Application and Benefits of Performance-Based Designed Fire Detection Systems

Issue 31: Application and Benefits of Performance-Based Designed Fire Detection Systems

By Christopher Marrion, P.E., FSFPE

Over the last several years, great strides have been made in the application of fire engineering. This has been facilitated in part by the development of codes and guidelines for the application of performance-based analysis and design of buildings and structures.1,2,3,4 Within the performance-based analysis and design process, various fire and life safety systems and features exist that may be incorporated into trial design(s), including fire detection systems. Fire detection systems serve multiple purposes, including monitoring other devices and systems, detecting products of combustion, activating smoke management and other systems, notifying people that there is an emergency, and summoning emergency responders (see Figure 1).
Figure 1 - Overview of Functions of a Fire Alarm Control Unit


The main objective of most fire detection and alarm signaling systems is detecting a fire early so as to initiate various actions. Being able to adequately assess the time when an initiating device may activate is therefore important, especially when undertaking a performance-based assessment and the overall development of a fire safety strategy. NFPA 72 1 contains information to assist in designing a detection system prescriptively. In addition, over the last several revision cycles, efforts have been made to help better address detection systems in the performance-based analysis and design process. These are reflected in 'Annex B – Engineering Guide for Automatic Fire Detector Spacing'. This Annex has continued to build upon the work that SFPE and NFPA have been supporting with respect to the development of codes, standards, handbooks, and guidelines as well as research work in both performance-based analysis and detector response. The use of performance-based analysis in the design of a detection system is allowed and encouraged in NFPA 72.

The following provides a general overview of the performance-based analysis and design process and highlights the benefits this approach brings to the performance-based design of fire detection systems.

 

Challenges

For many applications, designing fire detection systems in accordance with NFPA 72 on a prescriptive basis is typically adequate. However, there are various situations or applications when one should consider whether there may be value in undertaking a performance-based approach when designing fire detection systems. This may include situations in which:

  • Design objectives are different than those of a Nationally Recognized Testing Laboratory (NRTL) evaluation test;
  • Ambient conditions (temperature, humidity, room size/geometry, etc.) are outside the performance parameters;
  • Very early smoke detection is required (computer/IT rooms, clean work environments, etc.);
  • Challenges exist with the ventilation systems (high velocities, duct/opening placement, etc.);
  • Detection is needed to protect an individual piece of equipment, artwork, etc.;
  • Activation of suppression systems is required at a given time/fire size to meet fire safety objectives;
  • Complex or irregular ceiling configurations pose challenges (i.e., complex ceiling geometries, beams, sloped ceiling, ease of testing & maintenance, etc.);
  • High ceilings pose the potential for stratification;
  • Design goals require addressing potential visual obstructions (banners, flags, etc.) and the desire for flexibility in use of the space;
  • Reducing nuisance alarms; and
  • Harsh environments (dirt, dust, etc.) limit design options.

Even if the process and analysis is not undertaken on a quantified basis, there is value using this approach at a qualitative level.

Process
The overall process as outlined in Annex B of NFPA 721 has been adopted to a large extent from the SFPE Engineering Guide to Performance Based Fire Protection2 and the work of Custer and Meacham.3 This involves a number of steps:

  • Identify Goals
  • Identify Stakeholder's Objectives
  • Define Design Objectives
  • Define Performance Criteria
  • Develop Fire Scenarios/Design Fires
  • Develop Candidate Designs
  • Evaluate and Select Candidate Designs
  • Documentation

Goals/Objectives
Goals and objectives can be broken into two categories: "typical" fire protection goals and "other" goals. Under the typical fire protection goals, the desired outcomes in the event of a fire can be life safety, property protection, continuity of operations, and limiting fire's impact on the environment. Heritage/preservation goals are at times considered as well. With regards to designing a detection system, heritage preservation goals may pertain not only to limiting damage to the property, but also limiting impact to the historic fabric arising from the installation of a system or device. (See Figure 2)
Figure 2 – Detection goals may impact system selection,
design and installation in historic buildings


In addition to the typical fire and life safety goals noted, there may also be other goals that a fire detection system is intended to achieve. These may include goals related to:

  • Costs (design, equipment, installation, maintenance, etc.)
  • Reliability
  • Aesthetics
  • Maintaining flexibility in use of a space
  • Maintainability and serviceability

Design Objectives/Performance Criteria
The next step is to define design objectives and performance criteria. These more explicitly and quantitatively express the goals and objectives so one can determine whether the trial design meets the goals and objectives when exposed to the design fire scenario. An example of design objectives and performance criteria for a detection system is given in table 1.

Table 1 - Objectives/Goals/Performance Criteria1

 

Fire Protection Goal Provide life safety
Stakeholder Objective No loss of life within the room of origin
Design Objective Maintain tenable conditions within the room of origin
Performance Criteria Detect the fire and activate the smoke management system in sufficient time to maintain:
  • Temperatures below tenable limits
  • Visibility above established limits.

