A manhole refers to an opening structure for personnel to enter and exit equipment for installation, maintenance, and safety inspections. It is mainly composed of short筒节 (or short pipe), flanges, and handles on the manhole cover. From the perspective of safety inspections, pressure vessels require a certain number of manholes and their small dimensions, which can be found in "Inspection Holes." Manhole materials include carbon steel and stainless steel. Manholes are classified into atmospheric pressure and pressure manholes based on pressure conditions; into circular, 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 fixed methods of the manhole cover include rotating covers and hinged covers. The opening speed of the manhole can be ordinary or quick-opening. The manhole structure and dimensions are generally standardized and can be selected or designed according to relevant manuals.