Issue 12: Recent Developments in Smoke Management Systems
By James Milke, Ph.D., P.E., FSFPE
During the last five years, several of the recent technical developments in NFPA 92A1 and NFPA 92B2 were overshadowed by regulatory accomplishments in implementing these developments. The regulatory accomplishments are significant in that they reflect the recognition of the emergence of the smoke management field as having reached maturity (though similar to any 18-year-old individual, the field still can benefit from continued development).
The regulatory accomplishments began with the transition of NFPA 92B from a "guide" to a "standard" in 2005. In order to make the transition, the significant suggestions in the "guide" version of NFPA 92B had to be identified and then expressed as requirements. In the process, the NFPA Technical Committee had to be confident that the smoke management field had progressed to the point where suggested engineering approaches could be mandated. One significant aspect of changing NFPA 92B from a guide to a standard was that as a standard, NFPA 92B could be referenced by a code, such as NFPA 101®,3 NFPA 5000®,4 or the International Building Code® (IBC).5 A year later, the proposal to transition NFPA 92A to a standard was adopted by the NFPA.
While it was significant to develop the majority opinion within the Technical Committee that it was "time" to make the changes from guides and recommended practices to standards, the bigger tests would come from opinions of those outside of the Committee. The first result of those tests came with the approval by the NFPA membership and Standards Council to accept the proposal to transition
NFPA 92B from a guide to a standard. Less than one year later, the proposal to the
IBC to adopt
NFPA 92B by reference was accepted for the approach identified as the "exhaust method" in section 909 for atria, covered malls and similar spaces. While the model building codes had included excerpts from
NFPA 92B for many years, the systemic approach described by
NFPA 92B was adopted as the means to provide the exhaust method for smoke management.
During this same period, some technical aspects of NFPA 92A and NFPA 92B were improved as a result of research and experience with the use of the documents. The improvements principally address "details" of a smoke management system design, with the basic concepts remaining unchanged. Examples of the recent activities, some of which have already been incorporated into the last edition of NFPA 92B, include:
- Make-up air supply arrangements.
- Plugholing.
- Balcony spill plume.
Make-up air supply arrangements
The make-up air supply limit of 200 fpm (1 m/s) in NFPA 92B has long been debated. The basis of the requirement is that the velocity associated with entrained air for the range of design fires contemplated is on the order of 200 fpm (1 m/s). As such, the upper limit for make-up air velocity in the vicinity of the plume of 200 fpm (1 m/s) was established to avoid generating any additional turbulence that would result in additional smoke being created. While it was recognized that a greater air velocity would generate additional smoke, no research had been conducted to assess how much additional smoke would be greater with a greater velocity. Two research efforts have been conducted on this issue. One effort by Kerber and Milke6 used Fire Dynamics Simulator (FDS) to indicate that higher air velocities did lead to a significantly deeper smoke layer as a result of an air velocity that was 400 fpm (2 m/s), as illustrated in Figure 1. These simulations were applied to a 30 m cube atrium with a 5 MW fire located in the center of the space.
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| Figure 1. Smoke Layer Depth vs. Make-up Air Velocity (left = 1 m/s, right = 2 m/s).6 |
Of equal importance was the role of location of the air inlets, which NFPA 92B does not address. In the study by Kerber and Milke,6 an asymmetric provision of make-up air supply, e.g., providing make-up air from two of four sides surrounding the plume, resulted in the plume being deflected in a manner similar to a wind.
Plugholing
Plugholing refers to a situation where an exhaust fan has such a large capacity that it creates a hole in the smoke layer, exhausting clean air from below the smoke layer as well as smoke (see Figure 2).
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| Figure 2. Plugholing |
As a result of plugholing, part of the capacity of the fan(s) is used to exhaust air, which decreases the amount of smoke that is exhausted. As a result of the reduced amount of smoke exhaust, the smoke layer will be deeper and may result in design objectives not being met. The equation to estimate the maximum fan capacity to avoid plugholing was revised in the current edition of NFPA 92B. The current version of the standard limits the fan size to much smaller levels than in the previous edition, thereby mandating many small fans if the smoke layer is relatively thin.
Balcony spill plume
In addition, a change in the balcony spill plume correlation is under consideration, based on recent research at the National Research Council of Canada. A balcony spill plume is illustrated in Figure 3.
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| Figure 3. Balcony Spill Plume |
The research involved a set of large-scale experiments that were conducted to supplement the previous small-scale experiments that were used as a basis for the balcony spill plume equations in NFPA 92B.2 While the existing correlation was seen to provide reasonable agreement with the new experimental data for low heights above the balcony, once the smoke layer height above the balcony exceeds 15 m, the agreement is observed to suffer. As such, a two-equation approach is under consideration by the Technical Committee, one for heights under 15 m and one for heights in excess of 15 m.
James Milke is with the University of Maryland
References
1NFPA 92A, Standard for Smoke-Control Systems Using Barriers and Pressure Differences, National Fire Protection Association, Quincy, MA, 2006.
2NFPA 92B, Standard for Smoke Management Systems in Malls, Atria and Large Areas, National Fire Protection Association, Quincy, MA, 2005.
3NFPA 101®, Life Safety Code®, National Fire Protection Association, Quincy, MA, 2006.
4NFPA 5000®, Building Construction and Safety Code®, National Fire Protection Association, Quincy, MA, 2006.
5International Building Code®, International Code Council, Falls Church, VA, 2006.
6Kerber, S., and Milke, J., "Using FDS to Simulate Smoke Layer Interface Height in a Simple Atrium," Fire Technology, 43, 1, 45-75.
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