A manhole is an opening structure designed for personnel access to equipment for installation, maintenance, and safety inspections. It is mainly composed of short tube sections (or short pipes), flanges, and handles on the manhole cover. Considering safety inspections, pressure vessels require a certain number of manholes and their small dimensions are specified in the "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized by pressure conditions into atmospheric and pressurized manholes; by shape into round, oval, and rectangular manholes, with rectangular manholes suitable only for atmospheric pressure equipment; and by installation position into horizontal and vertical installations. The manhole cover fixing methods include rotating covers and lifting covers. The opening speed of the manhole can be ordinary or quick-release, and the manhole structures and dimensions are generally standardized, allowing selection or design according to relevant manuals.