Our atmosphere is mostly composed of the inert gas nitrogen (about 78%). Most of the remaining fraction is oxygen, which isn't flammable either—if it was, we wouldn't be here to chat about it! However oxygen does allow many other things to catch fire at a lower temperature, and then to burn both hotter and faster.
In most environments, the daily risk of fire may be small, but the effects of a fire once started can be catastrophic. Therefore most urban environments have dedicated firefighting crews stationed a short drive away. At one time those crews hauled water with them, as indeed they still do in many rural environments. But in an urban setting where water mains already run down the streets, it is more efficient to let the fire crews hook into the mains for their water supply. The trusty fire hydrant is the water access point for fire crews.
The 'fireplug' that we see above the ground in Canada and the U.S. is the tip of the iceberg, so to speak. A long standpipe runs below the hydrant deep into the ground to connect to the main. The connection, and the valve that allows water to flow into the hydrant, are normally set well below the frost line to prevent the hydrant freezing in winter. (Areas that seldom freeze may have water right at the hydrant at all times.) Most hydrants are either 'on' or 'off' with flow control in the 'on' state being the task of whatever is connected to the hydrant. A special tool is needed to activate the hydrant, to discourage casual misuse.
Most municipalities have by-laws that forbid any kind of encroachment on the hydrant. For example, in Toronto no building, vegetation, or other obstruction can be in front of, or within 1.2 metres (4 feet) of, a hydrant. Property owners are notionally required to keep the hydrant free of snow, although I have never seen anyone do that. In winter all of our hydrants are marked with reflective signs on long metal rods, meant to stick up above anything except a truly Buffalonian volume of snow.
It is illegal to park in front of a hydrant, and indeed tickets for this offense are very lucrative source of revenue for municipalities. Blogger David Brait reports, using public data, that the city of Toronto raised 3.3. million (Canadian) dollars in 2015 from 32,791 tickets issued for parking within 3 metres (10 feet) of a hydrant. Thus we can see that hydrants produce tax revenue as well as water. Of course, as those who recall the film Backdraft know, parking in front of a hydrant may also vex firefighters, who may take the expedient approach of smashing your car windows to run their hose to the hydrant. (Fire hoses are under a lot of pressure and take poorly to kinks, so sometimes this is truly the only answer.)
Hydrants may produce water at various pressures depending on the lines they are hooked into, and their own specifications. The hydrant cap may be painted a specific color to help firemen quickly identify how many gallons per minute (gpm) they can produce. Many municipalities have begun installing highly reflective 'flow rings' on the hydrants which are similarly color-coded, to help fire crews quickly spot suitable hydrants even in dark/smoky/snowy conditions. In the U.S., the National Fire Protection Association has standardized flow marker colurs as follows:
- Light blue — The most capable 'class AA' hydrants, which produce more than 1500 gpm.
- Green — 'Class A' hydrants, common in residential areas, produce from 1000 to 1499 gpm.
- Orange — 'Class B' hydrants which produce 500 to 999 gpm.
- Red — 'Class C' hydrants which produce up to 499 gpm.
Other colors may indicate inoperable hydrants (black), or hydrants with non-potable water (purple). The body of the hydrant is usually a highly visible chrome yellow, which makes the marker color stand out. Some areas also band the trunk of the hydrant with another color, which gives additional information. Sometimes hydrants are repainted to amuse, for example to look like Minions, which is cute, but must not interfere with the function of the hydrant nor its color coding.
In Canada and the U.S., standards for fire hydrants locations are usually set by municipal, provincial, or state governments. In urban residential areas, hydrants are usually located no more than 150 metres/500 feet apart, this being within reach of the standard 800 foot total length of a fire truck's supply of heavy fire hose. Hydrants must be as close or closer to the building(s) they serve and protect, preferably within 75 metres/250 feet.
All modern Canadian and U.S. fire hydrants have two standard hose outlets, one on each side, with a diameter of two and one-half inches. The front of the hydrant, which must face the street, has a main or 'steamer' outlet (a.k.a. `pumper port`) with a diameter of four inches, with a standard quick-connect adapter and cap. (An informed discussion of adapters would take another node and another half-day of research. Perhaps another time.)
References: Numerous manufacturer sites, municipal pages, and David Brait's page which is linked within the writeup.
My daughter asked why the hydrant had a blue ring on it.
4 hours later, the penultimate IN9 writeup has formed.