OK, so why are there giant steam pipes under New York City?

In the wake of yesterday’s violent steam release in NYC, which left a cratere in the middle of a street and killed at least one person, news reports seem to take it as a given that everyone knows why there are huge steam conduits criss-crossing the city. In several reports I’ve read so far, all I’ve found out about why those pipes are there have been some vague comments about the steam being used to “heat and cool buildings”. I’d like to know more. Who produces the steam, and where? What, precisely is it used for? If it’s used mainly for heating, why are they pumping steam through the pipes in the middle of summer? One story I read said another such release a few years ago killed three people. Just how often do these ruptures occur, anyway?

Thanks in advance.

Con Edison, which supplies electricity to all of New York City and gas to much of Manhattan and Queens, also supplies steam to almost all of Manhattan south of 96th Street. It’s the largest municipal steam network in the world.

The steam network was started in the 19th century before electricity was commonplace and has remained in use. The steam is produced from waste heat at power plants and is used for heating a number of buildings, air conditioning (via absorption coolers) and in some places even cooking.

Most of what it is used for could probably be replaced by electricity, but there’s not a huge incentive to do so, since steam service continues to be available. Cecil also wrote a bit about the steam system

This article has a good summary.

The upside is that it is more cost effective than electricity in a high density area, especially in the winter, as it is essentially putting waste heat to good use.

The downside is that the infrastructure is over 100 years old, and when you have conductivity issues, you may get a massive explosion (as opposed to an electrical fire).

AIUI, such systems were rather common in cities. Rochester’s utility company has some steam pipes downtown, still. And Kodak’s industrial park, until only very recently, donated waste heat to one of the local schools for their heating, cooking and ‘laundry’ uses. Manhattan’s system is the largest, but it’s far from being the only such system.

Just curious: why did you put quotes around “laundry”? Are you suggesting some kind of skepticism regarding the use of the term by the local schools?

Sorry, it’s a term from my Navy days, for what was called hotel load or laundry load steam. I didn’t mean that I doubted that they’d use the steam for laundry, just that it’s a useful source of heat for any number of housekeeping purposes, which I didn’t think were worth going through listing each and every one.

Pounds of steam?

What do we mean by this? Is it the weight of water produced if the steam were condensed?

We have steam radiators at my house. The water is boiled with oil in a furnace converted from a coal burner. I think it’s all from the 1920’s and still seems to run pretty efficiently.

Yes. It is also a way to measure the energy content of the steam, depending on the pressure. At a pressure of 1 atmosphere, 1 lb of steam condensing to water will give about 970 BTU’s of heat.

Because when they’re not exploding and killing people, they make toughguy detectives seem much suaver in the stark night air.

To make things more confusing people will talk about steam in terms of mass and pressure. As Dag Otto has mentioned a pound of dry steam has a specific heat value. There are other ways that people will think of steam pounds though:

When talking about steam people will talk about the pressure of the system, expressed in pounds per square inch gauge (PSIG). Or more commonly referred to as ‘pounds’ of steam.

Steam systems come several types. The most common systems that people will see are the home heating systems. Formally these systems are known in civilian practice as low pressure systems. They’ll operate at about 1-5 PSIG, and for many jurisdictions the limits for a low pressure system is that it can have an operating pressure no greater than 15 PSIG. This is likely what your house operates at, if it’s above atmospheric pressure at all.

Above 15 PSIG, and to about 125 or 150 PSIG, you get the ‘mid’ pressure systems. Not all jurisdictions recognize this, though. This is the realm of larger buildings, that need more driving power to get through all the floors they have. It’s also the beginning of the more dangerous steam accidents. Since we’re talking about saturated steam, here, the pressure also is a description of the temperature of the steam. It’s a lock-step relationship. The 15 PSIG system is going to be about 240 F, IIRC. The 150 PSIG system is going to be about 350F - with the potential for damage to people and property rising dramatically with the rise in temperature.

Above 150 PSIG is the realm of high pressure steam. The temperatures get pretty high, up to about 650 F, IIRC. (Above that you start dealing with plasma, not steam anymore.) The pressures get similarly high. A steam turbine plant, using saturated steam, will operate at about 600 pounds, for example. But they’re not the only ones who use high pressure steam. Most dry cleaners will have a boiler operating at around 150 PSIG.

The Navy ships tend to run 1200 PSI and my BIL worked for Con Ed for years and they had many if not most turbines running at over 600 PSI. I am surprised by your statement about most steam turbine plant running at 600 pounds.

Am I missing something?


You’re mistaking kinds of steam. There’s saturated (or sometimes called wet) steam, which is what I was talking about - where the steam is exactly at the temperature where it would change to water, releasing only latent heat. Superheated steam is what one calls steam that’s been heated above the temperature of the phase change. To get to 1200 pound systems the Navy has to superheat the steam.

Thank you, that makes sense. I only know about the Steam Cycle on the ship I served on. I had a feeling I was missing something.


A municipal heating system was common in the 1800’s. Even my small town with a current population of 10,000 had one in the downtown area. Just a litle smaller than Manhattan’s, it was 3 blocks long. I believe it was in use until just a few decades ago, and the tunnels are still there, connecting every building. Our local spelunker sometimes guides tours for the historically-minded.

College campuses have their own steam systems too, I guess. I’ve read about the steam tunnels under UCLA, and in the fictional TV show NUMB3RS one of the characters on the CalSci faculty who prefers to remain homeless sleeps in the ‘steam tunnels’.

My alma mater, the Colorado School of Mines has them (can’t validate anywhere else, though). “Exploring” the tunnels was a common pastime for students, although during my time there the administration started getting better at locking down access points, since you could get into any building on campus using the tunnels. The steam is generated by the Coors brewery, and IIRC is provided to the campus free of charge. I’d assume Coors also provides steam for other parts of Golden.

It’s also the weight of the steam, while it’s still steam. That whole conservation of matter business, you know.

How is the municipal steam network used for cooking?

Using it to heat steam kettles, sometimes even direct heat via pressure cookers - of course that requires that the steam meet cleanliness standards. Or the steam could be used to provide heat for a clean steam system reboiler to provide steam for direct heat applictions. I was told by a former Chief Engineer for Kodak Park’s facility that they couldn’t treat their condensate because some of the steam was being used for cooking purposes by the school.

ETA: By definition the steam is going to be sterile - no bugs in it. And preventing entrained particulate contaminants from being carried along is fairly easy. The biggest concern is that the chemicals that are used to minimize corrosion in condensate return lines don’t taste very good, and may be hazardous to ingest.