5.4　Skirts

5.4.1　The following factors shall be considered in material selection for skirt:

a)The ambient temperature in the region where column is constructed;

b)The design temperature for the column;

c)The effect to material performance of pressureparts after the skirt and the column shell(pressure parts)are welded,i.e.,the material of skirt shell shall present satisfactory weldability when the skirt is welded to column shell;

d)Safe and reliable operation,and cost effectiveness.

5.4.2　Unless otherwise specified in project design documents,the commonly used steels for skirt shells(excluding transition sections)are indicated in Table 6.

Table 6　Commonly used steels for skirt shells

5.4.3　Material of the skirt transition section shall be the same as that of the column shell(or column bottom head).

5.4.4　Design of skirt transition section shall comply with the following requirements:

a)The skirt shall be provided with a transition section for any one of the following cases(see Figure 1):

1)The design temperature for the column(or column bottom)is above 350℃,or below or at-20℃;

2)The skirt shell is welded to the column shell,possibly impairing the material performance of the column shell(head)(e.g.,the material of column bottom head is low temperature steel,Cr-Mo steel or stainless steel);

Figure 1　Structure of skirt

b)Material of the skirt transition section shall be the same as that of the column shell(or column bottom head);

c)The design temperature and allowable stress for the transition section shall be the same as those for the column shell(or column bottom head);

d)The length of transition section shall not be less than 300mm,however,when the design temperature for the column(or column bottom)is at or below-20℃,or above 340℃,the length of transition section shall be 4-6 times the thickness of external insulation and shall not be less than 500mm;

e)When the design temperature for the column bottom is ranged from 200℃to 340℃,the designer may decide whether a transition section shall be designed for the skirt considering techno-economic factors.

5.4.5　Material of the reinforcing components for the openings in skirt shell(e.g.,access pipe,access hole)should be the same as or similar to that of the skirt.