Manhole, column

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. 

Two maintenance workers had just removed the cover from a manhole near the top of a distillation column, which had been... 

Techniques have been developed to permit installation through a manhole. However, it is preferred and usually less costly to provide full shell flanges on either new or existing columns. 

If the liquid deposits a sediment, the plate column is more advisable. By fitting the column with manholes, the plate column can be cleaned of... 

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. 

Often, 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. 

To enable access from the bottom sump (where the manhole is often located) to the bottom tray, rungs are often installed at the column shell. Internal ladders are also sometimes used for the above purpose, but these may corrode and become unsafe. Corrosion-resistant materials are recommended if ladders are to be used. 

Care should be taken to ensure the analysis is correct. The sample should be taken from inside the column (e.g., using a sampling tube) and not from nesir the manhole. The testing should be repeated at reguleir intervals to ensure that a safe atmosphere is being maintained inside the column. The analytical instruments must be well maintained and frequently tested. 

A proper communication system must be devised. A three-way system with a connection to the ground, tower manhole, and work area inside the tower is considered most efficient (274). A person must never be inside the column unless another person stands by at the manhole. The standby person must always be aware of the exact location of the person inside, and be trained in emergency procedures. 

The risk of accident due to dropping nuts, bolts, or tools must be minimized. This is most important when two or more crews are working in the column at the same time at different levels. Loose articles in the vicinity of the upper manholes must be eliminated. Ropes and cables going into the tower must be secured in place. Guideline 10 in Sec. 10.5 elaborates on the preferred practices when more than one crew works inside the column. 

Whenever entry is not authorized (e.g., before column atmosphere is tested), the manholes should be blocked with barriers to ward off unauthorized entry (210). 

In large-diameter columns, when shutdown time is critical, and if manhole location permits, two tray installation crews are often used simultaneously inside the column. This practice suffers from the hazards of falling objects and/or poor ventilation and should be avoided whenever possible. If it has to be used, precautions are required for minimizing the risks. 

Usually, a wooden bulkhead is installed just below the middle manhole and securely wedged in position. The first tray above the manhole is then installed. If no manhole is available between the middle and bottom manholes, half the bulkhead is then removed to enable ventilation and passage of tray pieces to the section below. Trays are then installed from the bottom up by one crew and from the middle up by another crew in the usual manner. If an intermediate manhole is available between the middle and bottom manholes, the entire bulkhead is best left in place, and tray pieces passed to the section below through the intermediate manhole. Having the entire bulkhead in place improves personnel protection from falling objects, but requires separate ventilation for each column half. 

Safety considerations permitting, manhole doors should be shut immediately upon completion of column inspection and should only be kept open from then on while work (e.g., tray reassembly) is being performed inside. This is important in order to keep r2un, sand, dust, and animals out of the column. There have been instances (e.g., 238) where animal carcasses lodged in delicate internals and caused premature flooding. 

---