NFPA 92 defines design, testing of smoke control systems
NFPA 92: Standard for Smoke Control Systems provides fire protection engineers with guidance for the design and testing of smoke control systems.
Over the past few decades, building, fire, and life safety codes have been forced to continuously adapt to changing architectural trends. While smoke control systems are required to be provided in certain situations, they are sometimes provided as an alternative to having to comply with other requirements, usually for aesthetic or financial reasons. As the prevalence of design features such as large open spaces and open corridors without vestibules continues to increase, so do the number of new smoke control system installations and, consequently, the need for experienced individuals who understand and know how to correctly apply the applicable codes and standards.
NFPA 92: Standard for Smoke Control Systems is a standard published by the NFPA that provides requirements, recommendations, and guidance regarding the design, installation, acceptance testing, operation, and ongoing periodic testing of smoke control systems. An important distinction to recall is that a “code” tells us “when” or “where” something is required, while a “standard" tells us “how” it is designed, installed, tested, maintained, and so on. In this case, NFPA 92 tells us “how” to design smoke control systems such as stair pressurization, large volume exhaust, and elevator hoistway pressurization systems that are required to be provided in buildings by “codes” such as the International Building Code (IBC) or the NFPA 101: Life Safety Code.
Creation of NFPA 92
NFPA 92 was created during the NFPA Annual 2011 code cycle as a result of merging two predecessors: NFPA 92A: Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences and NFPA 92B: Standard for Smoke Management Systems in Malls, Atria, and Large Spaces. These two were maintained as separate documents from 1991 until 2009. The NFPA Technical Committee on Smoke Management Systems then decided to combine the two into a single document, in part to remediate the use of confusing terminology and duplicate provisions.
Much confusion existed due to the fact that NFPA 92A referred to pressurization systems as “smoke control systems” and NFPA 92B referred to systems used in large spaces such as malls and atria as “smoke management systems,” while at the same time, building codes and other standards recognized no distinction between these two terms. Building codes and standards simply referred to both pressurization (or “smoke control” as designated by NFPA) systems and systems used to maintain tenability in large spaces (or “smoke management” systems as designated by NFPA) as “smoke control systems.”
Therefore, to create consistency between the building codes and NFPA 92, the convention of referring to all systems used to address the impact of smoke from a fire as “smoke control systems” was adopted. Pressurization systems now fall under the smoke control sub-classification of “smoke containment systems,” while systems used in large spaces fall under the sub-classification of “smoke management systems.”
Chapters 1 through 4
The 2012 edition of NFPA 92 consists of 8 chapters and 13 Annexes. Chapters one through three cover the typical NFPA standardized introductory topics: Administration (scope, purpose, retroactivity, and units), Referenced Publications, and Definitions, respectively. Chapter 4, Design Fundamentals, contains exactly what the title implies, the fundamentals of smoke control design. The chapter walks users through a logical design process, which first involves selecting the desired smoke control method or methods to be used based on the selection of the specific design objectives.
As mentioned previously, the two smoke control “methods” (or sub-classifications) recognized by NFPA 92 include smoke containment, which involves establishing and maintaining pressure differences to contain smoke to the zone of origin, and smoke management, which involves removing smoke or managing smoke spread in large volume spaces to maintain tenable conditions. The ideal smoke control method for a particular application depends on the desired design objectives, four of which are listed in Section 4.1.2 (see Figure 1).
Three additional objectives are listed in Annex A and include providing increased visibility for fire department personnel, limiting the spread of toxic gases, and limiting the spread of combustion products to protect building contents. These are sometimes referred to as secondary objectives because, like anything contained in the Annex of an NFPA code or standard, they are not part of the mandatory requirements unless adopted so by the authority having jurisdiction (AHJ). (An example of this is where the AHJ indicates that the Annex is to be part of the mandatory requirements and the word “should” is to be replaced with “shall.”) Nevertheless, most of these secondary objectives are inherently met by systems designed to meet one or more of the primary required objectives. For example, a system designed to maintain the smoke layer interface at a predetermined elevation will usually meet all three of these objectives to some extent or for some specified period of time.
