Does the water supply meet the demands of the sprinkler system?
BY MATT KLAUS
Most of the time and effort spent on designing sprinkler systems is devoted to locating sprinklers and routing piping. But a critical aspect of the system design that typically does not get as much attention is the water supply. The effectiveness of a sprinkler system (and all water-based suppression systems) depends upon having a reliable and appropriately sized water supply.
NFPA 13, Installation of Sprinkler Systems, does not dictate what type of water supply a system needs, only that a reliable water supply capable of providing the minimum required flow and pressure for the system be provided. The type of supply selected is at the discretion of the design team. This choice is often driven by the availability and capacity of water provided by the local water purveyor. If the system demand can be met by the available water supply, typically that will be the approach of the design team.
Determining whether the water supply is capable of meeting the system demand requires a hydrant flow test to determine the system capacity or a review of the system’s hydraulic model (where available) to ascertain available flow and pressure. NFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants, provides a methodology for conducting a hydrant flow test and illustrates how the output data should be interpreted.
In some cases, the hydrant flow test may show that the water supply doesn’t meet the required flow and/or pressure demands of the system. In those instances or in areas where connecting to a water supply is not an option, a fire pump or a tank and pump system may be necessary. This requires the designer to go outside of NFPA 13 and look into NFPA 20, Installation of Stationary Pumps for Fire Protection, and NFPA 22, Water Tanks for Private Fire Protection, for additional design guidance on required system components and system sizing.
The trickier scenario is where the hydrant flow test indicates that the water supply can meet the system demand, but just barely. NFPA 13 does not cite a specific safety factor when considering the water supply, but sound engineering judgement should be used in these cases. The designer should consider several factors when considering the hydrant flow test data and may choose to make adjustments based on those factors, including the time of day and season when the flow test was conducted; typical usage in the water supply; maturity of the water supply, considering the potential for the community to grow significantly or whether most development is done; and planned modifications to the facility. Failure to make the appropriate adjustments or include the appropriate safety factor could lead to an under-supplied system.
It is important to remember that NFPA 25, Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, considers a system that does not have an adequate water supply to be impaired. This can lead to required fire watches until the system demand can be supplied or, in some circumstances, an order to temporarily close the facility. While those scenarios represent extreme examples of what can happen, they typically occur when the system was designed without consideration of future development or implementing a safety factor into the design. When such issues arise, the facility owner is responsible for remediation of the deficient water supply. As a result, it’s critical that the facility owner has a clear understanding not just of what’s required to meet the minimum demand, but also what the long-term assessment of the water supply looks like.
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