nitrogen tanks on NYC sidewalks


Visiting New York City this week, and I saw a bunch of stainless steel nitorgen tanks near power poles on sidewalks, with thin red hoses going a few feet out on to the road, then down into a manhole.

What are these for?


Of course, I meant Nitrogen.


To pressurize the phone line with dry nitrogen and keep out moisture till repairs can be made or the cable replaced.
Insures integrity of lines and maintains service.

spingears, asked and answered, thank you!

Almost. The dry nitrogen tanks are normally connected after a repair or splice has been made, to drive out any moisture which may have condensed inside the cabling while it was exposed. Once the nitrogen has done its job, the tanks are disconnected and the pressure in the cables is maintained by dry, filtered air pumped in from the central office. Moisture doesn’t physically affect the cabling - the plastic insulation is pretty well impervious to anything - but it affects transmission quality in various ways.

Holy crap! Are you telling us that the phone cables have dry air running through them from the offices? Surely this is flowing air to make up for leaks. What is the flow rate from the main office? Are there dry air substations? What is the pressure in the lines?

Sure, there’s small leaks, typically pinhole-sized, so there must be a constant flow of air in to maintain pressure. I don’t know specifically what the pressure level is, but it’s low; probably only a few PSI. The central offices each have a special pump with filters, dehumidifiers and whatnot to clean and dry the air and feed it to the cabling. I don’t know how the longer trunk lines are handled, but there are usually repeater substations on these, to periodically amplify the signals to make up for line losses, and it seems likely that these could have the pumps, as well. Some older lines are not continuously pressurized and have to be periodically dried with either compressed or liquid dry nitrogen whether repairs are made or not.

Yup! In every central office I’ve ever been in (a couple dozen?) there was a compressor, tanks, and an air drier immediate above the cable vault (where all the big cables enter the CO and get routed up to the frame).

I have no idea what the flow rate is, but there were certainly gauges on the equipment. Maybe someone who currently has access to a CO can give you more info.

The exact flow rate is going to depend on the number and size of leaks in the cabling. The more leaks and the larger they are, the more rapidly air needs to be pumped in to maintain the pressure. I’ll see if any of my telecom references have any data on cable pressure levels.

From here, a manufacturer of the aforementioned equipment. Looks like between 3 and 15 PSI is typical. I’ll go out on a limb and say typically 9 PSI.

I’m a bit puzzled. I’ve never seen stainless steel tanks for pressurized gas just the insulated cryogenic tanks for liquid nitrogen which I would think would not be suitable for pressurizing phone lines. Am I missing something?

I’ve seen both. The liquid nitrogen tanks work in a similar way: the liquid nitrogen evaporates gradually, keeping the pressure maintained. It’s just a way to store more nitrogen in the same volume, is all.

Mnay, many years ago when I was an army reservist, I worked full-time for a while at CFB Carp, which was then the location of the nuclear war bomb shelter and command centre for the Canadian government. While we were waiting for it to be put to its planned use, it was used as a communications centre. The radio antennas were all located at widely dispersed sites (to survive the A-bomb blast) and were connected through pressurized cables to the base. We would spend about half our time driving back roads to isolated manholes and humping fresh nitrogen tanks into them to keep the lines pressurized.