As you may have already guessed, the hierarchy of terminology used in this chapter is often misunderstood and misrepresented. After the selection of design objectives and methods, comes the selection of the design “approaches.” Smoke containment system “approaches” include stair, elevator, zoned, vestibule, and smoke refuge area pressurization. These approaches, along with the smoke management system approaches, are contained in Figure 1, which should help to clarify the major design terminology used in NFPA 92.
Criteria for smoke control systems
Chapter 4 also contains several design requirements and criteria for smoke control systems. You may wonder why the chapter is not simply divided into two sections, one containing criteria and requirements regarding smoke containment systems and the other regarding smoke management systems. This is due to the fact that a large number of the requirements and criteria apply to all systems, regardless of which smoke control method is used. For example, Section 184.108.40.206 specifies that the maximum pressure difference across doors shall not exceed the value stipulated in NFPA 101. This criterion applies to both smoke containment systems such as those that use the stair pressurization approach, and smoke management systems such as those that utilize mechanical exhaust within large-volume spaces.
Section 4.5 contains several requirements regarding system operations. This section requires that all smoke control systems be activated automatically, which is typically accomplished through the use of detection devices such as projected beam smoke detectors or spot-type smoke detectors and control relays, which send a signal to a control panel, which then signals the activation and/or shutdown of a number of devices that make up the complete smoke control system.
Power may be transferred to exhaust or pressurization fans, while at the same time, HVAC units may be shut down and dampers or vents may be opened or closed. Regardless of the components that are used as part of a particular system design, Section 4.5.3 requires that the entire smoke control system, including all of the devices just mentioned, reach full operating conditions before the design smoke conditions are reached (for example, when the design smoke layer depth is achieved).
The calculation of the system start-up time requires consideration of a number of factors in accordance with Section 220.127.116.11, including the time necessary for detection devices to activate (smoke must ascend to the device and reach a specific threshold before the device activates), the time for signals to be transferred, received, and processed, and also the time for mechanical devices to operate (HVAC equipment to shut down, exhaust or pressurization fans to ramp up to full capacity, etc.).
One of the primary reasons this requirement is contained in the standard is to ensure that the designer does not simply overlook these time delays as doing so could have a negative impact on the ability of the system to operate effectively in meeting the design objectives. While these and other requirements apply to all smoke control systems, NFPA 92 also contains some requirements and criteria that apply exclusively to either one type of system or the other. Selected criteria are discussed below.
Smoke containment systems
NFPA 92 Table 18.104.22.168.1 specifies a minimum pressure difference of 0.05 in. of water gage (in. w.g.) for all smoke containment system designs in sprinklered buildings. For nonsprinklered buildings, the minimum pressure difference depends on the ceiling height. Note that NPFA 92 also requires that factors such as wind forces, stack effect, and buoyancy be considered, and where the designer determines a higher minimum pressure difference is necessary, the higher minimum supersedes that contained in Table 22.214.171.124.1.
A numerical maximum pressure difference is not specified in NFPA 92; rather, it is calculated based on the maximum door opening force permitted by NFPA 101, as mentioned earlier. The 2012 edition of NFPA 101 requires that this force not exceed 30 lbf to set the door in motion and 15 lbf to fully open the door. Because the door is much easier to open once it is slightly opened and the pressure difference drops, the criteria used is the 30 lbf. Annex A.126.96.36.199 contains the calculation procedure used to determine the maximum design pressure difference.
Alternatively, the maximum pressure difference can be determined using Table A.188.8.131.52 for standard sized doors. Note that these requirements are not intended to apply to sliding elevator doors. While there is no maximum opening force specified in the standard for elevator doors, it is the intent that the pressure differential should not be sufficient to cause jamming of the door. Research has shown that this is not typically of concern because only a modest force is required to open elevator doors, even when significant pressure differentials are present. Keep in mind other codes may specify design criteria different from or in addition to that contained in NFPA 92, and whenever these codes are applicable, the more restrictive requirements must be used. Table 1 illustrates some of these differences.
Smoke management systems
Several criteria specified in Chapter 4 are written to apply exclusively to smoke management systems. Example requirements include a minimum smoke layer depth (20% of floor to ceiling height or based on engineering analysis) and a maximum make-up air velocity (200 ft/min near plume or based on engineering analysis). Most smoke management system designs are required by Section 4.5 to be based on tenability and egress analyses; however, these analyses are outside the scope of NFPA 92. In the current revision cycle, consideration has been given to creating a new Annex to address tenability.
