A manhole is an opening structure designed for personnel to enter and exit equipment for installation, maintenance, and safety inspections. It is primarily composed of short筒节 (or short pipe), flanges, and a manhole cover with a handle. In consideration of safety inspections, pressure vessels require a specified number of manholes and their small sizes, which can be found in "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, elliptical, and rectangular manholes, with rectangular manholes only suitable for atmospheric equipment; and by installation position into horizontal and vertical installations. The types of manhole cover fastening include swivel covers and lifting covers. The opening speed of manholes can be either standard or quick-opening. Manhole structures and dimensions are generally standardized and can be selected or designed according to relevant manuals.