Fire hydrant flow testing provides information about how well a fire hydrant system will function in an emergency. Engineers, sprinkler design companies, and commercial property developers use this data to design building fire protection systems.
Static Pressure
In fluid mechanics, static pressure is the total pressure that remains constant along any streamline. The pressure at a stationary point is independent of any fluid flow and is the same as the surrounding ambient pressure.
A hydrant flow test is a great way to see how well a water distribution system functions at any given time. It provides accurate data about fire hydrants’ pressure and flow-producing capabilities, which are used for various purposes, including training, marking hydrants, and system planning.
A minimum of two hydrants are used to conduct a fire flow test, one to record the static pressure and another to measure the residual pressure during a test flow rate. The data collected is then fed into formulas that calculate the available GPM at a specified residual pressure psi.
Calculations
Fire departments must understand the calculations required to determine needed fire flow. These calculations are used for many purposes, including marking hydrants, planning water system capacity, and training.
During fire flow testing, it is recommended that the hydrant be flowed for 10 minutes to obtain an accurate reading. The pressure drop and pitot gauge readings should be recorded for each hydrant tested. The AWWA recommends that the desired residual pressure be at least 25 percent of static pressure for better theoretical calculations of expected flows and rated capacities.
During training, have crewmembers practice using the NFA and fire flow formulas on buildings in their response area. It is also recommended that each department have a whiteboard where members can draw different-size building layouts and practice developing necessary fire flows. This will help them quickly develop the number of handlines and master streams needed to control a specific incident.
Residual Pressure
The pressure that remains in a fire hose or water main after the flow of water has been shut off. It is essential to understand this because it affects how firefighters use hydrants.
The difference between static and residual pressure can be found by measuring the pressure at the pumper nozzle (a 4-inch opening). The result will be the gallons-per-minute of available fire flow.
The pressure in a pipe during a fluid flow is related to the physical properties of the pipe, the friction loss caused by the movement of water, and the pressure drop from piping fittings. Engineers account for these losses in their design of water pipe systems by using an equation known as Bernoulli’s formula. It relates the upstream residual pressure (Ps) and the downstream velocity of the flowing liquid or gas, Pv, to their respective downstream pressures (Ps-Pv). The formula also accounts for head loss from the hydrant and water main valves.
Pitot Gauge Readings
Fire hydrant flow testing is necessary to ensure firefighters can use the water flowing through the system. It’s also a great way to uncover problems that could jeopardize firefighting operations if left unchecked.
When conducting a fire hydrant flow test, the test inspector takes a pressure reading and records it on a chart in gallons per minute (GPM) from the hydrant port using a pitot gauge. The test also involves identifying the orifice size and knowing that the hydrant outlet has a discharge coefficient that needs to be factored in.
During the fire flow test, the test technician ties the pitot gauge to a hydrant with a 2 1/2″ opening (also known as the flow hydrant). Then, the hydrant’s gate valve is opened. The crew at the flow hydrant then takes a second pressure reading and records it. The difference between the first and second readings is the residual pressure. Then, the flow hydrant is slowly closed.
