NFPA 92: Why it matters, how to use it, and changes to anticipate in 2018
NFPA 92: Standard for Smoke Control Systems provides fire protection engineers with guidance for the design and testing of smoke control systems, and there are critical changes underway to the 2018 edition.
- Explain that NFPA 92: Standard for Smoke Control Systems is the primary standard that guides fire protection engineers.
- How to comply with NFPA 92 to contain and manage smoke in a nonresidential building.
- Learn about the 2018 edition of NFPA 92.
NFPA 92: Standard for Smoke Control Systems is the industry standard that designers, code enforcers, and maintenance personnel turn to when confronted with a smoke control system. NFPA 92 is referenced in the International Code Council codes, the U.S. Department of Defense’s Unified Facilities Criteria, NFPA 101: Life Safety Code, and other local codes and industry design guides. As buildings become more complex, incorporating larger and more complex open spaces, smoke control becomes even more important.
In the past 5 years, NFPA 92 has evolved from two standards segregating the design of large-volume-space smoke management from the design of stairwell pressurization, zoned and elevator smoke control, vestibules, and smoke-refuge areas. In 2011, NFPA 92A Standard for Smoke-Control Systems Utilizing Barriers and Pressure Differences merged with NFPA 92B Standard for Smoke Management Systems in Malls, Atria, and Large Areas to form the current edition of NFPA 92.
The current year revision of NFPA 92 is the 2015 edition. The 2018 revision cycle has already begun, and the second draft’s public comments closed in November 2016. The second draft posting date is slated for August 2017.
Introduction to NFPA 92
As stated in the scope of the standard, NFPA 92 applies to the design, installation, acceptance testing, operation, and ongoing periodic testing of smoke control systems. How smoke is “controlled” in buildings can vary greatly and depends upon the specific traits of a facility. Therefore, NFPA 92 addresses the many means of controlling smoke, such as inhibition of smoke from entering stairwells, means of egress, smoke refuge areas, elevator shafts, or other similar areas where occupants may congregate or egress. It could also mean maintaining a tenable environment for the time required for safe evacuation, the inhibition of smoke migration by compartmentation, protecting and aiding emergency response operations, and improving life safety and reducing property loss. The design goals, methods, and criteria for smoke control systems are discussed in the next section.
NFPA 92 is a standard and not a code; therefore, designers and authorities having jurisdiction (AHJs) do not find the requirement for a smoke control system in NPFA 92. The locally adopted building code, such as the International Building Code (IBC) or NFPA 101, will require smoke control systems in certain cases and reference NFPA 92 as the governing standard for designing, installing, and testing of such systems.
Complying with NFPA 92
The term “smoke control” encompasses both the containment of smoke in a designated zone as well as the management of smoke within a large-volume space and adjacent connected spaces. NFPA 92 is structured around smoke containment and smoke management, providing approaches and criteria for the implementation of each.
Once the requirement for a smoke control system is established, the first step is to consult NFPA 92 and determine whether the system should be based on the smoke-containment concept or the smoke-management concept. Smoke management generally is used for large multistory spaces, such as atriums. Smoke containment, achieved using pressurization, is used for elevators, stairways, and zoned smoke systems. Additionally, a building may include smoke management as well as a smoke containment; the two methodologies are not mutually exclusive systems and both are often found in the same building. After the design methodology and smoke control objectives are identified, the design approach(es) should be selected.
For smoke-containment systems, the design approach includes one or more of the following: stairwell pressurization, zoned smoke control, elevator pressurization, vestibule pressurization, and smoke refuge area pressurization.
Smoke-management systems apply within large-volume spaces and their respective communicating spaces. The design approaches can include one or more of the following:
- Natural smoke filling and calculating/modeling of smoke layer descent to determine at what point occupants will be exposed if their egress inhibited.
- Mechanical smoke-exhaust capacity to remove enough smoke to maintain the smoke layer above the height where it affects occupants’ ability to egress.
- Mechanical-exhaust capacity to remove smoke to slow the rate of smoke layer descent for a period that allows adequate time for occupants to egress.
