NFPA 92 guides smoke control system design

NFPA 92 is the starting point for any smoke control system, but too frequently it is assumed to be the only standard by which to judge smoke control calculations.

05/11/2018


This article has been peer-reviewed.Learning objectives:

  • Know the limits of NFPA 92: Standard for Smoke Control Systems.

  • Better understand the role engineering judgment plays in applying NFPA 92.

  • Learn common misconceptions when applying NFPA 92.


NFPA 92: Standard for Smoke Control Systems is the gold standard for the design of smoke control systems in the United States. Referenced by both the International Code Council and NFPA codes and standards, it’s the starting point for any smoke control system design.

However, sometimes NFPA 92 is used as a panacea to solve any number of problems for which the standard may not be the correct prescription. NFPA 92 should be the starting point for any smoke control system design, but it’s important to recognize the situations where using only NFPA 92 is inappropriate. In these situations, it may be necessary to rely on computer smoke modeling, the ASHRAE Handbook of Smoke Control Engineering, the Society of Fire Protection Engineers’ (SFPE) Handbook of Fire Protection Engineering, or basic engineering judgment to design smoke control systems.

To start, what is NFPA 92 appropriate in a broad sense? The 2015 edition of NFPA 92 reads as follows regarding the scope of the document: “This standard shall apply to the design, installation, acceptance testing, operation, and ongoing periodic testing of smoke control systems … ” It then goes on to list the purposes of the document including preventing smoke from entering safe areas, such as stairs and shafts; maintaining tenability in the means of egress; preventing migration between smoke zones; providing conditions outside of the smoke zone to assist with emergency response; and mitigating the risk posed to life and property.

So, NFPA 92 can be used to design smoke control systems. Simple enough, and on the surface, this covers a very broad scope. However, inside these boundaries are gaps where the standard alone is insufficient to address every aspect of a smoke control design and require the engineer to rely on engineering judgment or an entirely different standard/process.

Figure 1: This graph shows the wide range of heat-release rates based on time and differing growth factors. All graphics courtesy: WSP USAWhat NFPA 92 does not do

Even when NFPA 92 provides the appropriate path, there are still things the document does not do. Most importantly, it does not specify fire characteristics for design fire events. These scenarios should be selected by an engineer who has experience in evaluating/determining fire scenarios. Appendix B does provide some information for common fire sizes, but it is still up to the engineer to determine which of those, if any, are appropriate.

Furthermore, the engineer must determine the growth rate for the fire, though often this is avoided because a steady-state fire is assumed. Growth rates can vary widely (see Figure 1) and significantly impact the fire size.

NFPA 92 also does not state how tenable or safe an environment will be. It provides a set of prescriptive requirements and calculations, and by meeting these, it’s accepted that a sufficient level of safety is provided. NFPA 92 will not tell you where smoke is nor how dense, hazardous, or hot the smoke is in the zone. Things like temperature can be calculated, but these are boundary values for the purpose of use in calculations. In a real fire scenario, the calculated smoke layer temperature will likely differ significantly from the calculated value in addition to varying inside the smoke layer itself.

NFPA 92 does not address environmental effects. Criteria like winter and summer temperatures, wind speed, and stack effect can all have a significant effect on the operation of a smoke control system, especially when it comes to determining make-up air for smoke-exhaust systems.

Because of these factors, not just any engineer can pick up a copy of NFPA 92 or use a calculation spreadsheet to determine the performance criteria for a smoke control system. NFPA 92 should be considered a supplement to, not a substitute for, experience and engineering judgment.

Figure 2: This example shows a complex geometry where NFPA 92 calculations are not robust enough to provide reasonable conclusions. This model was generated using PyroSim.How NFPA 92 is misapplied

This section details real-world mistakes when applying NFPA 92. This is not intended to be an indictment of anyone who has made one of these mistakes before, but rather as a guide to prevent engineers from making these mistakes in the future. Every person has blind spots and gaps and misses things at times, but engineers should seek to at least minimize, if not eliminate, these faux pas.

Fire size is possibly the most important variable for smoke control calculations, but unfortunately, it is an area of great uncertainty. While NFPA 92 provides some equations for determining some characteristics of the fire, the most important piece—heat-release rate—is not established prescriptively. Whereas previous code editions (and some jurisdictions with this still in their DNA) specified a minimum fire size of 5 MW, the current International Building Code and NFPA 92 do not.

While engineers are always looking for prescriptive requirements to reduce personal liability, NFPA instead defers to the engineer’s judgment while providing some helpful, though limited, examples. Fire sizes of 100 to 500 kW are sometimes proposed for calculations, which are on the order of magnitude of a trash can fire or a wooden chair with minimal padding, but there are few to no situations where this is a reasonably conservative fire size without including sprinkler activation.

ASHRAE suggests a minimum fire size of 2,100 kW for a transient fire, which is a good starting point, but ASHRAE cautions against using this for every scenario. This heat-release rate is approximately that of a two-seater foam sofa, but other pieces (or arrangements) of furniture can readily exceed this, especially when sprinklers are not present or are too high to control the fire. Furthermore, while furniture is the common culprit for the worst-case fire scenario, it is not the only possible scenario, which can include sources like hazardous material spills, kiosks, art exhibits, and Christmas trees.

Often a fast-growth fire is assumed, regardless of the fire source, and the fire grows until it is controlled by sprinkler activation, at which point the heat-release rate for the fire remains constant for the duration of the evaluation. This is a reasonable, if not overly conservative, approach, but how is sprinkler-activation time determined?

Table 1: This demonstrates the significant differences between sprinkler-activating times when inappropriately using Alpert correlations for quasi-steady fires. Commonly, the Alpert correlation (detailed in NFPA’s Fire Technology, volume 8, but referenced in SFPE’s Design of Detection Systems) is used to calculate sprinkler-activation times, but given the fast-growth fire situation described above, this is a mistake. The Alpert correlation should only be used for steady-state fires. Either the Beyler correlation (detailed in “A Design Method for Flaming Fire Detection,” Fire Technology, volume 20, issue 4 but referenced in SFPE) or a quasi-steady stepped method should be used. An example comparing the results of Alpert and Beyler correlations is shown in Table 1.

Note that for smaller fires with a fast growth factor, if the time to sprinkler activation has been calculated using Alpert, the fire size will exceed the initial fire size used in Alpert, indicating the sprinklers will never activate. For larger fires, the fire does not have time to reach the specified heat-release rate used in Alpert.

Compare this with Beyler in Table 1, where time to activation is based on growth rates and not predicted peak heat-release rates. The quasi-steady stepped method is not illustrated in this table, but models a fire using a series of Alpert correlations with small time intervals, essentially modeling a curved growth with discrete, stepped increases.


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