Hatch refers to an opening structure designed for personnel entry and exit for installation, maintenance, and safety inspections. It is mainly composed of short pipe sections (or short tubes), flanges, and a hatch cover with a handle. For safety inspection considerations, pressure vessels require a specified number of hatches and their small sizes are listed in "Inspection Ports." The materials for hatches include carbon steel and stainless steel. Hatches are categorized as atmospheric and pressure hatches based on the pressure conditions; as circular, elliptical, and rectangular hatches based on their shape, with rectangular hatches only suitable for atmospheric pressure equipment; as horizontal and vertical installation hatches based on their installation position; and as swinging covers and lifting covers for hatch cover fixing methods. The opening speed of hatches can be either normal or quick-opening. The hatch structures and dimensions are generally standardized, allowing for selection or design based on relevant manuals.