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 pipe), flanges, and a handle-equipped manhole cover. In terms 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 into atmospheric and pressurized manholes based on pressure conditions; into circular, elliptical, and rectangular manholes based on shape, with rectangular manholes being suitable only for atmospheric equipment; and into horizontal and vertical installation manholes based on installation position. Manhole cover fixing methods include revolving covers and hinged covers; opening speeds include standard and quick-open styles. Manhole structures and dimensions are generally standardized, allowing for selection or design according to relevant manuals.