A fire hydrant is a pipe with a valve that controls the water flow from a water main in order to quench a fire. The fire hydrants may be underground and above-ground. The underground fire hydrants were in use in Asia and Europe since XVIIIth century, but the above-ground ones are in use since XIXth century.

Before the invention of the fire hydrants, firefighters used the bucket brigade or hand pumping systems to get the water to extinguish fires. They had to dug a hole in the main, in order to retrieve water from the water main. The firefighters would pass buckets of water from the hole along a line of firefighters to help get water to the fire. The term “fire plugs” began to be used, since the newly dug holes had to be plugged up after the fire call.

With the fire hydrants, now the firefighters can connect a fire hose to the hydrants and then release a valve to get water from the water main. The different kind of valves on a fire hydrant allow it to be attached to different water sources that may be either pressurized or not pressurized. Most hydrants are designed to allow not less than 950 liters (250 gallons) of water to flow through the hydrant per minute.

Many different patents were used to create various fire hydrants. Each Country usually regulates the design and the technical specifics of the hydrants to meet fire district standards.

There are two kinds of fire hydrants: the wet barrel type, that holds a constant water supply, and the dry barrel type, that needs to have a valve release to let water in. The dry barrel hydrant avoids a frozen water supply in very cold climates. Some wet barreled hydrants may be made of bronze rather than iron, or they may have an inside finish that avoids rusting.

The barrel of a fire hydrant may also be known as “standpipe.” Some are rounded in style, while others have more angular lines. The outlets on a hydrant are usually made of bronze, but the caps may be made from other types of metal.