A manhole refers to an opening structure used for personnel entry and exit of equipment for installation, maintenance, and safety inspections. It is mainly composed of short筒节 (or short pipe), flanges, and a manhole cover with a handle. For safety inspections, pressure vessels require a specified number of manholes and their small dimensions, see "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized into atmospheric and pressure manholes based on pressure conditions; into round, elliptical, and rectangular manholes based on shape, with rectangular manholes only suitable for atmospheric pressure equipment; and into horizontal and vertical installation manholes based on installation position. The types of manhole cover fastening include swinging covers and lifting covers; the opening speed of manholes includes standard and quick-open styles. Manhole structures and dimensions are generally standardized, allowing for selection or design according to relevant manuals.