Inspection openings are required by the Code for all pressure vessels containing process environments that cause corrosion, erosion, or mechanical abrasion. Many pressure vessels will require access manways and inspection openings which are larger and more numerous than required by the Code. Vessels which contain internals usually require maintenance of the internals during the life of the unit. The inspection openings in the vessel should be designed to permit reasonable entry for personnel, welding equipment, and internals components. The Code requires a 15-inch manway in vessels having an inside diameter over 36 inches. The following guidelines may be followed in most cases:
1. For vessels 12-inch nominal diameter and smaller, means of inspection are often omitted.
2. Vessels from 12-inch up to 18-inch nominal diameter should have two inspection openings of 2½-inch minimum diameter. (The Code requires two 1½-inch size openings.)
3. Vessels from 18-inch through 36-inch nominal diameter should have two flanged 4-inch openings or a manhole. (The Code only requires two 2-inch openings.)
4. If replacement of internals is necessary, one end of the vessel should have a full diameter flange for vessel sizes through 24-inch diameter. From 24-inch through 36-inch vessel diameter, one of the following alternatives can be considered:
– A conical section and a 20-inch diameter flanged end (generally the least expensive and most satisfactory alternative)
– Full diameter flanged end
– A shell or head manway
5. Vessels larger than 36 inches in diameter:
All should have at least one manway. Recommended minimum diameter is 18 inches, although the Code allows a 15-inch diameter.
In relatively large vessels, two manways are frequently provided to facilitate
maintenance and to improve ventilation.
6. The following practices assume that access through internal tray manways is not restricted by appurtenances or by staggering of the tray manways:
Columns having 10 trays or less should be equipped with a manway below the bottom tray and above the top tray.
Columns having more than 10 trays but less than 30 trays should generally be equipped with a manway below the bottom tray, above the top tray, and near the middle of the column, preferably near the feed tray.
Columns of more than 30 trays present access problems which must be given special review. Manways should be provided below the bottom tray and above the top tray. Intermediate manways should, in general, be placed no further than 30 feet (about 15 trays) apart and located as close as economical to points of high corrosion, such as feed trays. If possible, project or plant design specifications should establish manway locations for each column.
You should note the relation between limitations on maximum ladder height without a break (30 feet), and the normal limit for number of trays between manholes, i.e., 15, and seek the economic balance between requirements for safety and requirements for maintenance. For example, in columns having between 10 and 15 trays, it may be possible to arrange the ladders and platforms so that a platform is not needed except at top and bottom. If this is the case, the cost of an intermediate platform to serve a “mid-point” or “feed tray” manway should be added to the cost of the manway when evaluating economic justification for the “third” manway. If two columns are served by common ladders and platforms, it may be possible to secure a more desirable location of manways and comply with the ladder length limitation if the number of trays between manways exceeds 15. The normal 15-tray limitation should not be exceeded without thorough considerations, but when better over-all design would result, it may be increased to a maximum of 20 trays if ladders can be arranged to meet the safety standard. Thus, in the final design of a column, the problem of manway location is one of balancing the 30-foot ladder limit with desired manway locations at points of high corrosion.
7. Normally, manways should be at least 18-inch nominal diameter. This diameter is sufficient to accommodate routine inspection and equipment for minor maintenance. However, if internal materials, equipment, and appurtenances, such as refractory lining, etc., require extensive maintenance or internal staging, a 24-inch manway should be provided to facilitate access of personnel and equipment.
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