506.3.2.2 Duct-to-hood joints.
MC § 506.3.2.2
Duct-to-hood joints shall be made with continuous internal or external liquid-tight welded or brazed joints. Such joints shall be smooth, accessible for inspection, and without grease traps. Exceptions: This section shall not apply to: 1. A vertical duct-to-hood collar connection made in the top plane of the hood in accordance with all of the following: 1.1. The hood duct opening shall have a 1-inch-deep (25.4 mm), full perimeter, welded flange turned down into the hood interior at an angle of 90 degrees (1.57 rad) from the plane of the opening. 1.2. The duct shall have a 1-inch-deep (25.4 mm) flange made by a 1-inch by 1-inch (25.4 mm by 25.4 mm) angle iron welded to the full perimeter of the duct not less than 1 inch (25.4 mm) above the bottom end of the duct. 1.3. A gasket rated for use at not less than 1,500°F (815.6°C) is installed between the duct flange and the top of the hood. 1.4. The duct-to-hood joint shall be secured by stud bolts not less than 1/4 inch (6.4 mm) in diameter welded to the hood with a spacing not greater than 4 inches (101.6 mm) on center for the full perimeter of the opening. The bolts and nuts shall be secured with lockwashers.
2.Listed and labeled duct-to-hood collar connections installed in accordance with Section 304.1.
