Learn when and how cities should perform hydrant flow tests
During a fire emergency, reliable sources of water can mean the difference between life and death. This is why the National Fire Protection Association (NFPA) outlines fire hydrant testing that measures real-world pressure and flow in a city’s water distribution system.
In this blog, we discuss how often flow tests are needed to ensure hydrants will work as expected during a fire. We also dig into the details of what should be done to ensure a water supply meets firefighting requirements.
When does NFPA require fire hydrant testing?
Regular fire hydrant testing ensures the ability to provide water at an acceptable pressure and flow rate for public health and firefighting operations. Most jurisdictions also require hydrant flow tests to design fire sprinkler systems for commercial or residential structures.
The 2022 edition of NFPA 291: Recommended Practice for Fire Flow Testing and Marking of Hydrants (4.2.2) recommends that fire hydrants should maintain a residual pressure of 20 psi (pounds per square inch), or 1.4 bar, for effective firefighting, as well as to prevent backflow that could contaminate the public water supply.
NFPA 291 stipulates hydrant flow tests every five years to ensure that changing conditions in the piping and system demands won’t impede hydrants’ ability to deliver water.
From the 2022 edition of NFPA 291
4.15.1 Public fire hydrants should be flow tested every 5 years to verify capacity and marking of the hydrant.
In the explanation that accompanies the section (A.4.15.1), NFPA clarifies the section’s intent. It states that it does not mean to mandate routine five-year testing of every hydrant—especially if there is no pressing need to test a specific hydrant or if test data less than five years old is available from an adjacent hydrant on the same grid.
Performed by city officials or professional contractors, fire hydrant testing verifies the performance of a city’s water distribution system, determining the pressure and rate of flow available at various locations. It measures static (non-flowing) and residual (flowing) pressure, as well as the rate of discharge in gallons per minute (GPM) of each fire hydrant.
The data that’s collected is used for two important purposes:
- Uncovering closed valves, heavy pipe-wall deposits, or other problems in a water distribution system. Reduced rates of flow often stem from blockages or other infrastructure problems.
- Properly designing fire sprinkler systems for commercial and residential structures. If water supply pressure and flow readings are off, it can lead to an underdeveloped system that requires additional fire pumps or an expensive overhaul of pipe fitting.
Besides delivering peace of mind that hydrants will work in an emergency, hydrant flow tests enable municipalities to color-code their fire hydrants according to their strength of output. The colors categorize hydrants by the GPM of their flow. For instance, the color-coding scheme recommended by NFPA 291 (5.1) and The American Water Works Association says light blue hydrants have a capacity of 1,500 GPM or more (“very good flow”) and red hydrants have a capacity below 500 GPM (“inadequate”).
This system allows fire departments to assess their water resource capabilities quickly when arriving on the scene of an emergency. You can check out our previous blog to learn more about fire hydrant colors and what they mean.
Preparing to conduct the test
Specific instructions for conducting fire hydrant testing can be found in NFPA 291’s Chapter 4, “Flow Testing.” NFPA 291 guidelines require identifying a residual (test) hydrant to measure static and residual pressure, as well as one or more flow hydrants.
Static pressure represents the pressure at a given point under normal distribution system conditions. It is measured at the residual hydrant with no hydrants flowing. Residual pressure is the pressure that exists in the water distribution system while water is flowing. It is measured at the residual hydrant at the same time flow readings are taken at the flow hydrants.
To determine how many flow hydrants are needed, keep in mind that NFPA 291 recommends flowing enough water to provide at least a 10% drop in residual pressure compared to the static pressure (4.4.6). Further, it states that testers may need to “declare an artificial drop in the static pressure of 10 percent” in “water supply systems where additional municipal pumps increase the flow and pressure as additional test hydrants are opened.” NFPA has updated this guidance from the 2019 edition, which recommended flowing the total demand necessary for firefighting purposes during the test, or enough water to provide at least a 25% drop in residual pressure compared to the static pressure. (In 2018, Sprinkler Age noted that the 25% drop was not necessary for a hydrant flow test used to design a fire suppression system.)
Both editions say that if the mains are small and the system is weak, only one or two hydrants need to flow (2022: 4.4.8/2019: 4.3.7). If the mains are large and the system is strong, as many as eight flow hydrants may be required (2022: 4.4.9/2019: 4.3.8).
Before starting a flow test, it’s important to notify the water company or water authority. Opening a hydrant could disrupt normal operating conditions for the water distribution system in an area.
Testers should also assemble the proper equipment, including:
- A flow test kit that includes a hand-held pitot gauge to take the pressure and rate-of-flow readings, as well as the correct nozzle size to attach to the hydrants
- An outlet-nozzle cap outfitted with a pressure gauge that’s used on the residual hydrant
- A simple ruler for measuring the inside diameter of each flow hydrant’s outlet nozzle
- A hydrant wrench for accessing hydrants to take residual and flow readings
A water diffuser and sock can also help prevent damage to landscaping and roadways, and redirect water to stop ice patches from forming on certain surfaces in the winter. It’s wise to check that local drains are not blocked by leaves or other debris to prevent water backup. Portable radios can also make testing easier when more than one hydrant flows.
