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1.7: Hydrants

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    In this chapter, we will examine water distribution fire hydrants and their uses.

    Student Learning Outcomes

    After reading this chapter, you should be able to:

    • Describe the different types of fire hydrants
    • Explain the different uses of fire hydrants
    • Identify the operational and maintenance criteria for fire hydrants

    From a very early age, we are exposed to fire hydrants. Many kids' storybooks have pictures of red fire hydrants and Dalmatian dogs.

    A red fire hydrant next to a wall in Santa Fe
    Figure \(\PageIndex{1}\): Photo by Ashim D’Silva on Unsplash
    Figure \(\PageIndex{2}\): Photo by Tim Zänkert on Unsplash

    We start recognizing the red fire plug and associate it with firefighting. However, most fire hydrants we see every day are not red. They are yellow. The use of fire plugs or fire hydrants as we know them today dates back to the 1600s. After a devastating fire destroyed a large portion of London, predrilled holes were add to new water mains and plugs were installed above ground for access. Since early water mains were made out of wood, the early hydrants were holes that had above ground plugs, hence the term “fire plug”. By the 1800s, cast iron hydrants replaced the traditional fire plugs.

    Yellow fire hydrant
    Figure \(\PageIndex{3}\): Image by Matt Gregory is licensed under CC BY-ND 2.0

    Fire Hydrant Uses

    The most common and understood use of a fire hydrant is fighting fires. Public fire protection is important for any community and it is the responsibility of water utilities to provide fire hydrants in required locations and the water required for fighting fires. There are other uses of fire hydrants. Some of these uses include flushing water pipelines, storm drains, and sewers, street washing, public landscape watering, and construction. Let’s take a look at some of these less obvious uses.

    Flushing Water Pipelines, Storm Drains, and Sewers

    When a new water pipeline is installed there are various things that need to be done before the pipeline can be connected to the distribution system and used for public water supply. New pipelines need to be disinfected and sampled to make sure the water in the pipe is safe for human consumption. After this disinfection and sampling process, the water needs to be flushed out and replaced with fresh water. This flushing process is commonly performed by the use of fire hydrants. Water in pipelines can also become stagnant if there is little or no use. An example of this would be a cul-de-sac where there is little to no use. Perhaps it is a long cul-de-sac with only one home and the homeowners are out of town for an extended period of time. The water in the pipeline could become stagnant and when the homeowners return may notice some taste and odor problems with the water. If the water utility is notified of these types of issues they would oftentimes flush the water out of the pipeline to replace it with fresh water.

    Storm drains and sewers also need to be cleaned from time to time. If debris blocks the flow in these systems, hoses are often connected to fire hydrants in order to push the debris and flush these piping systems. It is important to make sure the potable drinking water distribution does not become contaminated from storm drains and sewers. Often times certain valves known as back-flow devices are used or water trucks are filled through fire hydrants and then these piping systems are flushed.

    Street Sweeping and Landscape Watering

    In order for city streets to stay clean, vehicles such as street sweepers are commonly used. A street sweeper will connect to a fire hydrant in order to fill up a tank and then the water is discharged to clean the street. A lot of streets have center medians with landscaping which include trees and other plant material. If these medians are not equipped with irrigation systems, water trucks are often used to water these plants. The water to fill these trucks comes from fire hydrants.

    Water truck
    Figure \(\PageIndex{4}\): Image by Ildar Sagdejev is licensed under CC BY-SA 4.0


    Construction needs water for various purposes. One of the most common uses of water (supplied through fire hydrants) is for dust control. During grading operations, heavy equipment moves and removes dirt, sometimes over large areas. This process of dirt moving creates dust. In order to control the dust, water trucks collect water from fire hydrants and spray it over the entire area to wet the dirt controlling the dust.

    Is Water From Fire Hydrants Free?

    Unlike water served to customers such as home or business owners, water coming from a fire hydrant is not metered. When a fire truck pulls up to a fire hydrant, connects a hose to the hydrant, and starts to fight a fire, the water used is not metered. In other words, there is nothing to track the amount of water used and there is no one paying for the water being used. Since fighting fires is a public service, it actually makes sense that the water used is not metered. There are some uses from fire hydrants, which should be metered, and the water paid for by the user. Most other uses of water from fire hydrants besides fighting fires is metered for use. Portable hydrant meters are used to keep track of the water used. These meters, often referred to as construction or hydrant meters, are temporary meters rented to the user. Since these meters are temporary in nature and can be used in multiple locations the user is sometimes asked to provide the meter reads.

