Civil Engineering Reference
In-Depth Information
TABLE 27-1. Typical Range of Storage Requirements for Fire Protection
Storage Volume,
gallons
Storage
Volume, ML
Type of Development
Low-density residential, 2 hr at 500 gpm
60,000
0.23
Built-up residential, 2 hr at 1,000 gpm
120,000
0.45
Light commercial, 4 hr at 2,000 gpm
480,000
1.8
Commercial, 4 hr at 4,000 gpm
960,000
3.6
Emergency Storage
The emergency storage volume needed depends upon the risk of interruption of one
or more water sources and the length of time that may be needed to make repairs or
connect to an alternative source. Unplanned interruptions of supply may occur as a
result of well and or intake contamination, transmission main failures, or treatment
plant, pump, or power failures. Typically, with only one source of supply, a minimum
emergency storage volume would be enough to supply two days of average demand
in the area served by the storage facility. If the area is served by multiple supply
sources, this emergency volume could be reduced by the volume that could be provided
if only the largest source were out of service. The risks of emergency source outages
should be evaluated on a case-by-case basis.
Emergency storage can also provide additional water during planned source of
supply interruptions. Planned interruptions may be a result of intentional shutdowns
for inspection, maintenance, or improvement projects. The probable length of such
planned outages should be estimated and scheduled to minimize impacts. The neces-
sary emergency storage volume is then based upon the probable demands during this
period.
Dead Storage
It is important to consider that although a storage tank contains a certain total volume,
the bottom part of that volume may be unusable in the system. This is primarily
because as the storage volume is drawn down toward the bottom the tank, the water
pressure becomes inadequate to meet the needs of the system. This condition is es-
pecially true in standpipes where only the higher component of the storage volume is
usable. Therefore, depending upon service conditions, this dead storage must be con-
sidered when determining total beneficial storage.
Total Storage
The total storage required is typically the sum of operating storage, equalizing storage,
fire and / or emergency storage, and dead storage volumes. Rather than requiring both
fire and emergency storage, some local fire and state agencies will allow the use of
the larger of either fire or emergency storage volumes. These local agency requirements
