sprinklernotes
Refill Rates for Break Tanks
You asked the following question: ”I would appreciate some guidance determining
the fill rate from a municipal water supply when using a break tank. We have a
situation where a break tank has been specified per section 4.31.1(3). The
system demand is approximately 1,000 gpm for 60 minutes and the specified tank
is 40,000 gallons. Since the tank provides over 30-minutes duration, it falls
under the refill requirements of section 4.31.3.2. The refill line is a 6-in.
municipal water line that will have a backflow assembly and fill control valve
before emptying into an underground tank. We have a flow test at the street as
follows – Static: 45 psi, Residual: 30 Flow: 669 gpm. I also need to deduct 500
gpm for hose from the hydrants. The method that I came up with to determine the
fill rate was to input this information into my calc program and ask for a
fixed flow of 367 gpm (333 + 10 percent). It shows that I can satisfy the
requirement – and draws down the city main to 20.8 psi. Is this methodology
correct?
In response to your question, we have reviewed NFPA 20, Standard for the
Installation of Stationary Fire Pumps for Fire Protection, 2013 edition. Our
informal interpretation is that the fill rate only needs to satisfy the volume
requirements per NFPA 20. Any restriction on residual pressure from a municipal
source would be determined by the local authority or water purveyor.
In this case the break tank is sized to supply the system for 40 minutes. Per
NFPA 20, break tanks that supply over a 30-minute duration must meet the
requirements of section 4.31.3.2:
4.31.3.2 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 4.31.3.2.1 and 4.31.3.2.2.
4.31.3.2.1 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% X (total demand − tank capacity) / duration].
4.31.3.2.2 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% X (total demand − tank capacity) /
duration].
In this case the refill rate required by the municipal supply would equal:
If the refill arrangement is capable of providing 367 gpm, it satisfies the
requirements of NFPA 20. There is no other requirement for a minimum residual
pressure or to take a penalty for hose allowance.
It should be noted that the primary refill line is required to be listed and
automatic:
4.31.3 Refill Mechanism. The refill mechanism shall be listed and arranged for
automatic operation.
