Fire hydrants

Last updated: March 29, 2009.

Colourful, cute, even playful—fire hydrants nevertheless serve a serious purpose: they stand guard in our cities, towns, and villages, ready to supply huge amounts of water to fire engines, very quickly, whenever and wherever fires break out. These brightly painted street-side "characters" save properties—and lives. Ever wondered how they work? Let's take a closer look.

Photo: Fire hydrants are like faucets that stop up water supplies under high pressure. That's why it sprays out when they're opened. Photo by Stephen Schester courtesy of US Air Force and Defense Imagery.

Street faucet

A fire hydrant is really nothing more than a large outdoor faucet. It has up to four nozzles, one on each side, to which sturdy fire-hoses can be tightly screwed. To prevent tampering, the nozzles are held shut by pentagonal (five-sided) nuts that can be opened only with a special wrench. At the top, there is a similar nut and sometimes a wheel directly beneath it. Turning the nut and the wheel unscrews a valve inside the hydrant. This allows water to flow up from an underground pipe and out through whichever nozzles have been opened. Unlike an ordinary faucet, a fire hydrant is designed to operate either completely on or completely off.

It can take an enormous amount of water, sprayed for several hours, to put out a large urban fire—and time is always of the essence for fire-fighters. Ordinary faucets could not possibly supply enough water to do this job quickly enough: they are designed to supply small amounts of water over short distances and at quite low pressures. In this respect, fire hydrants are very different. Where a domestic faucet can provide just enough water to power a garden hose around 2 cm (0.75 inch) thick, a fire hydrant can fill a hose up to eight times thicker. Even the most powerful domestic faucet can deliver only a few gallons of water every minute. But a typical fire hydrant can supply water up to a thousand times faster. The water from a fire hydrant is at several times higher pressure than the water in your faucets at home: it can come out of the hydrant at 80 psi or more (roughly six times the pressure of the air we breathe).

Tanked up

Fire hydrants are the most visible part of our emergency water systems—but they are not the only part. The water that supplies hydrants comes from large tanks or reservoirs usually located on hilltops. Each of these is designed to supply enough water to operate fire hydrants for hours at a time. The tanks are connected to the hydrants by a system of pipes laid out in a grid pattern. This means the water can travel from any tank to any hydrant via several different routes, so a hydrant will continue to work even if one of the pipes springs a leak. When a fire-fighter opens the valve on a hydrant, the force of gravity makes water run downhill from the tank, through the grid of pipes, to whichever hydrant nozzles are open. The higher the tank is located, the more speed and pressure the water will build up, the quicker it will be delivered—and the faster the fire will be put out.