Establishing a water supply as part of a sprinkler system

Water supply is a critical piece of a sprinkler system and is often the first thing you need to establish when designing and installing a system. Without an adequate water supply, even the best-designed sprinkler system can fail.

But water supply can also be a tricky variable. Municipal water supply, for example, can vary with the seasons, and in summer it can be affected by an array of seasonal activities, including lawn and garden watering and filling swimming pools. Vacation destinations that see an influx of summer visitors also experience a corresponding impact on the local water supply. These are just some of the reasons that water supply was an important topic addressed in the 2019 edition of NFPA 13, Standard for the Installation of Sprinkler Systems.

When identifying the required water demand for a sprinkler system, three criteria must be assessed: required flow (gallons per minute), pressure (pounds per square inch), and duration (the number of minutes the sprinkler system must operate). The required pressure, flow, and duration can be calculated based on the specific approach selected and the protection requirements for each installation. This information can be found in Chapters 19 through 26 of NFPA 13.

Once you have determined the pressure and flow required by your system, you need to make sure your water supply is sufficient. For a municipal water supply, this can be determined by conducting a hydrant flow test in accordance with NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants, which includes the necessary water supply test data. If the hydrant flow test indicates that your municipal water supply is inadequate, then a fire pump installed in accordance with NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, will provide increased pressure, or an alternate water source can be used when the available flow is insufficient.

While a municipal water supply is the most common source, since it’s the most readily available in many areas, there are other possibilities if a municipal source isn’t an option. Static water supplies such as rivers, lakes, ponds, or reservoirs can also work, but sprinkler systems reliant on these sources must be installed with fire pumps to ensure adequate pressure and flow are met. Additionally, pumps need to be arranged to avoid the intake of sediment and require screens or strainers to help filter the water.

External tanks can also be used to contain a water supply for a sprinkler system. Some tanks are elevated and use gravity to help achieve the desired flow and pressure, while others need fire pumps or pressurized tanks to achieve this. Requirements for the design, construction, and operation of water storage tanks can be found in NFPA 22, Standard for Water Tanks for Private Fire Protection.

Since the reliability of water supplies is a critical part of a sprinkler system, those supplies must be properly maintained if they are to function when needed. NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, contains requirements for maintaining your water storage tanks.

It is important to consider all the variables that can affect your water supply—the potential for a community’s seasonal growth and the resulting daily fluctuations in water supply are just a couple of examples of the factors you need to take into account when planning and installing a sprinkler system. Failure to do so can lead to a system that is undersupplied and may not perform as designed. 

Brian O’Connor is a Fire Protection engineer at NFPA. NFPA members and AHJs can use the Technical Questions tab to post queries on NFPA 13 at nfpa.org/13.