GIVE ME A BREAK SIZING BREAK TANKS IN ACCORDANCE WITH NFPA 22
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KEVIN HALL, M.ENG., P.E., CET, CWBSP, PMSFPE | AMERICAN FIRE SPRINKLER ASSOCIATION hat is a break tank? NFPA 22, Standard for the Water Tanks for Private Fire Protection, 2018 edition simply states that it is a tank that supplies a fire pump but is not large enough to meet the system demand. The official definition in the standard is: 3.3.2.2 Break Tank. A tank providing suction to a fire pump whose capacity is less than the fire protection demand (flow rate times flow duration). Since the tank is not capable of supplying the fire protection system over its full duration, the next question might be: Why would a break tank be used? This question was common enough that annex language was added to the 2018 edition of NFPA 22: A.14.5 Break tanks have been used for one or more of the following reasons: 1. As a backflow prevention device between the city water supply and the fire pump suction
2. To eliminate pressure fluctuations in the city water supply and provide a steady suction pressure to the fire pump 3. To augment the city water supply when the volume of water available from the city is inadequate for the fire protection demand 4. To serve in situations where the building owner does not have room for a tank to meet the full demand of the fire protection system The last issue of Sprinkler Age focused on backflow prevention and testing, which is applicable in the first item listed in section A.14.5. Regarding water supplies, items two through four explain that break tanks can be beneficial when it comes to eliminating pressure fluctuations, supplementing municipal supplies, or saving some square footage as leasable space comes at a premium. Once a break tank is chosen as the preferred water supply arrangement, there are specific requirements in NFPA 22 to determine minimum stored water quantities and refill rates based on the duration provided by the volume of stored water. At a minimum, break tanks need to store 15 minutes of water based on 150-percent-rated capacity of the fire pump the break tank provides suction to. [NFPA 22, 4.1.7 & 14.5.1]
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12 SPRINKLER AGE | MAY/JUN 2021
This is a critical step. NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, permits fire pumps to utilize up to 150-percent capacity to meet system demand, so for a sprinkler system demand of 500 gpm, the designer could select a 400-gpm rated fire pump to optimize the minimum break tank size. By utilizing the lower capacity fire pump, the minimum size of the break tank is reduced from 11,250 gallons to 9,000 gallons. Once the minimum tank size is determined, there is another iteration to work through. The next benchmark occurs when the tank can supply a 30-minute duration based on the maximum system demand. If the break tank supplies the system for 30 minutes or more, NFPA 22 permits the refill rate to be determined by the following formula:
[NFPA 22, 14.5.2.2.1] Where: • Qr is the refill rate (gpm) • Vd is the volume of water required by the most demanding sprinkler system (gal)
