A manhole refers to an opening structure designed for personnel entry and exit for installation, maintenance, and safety inspections. It primarily consists of short pipe sections (or short pipes), flanges, and handles on the manhole cover. Considering safety inspections, pressure vessels require a specific number and size of manholes, which can be found in the "Inspection Holes." Manhole materials include carbon steel and stainless steel. Manholes are categorized by pressure conditions into atmospheric and pressure manholes; by shape into circular, elliptical, and rectangular manholes, with rectangular manholes suitable only for atmospheric pressure equipment; and by installation position into horizontal and vertical installation manholes. Manhole cover fixing methods include swinging covers and lifting covers. Manhole opening speed options include standard and quick-opening types. Manhole structures and dimensions are generally standardized, allowing selection from relevant manuals or custom design.