A fire hydrant (also known colloquially as a fire plug in the United States or as a johnny pump in New York City, because the firemen of the late 1800's were called Johnnies), is an active fire protection measure, and a source of water provided in most urban, suburban and rural areas with municipal water service to enable firefighters to tap into the municipal water supply to assist in extinguishing a fire. Buildings near a hydrant may qualify for an insurance discount since firefighters should be able to more rapidly extinguish a fire on the insured property.
The concept of fire plugs dates to at least the 1600s. This was a time when firefighters responding to a call would dig down to the water mains and hastily bore a hole to secure water to fight fires via bucket brigades or, later, via hand pumped fire engines. The holes were then plugged with stoppers, which over time came to be known as fire plugs. This is the source of the colloquial term fire plug still used for fire hydrants today. After the Great Fire of London in 1666, the city installed water mains with holes drilled at intervals, equipped with risers, allowing an access point to the wooden fire plugs from street level.
It has been claimed that Birdsill Holly invented the fire hydrant, but his 1869 design was preceded by many other patents for fire hydrants, and a number of these earlier designs were produced and successfully marketed. Numerous wooden cased fire hydrant designs existed prior to the development of the familiar cast iron hydrant. Although the development of the first above ground hydrant in the USA traces back to Philadelphia in 1803, underground fire hydrants — common in parts of Europe and Asia — have existed since the 1700s.
Most fire hydrant valves are not designed to throttle the water flow; they are designed to be operated full-on or full-off. The valving arrangement of most dry-barrel hydrants is for the drain valve to be open at anything other than full operation. Usage at partial-opening can consequently result in considerable flow directly into the soil surrounding the hydrant, which, over time, can cause severe scouring . A hose with a closed nozzle valve, or fire truck connection, or closed gate valve is always attached to the hydrant prior to opening the hydrant's main valve.
When a Firefighter is operating a hydrant, appropriate Personal protective equipment, such as gloves and a helmet with face shield, are typically worn. High pressure water coursing through a potentially aging and corroding hydrant could cause a failure, injuring the firefighter operating the hydrant or bystanders.
In most jurisdictions it is illegal to park a car within a certain distance of a fire hydrant. In North America the distances are commonly 3 to 5 m or 10 to 15 ft, often indicated by yellow or red paint on the curb. In the UK, yellow lines are used to keep cars from parking over underground hydrants. Parking restrictions are sometimes ignored (especially in cities where available street parking is scarce), however these laws are usually enforced. The rationale is that hydrants need to be visible and accessible in an emergency.
To prevent casual use or misuse, the hydrant requires special tools to be opened, usually a large wrench with a pentagon-shaped socket. Vandals sometimes cause monetary loss by wasting water when they open hydrants. Such vandalism can also reduce municipal water pressure and impair firefighters' efforts to extinguish fires. Sometimes those simply seeking to play in the water remove the caps and open the valve, providing residents a place to play and cool off in summer. However, this is usually discouraged as residents have been struck by passing automobiles while playing in the street in the water spray. In spite of this, some US communities provide low flow sprinkler heads to enable residents to use the hydrants to cool off during hot weather, while gaining some control on water usage.
In most US areas, contractors who need temporary water may purchase permits to use hydrants. The permit will generally require a Hydrant Meter, a gate valve and sometimes a clapper valve (if not designed into the hydrant already) to prevent back-flow into the hydrant. Additionally, residents who wish to use the hydrant to fill their in-ground swimming pool are commonly permitted to do so provided they pay for the water and agree to allow firefighters to draft from their pool in the case of an emergency. Municipal services, such as street sweepers and tank trucks, may also be allowed to use hydrants to fill their water tanks. Often sewer maintenance trucks need water to flush out sewer lines, and fill their tanks on site from a hydrant. If necessary, the municipal workers will record the amount of water they used, or use a meter.
Since fire hydrants are one of the most accessible parts of a water distribution system, they are often used for attaching pressure gauges or loggers or monitor system water pressure. Automatic flushing devices are often attached to hydrants to maintain chlorination levels in areas of low usage. Hydrants are also used as an easy above ground access point by leak detection devices to detect locate leak from the sound they make.
In areas subject to freezing temperatures, only a portion of the hydrant is above ground. The valve is located below the frost line and connected via a riser to the above-ground portion. A valve rod extends from the valve itself 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 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 larger 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, and a hydrant with such an outlet may be referred to as a "steamer hydrant" although this usage is becoming archaic. Likewise, an older hydrant without a steamer connection may be referred to as a "village hydrant."
