How to Perform a Fire Hydrant Flow Test?

Fire hydrant flow tests are essential for ensuring your community’s water supply is capable of supporting firefighting efforts during an emergency. 

These tests measure water flow (in gallons per minute) and pressure (in pounds per square inch), providing critical data for emergency planning and maintaining public safety.

What is the Purpose of a Fire Hydrant Flow Test?

Flow tests serve several purposes. The core idea of a flow test is to discharge water at a measured flow rate from the system at a certain location and observe how much the pressure drops in the mains.

Each flow test gathers data on three types of pressure:

• Static pressure: The water main pressure under normal, non-flowing conditions.
• Residual pressure: The water main pressure when water is flowing.
• Pitot pressure: The flow pressure at the outlet(s) of the flowing fire hydrant(s).

Flow tests also help communities color-code fire hydrants so firefighters can easily identify the water-flow capacity of each one. 

Here’s what the colors mean:

• Red: Less than 500 gpm 
• Orange: 500 to 999 gpm
• Green: 1,000 to 1,499 gpm
• Blue: 1,500 gpm or more

How Often Should Hydrant Tests Be Performed?

According to the NFPA, public fire hydrants should be flow tested every five years to ensure they have the proper capacity and are correctly marked. 

Fire hydrants should also be inspected annually to check their operation, handle any needed repairs, and confirm their reliability.

How to Prepare for a Fire Hydrant Flow Test

When preparing for a fire hydrant flow test, follow the guidelines in Chapter 4 of NFPA 291, “Flow Testing”. These guidelines outline the steps needed to measure both static and residual pressure at a designated test hydrant, as well as at one or more flow hydrants.

To decide how many flow hydrants are needed, NFPA 291 suggests that enough water should flow to cause at least a 10% drop in residual pressure compared to the static pressure. 

In some cases, you may need to declare an “artificial drop” of 10% in systems where pumps increase pressure when more hydrants are opened. This is an update from the 2019 edition, which previously recommended flowing enough water to cause a 25% drop in residual pressure.

Equipment Checklist

Before starting the flow test, notify the water company to avoid disrupting the water distribution system. 

Then, assemble the following equipment:

• A flow test kit with a hand-held pitot gauge for pressure and flow readings.
• An outlet-nozzle cap with a pressure gauge for the residual hydrant.
• A ruler to measure the inside diameter of each hydrant’s outlet nozzle.
• A hydrant wrench for access.
• A pitot gauge kit for measuring the pressure of flowing water.
• A water diffuser and sock to minimize damage and prevent ice patches in winter.

Also, ensure local drains are clear to avoid water backup, and use portable radios if testing multiple hydrants.

When to Conduct the Fire Hydrant Flow Test?

The NFPA recommends testing during peak demand periods for the most accurate results. Peak morning hours are ideal, as street pressures can fluctuate by up to 10 psi.

Record the following during the test:

• Date of the test
• Location of tested hydrants (street name)
• Time of day
• Static pressure at the residual hydrant (no flow)
• Residual pressure at the residual hydrant (during flow)
• Flow rate at the flow hydrant (using a pitot gauge)
• Water main diameter
• Hydrant outlet size and type
• Hydrant elevation

How to Conduct a Fire Hydrant Flow Test?

Fire hydrant flow testing should always be carried out by trained personnel in coordination with the local water department. 

For detailed procedures, consult the latest edition of NFPA 291, “Flow Testing”. 

Here’s a step-by-step guide for conducting a thorough flow test.

Step 1: Select the Hydrants

Choose a group of hydrants in close proximity. The size of the water system will determine how many hydrants to test (usually 2 or more). Inspect each hydrant for visible damage. Report any issues and avoid using defective hydrants.

Step 2: Check for Surrounding Hazards

Make sure the discharged water won’t cause flooding or damage. Uncover nearby drains and make sure the water can flow safely. Clear any leaves or debris to prevent drainage backups.

Step 3: Designate the Static/Residual Hydrant

Select one hydrant as the static/residual hydrant, where both the static (normal) and residual (flow) pressures will be measured. 

This hydrant should be positioned between the flowing hydrants and the main water supply.

Step 4: Set Up the Static/Residual Hydrant

Remove the 2½ inch cap from the static/residual hydrant and attach the hydrant cap with the 200 psi gauge and air relief valve. 

Open the hydrant valve slowly until water flows from the relief valve, then close it. Wait for the pressure to stabilize, then record the static pressure reading.

