Do you ever wonder why vapor comes up through the streets in big cities?

Many large cities throughout the U.S. have dozens of miles of buried pipes distributing steam to buildings for heating, cooling, and even sterilization. This is a smart and environmentally friendly way to use energy but the age of some of these systems and their efficiency is generating problems more and more often.

The age of underground pipes in these district heating systems can range from 20-100 years old. They are often susceptible to corrosion which can decrease the wall thickness of these pipes making them more vulnerable to leaks and failures. Leaks and failures will allow ground water to enter the piping system and it can contribute to the deterioration of existing insulation.

So, is the steam vapor that you see coming from the ground dangerous or a warning sign of a potential explosion? Most of the time the vapor you see rising out of manholes is just the condensation that results from cooler water, like rain or ground water, infiltrating through the casing and contacting the hot carrier pipe below street level. The white vapor is usually not the steam from the inside of the pipe. Does that mean that it is safe? The answer is no. While many times nothing will result, many other catastrophes have occurred due to a lack of insulation around the steam pipes.  This phenomenon is known as a “water hammer.”

A water hammer event occurs when cool water meets the exterior of a steam pipe, cooling the pipe and causing steam inside the pipe to turn to water. That water inside the pipe can travel as a very powerful force and rupture the steam piping at the next elbow or bend that it reaches within the piping system.  The spectacular amount of pressure and release of steam and water into the ground and surrounding atmosphere can be catastrophic and can harm nearby pedestrians, vehicles and buildings.