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Residual Fire Hydrant Flow Testing per NFPA 291


In addition to flow testing all hydrants in a system, HYDRANT.COM conducts residual fire hydrant flow testing throughout residential, commercial, and private water supply systems. A residual flow test helps determine water main capabilities before and after the installation of water lines and fire suppression systems.

Prior to the installation of fire suppression systems, water main pressures are gauged by conducting a residual flow test per NFPA. 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. Once the flow rates and static/residual pressures are noted, hydrants are shut down slowly, one at a time. Available GPM at a desired residual pressure (typically 20 PSI) can now be calculated using the Hazen-Williams formula.

Water flow test data  is gathered by hydrants in the vicinity of site developments. Fire flow calculations are done using static, residual psi and flow pressure test data. Results are both numerically and graphically calculated. Fire flow requirements for buildings can then be determined.

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 at the project site.

Residual Fire Flow Test Diagram

When measuring discharge from open flow hydrants, it’s best to measure from the 2.5 inch hose nozzle.

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.”[1]

NFPA guidelines on achieving satisfactory results are quoted below.

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.”[1]

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.

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.

HYDRANT.COM specializes in fire flow testing for private, residential and government entities. Call today to schedule residual flow tests.

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Flow tests for cities and private entities. We also provide fire hydrant and valve surveying for GIS and work order management systems