Trevor English
Fire hydrants are all around us. These above-ground pipe fittings can supply flows upwards of 1,500 gallons per minute to save lives and put out fires across the world. We all know what fire hydrants do, but how does a fire hydrant work? And why are they sometimes different colors?
Called fireplugs, fire pumps, johnny pumps, or just fire hydrants, these life-saving pipe fittings have 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 the fire department 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.
As for color choices, many fire hydrants around the US follow the NFPA 291 standards for coloring and markings, but water utilities also sometimes diverge from the guidelines and choose quirky color schemes, so it’s always best to consult your local fire department or a water professional to determine how they distinguish differences between hydrants.
Wet or dry? Aren’t they all wet??
In the US, there are two types of hydrants: wet barrel hydrants or dry barrel hydrants:
- Wet barrel hydrants are used where freezing temperatures are uncommon. In this design, water always fills the entire hydrant, and the valve to start the flow of water is placed above ground. This design is usually cheaper to construct and easier to maintain because most of its mechanism is easily accessible.
- Dry barrel hydrants are used where temperatures routinely drop below freezing. 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. This means you must turn a long valve that runs through the hydrant and the riser.
Another important aspect to note is that the flow of a hydrant isn’t variable during operation. Their valves are designed to be totally open or totally closed. Couple 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 distribute to stop a fire.
Going with the flow rates
Common flow rates for hydrants across the US range from less than 500 gallons per minute all the way up to above 1,500 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. The highest flow-rate hydrants can deliver enough flow to fill a backyard swimming pool in just 3 minutes!
Some fire hydrants are rarely used, but they still have to remain at the ready and connected to the water supply. Thus, fire marshals and other authorities perform regular inspections of hydrants, usually by attaching a pressure gauge and measuring the flow of the water. This is called measuring “fire flow”. Inspectors will check flow rate, valve functionality, and pressure in the pipe both before and during the flow.
However, this does not necessarily need to be performed manually. This is a situation where hydraulic modeling software can come in very handy. Our customers often use InfoWorks WS Pro or InfoWater Pro to perform fireflow simulations to ensure all hydrants in a network meet their necessary specifications.
However you measure it, 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, which could possibly contain microbes or harmful bacteria. If you ever receive a “boil water notice” at your house, it is highly likely that some unforeseen event caused pipes in your local water system to drop in pressure.
Raise a glass of crystal clear water to fire hydrants. These metal above-ground pipe fittings are actually lifesaving pieces of engineering that have to remain at the ready to supply water to firefighters across the country.
What are the parts of a fire hydrant?
Most fire hydrants are, in general, simply designed, with the same basic features:
- Bonnet: The hydrant cap is known as a bonnet. Within the bonnet is the stem nut. The bonnet usually has a pentagonal or hexagonal operating nut to prevent tampering and is typically color-coded to indicate the water flow rate available.
- Nozzle caps: These caps cover the outlets (or nozzles) where hoses are attached to a fire truck. They protect the threads from damage and prevent debris from entering the hydrant.
- Barrel: The body of the hydrant, which houses the internal stem and provides a passage for water. It extends above and below ground.
- Stem: This is an internal rod that connects the operating nut to the valve, so you can open or close the valve to allow water to flow.
- Valve: Located at the base of the hydrant, the valve controls the flow of water from the water main to the hydrant outlets. They are often closed by default.
There’s more to learn if you’re interested
- The NFPA has a method for Calculating the Required Fire Flow that’s a bit technical, but it’s very informative for engineers.
- Watch our video about how Arcadis uses Infowater Pro to map all of a city’s fire hydrants and simulate pressure flows to create extremely accurate fireflow results of an entire fire hydrant system without needing to manually test every single one.
- If you’re a budding engineer, you might want to know about this: Autodesk Water Infrastructure products now available to students and educators around the globe – for free