Hydrant flow tests are conducted to measure real world pressure and flow in the water system. Hydrant flow tests are required for fire sprinkler design and for water modeling purposes.  They are generally known as fire flow tests as the highest demands on the system occur when fighting a fire.

How a flow test is performed: 

A fire hydrant flow test is performed for a specific area by using three fire hydrants. A pressure gauge is placed on one hydrant to record the static pressure; this is hydrant #1. The remaining two hydrants are opened to allow water to flow out as fast as possible. A hand-held pitot pressure gauge is used to measure the residual pressure while water is flowing from hydrants #2 and #3. Both hydrants #2 and #3 will flow between 500 and 2,000 gallons per minute. While the flow is steady at hydrants #2 and #3, the residual pressure is recorded at hydrant #1, measuring the drop in water pressure.

The results of the fire flow test are reviewed by engineering staff and then recorded into the fire flow test database. Results of the tests are available upon request to customers who have a need for the results.

  1. How do I request fire hydrant flow test information?
    Fill out the Fire Hydrant Flow Test Request.  The most recent flow test information in the area requested will be sent to you within 1 – 3 business days.  If you need more recent information with a new test performed please request the Loudoun Water engineer to create an invoice.  Once the invoice has been paid the new flow test will be scheduled.
  2. Can I order my own independent fire hydrant flow test?
    No. Loudoun Water owns and maintains all of the fire hydrants within our system.  Independent companies are not allowed to operate hydrants within our jurisdiction. 
  3. What charges are associated with performing a new flow test?
    A charge of $325 per new flow test performed is charged to the requesting party.
  4. If a new test is needed how long will it take to get my results?
    A fire flow test typically takes two to three weeks after Loudoun Water receives payment to issue results. 
  5. How many fire flow tests are conducted per year?
    Loudoun Water performed approximately 50 fire flow tests in 2015.
  6. How do I use the results to calculate a pump in a water model?
    In order to use the flow test results in a water model you must create a 3-point pump curve. 

The shutoff flow will always be 0 gpm; the shutoff head is equal to the static pressure head at hydrant #1.  The design flow is the total flow from the test and the design head is equal to the residual pressure at hydrant #1.  The max operating flow is solved for by the following equation:

Q max = Q design/ [(H shutoff - H design )/ (H shutoff  - H max)]0.54

7.            How do I use the results to calculate the pressure at a specific location?

In order to calculate the pressure at a specific location you will need to know the length of pipes and pipe diameters between hydrant #1 and the location in question.  You will also need to know the elevation of hydrant #1 relative to the location you are looking for.  The elevation of hydrant #1 is listed on the flow test report.

First, calculate the head loss in the pipes between hydrant #1 and the location in question via the Hazen Williams equation.  Next, determine the absolute value of the difference in elevation between hydrant #1 and the location.  If hydrant #1 is lower in elevation add this elevation to the head loss to get total head loss.  If hydrant #1 is higher than the location subtract it from the head loss to get total head loss.  Finally, subtract the total head loss from the residual pressure at hydrant #1 to find the pressure at the location. Keep in mind you may need to convert between pressure/square inch (psi) and feet of head within the calculations.

8.            Who can I contact if I have questions?

Please email fireflow@loudounwater.org with any questions you may have, or call 571-291-7700.