NFPA 91-2020 Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Particulate Solids.
4.6 Hangers and Supports. 4.6.1 Duct supports shall be designed to carry the weight of the duct half ?lled with material. 4.6.2 Where sprinkler protection is provided or cleaning of the duct will be performed, the hanger’s design shall include the weight of any expected liquid accumulation. 4.6.3 Duct supports shall be designed to prevent placing loads on connected equipment. 4.6.4 Hangers and supports exposed to corrosive atmospheres shall be resistant to the corrosive atmospheres. 4.6.5 To avoid vibration and stress on the duct, hangers and supports shall be securely fastened to the building or structure. 4.6.6 Hangers and supports shall be designed to allow for expansion and contraction. 4.7 Duct Installation. 4.7.1 General Requirements. Unless the conditions stipulated in 184.108.40.206 or 4.7.2 exist, all duct systems and system components shall have a clearance of at least 6 in. (152 mm) from stored combustible materials, and not less than 1 ∕ 2 in. (13 mm) clear‐ ance from combustible construction. 220.127.116.11 Where stored combustible material or combustible construction is protected from ductwork by the use of materials or products listed for protection purposes, clearance shall be maintained in accordance with those listings. 4.7.2 Systems Conveying Combustible Materials. Unless the conditions stipulated in 18.104.22.168 exist, all duct systems and system components handling combustible materials shall have a clearance of not less than 18 in. (457 mm) from stored combustible materials or combustible construction. 22.214.171.124 When the ductwork system is operating at 140°F (60°C) or below and is equipped with an approved automatic extinguishing system designed for the speci?c hazard, the clearance shall be permitted to be reduced to 6 in. (152 mm) from combustible materials and 1 ∕ 2 in. (13 mm) from combusti‐ ble construction.
126.96.36.199 Ducts handling materials at temperatures in excess of 900°F (482°C) shall be lined with refractory material or the equivalent. 188.8.131.52 When stored combustible materials or combustible construction are protected from ductwork in accordance with 4.7.4, the clearance established in Table 4.7.3 shall be permit‐ ted to be reduced in accordance with Table 4.7.4, but not to less than speci?ed in 4.7.1. 4.7.4 Clearance Reduction Methods. It shall be permitted to protect stored combustible material or combustible construc‐ tion from ductwork in accordance with Table 4.7.4 and this section. 184.108.40.206 In no case shall the clearance between the duct and the combustible surface be reduced below that allowed in Table 4.7.4. 220.127.116.11 Spacers and ties for protection materials shall be of noncombustible material and shall not be installed on the duct side of the protection system. 18.104.22.168 Mineral wool batts (blanket or board) shall have a minimum density of 8 lb/ft 3 (130 kg/m 3 ) and have a minimum melting point of 1500°F (816°C).NFPA 91 pdf download.