Design Fire Scenarios
Design fire scenarios define the specific characteristics of the building, occupants and fire that are pertinent to the analysis/design. Some characteristics that may be more pertinent to the analysis/design of the operation of a detector include those in Figure 3.
Figure 3 - Development of Design Fire Scenarios


In developing design fire curves, a few things should be noted. Depending on the objectives and type of detector(s) used in the trial designs, it may not be necessary to define the various fire stages for all stages of a fire. For instance, for assessing activation of a smoke detector, it may only be necessary to look at the initial periods including ignition and growth.

 

It is important also to develop a design fire curve that defines additional characteristics and details that may not typically be addressed in other performance-based analyses and designs. This includes defining the "fire signatures" that may be produced by specific fires. Fire signatures are those fire induced by-products that one is looking to detect that may include smoke, heat, flame, CO, etc. For instance, if one had a fire that produced limited quantities of smoke, there could be substantial delays in detecting the fire using a smoke detector, or if one's objective were to detect a smoldering fire, then using heat solely as a fire signature may not be appropriate.

Once the design fire and the fire signatures are defined, then one should define the Design Objective Heat Release Rate (QDO) and the Critical Heat Release Rate (QCR) (see Figure 4). QDO represents the heat or product release rate which produces conditions representative of the design objective. This is not the point at which detection is needed. QCR represents that point on the fire curve and accounts for delays in response that may include system activation delays, fire department delays, etc.

It is important to differentiate between these as there may be delays associated with actions that must occur before extinguishment can begin. For instance, if the fire is going to be extinguished manually, and the fire department is to be notified by the detection and alarm system, actions including those in Figure 4 would need to be considered.
Figure 4 - Development of a Design Fire Curve and QCR and QDO for Fire
Department Response

Develop Candidate Designs
The candidate designs represent the detection strategies intended to detect the fire at QCR. There are several factors related to the selection of initiating devices including:

  • Type of detection
  • Location of detectors, including with respect to fire location
  • Number of detectors
  • Sensitivity to the expected fire signature(s)
  • Alarm threshold and duration needed at that threshold

Additional selection considerations may involve those that would also reflect various goals/objectives previously established:

  • Aesthetics
  • Accessibility – installation
  • Accessibility – inspection/testing/maintenance
  • Flexibility/functionality in use of the space
  • Cost (equipment, installation, testing, maintenance, etc.)

In terms of determining the location/spacing of detectors for the candidate design, the designer should consider the impact of the following as applicable regarding the ability and timeliness of the fire signature(s) to reach the sensing element of the detector:

  • Ceiling height
  • Ceiling shape and surface characteristics
  • Stratification
  • Obstructions
  • Configuration of contents/hazard location
  • Fire signatures and design fires
  • Compartment ventilation
  • Ambient conditions – temperature, humidity, etc.
  • Detection time(s)

Analysis
Analysis of the trial design(s) may fall into three broad areas to be assessed: production of fire signature(s), their transport from the fire to the detector, and assessment of the detector response to the fire signature(s) to determine whether the detector activates. Some of the more pertinent aspects of each of these components are highlighted in Figure 5.
Figure 5 - Analysis of Fire Detection Response

The modeling and analysis tools and data available for each of these steps varies depending on the compartment, fire signature and fire detector being assessed. For instance, there are more and better validated models to estimate the temperature at a proposed detector location and the response of a heat detector than to look at the production and changes to smoke during its transport to a smoke detector or the response of a smoke detector.

This further highlights the ongoing need for the development of more accurate performance metrics for smoke detector response. Designers should also review design methods and data available in research reports as well as analysis methods in NFPA 721 and the SFPE Handbook of Fire Protection Engineering4 and various computer models applicable to predicting detector response.

Documentation
Documentation of analyses and designs is critical. Documentation should address details regarding the overall performance-based analysis and design, including:

  • Scope of project
  • Goals
  • Performance objectives
  • Basis for the performance criteria
  • Design fire(s) and fire signatures
  • Modeling and computational analysis
  • Uncertainty analysis
  • Assumptions

The following additional items should also be well documented to help ensure the design matches the analysis, as well as ensure that any special installation or testing and maintenance requirements are noted. This should include development of:

  • Design specifications and drawings
  • Sequence of operation(s)
  • Special installation, testing/inspection/maintenance/commissioning requirements
  • Critical design assumptions

Chris Marrion is with Arup Fire.

References:

  1. NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 2007.
  2. SFPE Engineering Guide to Performance-Based Fire Protection, National Fire Protection Association, Quincy, MA, 2007.
  3. Custer, R. and Meacham, B., Introduction to Performance Based Fire Safety, National Fire Protection Association, Quincy, MA, 1997.
  4. Custer, R., Meacham, B. & Schifiliti, R., "Design of Detection Systems," SFPE Handbook of Fire Protection Engineering, Quincy, MA 2008.
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