Aside from being the historically-preferred location for canine bladder relief, fire hydrants serve an important function in providing access to a water supply system.
Types
Fire Hydrants fall within one of two types; wet and dry barrel.
Dry Barrel, as implied, is not water-filled until the hydrant valve is opened. Dry hydrants are overwhelmingly the most popular type of hydrant within the United States to provide insulate using depth to prevent freezing portions of the water supply.
Wet Barrel hydrants, though infrequent, are used in portions of southern California and Florida. These hydrants have one or more operating stems which run horizontal at each outlet. As implied, wet barrel hydrants are water-filled at all times.
Bonnet
The conical cap for the hydrant, or bonnet, holds the operating stem nut in place and protects the hydrant from mechanical damage and water penetration.
Branch
The branch pipe serving the hydrant from the city main is one restriction for the overall capacity of a hydrant. While older systems often connect hydrants with 4-inch branch pipe, a minimum of 6-inch pipe should be used to limit pressure loss and permit greater flow capacity. Our friction loss tool can be helpful in estimating loss through these pipes.
Flange
The flange at the base of the hydrant is the point of connection for the hydrant to the rest of the barrel.
While the dimension from the bonnet to the flange of the hydrant is standard, the height of the flange becomes important during installation as it determines the height of the outlets. Because hydrants need to be quickly accessed during an active fire, hydrant outlets need to be installed tall enough to allow a full-revolution of a hydrant wrench from the lowest outlet.
Hydrant Color
Some jurisdictions paint hydrants or hydrant bonnets to identify the capacity of the hydrant.
NFPA 291, the Recommended Practice for Fire Flow Testing and Marking of Hydrants, suggests hydrant colors as Red/Class C, Orange/Class B, Green/Class A, and Light Blue/Class AA for Less than 500 gpm, up to 1,000 gpm, up to 1,500 gpm, and 1,500 gpm and more, respectively (NFPA 291-2019 5.2.1.2).
Outlets
A
traditional dry barrel fire hydrant contains three outlets: two 2 1/2-inch (65 mm) side outlets and a single 4 1/2-inch (115 mm) or 6-inch (150 mm) "pumper" outlet. The latter outlet gets its name as it is often the preferred choice for the fire department to connect and feed pumper trucks.
The size and number of the outlets serve as one limit to the capacity of the hydrant. While the typical hydrant described above is the most common type, other combinations certainly exist - downtown St. Louis, for instance, have hydrants with only a single pumper outlet.
Stem Nut
The stem nut is the key to operating the valve within the hydrant. Typically shaped as a pentagon, the stem nut will turn the operating stem of the hydrant and raise the valve to an 'open' position when turned with a hydrant wrench.
Thrust Block
Unless mechanically restrained, thrust blocks serve as a way to distribute the
hydraulic force of the pipe network into the soil. Our thrust block calculator can be helpful in sizing these blocks.
Valve
When in the 'open' position, the valve at the bottom of a dry barrel hydrant rises to plug drain holes and simultaneously permit water to fill the barrel of the hydrant.
When in the 'closed' position, the valve lowers to block water passage and re-open drain holes at the bottom of the hydrant. These drain holes act as weeps which slowly drain the hydrant barrel and help prevent freezing.
A fire hydrant is a visible connection point placed in defined locations for firefighters to tap into a water supply. All buildings, parking areas, roadsides, mines, industrial areas, etc. must have fire hydrants with connection to a water service network. They are designed to provide the water required by firefighters instantly to fight and extinguish a fire. Till the 18th century, underground fire hydrants were used. However, from the 19th century onwards, above-ground pillar-type fire hydrants become popular and mostly used. A fire hydrant is basically a pipe with the control of a valve through which water flows from a water main in order to put out a fire.
Purpose and Uses of Fire Hydrants
The primary purpose of fire hydrants is to supply water for suppressing fire. However, they can be used for several secondary purposes like:
- Line Flushing: Due to its high flow capability and easy operation, fire hydrants can be used to flush main distribution system lines.
