Hatch refers to an opening structure for personnel to enter and exit equipment for installation, maintenance, and safety inspections. It mainly consists of short tube sections (or pipes), flanges, and a hatch cover with a handle. In consideration of safety inspections, pressure vessels are required to have a certain number of openings and their small sizes as seen in "inspection holes". The materials for hatches include carbon steel and stainless steel. According to the pressure condition, hatches are classified into atmospheric and pressurized hatches; according to shape, they are divided into circular, elliptical, and rectangular hatches, with rectangular hatches only suitable for atmospheric pressure equipment; according to the installation position, they are classified into horizontal and vertical installation hatches; the methods of fixing the hatch covers include rotating covers and hanging covers; the opening speed of hatches can be normal or quick, and the hatch structure and dimensions are generally standardized, allowing selection or design based on relevant manuals.