National Fire Academy FormulaThe NFA fire flow formula is: Required fire flow (gpm) = (length x width) ÷ 3. Using the same example, an IC arrives at a burning mercantile occupancy that measures 50 feet by 75 feet and is one story tall. The IC quickly determines that the area of the structure is 3,750 square feet.
The user attaches a hose to the fire hydrant, then opens a valve on the hydrant to provide a powerful flow of water, on the order of 350 kPa (50 pounds per square inch gauge (psig); this pressure varies according to region and depends on various factors including the size and location of the attached water main).
- Step 1: Multiply the constant 29.84 times the coefficient of discharge (cd).
- Step 2: Square the measured diameter.
- Step 3: Multiply those two numbers: 26.856 x 6.5025 = 174.6.
- Step 4: Find the square root of 62 psi, which is 7.8740.
- Step 5: Multiply the final two numbers to solve for Q, the discharge in GPM.
PSI can't be directly converted to GPM; these are two different units of measure. PSI measures pressure and GPM measures flow rate. Additionally, how do you calculate psi from flow rate? to get this, divide the total pressure drop by the pipe length and multiply by 100.
The flow rate, in turn, at a known pipe cross-sectional area, determines the fluid's flow rate. Subtract static pressure from the total pressure. If the pipe has a total pressure of 0.035 kilopascals and a static pressure of 0.01 kilopascals: 0.035 - 0.01 = 0.025 kilopascals.
Static pressure is measured when water is not flowing from a cap gauge attached to a fire hydrant. Residual pressure is measured from the same cap gauge while water is flowing. Gallons per minute is measured by a diffuser with a pitot and a GPM gauge while the hydrant is flowing.
Metric PVC Pipe
| | Assume Average Pressure. (20-100PSI) About 12f/s flow velocity |
|---|
| 1" | 1.00-1.03" | 37 gpm |
| 1.25" | 1.25-1.36" | 62 gpm |
| 1.5" | 1.50-1.60" | 81 gpm |
| 2" | 1.95-2.05" | 127 gpm |
If a hydrant doesn't have an adequate flow, it has to be painted black or covered with a black sack. House Bill 1717 states than if a fire hydrant doesn't provide an adequate flow of water -- at least 250 gallons per minute -- it must be painted black or covered with a similar-colored sack.
The tops of hydrants are painted in colors to indicate how much flow they can produce in gallons per minute (gpm). Some older hydrants have been reclassified to a different flow rating. Most of the hydrants in the downtown area have been upgraded to “Blue” (the highest rating).
The gray colour is primer. They will be red again soon. The colour on the bonnet (top of the hydrant) identifies the flow rate. Blue hydrants can move the largest volume of water, able to pump out 95 litres per second, while a red hydrant is the weakest, only able to pump less than 31 litres a second.
Yellow indicates that the water comes from a public supply system. Violet means the water comes from a lake or pond. While most areas follow this color scheme, some choose to make up their own system. Either way, fire hydrants may stick out like a sore thumb but those bright colors are chosen with your safety in mind.
The colors indicate the rated water-flow capacity of that particular hydrant: Red indicates water-flow capacity of less than 500 gallons-per-minute (GPM). Orange indicates water-flow capacity of 500 to 999 GPM.
An example of this would be painting the body White to show the hydrant is a Public system hydrant, Yellow for Private hydrant connected to a public water system, Red for a special operations, meaning for special purposes and situations only, and Violet suggesting that the water is non potable.
Hy. Hydrants are color coded for a variety of reasons . Normally they are painted purple to show the fire department and other users the hydrant piping is on a “ reclaimed “ water system ( not safe for drinking ) instead of the regular “potable or drinking “ water pipes.
Hydrants usually operate at local water system pressure. Pressure will vary depending on the time of day, due to water usage, and by elevation. The hydrants at the lowest levels of a system will have the highest pressure because gravity gives the pressure a boost, roughly half a pound per foot of elevation.
Open the shutoff mechanism, and bleed the air until the nozzle reaches its minimum flow rate within ± 2%. Record the base pressure which should read within 15 psi of the rated pressure to continue. Increase the flow to the maximum rated flow and record the base pressure reading on the pressure gauge.
A water flow test, also known as a hydrant flow test, is a way to measure the water supply available at a building site, usually for the purposes of installing a water based fire protection system (fire sprinkler system).
Measure the diameter of the orifice on the nozzle. The distance the pitot blade should be placed in front of the tip is half the diameter of the orifice. Example: For a 2.5” (65mm) orifice the pitot blade would be 1.25” (32mm) from the tip. The blade is marked in 1/4” increments from 1/2” - 3” tip size.
Don't touch the hydrantsAn important reason. It's because different colours are used to let firefighters know how much water is coming out of the hydrant. The color codes are standards set by the National Fire Protection Agency (NFPA) and each municipality is required to maintain hydrants according to those standards.
Fire hydrants can typically supply a large volume of water. This water is pumped through hoses to the fire truck, where it is pressurized and divided into several streams to supply water to multiple fire hoses at once. In these hydrants, the valve is located underground below the freeze line.
Most fire hydrants have two 2.5 inch hose connection outlets with 7.5 threads per inch (TPI) and one 4.5 inch-pumper connection outlet with 4 threads per inch.
The formula to find GPM is 60 divided by the seconds it takes to fill a one gallon container (60 / seconds = GPM). Example: The one gallon container fills in 5 seconds. 60 / 5 = 12 GPM. (60 divided by 5 equals 12 gallons per minute.)
