By Todd Smith, P.E.
The intended application of standards for water-based fire suppression is to address a single occurrence of fire. Both NFPA-13 and NFPA-14 address minimum fire water requirements and minimum fire flow durations. Scenarios involving multiple fires, multiple degrees of failure to normal power or water infrastructure, or orchestrated acts of man may be beyond the scope of effort to establish autonomy through application of minimum requirements of current codes and standards. It is vital to remember this when evaluating realistic threats and making engineering decisions to prepare a critical facility to respond to fire events during or after a disaster. How vulnerable is your facility and what are the specific threats? Is shelter-in-place capability a program requirement?
True fire water autonomy means possessing an independent, sufficient, reliable source of fire water and a reliable means to deliver it to a fire. All components of the fire water source and ancillary systems, including fire water storage, the fire pump and its secondary source of power, must be capable of withstanding disaster and remain fully operable. This can be achieved by any number of configurations which may include a water tank, water tower, natural or manmade body of water, and a fire pump. Combinations might include recycled or reclaimed water obtained from sources such as swimming pools and rainwater cisterns.
Justifying Water Autonomy
The value of an autonomous fire water supply is apparent when you consider the possibility of impaired municipal water and/or power supplies for any length of time. Consider a seismic event and the subsequent loss of power or impairment of the municipal water supply. Or consider coastal flooding and loss of power following a hurricane. Flooded streets and streets damaged by earthquake, moving water, or sub-surface erosion from broken water mains can delay or prevent fire apparatus from reaching a critical facility in need. Existing facilities may be equipped to fight fire and still find themselves faced with the challenge of relocating fire pumps and sources of emergency power to higher elevation.
Facilities that can most easily justify the need for an autonomous fire water supply are typically located in populous areas that are prone to natural disaster. If fire water autonomy has not been mandated to meet shelter-in-place requirements, consider the advantage that an independent fire water system gives to the responding fire department during or after a disaster. Financial incentives to facility owners may include federally subsidized construction and reduced insurance premiums.
Quantifying Fire Water Storage Volume
Fire water storage is not a one-size-fits-all application. The necessary volume of water storage depends on the water-based fire suppression systems specific to a particular facility. To determine storage volume, the flow demand of each water-based fire suppression system must be known. Each flow demand, when multiplied by the respective flow duration, will yield the minimum storage volume for that system. Compare the totals for each, and the largest single volume will be the minimum water storage volume necessary. A cumulative volume is not required.
For facilities without standpipe systems, NFPA-13 provides guidance on flow duration based upon hazard classification. Hydraulic demand can be determined from hydraulic calculations or existing hydraulic plaques. For facilities with standpipe systems, NFPA-14 provides guidance for both the flow demand and duration. The latter standard also addresses an allowance for fire sprinkler flow in buildings with full and partial fire sprinkler systems and combined standpipe/sprinkler systems. The refill rate for fire water storage tank should be neglected when calculating fire water storage volume because an autonomous design must be fully independent.
Finding Space For Water Storage
Looking at fire water storage in the context of finding the necessary space within a building footprint will make it clear that even for an ordinary hazard/sprinkler-only application, both the volume and the corresponding weight of fire water storage are enormous. The required minimum flow durations push storage volume into the tens of thousands of gallons.
Remember that water has a mass of over eight pounds per gallon at ambient temperature. The mass of an adequate storage volume will involve structural considerations in all instances where natural bodies of water are not employed. New facility designs allow flexibility for designing water storage at or below grade or elevated on the roof. Retrofit scenarios require more creativity, particularly when limitations are posed by property boundaries, historically significant architecture, or inadequate structure. Giving up program space or parking space at critical facilities may not be an option. In all cases, consider unused and under-used real estate. Perhaps a master plan involves eventual demolition of an existing structure. Green spaces and storage spaces at grade or voids beneath the lowest level of elevated parking structures may provide the needed space.
Fire water towers provide both storage and static pressure, but they occupy skyline and real estate. Fire water storage tanks on top of a high rise may be an alternative, particularly when designing the structure of a new building. Existing structures are unlikely to be designed to accommodate the weight and require a structural analysis in order to entertain rooftop water storage.
Considerations For Pumping Fire Water
Pumping is necessary where sufficient static head is not available from a reservoir or elevated water storage tank. When fire pumps are employed, a reliable or secondary source of power is required. This can be a secondary electrical source, or diesel or steam turbine fire pump drivers. In all cases, full reliability and survivability of secondary sources are necessary in order to achieve autonomy.
Specific to retrofitting a fire water storage tank to an existing facility, note that this typically involves providing a new fire pump. It is important to evaluate what changes have occurred during the life of the facility that followed the original fire pump installation. Changes to flood maps, codes, and standards are also a factor. Has a story been added to the hospital, or a heliport with fire suppression on the roof of a high-rise? Have exit stairs with standpipes been added to the original building? Is the hazard classification of the original hydraulically-remote area still sufficient for the current hazard? Might there be a more hydraulically-remote area now? Such possibilities justify a fresh look at existing fire pump location and capacity. If a new fire pump is employed as part of an autonomous system, and an existing fire pump will remain, a hierarchy of fire pump operation must be established.
Smith has worked as an engineer with RMF Engineering over the past 19 years. He has 26 years of combined construction, design, fire protection, contract administration, and commissioning experience on commercial building projects.