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Q: Liquid nitrogen into sewers - why? ( Answered 5 out of 5 stars,   2 Comments )
Subject: Liquid nitrogen into sewers - why?
Category: Science > Earth Sciences
Asked by: stevedrosie-ga
List Price: $10.00
Posted: 22 Feb 2004 11:17 PST
Expires: 23 Mar 2004 11:17 PST
Question ID: 309530
I noticed tanks of liquid nitrogen on the streets of new york, with
tubes leading down into the storm drains, before a parade a year or so
ago.  (I don't know if the parade was a red herring or not.)  Why was
this done?  To asphyxiate rats?  To reduce odors? Something else?
Subject: Re: Liquid nitrogen into sewers - why?
Answered By: pinkfreud-ga on 22 Feb 2004 13:12 PST
Rated:5 out of 5 stars
Your guesses about rats and odors are good ones, but although liquid
nitrogen is piped down the storm drains, the NYC storm sewer system is
not its ultimate destination. The nitrogen is not dispersed into the
atmosphere of the storm sewers, but is closely targeted. Both Con Ed
and New York Telephone use liquid nitrogen in various ways in the
maintenance of cables and fiber bundles.

Here are some explanations from the nyc.general newsgroup:

"Cables are pressurized with nitrogen to reduce corrosion and prevent
combustion. They transport it as liquid because they can get more of
it in a tank that way."

"It's used to pressurize important fiber and cable conduits to keep
out the elements.  Various alarms go off when the conduits
depressurize, alerting them to a crack/break in the system."

Posts from nyc.general newsgroup

From the sci.chem newsgroup:

"Re: liquid nitrogen...
Con Ed in New York City puts 160 L dewars on the street and uses the
cold gas to cool transformer vaults in the summer. The Dewars sit on
the street all over Manhattan."

Post from sci.chem newsgroup

"What are the (nitrogen??) canisters near the Con Ed manholes used
for? I've heard at least three different versions...

Answer... Con Edison sometimes has tanks of liquid nitrogen placed on
street corners so that their employees can freeze cables, which make
them easier to cut."

Experts: New York

"Rain water is like little birds who don't make a circuit. During
severe rain in NY city, underground lines are covered with acidic
freestanding water but they still work just fine. The secret is good
insulation. This is why the phone company uses liquid nitrogen to seal
its cables in NYC. If you've ever been here you would see freestanding
liquid nitrogen tanks on street corners every now and then with the
label of 'NY Telephone' on their side. They are used in situations
where pressure drops and leaks are detected. In the end, freestanding
water does not like liquid hydrogen and the wires are kept nice and

Cinematography Mailing List

A detailed description of a similar phenomenon in Los Angeles:

"Several days ago a very large number of trucks and men from the Los
Angeles Department of Water and Power descended on my neighborhood.
They removed large sections of Pershing drive to a depth of 15 feet or
so over a stretch of about a city block. I assumed they had a problem
with a water main or something.

When they started building semi-permanent structures over the holes I
knew something really big was up. When the large trucks full of
strange power tools, mega-welding machines, breathing equipment, and
racks of test equipment came I started wondering. Driving by a couple
nights ago (11 PM), I noticed that the pace hadn't slowed - they were
at it 24 hours a day.

My curiosity got the best of me yesterday when they brought in the
giant tanks full of liquid nitrogen. LN-2 for the DWP? I parked my car
and played the lookie loo.

It turns out they have a problem with an underground wire. Not just
any wire but a 230 KV, many-hundred-amp, 10 mile long coax cable. It
shorted out. (Lotta watts!) It feeds (fed) power from the Scattergood
Steam Plant in El Segundo to a distribution center near Bundy and S.M.

To complicate matters the cable consists of a copper center conductor
living inside a 16 inch diameter pipe filled with a pressurized oil
dielectric. Hundreds of thousands of gallons live in the entire length
of pipe. Finding the fault was hard enough. But having found it they
still have a serious problem. They can't afford to drain the whole
pipeline - the old oil (contaminated by temporary storage) would have
to be disposed of and replaced with new (pure) stuff which they claim
takes months to order (in that volume). The cost of oil replacement
would be gigantic given that it is special stuff. They also claimed
the down time is costing the costing LA $13,000 per hour. How to fix
it and fast?

That's where the LN-2 comes in. An elegant solution if you ask me.
They dig holes on both sides (20-30 feet each way) of the fault, wrap
the pipe with giant (asbestos-looking) blankets filled with all kind
of tubes and wires, feed LN-2 through the tubes, and *freeze* the oil.
Viola! Programmable plugs! The faulty section is drained, sliced, the
bad stuff removed, replaced, welded back together, topped off, and the
plugs are thawed. I was amazed."

LiveJournal: JWZ

Google Groups Search: "sewers" + "liquid nitrogen"

Google Web Search: "cables" + "liquid nitrogen" + "new york"

I hope this information is helpful. If anything is unclear, or if a
link does not function, please request clarification; I'll be glad to
offer further assistance before you rate my answer.

Best regards,
stevedrosie-ga rated this answer:5 out of 5 stars
Well done, including the followup to the hydrogen comment.  My bet is
that the transformer cooling was the dominant reason when I saw them
(hot summer day, heavy load on the grid due to A/C).

Subject: Re: Liquid nitrogen into sewers - why?
From: neilzero-ga on 22 Feb 2004 15:58 PST
Just before the 13 th blank line of the answer is a typo where
hydrogen is mentioned instead of nitrogen. Since licquid hydrogen is
much colder than licquid nitrogen it would do the job, but it would be
a serious fire and explosion hazzard.  Neil
Subject: Re: Liquid nitrogen into sewers - why?
From: pinkfreud-ga on 22 Feb 2004 16:08 PST
Regarding the comment made by Neilzero (who is not a Google Answers
Researcher), please note that this material was quoted verbatim from a
website. The link for the site appears immediately under the quote. I
agree that the substitution of the word 'hydrogen' for 'nitrogen' is a
typo. This error was not mine, but appeared on the site from which I
excerpted the quote.


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