Step 5: Set Up the Flow Hydrant(s)

Remove the caps from the flow hydrants. 

If using a hand-held pitot gauge, note the outlet port type (rounded, square, or projecting) to determine the discharge coefficient. Attach any necessary flow equipment (like stream straighteners or diffusers) to ensure water discharges safely.

Open the flow hydrants to clear debris, allowing enough time for a steady flow.

Step 6: Measure Flow and Residual Pressure

Use a pitot gauge to measure the velocity pressure of each stream from the flow hydrants. Position the gauge in the center of the stream, about half the outlet diameter from the port. If using advanced flow equipment, the built-in gauge will simplify this process.

Simultaneously, take another pressure reading at the static/residual hydrant. This represents the residual pressure.

Step 7: Close the Flow Hydrants

After recording all pressures, slowly close the flow hydrants.

Step 8: Take a Final Static Pressure Reading

Take another static pressure reading at the static/residual hydrant. This value should be close to the initial static pressure reading. 

If there’s a significant difference, consult with the water department. If the readings are consistent, close the hydrant slowly.

How to Assess the Results?

After conducting a hydrant flow test, you need to use two key equations to assess the results. 

Here’s a breakdown of each equation and the steps to follow.

Step 1: Calculate the Discharge Flow Rate (GPM)

The first formula calculates the flow rate, or gallons per minute (GPM), of water coming from the hydrant during the test. This is based on the pitot gauge pressure reading and other factors like the size of the outlet and the shape of the hydrant’s internal structure.

Here’s the formula:

Symbol meaning:

• Q = Discharge flow rate (GPM) – how much water is flowing out of the hydrant.
• c = Coefficient of discharge – a value that accounts for friction loss inside the hydrant. This depends on how smoothly the water flows from the hydrant’s vertical barrel to the horizontal outlet. Most hydrants have a coefficient of around 0.90, but this varies if the hydrant’s internal structure isn’t smooth.
• d = Diameter of the outlet (in inches).
• P = Pressure at the pitot gauge (in psi) – the pressure of the water flow at the outlet during the test.

This formula gives you the flow rate in gallons per minute based on the pitot pressure and the design of the hydrant.

Step 2: Estimate the Available Fire Flow (AFF)

Once you’ve calculated the flow rate (Q), you can use that value in a second equation to estimate the available fire flow (AFF). This tells you how much water would be available at a given desired residual pressure (usually set at 20 psi, the minimum pressure needed to prevent backflow and ensure proper fire suppression).

Here’s the formula:

Symbol meaning:

• QR = Flow predicted at the desired residual pressure, or AFF.
• Q = The discharge flow rate (GPM) from the first formula.
• S = Static pressure – the fire hydrant water pressure measured before any hydrants are flowing.
• 20 = The minimum residual pressure (in psi) required for fire protection and backflow prevention.
• R = Residual pressure – the water pressure while the hydrants are flowing during the test.
• 0.54 = A constant from the Hazen-Williams equation (used for fluid flow calculations in water pipes).

Using the first equation, you may calculate the AFF or the amount of water that would be available at the specified pressure in an actual fire emergency, by entering your observed values for static pressure (S), residual pressure (R), and flow rate (Q).

Avoid These 5 Common Mistakes When Doing the Test

1. Using Only One Hydrant Instead of Multiple Hydrants 

Flow testing requires at least two hydrants to gather accurate data and assess the available water supply in the main. 

According to the NFPA, the number of hydrants used in a test depends on the strength of the distribution system. If the system is weak with small mains, 1-2 hydrants may suffice. 

However, stronger systems with larger mains may require 7-8 hydrants to flow in order to achieve the necessary 10% pressure drop between static and residual pressures.

2. Improper Hydrant Test Layout 

The static/residual hydrant must be placed between the flow hydrants and the water supply to guarantee accurate measurements. 

3. Inadequate Pressure Drop 

A 10% drop in pressure between static and residual readings is required for valid results. If the pressure drop is less than this, the test should be redone with additional hydrants or ports open to create a larger flow.

4. Incorrect Coefficient of Discharge 

The coefficient of discharge varies based on the type of hydrant port and equipment used. Make sure to use the correct coefficient to ensure accurate calculations. 

If you’re using different equipment than what’s recommended, follow the manufacturer’s specifications for accurate readings.

5. Poor Record-Keeping 

It’s important to keep standardized records of each test. Ensure all results are documented and stored for future reference. 

Use a hydrant flow test form that helps organize and standardize results, making it easier to track data from different tests.