Shell manholes

Item 4. Dimensions in Inches for 20-in. Shell Manhole, Recommended by API Standard 12 C See Item 3 and Fig. 3.15... 

Shell manhole. The shell manhole shall be circular, 24 inches in diameter, and shall include an extended neck for ease in cover removal and replacement. If the manhole cover weighs more than 50 lb, a cover hinge shall be provided. 

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. 

The purchased cost for plate and packed towers can be divided into the following components (1) cost for shell, including heads, skirts, manholes, and nozzles (2) cost for internals, including trays and accessories, packing, supports, and distributor plates and (3) cost for auxiliaries, such as platforms, ladders, handrails, and insulation. 

The cost for fabricated tower shells is quite often estimated on the basis of weight. Figure 16-24 indicates the approximate cost for fabricated steel towers based on weight. This cost is for the tower shell only, without trays, packing, or connections. The cost for installed connections, such as manholes and nozzles, may be approximated from the data in Fig. 16-25. 

Construction All welded mild steel with flanged and dished heads. Head thickness in., shell thickness 3- in., internal lining in., acid-resistant rubber Connections Manhole 18 in., skimmer 6 in., drains 3 in. (2 required), gauge connections 13- in. 

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. 

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. 

The f.o.b. purchase cost, at a CE index = 394, of the empty vessel, Cy, but including nozzles, manholes, and supports, based on the weight in pounds of the shell and the two heads, W, depends on orientation, as shown in Figure 16.13. The correlating equations are... 

Under conditions of high temperature and in the presence of oxygen, an exothermic reaction can occur, leading to the risk of fire or explosion of the tanks. Flence, manholes in bitumen tanks should be kept closed and access to tank roofs should be restricted (Shell Bitumen 2003). 

---