sprinklernotes
Refill Rate for a Break Tank in an ESFR System
You asked the following question: “For a suction tank and fire pump supplying
an ESFR sprinkler system, is the tank permitted to be designed at a lower
volume if the refill rate from the municipal supply is able to meet system
demand and duration?”
We have reviewed NFPA 20, Standard for the Installation of Stationary Fire
Pumps, 2019 edition as the applicable standard. Our informal interpretation is
that break tanks are permitted to be installed per NFPA 20 when they are sized
in accordance with NFPA 22. The minimum size would be 150 percent of the fire
pump capacity for 30 minutes as long as the refill rate is sufficient to supply
the system for the remaining duration.
Break tanks are defined in NFPA 20:
4.33 Break Tanks. Where a break tank is used to provide the pump suction water
supply, the installation shall
comply with NFPA 22.
The specific sizing requirements of the refill rate are determined by NFPA 22,
Standard for Water Tanks for Private Fire Protection, with the applicable
sections extracted below:
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).
The tank meets the definition of a break tank per NFPA 22 and the minimum
sizing requirement is prescribed in section 14.5.1 and the refill mechanism
must be listed and installed per section 14.5.2:
14.5.1 Break Tank Size. The tank shall be sized for a minimum duration of 15
minutes with the fire pump operating at 150 percent of rated capacity.
14.5.2 Refill Mechanism. The refill mechanism shall be listed and arranged for
automatic operation.
Where the minimum size is determined by the fire pump rating, the specific
arrangement of the refill mechanism is based on the stored volume as it related
to the system demand vs. available duration. For tank capacities that supply
system demand for less than 30 minutes, section 14.5.2.1 applies:
14.5.2.1 If the break tank capacity is less than the maximum system demand for
30 minutes, the refill mechanism shall meet the requirements in 14.5.2.1.1
through 14.5.2.1.5.
14.5.2.1.1 Dual automatic refill lines, each capable of refilling the tank at a
minimum rate of 150 percent of the fire pump(s) capacity, shall be installed.
Continued on Page 2
14.5.2.1.3 A manual tank fill bypass designed for and capable of refilling the
tank at a minimum rate of 150 percent of the fire pump(s) capacity shall be
provided.
14.5.2.1.4 If available supplies do not permit refilling the tank at a minimum
rate of 150 percent of the rated pump capacity, the manual fill bypass shall be
capable of refilling the tank at a rate that meets or exceeds 110 percent of
the maximum fire protection system design flow.
14.5.2.1.5 A local visible and audible low liquid level signal shall be
provided in the vicinity of the tank fill mechanism.
For tank capacities that supply system demand for 30 minutes or more, section
14.5.2.2 applies:
14.5.2.2 If the break tank is sized to provide a minimum duration of 30 minutes
of the maximum system demand, the refill mechanism shall meet the requirements
in 14.5.2.2.1 through 14.5.2.2.5.
14.5.2.2.1 The refill mechanism shall be designed for and capable of refilling
the tank at 110 percent of the rate required to provide the total fire
protection system demand [110% (Total Demand – Tank Capacity) / Duration].
14.5.2.2.2 A manual tank fill bypass shall be designed for and capable of
refilling the tank at 110 percent of the rate required to provide the total
fire protection system demand [110% (Total Demand – Tank Capacity) / Duration].
14.5.2.2.3 The pipe between the city connection and the automatic fill valve
shall be installed in accordance with NFPA 24.
14.5.2.2.4 The automatic filling mechanism shall be maintained at a minimum
temperature of 40°F (4.4°C).
14.5.2.2.5 The automatic filling mechanism shall activate a maximum of 4 in.
(102 mm) below the overflow level.
The major trade off for providing a larger tank is that the minimum 30-minute
duration permits the use of 110% of the system demand, rather than 150% of the
fire pump capacity. This permits smaller flows for the refill lines and a more
economical installation as dual refills are no longer required.
