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.
A typical U.S. dry-barrel hydrant has two smaller outlets and one larger outlet. The smaller outlet is often a Storz connection if the local fire department has standardized on hose using Storz fittings for large diameter supply line. The larger outlet is known as a "steamer" connection, because they were once used to supply steam powered water pumps, and a hydrant with such an outlet may be called a "steamer hydrant", although this usage is becoming archaic. Likewise, an older hydrant without a steamer connection may be called a "village hydrant."
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