at 138 degrees C) by volume, meaning 4.4% of the total volume of the air consists of methane. At 20 degrees C the LEL is 5.1 % by volume. If the atmosphere has less that 5.1% methane, an explosion cannot occur even if a source of ignition is present. When methane concentration reaches 5.1% an explosion can occur if there is an igniton source.” Surely the effect of introducing ambient air into the gas pipe would just lower the concentration of natural gas, possibly to a point below which it can ignite?
I actually linked to the part referring to the upper explosive limit (UEL), not the lower explosive limit (LEL).
Pure natural gas, with no ambient air present, will be above the UEL. If ambient air is drawn into the gas line, it may be possible to get below the UEL, I would think. Certainly, to get below the LEL, you have to get to a point between the UEL and LEL, at which point an explosion is possible.
However, I’m not an expert in this field. Much of this is a bit of conjecture on my part, in particular the part about air being drawn into the gas line. I’m not sure if this is a contributing factor to an explosion or not.
A backhoe doesn’t necessarily break a gas line at the point it hits it. For smaller lines, it’s more likely to break at a connection, which could be well away from the point of contact. And depending on the fill used around any lines in the area, there can definitely be preferential paths along these lines. When they put in a line, they aren’t filling the excavation with clay. It’s probably going to allow air flow.