NFPA 92 defines design, testing of smoke control systems
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 126.96.36.199 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 188.8.131.52, 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 184.108.40.206.1 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 220.127.116.11.1.
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.18.104.22.168 contains the calculation procedure used to determine the maximum design pressure difference.
Alternatively, the maximum pressure difference can be determined using Table A.22.214.171.124 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.