Hydrant Flow Testing Verifies the Water Mains Capacity
Hydrant flow testing verifies water main capabilities. Hydrant flow tests are required prior to new construction and the installation of fire suppression systems. Static, flow, and residual pressure data is used to calculate test results both numerically and graphically.
Water main pressures are recorded by conducting a residual fire flow test per NFPA 291. Residual testing requires at least 2 hydrants. Static pressure is taken on a Residual Hydrant with other hydrants closed. Then, an adjacent Flow Hydrant is opened and flow rates are measured at the same time the residual pressure is recorded on the Residual Hydrant. Available GPM at a desired residual pressure (typically 20 PSI) can now be calculated using the Hazen-Williams formula. Fire flow results can then be used to verify a buildings water demand requirements.
A minimum of two hydrants are used during a residual flow test. Typically, the two hydrants nearest to the water main tie-in location are used to conduct the test.
According to NFPA, when measuring discharge from open flow hydrants, it’s best to measure from the 2.5 inch hose nozzle. Guidelines on achieving satisfactory results are quoted below.
NFPA 291 4.6.2 states:
“In practically all cases, the 2 1/2 in. outlets are filled across the entire cross-section during flow, while in the case of the larger outlets there is very frequently a void near the bottom.”
NFPA 291 4.3.6 states:
“To obtain satisfactory test results of theoretical calculation of expected flow or rated capacities, sufficient discharge should be achieved to cause a drop in pressure at the residual hydrant of at least 25 percent, or to flow the total demand necessary for fire-fighting purposes.”
Water main and hydrant flow capabilities impact decisions as to what fire protection and fire resistance features are required for new developments and where priorities should be placed with respect to upgrading older, smaller water mains. Before new lines are installed, a residual fire flow is performed near the project site.
All residual flow tests should include static, residual and flow pressures as well as a 50+ point inspection of each fire hydrant tested. Always remember to flow test and inspect fire hydrants annually to ensure the operation of the hydrants in your system.
1. “Chapter 4 Flow Testing.” NFPA 291: Recommended Practice for Fire Flow Testing and Marking of Hydrants. Quincy, Ma: Association, n.d. N. pag. Print.