Fire hydrant flow tests help uncover blockages or infrastructure problems in the water distribution system. These tests also aid in properly designing fire sprinkler systems for commercial and residential buildings.
Available fire flow data is important to city planners, firefighters, sprinkler contractors and engineers for various reasons. There are several methods to calculate fire main capacity, each with pros and cons.
Hydrant Capacity Test
The Hydrant Capacity Test is a great way to identify inoperable fire hydrants or those without the necessary flow rate. It requires a tester to gather key information and plug it into two sequential formulas: the flow rate during the test (gpm) and the predicted flow at the desired residual pressure (gpm).
Engineers, sprinkler contractors, and commercial property developers can then use this data to design fire protection systems for their buildings properly. City planners also use it to verify that public fire hydrants and water mains have sufficient capacity to meet emergency demands.
After completing the flow test, slowly close the hydrant gate valve and record the pressure reading. If the second static pressure reading is significantly lower than the first, there may be a water main break near the test hydrant. This must be discovered quickly, as this could affect other hydrants in the area and cause a fire suppression system problem for those buildings connected to the affected pipe.
Hydrant Flush Test
The City of Boca Raton’s Utility Services staff conducts an aggressive two-week system flushing program every six months. This involves changing the normal water disinfection process and opening fire hydrants throughout the City to allow the water to flow through all of our water distribution pipes.
Hydrant flushing helps to ensure adequate water pressure for firefighting and removes sediment from the fire hydrant and main lines that can restrict water flow and reduce water pressure. It also helps to uncover any closed valves or heavy pipe wall deposits that may have occurred in the system over time.
During the hydrant flush test, a pressure gauge is marked at hydrant #1 and hydrants #2 and #3 are opened to allow water to flow out as fast as possible. The static pressure is recorded from hydrant #1, and the residual pressure is assessed at hydrant #2 (along with the total pressure drop between the static and residual hydrants). A water customer may experience discolored water during this period as iron or mineral deposits are stirred up during the increased flow. The water is safe to drink but can have an unpleasing appearance and may stain laundry.
Main Flush Test
The underground fire service line from the water supply to the sprinkler system risers should be flushed before the installation of lead-in connections and an aboveground fire protection system. This flushing is required under NFPA 24 and local Authority Having Jurisdiction requirements. This flushing requires thousands of gallons, so coordination to minimize impacts on the municipal water system is advisable.
Flushing at the proper velocities will help remove loose sediment deposits, improve water quality and reduce biofilm. Biofilm is a slimy coating of microorganisms that can grow on pipe walls and reduce the capacity of pipes to carry fire flow testing.
During this type of flushing, it is necessary to sample and observe the water quality. Water samples should be taken every five minutes and observed for particulates (styrofoam cups work well). The flushed hydrants should be opened and closed slowly to avoid water hammer. Testing rigs should be secured with a sand tarp to prevent them from being moved during the high-volume flushing.
Residual Pressure Test
The Residual Pressure Test allows fire departments to gather actual pressure and flow data from specific fire hydrants (FH) connected to the County water system. This information is used to determine fire protection water availability.
This test involves opening the “test FH” and measuring its static and residual pressure at a fixed test flow rate. Then, the pitot pressure and size of the nozzle opening (measured in gpm) are recorded and entered into a formula called the Hazen-Williams equation.
This formula relates the upstream flow velocity (v1), upstream elevation (z1) and head losses due to friction in the pipe system to the downstream flow pressure and flow rate (P). This equation also accounts for the upstream residual stress in the case of liquids like water. These six pieces of information are then fed into the formulas to determine the expected flow rate at a specified residual pressure (psi). The FH can be designed appropriately for a given water demand using this information.
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