    U. S. Air Force Airman 1st Class Zach Fowler, 20th Civil Engineer squadron fire department firefighter, releases water from a fire hydrant into a hose at Shaw Air Force Base, S.C., May 16, 2013.  The firefighters train weekly on structural firefighting, aircraft firefighting, hazardous materials, rescue operations including confined space and also vehicle extrication. (U.S. Air Force photo by Airman 1st Class Nicole Sikorski/Released).
    Figure \(\PageIndex{5}\): Image by the U.S. Air Force is in the public domain

    By charging for water used from fire hydrants there is accountability of the water and it also discourages the wasting of water. It is important for water utilities to account for all the water within a water distribution system. It costs the utility to pump the water throughout the distribution system, so when water leaves the system unaccounted for, there is lost revenue. This lost water is measured by utilities and is referred to as water loss.

    There are several ways water utilities monitor and control the use of water from fire hydrants. Specific meters are assigned and used only for connecting to fire hydrants. These meters are commonly referred to as construction or hydrant meters. Often times, deposit fees are charged to “rent” these meters. Deposits are designed to cover cost of the meter in case they are lost or damaged. These fees can also be used to maintain and service these meters to make sure they work and measure flow accurately. Some utilities use a permitting process to issue these meters. This process is designed to help track the meters being issued and often includes a fee, which provides the same coverage for maintenance and replacement costs.

    Parts of a Fire Hydrant

    There are several parts of a hydrant and can be broken into the upper and lower section. The upper section consists of the nozzle and head. The nozzles are the areas where hoses or meters can be connected and where the water flows from the hydrant. These connections are commonly 2 ½” and 4” in diameter, threaded, and caps are usually provided to cover and protect the threads.

    Caps and nozzles labeled on fire hydrant
    Figure \(\PageIndex{6}\): Image by Ben Schumin is licensed under CC BY-SA 2.5 (modified by COC OER)
    U.S. Air Force Airman Jacob Molden, 23d Civil Engineer Squadron firefighter, closes a fire hydrant during a joint live fire training exercise, Aug. 24, 2016, at Moody Air Force Base, Ga. After the firetrucks deplete their water supply, they’re refilled by the on-site hydrant.  (U.S. Air Force photo by Airman 1st Class Janiqua P. Robinson)
    Figure \(\PageIndex{7}\): Image by the U.S. Air Force is in the public domain

    The top section of a hydrant can also be referred to as the bonnet. The bonnet is the top cover or enclosure. There is an upper barrel portion that sits above the ground and a lower buried portion, oftentimes referred to as the “bury-el”. This buried elbow portion is shown below.

    A workman is shown preparing a new fire hydrant for installation.
    Figure \(\PageIndex{8}\): Image by FEMA is in the public domain
    Fire hydrant labeled (bonnet, ground level, nozzles, barrel)
    Figure \(\PageIndex{9}\): Image by FEMA is in the public domain

    Types of Fire Hydrants

    On the surface, most fire hydrants look similar. They are usually molded in cast or ductile iron, bronze, and sometimes steel. They are typically constructed above ground with threaded openings for the attachment of hoses. However, there are a few different types of fire hydrants. The first two we will look at are the dry barrel and wet barrel hydrants.

    Dry Barrel Hydrants

    A dry barrel hydrant is exactly that…a hydrant with the barrel dry. The barrel of a hydrant is the body above ground and a section below ground. The picture below depicts the barrel. This type of fire hydrant is very common in areas where the weather drops below freezing. In these areas, the barrel of a hydrant needs to be dry in order to prevent the water in the barrel from freezing. If the water in a barrel of a fire hydrant freezes, then the ability of the fire department to access water in order to fight fires is impeded.

    Dry barrel fire hydrant diagram
    Figure \(\PageIndex{10}\): Dry Barrel Fire Hydrant - Image by the U.S. Fire Administration is in the public domain

    There is a nut at the top of a dry barrel fire hydrant, which is connected to a stem. This stem connects to and operates a valve, which allows water to enter the hydrant barrel, and water can flow. There are two types of dry barrel fire hydrants: wet-top and dry-top. A wet-top dry barrel hydrant is constructed such that the threaded end of the stem and the operating nut sealed from water when the valve is open. A dry-top dry barrel hydrant has the threaded end of the stem sealed off from water in the barrel when the hydrant is filled with water and in use. This design reduces the possibility of the threads becoming fouled by sediment or corrosion.

