A manhole is an opening structure used for personnel to enter and exit equipment for installation, maintenance, and safety inspections. It is mainly composed of short筒sections (or short pipes), flanges, and manhole covers with handles. For safety inspection considerations, pressure vessels require the number and small dimensions of manholes to be opened, see "Inspection Ports." Manhole materials include carbon steel and stainless steel. Manholes are categorized into atmospheric and pressure manholes based on the pressure conditions; into round, elliptical, and rectangular manholes based on shape, with rectangular manholes suitable only for atmospheric pressure equipment; and into horizontal and vertical installation manholes based on installation position. The methods of fixing the manhole cover include revolving covers and lifting covers; the opening speed of the manhole can be ordinary or quick-opening. Manhole structures and dimensions are generally standardized, and can be selected or designed according to relevant manuals.