https://hackaday.com/2015/07/21/why-is-there-liquid-nitrogen-on-the-street-corner/
Any NYC hackers may have noticed something a bit odd this summer while taking a walk… Giant tanks of the Liquid Nitrogen have been popping up around the city. There are hoses that go from the…
Nitrogen makes up most of the atmosphere and is a relatively cheap gas to capture and compress.
Also seems like it's only during construction and repair. They're not constantly pressurized. (Or rather, Nitrogen is used to sypplement)
Not entirely clear but I think they mean they flush the sewers with the nitrogen, not actually pressurize them with the nitrogen?
The cables have a protective sheath covering them, but during construction and repairs, the steam build up in some of the sewers can be too much for them — so they use liquid nitrogen expanding into gas to supplement the pressurized cables in order to keep the them dry.
Think a nitrogen generator is a few thousand?
A nitrogen generator is like 30k, which in building projects is a drop in the bucket. They probably prefer tanks cause they are more mobile and simpler, anybody can run a line and twist a valve.
Lots of armchair speculation incoming: In the article, it mentions the tanks some times hit their pressure release causing “snow”, so they must not be pushing much gas as I’m guessing the heat of the sun is causing the tank to over pressure. That leads me to believe they aren’t trying to pressurizing the whole sewer system, but create a positive pressure in the conduit or sleeve the lines run through in the sewer system.
They must have to do a ton of venting and gas checks before climbing down into any of the manholes when this system is running anywhere in the vicinity.
It would be pretty economical to own a generator and the tanks as required, not have a generator at each site