After I had my house built, I noticed that the natural gas line to the
dryer did not have a shutoff valve. The builder sent over a plumber to
install one. Without turning off the gas, he unscrewed the end cap,
doped up the threads, and screwed in a shutoff valve. Although it only
took less than a minute, was this safe? This was located in the
basement near the furnace and water heater, which could have provided
an ignition point. I never had to shut off the gas to the whole house.
For my information, where is the main shutoff for the gas?
mainm shutoff should be at meter.
yes its safe provided its completed fast. just like working with live
electric lines, that safe too if done properly.
natural gas is lighter than air, and the amount lost small. so dont
On Mon, 3 Dec 2007 08:08:03 -0800 (PST), " firstname.lastname@example.org"
Recently the gas company was out to put a new meter on a new home. She
installed the meter and opened a gas line inside the garage to purge
the line. About thirty feet away was a lit cigarette.
The tech told me that the cigarette was "not hot enough" to ignite the
gas. I was surprised by this comment.
A smouldering cigarette isn't very hot - puffing it makes it a lot
hotter, and apparently methane has a rather high ignition energy compared
to other gasses.
However, the explanation is probably not that.
Methane has a LFL (lower flammability limit) of 5.3% and a UFL
(upper flammability limit) of 15%. It also has LEL (lower explosive
limit) of 5.53%.
Which means that unless the methane concentration in air is between LFL
and UFL, it won't ignite.
Gas lines past the regulator are also fairly low pressure, so opening
it doesn't spew gas very quickly. Secondly, methane is lighter than
air, so it goes up, rather than stick around.
An residential gas line venting in open air, is quite unlikely to
ignite from something 30' away. Even enclosed in an open garage,
it'd take a while to build up the concentration to the danger
point - if there's any ventilation at all, it might not ever reach
that concentration more than a few feet away from the vent point.
You still have to be careful - there could be wind patterns
that push a plume to exactly the right place. And it depends on
the gas. Propane is heavier than air. It will "puddle" in
depressions and stick around for quite a while in low wind
conditions. There was a propane tank car derailment around 40
years ago in the north end of Toronto, where the tank fully
vented. Everything was pretty much under control, until the
propane flowing down a small creek valley (very still air
conditions) finally reached an open flame about half a mile away.
Can't happen with methane.
Age and Treachery will Triumph over Youth and Skill
Besides UFL and LFL flash point is another part of the equation.
Methane's is far below zero and a cigarette or match would readily
ignite it. Static electricity, like from walking over a nylon carpet
could ignite it. There is also autoignition temperature and it takes a
very hot surface to ignite methane. The tech's comment about the
cigarette was downright stupid.
Not quite. Flash point is the point at which a liquified gas can
produce enough vapor to flash when ignited with a flame. By
definition, methane in a home is well above flash - all flammable
gasses are above flash point when they're a gas. The issue
raised by the OP was the temperature of the ignition point.
The autoignition temperature of methane is 537C. Which theoretically
means that methane won't ignite unless it comes in contact with something
On Dec 3, 12:31 pm, email@example.com (Chris Lewis) wrote:
I have been told that natural gas is not chemically pure. It is a
mixture of various natural gases - methane, ethane, etc. What the
utility company does to ensure a uniform BTU, I don't know.
As for the low pressure of the gas line, I know that I have turned on
the gas at a stove where it did not ignite immediately. The gas
buildup from just waiting a couple of seconds before finally igniting
produces a mini explosion that would singed my eyebrows if I were
close enough. A plumber's face would be closer to an open pipe. I
guess a good plumber will still have hair on his face.
stove burners are mixers too, designed to mix air and gas for good
burning. thats not the case with a open gas line.
gas companies do it all the time, as proof whens the last time your
gas was shut off when they did repairs?
never in my case, really only occurs when water gets into gas lines.
other than that gas up time must be 100%
For what it's worth, I was watching TOH a few weeks ago where they
tapped into the gas main to run a new line to a customer. They used
something very similiar to those needle valves you use to tap into a
water line. It was more sophisticated in that they drilled the line
first and then attached the valve, but the device they used kept the
gas line sealed at all times. I know that's not a repair, but the
point is that they kept the system both pressurized and sealed at all
However, I do have a question about your repair scenario: If they
actually opened the lines and vented the gas to the open air during a
repair, wouldn't there be a "gap" in any downstream gas delivery that
would extinguish any pilot lights that were burning?
That was the new service line to the residence that was being purged,
not the main, so the answer is "no", they wouldn't have noticed.
As for the tap-in, sure they seal and are careful of sparks, too, but
the main lines are at much higher pressure/volume than a single service
line so they lose much more gas than the scenario here.
If you were responding to my post, you may have misunderstood my
I was asking about his repair scenario, not the new service line to
hallerb said "gas companies do it all the time, as proof whens the
last time your gas was shut off when they did repairs?"
I am assuming he means that the gas company does not shut off the gas
every time they repair a gas line. Here's my question: If they open a
line and vent it to the air during the repair, then wouldn't any
downstream customers lose pressure and therefore lose their pilot
lights? I'm not questioning the fact that they do or don't shut off
the gas, but it seems to me that to any customers downstream of the
repair, it's going to look like the gas was off if they open the
Perhaps they tap around the repair before opening the line? If that's
the case, no one would lose service, but they are not really doing the
same thing as a guy installing a valve or T in a house like the OP
(and I) have seen them do.
OK, yes, I did misread your question and answered the unasked one... :)
If they were, indeed to completely open a line, then yes, unless there
were alternate supply there would be a loss of service downstream. IME
anything that required that level of work _would_ entail a notification
and a cessation of services. Not many residential branches have
alternate supply mains although I suppose if the work were extensive
enough they might bypass an area to not have extended outage.
In general, installed pipelines are pretty maintenance free for quite
long time periods so the consumer observation of loss of service is
quite rare. Our tap comes directly off a main (36") cross-country line
that crosses our place east of the house about a quarter mile away. To
the best of my knowledge that line hasn't had a service outage in
20-something years since they redid a section. It is flown by aerial
surveillance once/month to spot any problems. The other line just to
the west of the house that was more recently laid was worked on just
last year to lower it under the road because the county has been making
noises of re-elevating these roads sometime and if they were to do so
they just might have come close to the existing depth. It's a 42" line
and was shutdown for a period while the new section was fitted in and
welded in place. They only dug access holes to the ends on each end
about 40-ft away from the road and then a little longer trench on one
side. The then punched the new through under the road w/o trenching the
100-ft or so from one access hole to the other. The did a local
pressure test on the new section before inserting it then a pressure
test on the last connections and done. On those big lines they
don't/can't flush them anyway because of the length the amount of air in
a 100-ft section out of the 1500 miles before it gets to its ultimate
destination is inconsequential as it gets mixed up during the compressor
booster stations anyway.
For smaller side distribution lines I don't know whether they would
bother to try to purge or not or whether they just let the end users
deal with it until the air slug is gone...
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