The earth is a good insulator that if you dig a few feet underground it will always be the average year-round temperature. Hydrants take advantage of this by burying the water pipe and valve deep enough in the ground that they can't possibly freeze. After the hydrant has been used, a drain valve removes any water above the frost line, ensuring water will remain available for the next fire. The "hydrant" you see on the street corner is just the stand pipe. The rest of the device is buried several feet underground.
It also means that this won't happen:
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Water only enters the top of the hydrant when the valve, controlled by the operating nut on top, has been opened. It is possible for a car accident to break the valve from the impact on the stem connecting the operating nut and the valve, but often such impacts only breaks off the stand pipe. Taking off the nozzle cap won't result in a massive spray unless the valve has been opened.
In environments with severe cold weather hydrants are pressurized with air to keep the water below the frost line. When in use the line is hooked to a fire truck and the water is pumped out at a rate that prevents freezing. After use the hydrant is sealed and repressurized by emptying the air from a scuba tank through a special valve. While we mainly associate scuba gear with deep sea diving, firefighters use similar equipment for breathing in oxygen deprived areas. The availability of tanks makes this method of sealing the hydrant convenient. Unused hydrants have to be repressurized about once a year.
Houses in rural areas also have hydrants but they look and function like regular outdoor spigots. Like their city versions, these also have their major components placed deep underground. If a water system isn't available these hydrants may be connected to a pond. This type of hydrant is little more than a capped pipe: No valve is needed since the water isn't pressurized.
Posted 4676 day ago