- Gravity smoke venting to maintain the smoke layer interface at a predefined height for a design time.
- Gravity smoke venting to slow the rate of smoke layer descent for a period that allows occupants to egress.
- Opposed airflow to prevent smoke movement between a large-volume space and communicating space.
Keeping the design basis in mind, the smoke control approaches should be clearly defined and documented from the beginning. The report should also document the next step, establishing the design criteria. The criteria may apply to one or both of the smoke control methodologies and can help assess the effectiveness of a smoke control system; they can be found in Section 4.4 of NFPA 92.
Once the general design requirements are established, the next step is to perform the design calculations. A number of techniques including hand-calculation methods, scale models, or computer models can be employed at this stage. Special consideration should be given to the design fire, including likely fuel materials, location, intensity, and growth. It often helps to develop several fire scenarios to determine a conservative, yet reasonable, fire situation and ensure the smoke control system can operate effectively under a variety of circumstances. Chapter 5 of NFPA 92 outlines the calculation procedures related to smoke management. It is important to note that NFPA 92 does not provide guidance for design calculations for smoke-containment systems.
Once the calculations are completed, the design of the system can begin. Chapter 6 of NFPA 92 outlines the building equipment and controls requirements. One of the most widely recognized and discussed stipulations of this chapter is the requirement for control systems to be listed in accordance with UL 864: Standard for Control Units and Accessories for Fire Alarm Systems, which includes the UUKL listing that establishes a rating for the equipment used within a smoke control system, for their intended purpose.
After establishing the design approach, equipment layout, and controls specification, the last step of the design process involves the documentation of the system. Detailed requirements for the design report can be found in Chapter 7 of NFPA 92. Careful records of the design approach, methodology, design criteria, and other relevant information are crucial to the commissioning and acceptance-testing process of smoke control systems.
Additionally, the operations and maintenance manual should be developed, using Chapter 8 of NFPA 92 as guidance for the testing and maintenance throughout the life of the system. Compared with other systems, where the testing and maintenance is mainly prescriptive and based on code requirements or the equipment installed, the designer is obligated to be involved in the development of the testing and maintenance procedures because each smoke control system must be tested against its exact design criteria. Therefore, the designer must ensure the proper procedures are followed to test and maintain each unique smoke control system configured to a specific building by providing procedures for commissioning, testing and inspection (including required frequency), critical design assumptions and limitations, and the overall purpose of the system.
UL 864—UUKL and weekly self-testing
On the subject of testing and maintenance, one of the most significant changes to the 2015 version of NFPA 92 included the departure from the UL 864 UUKL listing requirements for equipment with smoke control applications.
Section 50.7 of UL 864, the third-party testing standard used to verify the effectiveness of smoke control equipment, requires a weekly self-testing function for dedicated smoke control systems. The self-test involves automatic command activation of each associated function, and any function that fails to operate within a specified time period shall be annunciated audibly and visibly at the firefighters’ smoke control station. The intent of the self-test is to quickly diagnose any operational failures of the smoke control system and alert the owner/manager of the system via supervisory signal so repairs can be implemented promptly.
The 2012 edition of NFPA 92 states in Section 22.214.171.124 that the “operational capability of dedicated smoke control equipment shall be verified using the weekly self-test function provided by the UUKL-listed smoke control panel mandated by 6.4.1.” In the 2015 edition, this section was changed to read, “operational capability of dedicated smoke control equipment shall be verified as specified by the registered design professional (RDP) and approved by the AHJ.”
Proponents of the 2015 deletion of the UUKL requirement argue that the weekly testing function placed undue hardship on the systems that did not increase added safety. The additional functionality cost more to install and maintain and served a redundant purpose to the continual fire alarm system monitoring of various smoke control system components.
