March 28, 2016
Overflow Pipe Size for Underground Installations
"There is a question about the overflow pipe size and discharge location. In
this case, a tank room or water vault is being built inside a high-rise
building located five stories underground. The waterlines coming into the tank
are two 8 in. automatic fill pipes and one 8 in. manual fill pipe. What
calculation would be performed to determine if an 8 in. versus a 10 in. pipe
could be used for the overflow? If an 8 in. pipe supplies the tank, why would
an 8 in. overflow not be sufficient? With the tank and pump room being located
so far below grade level, there is no good way to discharge the water of the
overflow pipe in the scenario above to a drain. Is it implied that the overflow
pipe should be routed through the building and up to grade level to discharge
the water?"
We have reviewed NFPA 22, 2008 edition that you indicated as the applicable
standard. Our informal interpretation is that overflow design should be
coordinated with other trades such as plumbing.
NFPA 22 does not provide guidance on calculating the size of the overflow pipe.
Section 14.5.2.3 indicates that unless the maximum fill capacity is known and
the overflow capacity is calculated to be at least equal to the fill capacity,
the overflow pipe shall be at least one pipe size larger than the fill line and
shall be equipped with an inlet such as a concentric reducer, or equivalent,
that is at least 2 in. larger in diameter. Theoretically, the same size pipe
should be sufficient. However, a search of studies on the Internet showed that
the configuration of the overflow pipe in the vertical orientation addressed
the incoming flow whereas the horizontal orientation with elbows did not
perform as well. A Weir box or funnel for the overflow pipe also assisted with
the drainage. The overflow pipe has to contain a screen to prevent animals from
entering the tank where that will reduce the surface area of the pipe thus
affecting the flow. NFPA 22 appears to compensate for that by upsizing the
overflow pipe. The pipe should discharge to atmosphere or air break with a
drain. An overflow that is piped five stories above the level of the tank would
pressurize the tank and its components during overflow. As such, the plumbing
engineer should size a drain to accommodate in case of tank overflow. This is
also beneficial from a risk management perspective if there is a breach of
supply piping into the underground structure. The drain should be sized to
prevent flooding of the structure at the lower levels. The maximum fill
capacity should be determined and communicated to the engineer of record. They
should apply the applicable building or plumbing code to dispose of the
overflow water at the tank location.
Technical Update is prepared by the Technical Services Dept. of the AFSA: Vice
President of Engineering and Technical Services Roland Huggins, a PE registered
in fire protection engineering; Phill Brown, a NICET IV certified automatic
sprinkler technician and NFPA Certified Fire Protection Specialist (CFPS) and
Tom Wellen, a PE registered in fire protection engineering. This is provided
with the understanding that the AFSA assumes no liability for this opinion or
actions taken on it and they are not to be considered the official position of
the NFPA or its technical committees.
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