Eli5: How do fire hydrants work?


How do they build up so much pressure to push out through fire hoses, yet they do not have enough pressure to break the fire hydrant? Where does the pressure come from?

In: 8

Fire hydrants are made of thick steel strong enough to withstand the pressure from the flow of water. The same is true of all water pipes that connect to the hydrants (and every building that uses municipal water). The plumbing is designed by engineers to withstand the pressure found throughout the systems. Fire hoses are made of (typically) steel and nylon reinforced fabric over a rubber liner. They are narrower than the diameter of the hydrant pipe, so water pressure increases as it flows from a wider pipe to a smaller hose. The nozzles at the end of the hoses can further decrease the diameter of the water flow, increasing pressure even more. The pressure in the system is just the normal water pressure created by downward water flow (*i.e.*, gravity), supplemented where needed by water towers and possibly in-system pumps.

A fire hydrant is only an access point to a main. The 2 feet you see above ground goes 6-8 feet underground and hooks up with a water main. The steel is incredibly thick, and not to mention there is a long stem valve that goes down to the main, a fire hydrant doesn’t perpetually have water in it, only when the fire department hooks up to it. Not to mention, most fire hydrants are relatively low pressure, 60-100psi. Fire engines have very powerful pumps that take that water and increase the pressure. The fire department doesn’t use a fire hydrant to supply attack hose lines directly, but rather to supply an engine with water that then increase the pressure and puts it in specialized hoses for fire suppression. Hopefully that makes sense.

Tl;Dr, fire hydrants are relatively low pressure and don’t constantly have water in them.

There’s a plant that makes thee water drinkable. The plant sends the water through a series of pipes all over town with high pressure. When the water gets to your house it reduces the pressure through devices like a pressure regulator.

Fire hydrants don’t have pressure regulators, so the water has high pressure. And the hydrant itself is just heavy duty so it doesn’t break from the high pressure.

The house closest to the plant has the highest pressure, the house farthest away from the plant has the lowest pressure. So the pressure from the plant has to be high enough so the water can reach really far (but kinda screws over the ppl in closer homes)

Pump stations regulate the pressure in the water mains. 60-80 PSI is normal for around me. In a lower demand area, a water tower is more efficient. They fill the tower with pumps that shut off when the tower is full. As water is used, and the water pressure drops slightly as the level in the tower drops. A 120 ft tower would get you about 60 PSI. The pumps kick on to refill the tower before the pressure drop becomes enough for you to notice.

A fire hydrant is made of cast iron and is roughly 1″ thick. The reasoning for this is not because of the water pressure, but potential damage where they are located. This way you have to strike a hydrant with decent force even with a car to damage it. A common water pipe material is copper, and those lines are only 1/8 of an inch thick. 100 PSI isn’t an incredibly high pressure.

Power generation stations often have pipes with hundreds of pounds of pressure.