Hatch refers to an opening structure used for personnel entry and exit, installation, maintenance, and safety inspections. It is mainly composed of short筒 sections (or short pipes), flanges, and hand-operated hatch covers. From a safety inspection standpoint, pressure vessels require a specific number of hatches and their small size, which can be found in "Inspection Holes." The materials for hatches include carbon steel and stainless steel. Hatches are categorized by pressure conditions into atmospheric and pressure hatches; by shape into round, elliptical, and rectangular hatches, with rectangular hatches only suitable for atmospheric pressure equipment; by installation position into horizontal and vertical installations; and by hatch cover fixing methods into swinging covers and lifting covers. The opening speed of hatches can be standard or quick-release. Hatch structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.