Water tanks for fire protection (whether suction tanks for fire pumps, pressure tanks or gravity tanks) are sometimes needed where: a water utility cannot provide adequate flow, pressure and/or duration for a site, if it is not economically feasible to connect to the utility (think of a remote site) or, if underwriting concerns drive the need for a second water source.
The tank needs to be designed to provide sufficient capacity for sprinkler and hose stream demand for a site. This includes design duration of the fire. In the case of gravity tanks the height of the tank will have to be determined to develop sufficient pressure. Break tanks are designed based upon a less than full duration (this subject has been earlier discussed in our April 2014 technical article). If there are multiple hazards at a site then each one should be analyzed to determine the maximum capacity of the tank.
Generally the tank should be designed per NFPA 22, Standard for Water Tanks for Private Fire Protection, with consideration towards FM Global Property Loss Prevention Data Sheet 3-2, Water Tanks for Fire Protection. Both standards deal with structural design loads (including foundations), protection against corrosion and protection against freezing.
While it is preferable that the tank be dedicated solely for fire protection, there are cases where the tank is also used for domestic and/or operational purposes. In that case, the tank needs to be designed for additional capacity, with a dedicated fire protection capacity previously determined. The outlet for the domestic/operation demand should be located above the point of the tank that is reserved for fire. Any treatment and distribution of the non-fire protection water should be independent of the fire protection distribution.
In the event that the tank as a private water source is interconnected with a public water source a backflow prevention device would be needed. The type of device (method and in some cases make and model) will be driven by the water utility.
Risk Logic can provide assistance in the capacity, design and location of fire protection water tanks.