Hydrant coloring may be due either to purely practical criteria or more artistic. In America, the AWWA and NFPA recommend hydrants be colored chrome yellow for rapid identification apart from the bonnet and nozzle caps which should be coded according to their available flow. Class AA hydrants (>1500gpm) should have their nozzle caps and bonnet colored light blue, Class A hydrants (1000-1499gpm), green, Class B hydrants (500-999gpm), orange, and Class C hydrants (0-499gpm), red. This aids arriving firefighters in determining how much water is available and whether to call for additional resources, or locate another hydrant. Other codings can be and frequently are used, some of greater complexity, incorporating pressure information, others more simplistic. In Ottawa, hydrant colors communicate different messages to firefighters; for example, if the inside of the hydrant is corroded so much that the interior diameter is too narrow for good pressure, it will be painted in a specific scheme to indicate to firefighters to move on to the next one. In many localities, a white or purple top indicates that the hydrant provides non-potable water. Where artistic and/or aesthetic considerations are paramount, hydrants can be extremely varied, or more subdued. In both instances this is usually at the cost of reduced practicality.
In Germany, most hydrants are located below ground (Unterflurhydrant) and are accessed by a Standrohr which provides the connections for the hoses.
In the UK, hydrants are located in the ground. Yellow "H" hydrant signs indicate the location of the hydrants, and are similar to the blue signs in Finland. Mounted on a small post or nearby wall etc, the two numbers indicate the size of the water main in millimetres (top number) and the distance in metres from the sign (lower number). Older signs may use Imperial units.
In areas of the United States without winter snow cover, blue reflectors embedded in the street are used to allow rapid identification of hydrants at night. In areas with snow cover, tall signs or flags are used so that hydrants can be located even if covered with snow. In rural areas tall narrow posts painted with visible colours such as red are attached to the hydrants to allow them to be located during heavy snowfall periods.
In Australia, Hydrant signage varies, with several types displayed across the country. Most Australian hydrants are underground, being of a ballcock system, and a standpipe with a central plunger is used to open the valve. Due to this, hydrant signage is essential, due to their concealed nature.
In most areas fire hydrants require annual inspections and maintenance - they normally only have a one year warranty, but some have 5 year warranties. These inspections are generally performed by the local municipalities but they often do not inspect hydrants that are identified as private. Private hydrants are usually located on larger properties to adequately protect large buildings in case of a fire and in order to comply with the fire code. Some companies are contracted out to inspect private fire hydrants unless the municipality has undertaken that task.
Fire Hydrant manufacturers recommend lubricating the head mechanism and restoring the head gaskets and o-rings annually in order that the fire hydrant perform the service expected of them. Lubrication is generally done with a food grade non-petroleum lubricant to avoid contamination of the distribution system.
Occasionally a stone or foreign object will mar the seat gasket. In this case, most hydrants have a special seat wrench that allows removal of the seat to replace the gasket or other broken parts without removing the hydrant from the ground. Hydrants extensions are also available for raising a hydrant if the grade around the hydrant changes. Without extending the height, the wrenches to remove caps would not clear and the break flanges for traffic models would not be located correctly in case they were hit. Hydrant repair kits are also available to repair sacrificial parts designed to break when hit by a vehicle.
Many departments use the hydrants for flushing out water line sediments. When doing so, they often use a hydrant diffuser, which is a device that diffuses the water so that it doesn't damage property and is less dangerous to bystanders than a solid stream. Some diffusers also dechlorinate the water to avoid ground contamination. Hydrants are also sometimes used as entry or exit points for pipe cleaning pigs.
In rural areas where municipal water systems are not available, dry hydrants are used to supply water for fighting fires. A dry hydrant is analogous to a standpipe. A dry hydrant is usually an unpressurized, permanently installed pipe that has one end below the water level of a lake or pond. This end usually has a strainer to prevent debris from entering the pipe. The other end is above ground and has a hard sleeve connector. When needed, a pumper fire engine will pump from the lake or pond by drafting water. This is done by vacuuming the air out the dry hydrant, hard sleeve, and the fire engines pump with a primer. Because lower pressure exist at the pump intake, atmospheric pressure on the pond or lake forces water into part of the dry hydrant above water, into the hard sleeve, and finally into the pump. This water can then be pumped by the engine's centrifugal pump.