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 into atmospheric and pressure manholes based on pressure conditions; into circular, 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. The methods of fixing the manhole cover include swing covers and lifting covers. The opening speed of manholes can be either standard or quick-open. The manhole structures and dimensions are generally standardized, allowing for selection or design according to relevant manuals.