The diameter of the hand hole is generally 150mm or 250mm, and its structure is generally similar to the manhole. When the diameter of the device exceeds 900mm, it should open manhole. There are two types of manholes: round and elliptical. The short axis of the elliptical manhole should be parallel to the axis of the cylinder of the pressure vessel. The diameter of the circular manhole is generally 400mm. When the pressure of the container is not high or there are special needs, the diameter can be larger. The nominal diameter of the circular standard manhole is 400mm, 450mm, 500mm and 600mm.

　　Types of manholes

　　Depending on whether it is under pressure, there are two types of manholes: normal pressure and pressurizing.

　　According to the type of flange, the pressure manhole has plate flat welded flange manhole, neck welded flange manhole and neck welded flange manhole.

　　According to the opening method of the manhole and the location of the manhole cover after opening, the manhole is divided into a manhole with a rotary cover, a manhole with a vertical cover, and a manhole with a horizontal cover.

　　Manholes and hand holes are both components with a large amount of using. In order to reduce the workload of designers, there are standards. When using, the size of the pressure piece can be obtained according to the relevant standard of nominal pressure.

　　Pressure vessels that meet one of the following conditions may not have inspection holes

　　(1) Pressure vessel with cylinder Di≤300mm

　　(2) The container is provided with a removable head, cover plate or other cover that can be opened and closed, and their size is not less than the standard

　　(3) Pressure vessels without corrosion or slight corrosion that do not require internal inspection and cleaning

　　(4) Pressure vessel for refrigeration equipment

　　(5) Heat Exchanger

　　If it does not belong to the above five situations, but the inspection hole cannot be opened due to some special reasons, the following measures should be taken:

　　(A) 100% non-destructive testing of all longitudinal and circumferential welds of the container

　　(B) Indicate the calculated thickness on the design drawing, and focus on the thickness measurement inspection during the use or inspection of the pressure vessel

　　(C) Shorten the inspection period.