Hatch refers to an opening structure designed for personnel entry and exit, facilitating installation, maintenance, and safety inspections. It is primarily composed of short tube sections (or short pipes), flanges, and a hatch cover with a handle. For safety inspections, pressure vessels are required to have a specified number of hatches and their small size, which can be found under "Inspection Holes." The material for hatches includes carbon steel and stainless steel. Hatches are categorized as atmospheric pressure and pressure hatches based on their pressure conditions; as circular, elliptical, and rectangular hatches based on their shape, with rectangular hatches being suitable only for atmospheric pressure equipment; as horizontal and vertical installation hatches based on their installation positions; and as swivel covers and lifting covers for the hatch cover's fixation method. The opening speed of hatches can be ordinary or quick. Hatch structures and dimensions are generally standardized, and can be selected or designed according to relevant manuals.