2.3.3 Fan housing openings for pipe connections shall be at least NPS 3/4 and shall be flanged or machined and studded. Where flanged or machined and studded openings are impractical, threaded openings in sizes NPS 3/4 through 11/2 are permissible. The threaded openings shall be installed as specified in 2.3.3.1 through 2.3.3.7.