1. Protruding Manhole Covers
In my country's shipbuilding industry, protruding manhole covers are commonly classified into three types based on their structure: Type A, Type B, and Type C.
Type A manhole covers have welded enclosures and seat rings, protruding 100 mm above the opening plane. Common specifications are based on the through-hole size (L×B): 450 mm×350 mm, 500 mm×400 mm, 600 mm×400 mm, and 600 mm×450 mm. Enclosure thickness (S) ranges from 4 to 14 mm. The seat ring and cover plate thickness (S) should be 2 mm greater than the enclosure thickness.
Type A manhole covers have a higher enclosure, preventing sewage or garbage from entering the hold. They are commonly used in engine rooms, boiler rooms, and other areas prone to water accumulation. Type A manhole covers can also be installed on curved surfaces, such as cylindrical crane bases or spherical structures.
Type B manhole covers are oblong protruding manhole covers. The seat ring thickness is 20 mm, and its commonly used specifications and cover thickness are the same as manhole covers. It is commonly used on decks, inner floor plates, platforms, and bulkheads. When used in cargo hold bilges, a steel frame is added around the manhole cover, followed by a steel or wooden cover plate, gradually transitioning to flush with the wooden planks.
Type C manhole covers are round, protruding manhole covers. The seat ring thickness is 20 mm, and commonly used specifications are Φ450 mm and Φ600 mm based on the through-hole size (D). The cover plate thickness (S) is 4–14 mm. These round, protruding manhole covers can be used on decks, inner floor plates, platforms, and containers.
2. Recessed Manhole Covers
Recessed manhole covers commonly used in my country's shipbuilding industry can be divided into two types according to their structure: Type D and Type E.
Type D manhole covers have a cover plate, and common specifications based on the through-hole size (L×B) are 530 mm×430 mm, 630 mm×430 mm, and 630 mm×480 mm. The cover plate thickness (S) is 10 mm and 12 mm.
Type E manhole covers have the same specifications as Type D, but their cover plate thickness (S) is 14 mm and 18 mm.
Both Type D and Type E manhole covers can be used in areas requiring a flat cargo hold bottom and deck passageways for cargo loading/unloading and personnel movement.
