Loudoun Water hydrant flow tests are conducted to measure water flows and pressure in the water system. Hydrant flow tests are required primarily for fire sprinkler design and insurance purposes. 

Loudoun Water recommends modeling new developments using a supply reservoir with an elevation set to the low HGL of the pressure zone. Hydrant flow tests will be used as confirmation of local system characteristics as needed.

Hydrant flow tests measure the highest demands on the system that occur when fighting a fire.

How a flow test is performed: 


A 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 hydrant flow test are reviewed by Loudoun Water staff and then recorded into the hydrant 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 hydrant flow test information?
    Fill out the Fire Hydrant Flow Test RequestWith the completion of this form, Loudoun Water will provide you with any flow test that has occurred within the last five years in the vicinity of the requested address at no charge.  If there is no existing flow test or the existing flow test does not meet your needs, Loudoun Water will perform a new flow test upon your verification and payment of the invoice.
  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 $350 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 hydrant flow test typically takes four weeks after Loudoun Water receives payment. 

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

5.            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.


6.          Who can I contact if I have questions?

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