The 2022 edition of NFPA 291 (4.3.1) now suggests conducting hydrant flow tests during periods of “periods of peak demand, based on knowledge of the water supply and engineering judgment,” which is an update from the 2019 edition’s guidance to do it during periods of “ordinary demand” (NFPA 2019: 4.2.1) That’s likely because many fire protection professionals recommend performing flow tests during peak morning hours to reflect the worst-possible scenario during an emergency. Street pressures can fluctuate as much as 10 psi in the morning, compared to later in the day when demand is typically less.
Be prepared to record the following information during the test:
- Date of hydrant flow test
- Location of hydrants being tested (name of the street)
- Time of day testing was performed
- Static reading at the residual hydrant (pressure in the system with no flow)
- Residual reading at the residual hydrant (pressure in the system during flow)
- Flow reading at the flow hydrant, using a pitot gauge
- Water main diameter in inches
- Hydrant outlet size and type (determining the coefficient of discharge)
- Hydrant elevation
How to conduct a hydrant flow test
Here’s an overview of how to perform a hydrant flow test. Read this carefully: It is not meant as a comprehensive step-by-step guide. Testers should always consult their local authority having jurisdiction (AHJ) and fire department guidelines for the most accurate information.
- Determine the location of the test by selecting a group of hydrants in the same vicinity. Remember, as many as eight hydrants may be required for robust systems with large water mains.
- Mark the hydrant measuring pressure as the residual hydrant. Both static pressure (when flow hydrants are closed) and residual pressure (when flow hydrants are open) are assessed from this hydrant. The residual hydrant should be between the hydrant(s) to be flowed and the large mains that supply water to the area.
- Flush the residual hydrant to remove any sediment and attach a nozzle cap with a gauge to the hydrant’s outlet.
- Slowly release the main valve until air is vented. Take a static pressure reading.
- Measure the inside diameter of the outlet nozzle or hydrant outlet where flow occurs. A hydrant’s inside diameter is usually 4”.
- Field personnel should slowly open each flowing fire hydrant, one at a time, to avoid pressure surges.
- After the residual pressure read from the outlet cap stabilizes, take readings at each flow hydrant using a pitot gauge. Residual pressure and pitot gauge readings must be taken simultaneously. For accurate pitot gauge readings, the pitot tube should be held downstream and in the center of the nozzle.
- Record both the residual pressure at the residual hydrant and pitot gauge readings at the flow hydrant(s).
- Slowly close each fire hydrant.
- Use the PSI readings from the residual hydrant’s static and residual pressure, the coefficient determined by measuring the inside diameter of the hydrant’s outlet nozzle, and other factors to determine two sequential numbers using two sequential formulas: the discharge, aka the gallons flowing during the test (gpm), followed by the flow predicted at the desired residual pressure, aka the available fire flow.
We get into the equations of step 10 in the next section, but you can first watch this video to witness a hydrant flow test:
The math needed to assess the results
As mentioned above, the tester needs to gather key information to run two equations in sequence.
The first equation determines the flow (gpm) from the tested fire hydrants based on the pitot gauge pressure readings. A version of it is found in section 4.93 of NFPA 291:
Q = 29.83 * c * d2 * √P
Q = discharge; the gallons flowing during the test (gpm)
c = coefficient of discharge, which represents friction loss. It’s determined by assessing the shape of the transition between the vertical barrel of the hydrant and the horizontal outlet. Most hydrants have a smooth and rounded transition resulting in a .90 coefficient of discharge but not all of them (as shown below):
d = diameter of the outlet
P = the pressure reading at the pitot gauge during the test (PSI)
You can read a more in-depth look at calculating this gpm value plus access a handy calculator in our previous blog: “Pitot Gauges: How Do I Calculate the PSI to GPM Conversion?”
The second formula estimates the “flow predicted at desired residual pressure,” which is sometimes called the “available fire flow” (AFF). This essential equation is found in section 18.104.22.168 of NFPA 291, but here is a version with more steps broken out:
QR = Q * (((S – 20)0.54) ÷ ((S – R)0.54)))
QR = flow predicted at desired residual pressure/available fire flow
Q = the discharge (gpm) measured during the test (the result of the first equation)
S = the static pressure measured during the test
20 = the amount of minimum pressure (in psi) required for most municipal water supplies to prevent backflow and achieve fire protection objectives.
(NFPA 291 calls the “S – 20” calculation above “hr,” which equals “pressure drop to desired residual pressure”)
R = the residual pressure measured during the test
(NFPA 291 calls the “S – R” calculation above “Hf,” which equals the “pressure drop measured during test”)
0.54 = a constant within the Hazen-Williams equation
After conducting a hydrant test, testers plug in their measurements to the two formulas above, completing one after the other.
Fire hydrant testing ensures that hydrants and sprinklers can provide adequate protection
The middle of a fire emergency is not the right time to find out that fire hydrants or sprinklers don’t have enough flow and pressure. Regular fire hydrant testing ensures that this vital equipment works as intended. And when requirements aren’t up to par, these assessments enable repairs to be proactively scheduled so problems can be dealt with before they could lead to loss of life or property.
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