    The type of operating valve can further classify a dry barrel hydrant. These types of valves include standard compression, slide gate, and toggle.

    • Standard compression – This type of hydrant valve closes the water against the seat of the valve to aid in providing a good seal
    • Slide gate – A slide gate is a gate valve similar to a distribution system gate valve
    • Toggle – This type of valve closes horizontally and the hydrant barrel extends well below the branch line. This type of valve is also called a “Corey” valve.

    Wet Barrel Hydrants

    Wet barrel hydrants are used in warm climates where the risk of freezing is minimal. Since most fire hydrants are above ground and exposed to the elements, it is important that the water inside the hydrant doesn’t freeze when the temperatures dip below freezing temperatures. However, it is also important that a fire hydrant flows water when it is opened. This is the design of a wet barrel hydrant. When the valve stem adjacent to a threaded opening is twisted open, water flows directly out of the hydrant.

    wet barrel fire hydrant diagram
    Figure \(\PageIndex{11}\): Wet Barrel Fire Hydrant - Image by the U.S. Fire Administration is in the public domain

    Locating Fire Hydrants

    Fire hydrants need to be properly located in order for ease of access and properly spaced in order to give proper coverage for firefighters. Hydrants are typically located at street intersections and spaced between 350 and 600 feet apart. The spacing is dependent on the type of structures in the area (residential, commercial, etc) and the density of the buildings. Commonly they are placed about two (2) feet from the back of curb face and located far enough away from buildings to allow the fire department to gain access without being too close to fire. When installed along public roadways on sidewalks, care must also be taken to install them so they do not impede foot traffic or wheelchairs.

    Since fire hydrants are commonly installed where there is vehicular traffic, the potential for one to get hit by are a vehicle is likely. Therefore, proper installation must be considered in order to provide the least amount of damage. Several methods are commonly used. One method is to use hollow bolts, which connect the flange above ground to the flange below ground. Another method employs the use of a “breakaway” flange. This type of flange is manufactured to split in the center. Both of these methods allow for the hydrant to break away from the below-ground barrel. This prevents the buried components from being damaged and allows for the hydrant to be reinstalled relatively easy without any excavation.

    Potential Problems

    As discussed above, hydrants are not just operated by water utility operators. Fire departments, City workers, construction workers, and others can and often do operate a fire hydrant for various reasons and uses. This presents a potential problem. Older hydrants may not seat (close) properly and leak after it is closed. Opening a hydrant at the end of a cul-de-sac may stir up sediment in a pipe causing water quality issues and concerns. Proper traffic control must be taken when a hydrant is open on public streets with traffic. When a hydrant is open, it can disrupt the flow of traffic by spraying water across lanes. Whenever a hydrant is open to flow onto the ground, proper drainage needs to be available. If storm drains and gutters are filled with debris, water may not be able to drain properly and some of this debris can be flushed into storm drains ending up in local waterways. Another problem that can occur when a hydrant is improperly opened and closed is something called “water hammer”. Opening or closing a hydrant too fast can cause the flow of water to move or stop too rapidly causing damage to pipes and other appurtenances.

    It is important to understand the need and various uses of fire hydrants. They play an important role to the surrounding community, public safety, safety to property, and a variety of other uses.

    Sample Questions

    1. A fire hydrant referred to as a “Corey” style would be what type?
      1. Flushing
      2. Wet Barrel
      3. Dry Barrel
      4. None of the above
    2. The base or buried portion of a hydrant is commonly called the ___________.
      1. Bonnet
      2. Cap
      3. Nozzle
      4. Bury-El
    3. Approved use of a fire hydrant includes all of the following except?
      1. Dust control
      2. Fighting fires
      3. Cooling the public during heat waves
      4. Flushing sewers
    4. Which of the following is an advantage of a dry-barrel hydrant?
      1. Water is easily withdrawn by opening an operating nut
      2. Water will not flow if the hydrant gets hit and knocked off its base
      3. Water will not freeze in the hydrant body
      4. Both 2 and 3
    5. A wet-top fire hydrant is a type of ___________.
      1. Wet barrel hydrant
      2. Flushing hydrant
      3. Dry barrel hydrant
      4. All of the above

    This page titled 1.7: Hydrants is shared under a CC BY license and was authored, remixed, and/or curated by Mike Alvord (ZTC Textbooks) .

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