Manholes for Column Access

Entry into the shell of a distillation tower is made via manholes. These are usually fitted in the column so that each serves 10 to 20 trays (48, 177, 354). When the service is clean and noncorrosive, up to 30 trays or more may he served hy one manhole. When frequent cleaning is anticipated, or if the trays are large and the process of removing them through the hole is slow, the smaller number above should be used. This enables multiple crews to work on removal or installation. If the column diameter is too small to admit personnel, cartridge trays (Sec. 7.13) should be used. 

In packed columns, manholes should be positioned so that distributors, redistributors, and other internals can be accessed. Usually, manholes are located above each bed support plate, above the top distributor, and at the bottom of the column. Locating manholes above the support plates also permits removal of packing from the bed above. 

tray spacings must be locally increased to be larger than the manhole diameter. For this reason, and whenever practicable, it is a good policy to install manholes in the space above the feed trays where the tray spacing is normally extended (for reasons given in Sec. 2.3). When this is done, care must be taken to ensure that feed nozzles and distributors do not impede entry by personnel into the column. One designer (48) recommends that tray spacing at the manhole be at least 36 in to provide adequate work space. 

Whenever possible, all manholes should be oriented in the same direction. It is also preferable that the manholes face the main accessway to the column. The aligned manholes will occupy a segment of the total tower circumference that should not be occupied by any pipe runs. This minimizes the difficulties in lowering tower internals to the ground (190). 

To minimize possible damage to column internals, care should be taken not to locate the manhole in the downcomer seal area. 

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