A fire hydrant, also called a fireplug, fire pump, johnny pump, or simply pump, is a connection point by which firefighters can tap into a water supply. It is a component of active fire protection.
In areas subject to freezing temperatures, at most, only a portion of the hydrant is above ground. The valve is located below the frost line and connected by a riser to the above-ground portion. A valve rod extends from the valve up through a seal at the top of the hydrant, where it can be operated with the proper wrench. This design is known as a “dry barrel” hydrant, in that the barrel, or vertical body of the hydrant, is normally dry. A drain valve underground opens when the water valve is completely closed; this allows all water to drain from the hydrant body to prevent the hydrant from freezing.
In warm areas, hydrants are used with one or more valves in the above-ground portion. Unlike with cold-weather hydrants, it is possible to turn the water supply on and off to each port. This style is known as a “wet barrel” hydrant.
Both wet- and dry-barrel hydrants typically have multiple outlets. Wet barrel hydrant outlets are typically individually controlled, while a single stem operates all the outlets of a dry barrel hydrant simultaneously. Thus, wet barrel hydrants allow single outlets to be opened, requiring somewhat more effort, but simultaneously allowing more flexibility.