Those in favor of the 2012 edition text argued that allowing systems to operate without meeting all the UUKL listing requirements—most importantly, the weekly self-test of otherwise dedicated equipment that would only operate in a smoke-control mode—greatly diminished the overall reliability of the system and the confidence that the system would be capable of operating properly when called upon to do so in an emergency situation. Furthermore, removing the frequency of testing requirements of the dedicated equipment and leaving it up to the RDP and AHJ to decide would result in inconsistencies predicated on the experience of the professionals involved.
The first revisions report of the 2018 edition of NFPA 92 lists a possible compromise between the two factions. As proposed, the text of the 2012 edition will be reinstated, with the addition of an “except as noted” clause, which allows the adoption of an alternate method of verification to the weekly self-test requirement. The proposed alternatives are consistent with testing requirements outlined in the IBC and the International Fire Code. These include weekly verifications of power downstream of all disconnects, with semi-annual complete testing of bypassed components including testing under standby power.
Inclusion of tenability guidance in NFPA 92
Another long-discussed topic in the development of the NFPA 92 standard is the criteria for what constitutes a “tenable environment.” In recent years, there has been an increased use of computational fluid dynamics (CFD) models for evaluating smoke control system designs using the tenability method of smoke control. This type of approach is predicated on providing a smoke control system that prohibits the development of an untenable environment while occupants are still in the space protected by the smoke control system. However, the recently published editions of NFPA 92 provide no guidance on acceptable tenability criteria.
Typically, as the standard is now, acceptable criteria for tenable environments must be proposed by the design professional or set forth by the AHJ. This leads to greatly varying results and verification dependent upon the experience of the registered design professional and AHJ.
As a whole, the fire protection field has been reluctant to state definite parameters for a definitively tenable environment. This is not without good reason—tenability depends on multiple factors that can be difficult, if not impossible, to predict accurately, including but not limited to ambient temperature, smoke toxicity, and visibility. The Society of Fire Protection Engineers Guide to Human Behavior in Fire is a detailed resource for qualitative and quantitative data related to the effects of fire on occupants. Although dated (originally published in 2003), a second edition is expected to be issued for public comment soon.
The NFPA 92 Committee’s first-draft meeting of the 2017 fiscal year code cycle references the formation of a task group to consider the criteria for tenable environments for inclusion in the next edition of the code.
One such example can be found in Appendix B of NFPA 130: Standard for Fixed Guideway Transit and Passenger Rail Systems. Provision of emergency ventilation of passenger rail tunnels and smoke control systems for transit stations is crucial to the life safety of transit customers, and for tunnels specifically. CFD is the only design tool that adequately assesses the performance of the ventilation system, necessitating a discussion of appropriate tenability criteria. Thus, NFPA 130 includes an annex that discusses tenability requirements. This annex material was not added to the standard as a requirement, as listed in the body of the standard, but as appendix material intended to be used for informational purposes only. However, in the absence of other criteria, the appendix language in NFPA 130 is used as the de facto tenability criteria by design professionals designing passenger rail systems.
Appendix B of NFPA 130 covers heat effects over time; air carbon monoxide content; smoke-obscuration levels; air velocities (minimum and maximum); noise levels (which may not be as relevant in buildings as in trainways); geometric considerations including smoke layer height, zone of application for tenability criteria, effects of ventilation, and short-term transient effects; time considerations; and modeling accuracy and application of safety factors.
Incorporation of some, if not all, of these tenable environmental criteria in the 2018 edition of NFPA 92 would be a welcome first step to defining and refining acceptable tenable environment criteria. Further development would be required, however, to reach a widely accepted process for determining whether an environment is tenable based on as many of the multiple, ever-changing variables as possible.
It is important to note the possible changes to the 2018 edition of NFPA 92 are still in the developmental phases. The proposed changes are not guaranteed to be agreed upon, and speculation as to the form they may take is simply that: speculation. Much can change over the course of the next year as the development committee finalizes the second draft.
Nevertheless, NFPA 92 continues to evolve and adapt to changing technology and the demands of modern building systems. It continues to provide direction and make improvements on the design as well as operation and maintenance of smoke control systems.
Kelly Kidwell is a security and fire protection engineer at JENSEN HUGHES.