Fire hydrants are all around us. These above ground pipe fittings can supply flows upwards of 1500 gallons per minute to save lives and put out fires across the world. We all know what fire hydrants do but how exactly do they work?
Called fireplugs, fire pumps, johnny pumps, fire hydrants, these life-saving pipe fittings shave been around since 1801. Their designs vary by country of origin, but they usually have a connection point to hook up a fire hose and a nut or bolt to turn that will start the water flow.
Essentially every fire hydrant is just an attachment to a main potable water line. If you see one on the side of the road, there’s likely a potable water line running underneath that connects to the hydrant valve through a pipe called a “riser.” It’s important to note that hydrants don’t alter the pressure or flow of the water in any way, they simply function as valves so firefighters can utilize the already present pressure in the water pipes. While all of this may sound simple, the internal mechanics of a fire hydrant are a little more complex and can vary by region.
In the US, there are 2 types of hydrants, wet barrel hydrants or dry barrel hydrants, which we’ll focus on primarily in this article.
Wet barrel hydrants are used where freezing temperatures are uncommon. In this design, water fills the entire hydrant at all times, and the valve to start the flow of water is placed above ground. This design is usually cheaper to construct and easier to maintain due to the majority of its mechanism being easily accessible.
Dry barrel hydrants are used where temperatures routinely drop below freezing and thus, to keep the water in the hydrant from freezing, the valve is placed below the area’s frost line. The nut to turn the valve on is usually placed on top of the hydrant in this design to turn the long valve running through the hydrant and the riser.
These two hydrant designs encompass the majority of fire hydrants in the US and even many parts of the world. However, their complexities and intricacies don’t stop there.
Image Source: Concerning Reality
Other important aspects to note would be that the flow of a hydrant isn’t variable during operation. Their valves are designed to be totally open or totally closed. Coupling this with the fact that each hydrant is only a valve and usually not a pump, the output of water is solely dependent on the pressure available to the hydrant at that point in the water line. The higher the pressure, the more water the hydrant can output to stop a fire.
Common flow rates for hydrants across the US range from less than 500 gallons per minute all the way up to above 1500 gallons per minute. Higher flow rate hydrants are usually needed in commercial areas and dense residential areas – essentially where there’s the potential for larger fires. Higher flow rate hydrants deliver enough flow to fill a backyard swimming pool in just 3 minutes.
In the US, the colors of hydrants can also indicate their type. White means public system hydrant, yellow means it’s connected to a public water main, red means it is for use in special operations – not for other use, and violet means non-potable supply – water from an untreated source like a lake.
Image Source: Concerning Reality
Some fire hydrants are rarely used, which is obviously preferable, but they still have to remain at the ready. This importance means that the Fire Marshal and other authorities perform regular inspections of hydrants, usually by attaching a pressure gauge and measuring the flow of the water. Inspectors will check flow rate, valve functionality, and pressure in the pipe both before and during the flow.
It’s important that pressure in the surrounding pipes doesn’t drop below 20 psi during hydrant flow. If the pipe pressure does this, then certain areas of pipe in the water network can drop to negative pressure and suck in water from the ground possibly containing microbes in bacteria. If you ever get a “boil water notice,” it is highly likely that some unforeseen event caused pipes in your water system to drop in pressure.
So, these metal above ground pipe fittings that often meet the rear ends of man’s best friend are actually lifesaving pieces of engineering that have to remain at the ready to supply water to firefighters on the side of roads across the country.