Published: 7 November, 2002

How a Fire Hydrant Works

How does a fire hydrant work?

Fire hydrants are actually pretty simple in the way they work. Each fire hydrant is connected to a large underground pipe that carries water. To get water out of the hydrant, firefighters use a five-sided wrench to open up one or more of the covers and connect hoses to the openings, which are like the spigots on your faucets at home. Then, they use the same wrench to turn the 'stem nut', which is like your faucet's cold or hot water knobs. Water comes up through the pipe, through the openings, and into the hoses that lead to the fire engines.

The water that comes out of the hydrant is the same water that comes into homes, businesses, and schools. This water has a water pressure of about 50-80 psi (pounds per square inch). This pressure is high enough for everyday use, but is not high enough for use by firefighters. So, the pumps on the fire engines increase the pressure. Then, there are smaller hoses that attach to the engines that firefighters use to fight fires.

Fire hydrants in the cooler states have an added level of complexity because the temperature often goes below freezing in the winter. Since water expands when it freezes and turns to ice, the hydrant could crack if there is water in the hydrant when it freezes and expands. Also, if there was a fire during the freezing weather, the hydrants wouldn't work, because there would be a huge chuck of ice clogging the hydrant.

To avoid the problem of freezing water, many hydrants are 'dry barrel hydrants', which means that no water stays in the upper section of the hydrant when the hydrant valve is turned off. The valve that controls the water flow is below ground and there is a long rod that connects the stem nut to the valve. Next to the valve, there is a drain hole to let water drain out of the barrel of the hydrant after firefighters turn the valve off. The pipe that holds the water is buried deep enough so that it never freezes. (A bit of trivia: Because of this mechanism, it's near impossible for a car to run over a fire hydrant and cause it to gush water as they do in the movies, since there usually is no water in the upper part of the barrel.)