3 ms·

Couple of thoughts about gas and electricity and the similarities and differences: - an overpressure situation is specifically trying to blow through whatever

Couple of thoughts about gas and electricity and the similarities and differences: - an overpressure situation is specifically trying to blow through whatever "off" means (e.g. a ball valve or whatever the valves are). If the pressure rises too much, that valve is going to fail. That's true of circuit breakers too, but the "overvoltage fault voltage" of a circuit breaker is likely going to be way way way higher than anything you could reasonably expect from the grid, although lightning could probably flash over it. - Your breaker box will turn off during certain forms of fault, but not all. A line overvoltage probably wouldn't trip a breaker until the line voltage gets high enough to cause a fault in some piece of equipment connected downstream of the breaker (e.g. it blows up a switching power supply in your house and that fault current trips the breaker) - In the gas overpressure case, if all of the houses had gas meters that automatically turned off... the pressure in the community gas line would rise higher and higher. This is going to sound a bit cold, but it's likely better to have some houses blow up due to overpressure than to have a gas main explode after all of the houses have already shut off the gas. Bad situation all around :(. While acknowledging that I'm not a Mech E, my guess would be that the best solution would be to have periodically-placed blow-off valves on the gas main that could flare the gas if there were an overpressure situation. You don't really want to just release it, because then you've got a cloud of gas floating around that could get ignited by... whatever random ignition source.

maxsilver 8y ago
This is probably a stupid idea, (I know absolutely nothing about natural gas) but why can't we emergency vent the gas outside? My gas meter is already outside the house, more than 30 feet away from anything flammable. In an emergency overpressure like this, shouldn't I be able to pull some sort of emergency lever to vent the gas outside. Yeah, that gas is flammable, which is risky. But that has to be less risky than just sitting around waiting for my house to blow up, which seems to be my only option today.
- tonyarkles 8y ago
  If your gas installation is anything like mine, you could go outside and shut off the feed into your house. There's a valve that you could turn with a crescent wrench. Mine, at least, has a hole in it that the gas company can put a lock through when the valve is shut (which is what they do when they cut off your gas service). That way you're at least not going to have the inside of your house fill with gas. The trick with an overpressure gas main is that pulling that lever isn't going to be a "gas vents off for a few seconds and then things are good again" event. You won't be just reducing the pressure for your house, you'll be reducing the pressure for your entire neighbourhood, and there's going to be a pretty significant amount of gas that needs to vent. Also, I'm not sure if this is the case with methane, but propane has a really fun property... Going from compressed/liquified propane to gas propane has a high enough latent heat of vaporization that it can cool the surrounding liquid enough to freeze it (!!!). There was a court case around here a few years ago with a propane truck that blew a valve and started venting propane to the atmosphere. The liquid->gas transition cooled off the valve so much that a solid propane plug formed and stopped the leak (-188 degC!). Temporarily, of course... the plug eventually thawed, the gas continued escaping, and things got bad again. Having devices that could experience similar behaviour installed at peoples' homes seems pretty dangerous to me. It's all a shitty situation. Really, this is on the gas company. Their pumps should have never gotten the lines pressurized like this, and they should have had mechanisms in place to vent it in a controlled and safe fashion.

maxsilver 8y ago

This is probably a stupid idea, (I know absolutely nothing about natural gas) but why can't we emergency vent the gas outside? My gas meter is already outside the house, more than 30 feet away from anything flammable. In an emergency overpressure like this, shouldn't I be able to pull some sort of emergency lever to vent the gas outside. Yeah, that gas is flammable, which is risky. But that has to be less risky than just sitting around waiting for my house to blow up, which seems to be my only option today.

tonyarkles 8y ago
If your gas installation is anything like mine, you could go outside and shut off the feed into your house. There's a valve that you could turn with a crescent wrench. Mine, at least, has a hole in it that the gas company can put a lock through when the valve is shut (which is what they do when they cut off your gas service). That way you're at least not going to have the inside of your house fill with gas. The trick with an overpressure gas main is that pulling that lever isn't going to be a "gas vents off for a few seconds and then things are good again" event. You won't be just reducing the pressure for your house, you'll be reducing the pressure for your entire neighbourhood, and there's going to be a pretty significant amount of gas that needs to vent. Also, I'm not sure if this is the case with methane, but propane has a really fun property... Going from compressed/liquified propane to gas propane has a high enough latent heat of vaporization that it can cool the surrounding liquid enough to freeze it (!!!). There was a court case around here a few years ago with a propane truck that blew a valve and started venting propane to the atmosphere. The liquid->gas transition cooled off the valve so much that a solid propane plug formed and stopped the leak (-188 degC!). Temporarily, of course... the plug eventually thawed, the gas continued escaping, and things got bad again. Having devices that could experience similar behaviour installed at peoples' homes seems pretty dangerous to me. It's all a shitty situation. Really, this is on the gas company. Their pumps should have never gotten the lines pressurized like this, and they should have had mechanisms in place to vent it in a controlled and safe fashion.