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 inspection considerations, pressure vessels require a specified number of manholes and their small sizes are referred to as "inspection holes." Manhole materials include carbon steel and stainless steel. Manholes are classified into atmospheric and pressurized manholes based on pressure conditions; into round, elliptical, and rectangular manholes based on shape, with rectangular manholes only suitable for atmospheric equipment; and into horizontal and vertical installation manholes based on installation position. Manhole cover fastening methods include swivel covers and hinged covers. The opening speed of manholes can be normal or quick. Manhole structures and sizes are generally standardized, and can be selected or designed according to relevant manuals.