Consulting-Specifying Engineer - August 2008 - (Page 53) Codes & Standards set. These model codes specify standards and listings developed by independent testing groups such as UL and ASTM International. Basic construction of grease ducts serving Type I hoods require 16-gauge carbon steel or 18-gauge stainless steel. The referenced codes are: 2006 IMC – Section 506.3.1.1, 2004 NFPA 96 – Section 7.5.1, and 2006 UMC – Section 510.5.1. Both types of construction require a minimum of 18 in. of clearance to combustibles when unprotected. The referenced codes are: 2006 IMC – Section 506.3.6, 2004 NFPA 96 – Section 4.2.1, and 2006 UMC – Section 507.2.1. For grease duct applications that do not require a fire-rated system, but do come within closer proximity to combustible materials, the codes allow for means to reduce the minimum air space between grease duct and combustible: 2004 NFPA 96 – Section 4.2.3 and 2006 UMC – Section 507.2.1 Exception 2. Initial protection, where structural or other considerations did not allow room for clearances, was provided by a fire-rated shaft enclosure as defined in model building codes. A one-hour fire-resistive shaft is constructed of a single layer of fire-rated gypsum board applied to both sides of metal studs in the manner described by the building codes. Two-hour rated shafts require an additional layer of fire-rated gypsum board. Proper construction of fire-resistive enclosures was very time-consuming, especially where offsets were required. In the early 1990s, SBCCI created the first alternative fire-resistive enclosure test criteria based on ASTM E119, ASTM E814, and the internal fire test section of UL1978 “Test Standard for Grease Ducts.” Flexible wrap systems were tested to the model code acceptance criteria, and regional acceptance of these systems was achieved. Soon after, BOCA and the International Conference of Building Officials evaluation services created their own separate acceptance criteria for evaluating alternative fire-resistive systems as pertaining to kitchen exhaust. Flexible wrap systems This site-welded grease duct has a flexible wrap. Photo: Metal-Fab Inc. A factory-built grease duct was recently installed in a terminal at the Dallas-Fort Worth International Airport. Photo: Metal-Fab Inc. were tested to these model code acceptance criteria and, as a result, achieved national acceptance as an alternative to a code prescribed fire-rated shaft protecting a welded grease duct. As early as 1994, several manufacturers introduced factory-built grease duct systems composed of completely assembled components designed to easily connect in the field for complete systems. Standard UL1978 provides reduced clearances to combustibles and evaluates nonwelded joints as in compliance with NFPA, ICC, and IAPMO requirements. UL1978 does not evaluate fireresistive enclosures or building materials. It is used solely for factory-built ducts that are completely assembled in the factory, listed and labeled, and subject to periodic inspections by the listing agency to ensure materials and final construction are equivalent to the products tested per the UL1978 standard. Clearances must be displayed on the UL label and installed in accordance with the clearances specified. UL1978-listed grease ducts must be installed per the manufacturer’s installation instructions that meet the minimum requirements of the standard. . Ray is a principal member of NFPA 96, a member of the ASTM E05 committee responsible for the development of ASTM E2336, and a member of the UL Standards Technical Panel for UL1978 and UL2221. Consulting-Specifying Engineer • AUGUST 2008 53
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