Selecting a fire protection system
The decision-making process
The consulting engineer needs to possess a thorough knowledge of the limitations and uses of the various types of “alternative” systems he or she might consider as part of the overall fire protection package. It often falls upon the individual consulting engineer to develop his or her own method or application guide for selecting and recommending the most appropriate “alternative” system to meet the overall project objectives. Information from each of the individual reference standards as well as from other sources, such as system manufacturer materials listing protocols and fire tests, is essential in developing such an application guide. Ideally, a comprehensive fire risk assessment should serve as the basis for structuring any application guide or method for recommending a fire protection system. At a minimum, the decision process should be risk-informed.
A fire risk assessment is a process used to characterize the risk associated with fire that addresses the fire scenario or fire scenarios of concern, their probability of occurring, and their potential consequences. Within the context of the risk assessment, fire protection systems serve to mitigate the consequences. In undertaking a fire risk assessment, the level of acceptable fire risk needs to be sufficiently considered and articulated. The fire risk assessment will help crystallize the overall intent and purpose of any fire protection system, and how it fits into the overall fire safety strategy. Preferably, the fire protection systems need to be linked to the overall goals and objectives of not only the building owners, but also other key stakeholders involved with the project.
Certain fire protection standards specifically call out the use of fire risk assessments. For example, NFPA 75: Standard for the Fire Protection of Information Technology (IT) Equipment indicates that a fire risk analysis can be used to determine the construction, fire protection, and fire detection requirements for IT equipment, rooms, and areas. NFPA 75 identifies—among other things that need to be considered to determine the level of acceptable fire risk—factors such as the effect of loss of function of IT equipment on life safety, for example, process controls; threat of burning equipment to occupants and other property; and economic impact from loss of function, records, or physical assets. Numerous resources on fire risk assessments, including several chapters in the SFPE Handbook of Fire Protection Engineering and NFPA 551: Guide for the Evaluation of Fire Risk Assessments, are available to the consulting fire protection engineer in this regard.
Another resource available for structuring the decision making process is NFPA 550: Guide to the Fire Safety Concepts Tree. The “tree” can be used to develop and analyze the potential impact of fire safety strategies, and help identify gaps and areas of redundancy. The logic of the “tree” is directed toward the achievement of specified fire safety objectives that need to be clearly articulated. Strategies for achieving the objectives are divided into two categories: “prevention of fire ignition” and “managing fire impact.” Active fire protection systems can be employed to accomplish both: preventing a fire from starting, for example, inerting the atmosphere once the flammable limits of a particular fuel are sensed; and by managing the impact of the fire once ignition has occurred, for example, suppressing or controlling the fire, or safeguarding the exposed. Additionally, the system can be used to protect the entire building or just specific areas or operations.
Articulating goals, objectives
Ideally, the objectives necessary to achieve the stated goals will be quantified in some manner such as a maximum permitted fire size or concentration of products of combustion. In other words, how big of a fire and for what duration can the owner tolerate and still achieve his or her life safety or property protection goals? From a fire protection engineering perspective, especially through the application of performance-based design approaches, the fire can be quantified in terms of heat release rate as a function of time.
Tolerable fire size and growth rate—factors not explicitly described in building regulations and most design standards—will help inform the decision as to whether extinguishment, suppression, or control of the fire is needed; how soon after ignition system activation must occur; and what quantity of agent will be needed. The type of fuel and its location and orientation, ignition source, and room ventilation greatly influence a fire’s growth and heat release rate.
With respect to the various types of systems that can be used, some systems are more appropriate for fire suppression after a relatively short period of agent discharge followed by a longer time period in which the concentration of agent is held in the vicinity or room of the fire. Other systems are better suited for fire control in which the agent is directly applied to the burning and adjacent surfaces for an extended period of time. For many of these systems, a supplemental detection system is necessary to activate and control the system. Such detection systems and devices need to be integrated into the overall fire safety strategy, and selected and designed so that they initiate fire protection system discharge within the time period necessary to achieve the overall fire safety goals and objectives.