Why do fire hydrants not freeze and burst in the winter? The water is theoretically above the freezing line, no?
In: Engineering
Once you close the valve (which is located about two feet below the ground level, or more), that water that is in the top part of the hydrant drains away into the soil nearby.
If you do not open the valve, and you drilled into the side of the part of the hydrant that is above ground, there would be only air in the part where you drilled the hole, and a rod that connects the top handle part to the subterranean valve.
> **The water is theoretically above the freezing line, no?**
In cold climates, it is below the freezing line. The design is simply a valve below ground and long rod that extends up and you can turn from the top. [Fire hydrant design.](https://qph.fs.quoracdn.net/main-qimg-24c937de0553bd9abb5398e15f423f12)
If I recall correctly, there is a feeder pipe up to the hydrant from below the frost line, and an activator device which keeps the water below the frost line.
>The water is theoretically above the freezing line, no?
No, actually! The part of the hydrant you see is only part of it; there’s a long pipe that extends underground to the water main, where it’s connected with a valve. The valve is connected to the nut on the top of the hydrant (that you twist to turn it one) by a long shaft. The whole thing is kept empty until it’s turned on.
The reason for this is exactly the issue you’re thinking of: to keep the working parts of the hydrant buried so deep that they never freeze.
One I can answer!
They can and do freeze if there is water past the shutoff valve. We prevent this by systematically and regularly checking the water level of hydrants. If water is found, we turn on and then turn off the hydrant to make sure it is fully closed and not leaking. Then we pump out the water that remains in the space between the shutoff and the street. Hopefully, when they are checked two weeks later, they are still unfrozen and dry. I have tried to open a hydrant that was full and frozen and it was not fun. Fortunately another truck had a working hydrant.
It may work differently in different jurisdictions.
Source: am a firefighter
Called a “dry charge system,” a remote valve is actioned at the port above ground. The system is charged 6 or more feet below the local frost line. Either 10″ or 12″ in diameter, a fire water main can deliver almost instantly to a reduced 4″ hose.
I’m sure the pipe could withstand freezing. This is just a quick recall from memory so I’m sure their might be errors. Water can not freeze if it doesn’t have the space to expand. An increase in pressure drops the freezing point. As the water starts to expand, the pressure held back in the pipe restricts the water from forming ice. This might also correlate with waters boiling point at different altitudes, which IIRC, is also pressure related.
There are two types of pressurized fire hydrants. Wet barrel and dry barrel. Wet barrel hydrants are found in warm climates only and the shut-off valve is on the hydrant itself. These hydrants are always full of water, or wet. Dry barrel hydrants are found in climates with freezing temperatures and the shut-off valve is located many feet below ground (below the frost line), but still controlled on the hydrant by a large rod internally that goes into the ground. These hydrants are only full of water when they’re turned on and they drain when turned off. They don’t freeze when not in use because there is no water in them.
If a wet barrel hydrant was installed in a cold climate, it absolutely would freeze! Wet barrel hydrants are also the ones that turn into fountains in the movies when hit by a car.
Google “fire hydrant out of the ground”. Only a small portion is above the freeze line and the actual valve is all the way at the bottom.
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