A manhole refers to an opening structure designed for personnel entry and exit, facilitating installation, maintenance, and safety inspections. It is primarily composed of short筒节 (or short tube), flanges, and a handle-equipped manhole cover. In consideration of safety inspections, pressure vessels require a specified number of manholes and their small dimensions, 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 circular, elliptical, and rectangular manholes, with rectangular manholes being suitable only for atmospheric equipment; and by installation position into horizontal and vertical installations. The manhole cover fixing methods include swivel covers and hinged covers; the opening speed of manholes is available in standard and quick-open styles. Manhole structures and dimensions are generally standardized, allowing for selection from relevant manuals or custom design.