- Testing System: To test the hydraulic capacity of the distribution system, fire hydrants can be used.
- Other Common uses: Fire hydrants are also frequently used as a water source for commercial construction work, sewer cleaning, street construction, street cleaning, etc.
Fire hydrants with a variety of valves and connection points are seen in many places. In the event of a fire breakout, firefighters locate the fire hydrants, connect their hoses and then pump a large volume of pressurized water to put out the fire. A special pentagonal wrench is used to remove the valve cover of the hydrant. Then after attaching the hoses, the firefighters open the valve for the water to flow.
They usually have a connection point to hook up a fire hose and a nut or bolt to turn on what will start the flow. Every fire hydrant is essentially just an attachment to the main water line. Underneath that connects the hydrant valve through a pipe called a riser. However, the normal hydrants don’t change the water pressure or flow in any way. They 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.
Types of Fire Hydrants
There are two types of Fire Hydrants; Wet hydrant and Dry hydrant.
Wet Hydrant
Wet hydrants are widely used in places where there is no problem of freezing. In such types of fire hydrant systems, the water in the main supplies the hydrant close to the surface. So, in cold weather conditions, it is susceptible to freezing.
Dry Hydrant
A dry hydrant system stores the water below ground. The Earth’s temperature is usually higher than the cold environment temperature in cold regions. So, the possibility of freezing can be prevented by this arrangement. When the dry hydrant system is required to be used, firefighters open a valve on top of the hydrant and engage their hose in it. This causes the drain valve to open inside of the hydrant. This allows the water to come through which the firefighters use against the fire.
Difference between Wet barrel Hydrant and Dry barrel Hydrant
The main differences between a wet hydrant and a dry hydrant are mentioned below:
| Wet Hydrant or Wet Barrel Hydrant | Dry Hydrant or Dry Barrel Hydrant |
| Wet Hydrants are used where water freezing issues are not present | Dry barrel hydrants are used in cold regions where the temperature routine drops below water freezing temperature. |
| In this design, the water is placed aboveground | The water in the dry barrel design is kept below ground to avoid freezing. |
| Wet Hydrant is easier to construct and cheap. | On the other hand, dry barrel hydrants are costlier and difficult to construct. |
| Maintenance of wet barrel hydrants is easier due to easy access. | Maintenance is comparatively difficult. |
Components of a Fire Hydrant System
The main components that constitute a fire hydrant system are:
- Fire Fighting Pumps & Accessories
- Piping
- Panels
- Landing Valves
- Hoses
- Couplings
- Hose Reel
- Branch Pipes & Nozzles
- Fire Brigade Connections
- Wiring & Instrumentations
- Maintenance Valves
Color Coding of Fire Hydrants
Following NFPA standards, all fire hydrants are color-coded. These colors indicate the expected flow during the operation. Usually, the top caps of the fire hydrants are painted. The following table provides the common colors according to the flow.
| Color-Coding | Flow | Meaning |
| Light Blue/Blue | >1500 GPM | Very good flow, suitable for industrial applications. |
| Green | 1000-1500 GPM | good for residential purposes |
| Orange | 500-999 GPM | marginally adequate |
| Red | <500 GPM | Low flow, inadequate |
Codes and Standards of Fire Hydrants
The codes and standards that are used as guidelines for designing fire hydrant systems are:
- NFPA 1
- NFPA 25
- NFPA 291
- AWWA
- A112.21.3M
- BS EN 14384
- BS 750
- DIN 3222
- DIN EN 14339
- AS 2419
- FP-009
- IS 3844
- IS 13039
Fire Fighting System Online Courses
To learn more details about the fire fighting systems, their operation, application, etc you can join the following online in-depth video courses:
- Basic Fire Fighting
- Fire Safety: Become A Fire Safety Expert
- Fire Safety Master Course: The Untold Secrets Of Fire Safety
- Design Fire-Fighting Systems
- Become A Fire Protection Design Engineer – Basic to Advanced