Rate-of-flow formulas for larger fires
Firefighters assigned to a pumper must perform three basic fire attack tasks:
- Place small attack lines in service for a fast initial attack.
- Operate a standard 250 gpm 2.5 in attack line with pressure supplied by the pumper.
- Operate one heavy stream nozzle flowing at least 400 gpm.
Now let’s consider the second operation that firefighters perform at a fire, namely supplying water with needed pressure and volume. The fire department is almost completely dependent upon the capacity of the water system in the urban area. This is the available fire flow that may vary somewhat in different parts of the city. What we are talking about is the amount of water that is available for the fire department over and above the normal supply for the systems customers. The available fire flow is usually several thousand gpm even in small towns. It is the responsibility of the fire officers to know exactly what the available fire flow is in each area of the city.
The cause is almost always the fault of the fire department attempting to pump more water than is available. This knowledge is extremely important because if a fire officer attempts to pump more water than that available, then the entire water supply operation may be disrupted. Exceeding the available fire flow by a small percentage can lower the pressure needed for each fire stream being used. This in turn reduces the volume of flow which could drop below the needed fire flow. Fire officers sometimes blame the water system for “low water pressure”, but the fault does not lie with the water system. The cause is almost always the fault of the fire department (brigade) attempting to pump more water than is available.
Hydrants that flow around 500 gpm, or less, are not capable of supplying the water needed for larger fire The second limit on water supply is the available flow from a single fire hydrant. It is the responsibility of the fire department (brigade) to test this hydrant to find out what the maximum flow is. It is also the fire department’s responsibility to test the hydrant when other hydrants in the vicinity are flowing. Sometimes this may reduce the available flow well below the maximum flow. Color coding hydrants, and other means, are used to help fire officers determine how much water is available from each fire hydrant. Hydrants that flow around 500 gpm, or less, are not capable of supplying the water needed for larger fires in larger building. Hydrants flowing from 1,000 gpm to 1,500 gpm usually will supply enough water to meet the needed fire flow for a larger fire.
Present day pumpers with capacities ranging from 1,000 gpm on up to 2,000 gpm usually provide more than enough pumping capacity to handle the available fire flow. In fact at multiple alarm fires, each pumper on the average will supply only about 500 gpm. The reason for this is that as hydrants are used farther away from the fire, friction loss becomes so great that less and less water can be supplied to the fire.
The final limiting factor is personnel, the number of firefighters on the fire ground. The final limiting factor is personnel, the number of firefighters on the fire ground. For most multiple alarm fires, it will turn out to be true that the number of firefighters is the ultimate limiting factor for fire flow. The standard first alarm response provides 13 firefighters producing a flow of 400 gpm to 500 gpm. With one heavy (master) stream this flow can be increased to 1,000 gpm. Doubling this fire flow to 2,000 gpm requires twice as many firefighters, or 30. Increasing to 3,000 gpm requires the same increase in firefighters, 15 more for a total of 45, and so on. Increasing fire flow by 1,000 gpm means an additional 15 firefighters. This is a linear relationship between fire flow and manpower. As shown in the following table.
| Fire Flow Applied | Total Personnel |
| 1,000 gpm | 15 |
| 2,000 | 30 |
| 3,000 | 45 |
| 4,000 | 60 |
| 5,000 | 75 |
| 6,000 | 90 |
Of course, flows greater than 3,000 gpm are beyond the capability of most fire departments (brigades). Twelve pumpers would be needed to provide 6,000 gpm at an average flow of 500 gpm per pumper. Only the largest cities can comfortably handle these large fire flows to battle the largest fires.
