What keeps fire hydrants from freezing in winter?

The fire hydrants are a common sight in cities, especially in shopping areas. They are a blessing to the firefighters. The hydrants help the firefighters to access the underground water through hoses.
The hydrants, as we see them today, have been around for about two centuries. Frederick Graff Sr., chief engineer of the Philadelphia Water Works, is credited with the invention of the current fire hydrants at the turn of 19th century. This design is known as pillar or post type. He is said to have obtained the first patent for a hydrant, but it is unconfirmed because the United States patent office itself was decimated in a fire in 1836.
Hydrants are of two types: wet barrel hydrants and dry barrel hydrants. The one that was developed by Graff was wet barrel kind which had the main valve on top of it. It had a constant flow of water into the hydrant. The problem with wet barrel hydrants is that they can be useful only in warmer climates. It is the dry barrel hydrant that is used in areas very much affected by cold.
In dry barrel hydrants, the water doesn’t always flow into the hydrant. Only a small part will be above the ground. This is used to connect the hose to the hydrant when necessary. These hydrants would have two valves: the main valve and the drain valve. Only one valve will be open at a time. The main valve allows water flow into the hydrant while the other helps to drain out the water into ground after the use. Unlike the wet barrel design, in which the main valve is above the hydrant, the valve connecting the hydrant to the water main is very much deep under the ground in dry barrel hydrants. The design doesn’t allow the water to remain in the hydrant. Since there is no water in the hydrant, there is nothing to freeze.
The water pipe and valve in dry barrel hydrants will be under the frost line, the maximum depth to which the ground freezes in winter. The part above the ground is a simple metallic pipe. Another advantage with the design is that even if an accident takes place, it doesn’t result in a leakage of water. However, dry barrel hydrants are not completely free from hazards. In unusually cold weathers the frost line can move much deeper and cause damage to the water pipes.
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