Fire Water Supplies
There are two critical factors to consider when designing the water supplies for a chemical facility. These are adequacy and reliability. Adequacy of the system is determined by the capability of the supply to meet the flow and system pressure requirements for the anticipated fire duration, typically 2 to 4 hours. The system must also be reliable under adverse conditions such as those following an explosion, so redundancy is often provided. Also, the onsite dedicated fire water storage should have the capability of replenishment within 24 hours.
The water supply and distribution system must be able to deliver large quantities of water to many areas of a facility. The actual volume and pressure required depends on the specific hazard to be protected. The water supply should be capable of supplying the maximum instantaneous and continuous demands which may result from an explosion or fire within the facility being protected. When evaluating the demands for a specific hazard, consider the maximum number of fixed systems that may be expected to operate simultaneously (e.g., multiple water spray, deluge, or foam systems) and the water to be used for manual fire fighting from monitors and hose lines. The total demand could be as low as 1000 gpm for administrative areas to 10,000 gpm and higher for large process areas. These flows should be available at a minimum of 100 psi within the protected area in most cases.
Reliability of the system can be enhanced in several ways. First, fire protection water supplies should be dedicated for fire protection and not combined with process water needs. Many facility water supplies are not capable of providing adequate fire water protection from existing water systems without reinforcement, such as from booster pumps. Process water systems may not be designed for the pressures required for fire fighting. Combined systems are much more likely to be out of service when needed, and when large quantities of water are used for fire fighting, it can rob critical cooling water from processes, resulting in unstable process conditions, increasing the severity of an emergency condition.
Second, the facility supply should be capable of providing much of the anticipated demand with any “impairment” to the system. An impairment could be the loss of a single fire pump, reduced suction supply, a break in the distribution piping, a shut valve, or other adverse condition. This means that looped and redundant water supplies should be provided to minimize any single failure.
In all but the most unusual circumstances, dedicated fire water pumps and dedicated water storage facilities should be provided to meet the maximum fire water demand. Most public water systems are inadequate to supply the volumes and pressures required. Gravity tanks are also usually of inadequate volume and pressure to be of significant benefit in all but small facilities. Due to the large volumes of water required, many facilities arrange their pumps to take suction from rivers, lakes or ponds. Reliance should not be placed on water supplies located in adjacent plants (i.e., those owned by another company) except in the most unusual circumstances.
Where fire water pumps are installed, they should be provided with all necessary features to allow for independent and reliable operation should other systems fail. This is achieved by providing self-contained diesel engine driven pumps. Loss experience has shown (Garrison 1988) that when fire pumps fail during an incident, more than 90% of the failures involve electric motor or steam turbine driven pumps. NFPA 20,” Standard for the Installation of Centrifugal Fire Pumps” (1990) should be reviewed relative to system requirements.
Fire pumps should be located to avoid the potential loss of all pumping supplies at the same time. Locate fire pumps away from severe process hazards so that they will not be damaged by explosion overpressures. Explosion debris has landed on fire pump buildings and destroyed suction tanks making the systems inoperable even when located remotely. At least two pumping stations are desirable. Fire department connections should be provided in convenient locations to boost the pressure for sprinklers and other fire protection systems which may become overtaxed or somehow isolated from the water supply.