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The adage “water is life” holds a different connotation in fire protection engineering. Water is not only vital for sustenance but also essential for fire safety. Just as a building’s foundation bears the weight of its structure, a fire protection system’s water supply shoulders the responsibility of delivering the required quantity and quality of water throughout the life of the fire protection system.
This core principle is emphasized in NFPA 13 (2022 edition), Standard for the Installation of Sprinkler Systems, in the Annex to section 5.2.2:
“Care should be taken in making water tests to be used in designing or evaluating the capability of sprinkler systems. The water supply tested should be representative of the supply that might be available at the time of a fire. For example, testing of public water supplies should be done at times of normal demand on the system. Public water supplies are likely to fluctuate widely from season to season and even within a 24-hour period. Allowance should be made for seasonal or daily fluctuations, for drought conditions, for the possibility of interruption by flood, or for ice conditions in winter.
“Testing of water supplies also normally used for industrial use should be done while water is being drawn for industrial use. The range of industrial-use demand should be taken into account. In special situations where the domestic water demand could significantly reduce the sprinkler water supply, an increase in the size of the pipe supplying both the domestic and sprinkler water can be justified.
“Future changes in water supplies should be considered. For example, a large, established urban supply is not likely to change greatly within a few years. However, the supply in a growing suburban industrial park might deteriorate quite rapidly as greater numbers of plants draw more water.”
The Annex material in NFPA 13 is explanatory and not part of the document’s requirements; therefore, it is not enforceable.
However, if you design a fire sprinkler system and the applicable jurisdiction enforces the International Building Code, then Section 903.3.5 (in every edition since 2009) is enforceable and states: “For connections to public waterworks systems, the water supply test used for design of fire protection systems shall be adjusted to account for seasonal and daily pressure fluctuations based on information from the water supply authority and as approved by the fire code official.”
Going Beyond a Simple Flow Test
While a hydrant flow test gives us a snapshot of the water supply at a particular moment, it doesn’t paint the entire picture. Think of it as taking a single photo of a bustling city. It won’t tell you about the peak hour traffic or the early morning calm. To truly comprehend a city, one would consider its history, residents, landmarks and potential future.
Similarly, for a comprehensive understanding of water supply, engineers must consider factors beyond the raw flow test data. A truly effective water supply evaluation is holistic, encompassing a multitude of factors:
• Identifying water sources. Elevated and ground-level water tanks, accompanied by booster pump stations, are fundamental components of waterworks systems. These water sources are designed with the purpose of ensuring a consistent and reliable water supply. Elevated tanks use gravity to maintain pressure, while ground-level tanks often rely on booster pump stations to supply water and maintain the desired pressure in the distribution system.
• Understanding large water users. Sites such as processing plants, amusement parks, golf courses and airports are not only large in scale, but also in their water use. Their operations often require substantial amounts of water, whether for cooling machinery, maintaining landscape aesthetics, ensuring the functionality of sanitation systems, or providing water for rides and attractions.
This demand becomes especially significant during peak operational hours or during special events. As a result, when evaluating the integrity and reliability of a water supply system for fire protection, it’s crucial to factor in the consumption patterns of these large-scale users. Understanding their water usage schedules, seasonal peaks and special event demands can provide a clearer picture of the potential challenges and needs of the waterworks system.
• Researching historical data. The importance of understanding the past to predict the future is vital when it comes to water supply systems. Historical flow tests serve as a record of the water source’s capacity, reliability and performance over time. These tests can uncover trends such as gradual reductions in flow rates or periodic fluctuations that might correlate with seasonal changes or urban development.
For instance, a decade-old flow test might show a higher water yield than a more recent one, indicating a gradual depletion or strain on the source. Or perhaps during a particular season or year with significant industrial growth or housing development, there might have been unusual spikes or drops in flow rates.
By analyzing this historical data, engineers can make informed decisions, ensuring that designs are not only based on the current status of the water supply but are also adaptive and relevant, considering potential future scenarios and challenges.
• Adjusting for variability. Water supply systems constantly evolve, responding to an array of factors that can change on a daily, seasonal or even hourly basis. It is one of the reasons some jurisdictions require, in addition to the flow test, a 24-hour static pressure recording. A consistent flow and pressure today may not necessarily mean the same will be true tomorrow. Daily activities, from morning routines to industrial shifts, can lead to noticeable fluctuations in water demand.
Moreover, the environment plays a pivotal role in shaping these demands. Prolonged droughts, for instance, can diminish water availability, and the water authority may not allow hydrant flow tests during a drought.
And during the summer, a unique set of challenges emerges. Rising temperatures naturally increase water consumption. People not only drink more but also use water extensively for gardening, cooling and recreational activities such as swimming. Additionally, public water facilities, including community pools and water parks, often experience heightened usage, further amplifying the overall demand.
Adjusting the Flow Test Data
Relying solely on raw flow test data is inadequate. Adjustments to this initial data are crucial to account for potential external influences and weaknesses in the distribution system. By incorporating insights from the water supply evaluation, engineers can adjust the design basis water supply to a level they believe the system can consistently maintain, regardless of seasonal variations and external challenges.
For practical purposes, it’s essential to select a water supply effective point situated near the site, typically at the point of connection to the waterworks system. This datum, or reference point, lays the groundwork for all hydraulic calculations.
It is established after adjusting raw water flow data, considering elements such as friction loss and elevation differences between the location of the hydrant flow source data and the chosen effective point. By pinpointing an accurate effective point, engineers guarantee that all parties involved in the project’s fire sprinkler development are in unison.
After adjustments to the raw water flow data, a design basis water supply is established at the water supply effective point (see Figure 1).
Typically included in the construction criteria documents prepared by the engineer of record, the design basis water supply indicates parameters such as static pressure, residual pressure, flow at the residual pressure and theoretical flow at the lowest permissible pressure in the water purveyor’s system (typically 20 psi) at the effective point.
The design basis water supply should also include the maximum anticipated static pressure to determine if high anticipated pressures exceed the pressure limitations of the sprinkler system (typically 175 psi). The highest expected static pressure (often reported at the fire pump suction flange) is necessary to ensure that the fire and jockey pumps are appropriately sized.
If you conduct flow testing in accordance with NFPA 291, Recommended Practice for Water Flow Testing and Marking of Hydrants, and only report the raw water flow test data on fire sprinkler construction criteria documents (what some call a “flow and go”), then you are doing it wrong.
With the design basis water supply accurately established at the effective point, the design of the fire sprinkler system (and fire pump, if necessary) can be performed to adequately protect a building. This foundational step in the design process is crucial, paving the way for an effective, responsive and dependable fire protection system.
John Dreher, EIT, is a fire protection consultant with the Harrington Group. His areas of expertise encompass fire alarm and fire suppression system design, along with acceptance testing. Dreher is a member of the Society of Fire Protection Engineers (SFPE) and the National Fire Protection Association (NFPA).
Tom Gardner, PE, FSPE, LEED AP, is a senior vice president/director of business development with the Harrington Group. He is a registered Professional Engineer (Fire Protection Engineering) in 17 states and has more than 43 years of experience. Gardner is the past chair of the SFPE’s Engineering Education Committee, past chair of the NFPA’s Health Care Section, and is an SFPE Fellow.