Section 184.108.40.206 requires an egress analysis to be conducted when the smoke management system design objectives include maintaining tenability for the time necessary for occupants to exit the building or preventing occupants from being exposed to smoke. This requirement applies to the majority of smoke management system designs, as three of the four possible design objectives contained in Section 4.1.2 fit this description. Section 220.127.116.11 also requires that these systems remain operational for the calculated duration of egress. This requirement coincides with that of section 4.2.3, which together require that equipment must be capable of operating under exposure to the anticipated elevated temperatures for the calculated duration of egress.
Section 18.104.22.168 states that systems designed in accordance with objectives 2 or 3 from Section 4.1.2, which involve maintaining tenability for the duration of egress, are permitted to use design approach 3 or 5 from Section 4.3.2, which involve controlling the rate of smoke layer descent. Section 22.214.171.124 permits flexibility in the design in that occupants are permitted to be exposed to smoke, so long as conditions remain tenable for the duration of egress.
Chapters 5 through 8
Chapter 5 contains calculation procedures for smoke management system designs. Section 5.1 specifies three different methods that can be used for the design of a smoke management system:
- Algebraic equations (see the remainder of Chapter 5)
- Scale modeling (not very common)
- Compartment fire models (includes zone fire models such as consolidated model of fire and smoke transport (CFAST) and computational fluid dynamics (CFD) models such as fire dynamics simulator (FDS)).
NFPA 92 does not contain calculation procedures for smoke containment systems. The SFPE Handbook of Fire Protection Engineering and ASHRAE/ICC/NFPA/SFPE Handbook of Smoke Control Engineering are two commonly used resources for calculation procedures regarding these systems. These handbooks also contain additional information regarding smoke management system design.
Chapter 6 contains requirements regarding equipment and controls that are used as part of, or may affect the operation of, the smoke control system, such as HVAC controllers, firefighters’ smoke control stations, smoke detectors, or dampers. Chapter 7 contains requirements regarding the two documents required to be generated during the design process, the Detailed Design Report and the Operations and Maintenance Manual. Chapter 8 contains smoke control system testing requirements.
As noted earlier, the annexes are included for informational purposes only, and are not part of the requirements of NFPA 92.Information in the 13 annexes includes additional calculation procedures and examples, assistance with choosing a design fire and associated heat release rate, and additional information regarding CFD and zone modeling, HVAC air-handling and stairwell pressurization system types (compensation types), and testing.
Upcoming NFPA 92 changes
NFPA 92 currently is being revised as part of the fall 2014 NFPA code cycle. NFPA is still accepting public comments on the first draft report (visit www.nfpa.org/92 for information about the next edition or to submit a notice of intent to make a motion); therefore, nothing has been set in stone. Nevertheless, it is certain that the 2015 edition will feature several editorial revisions and minor revisions to comply with the NFPA Manual of Style and clarify the intent of the standard.
For example, Section 126.96.36.199 is slated to be reworded to clarify that smoke control system operational capability does not have to be verified by weekly tests; rather, it can be verified by other means such as electrical monitoring (supervision) of the control equipment. The committee also has proposed to incorporate references to the 2015 edition of NFPA 4: Standard for Integrated Fire Protection and Life Safety System Testing. In the 2015 edition of NFPA 92, tenability threshold guidance may potentially be brought over from NFPA 130: Standard for Fixed Guideway Transit and Passenger Rail Systems for incorporation into Annex D.
One of the only major changes that has been proposed and is currently under consideration is a substantial revision of the balcony spill equations contained in Chapter 5 (Section 5.5.2). New correlations have been proposed as a result of significant research in the area by Roger Harrison at the University of Canterbury, New Zealand. These new correlations have the potential to yield more accurate calculations and cover a wider range of scenarios than the existing correlations.
William E. Koffel is president of Koffel Associates and is a member of the NFPA Technical Committee on Smoke Management Systems. Nicholas Sealover is a fire protection engineer with Koffel Associates.