4.4　Additions to the Thickness

4.4.1　The additions to the thickness shall be determined by Equation(2).

where　C—additions to the thickness,mm;

C1—undertolerance of steel,mm;

C2—corrosion allowance,mm.

4.4.2　Undertolerance of steel plates or steel pipes,C1,shall comply with applicable steel standards.The undertolerances of commonly used steels are indicated in Appendix A.

4.4.3　The corrosion allowance,C2,shall be determined based on the following principles:

a)If corrosion allowance,C2,is otherwise specified in project design documents or is proven by operating experiences,it may be determined according to project design documents or the operating experiences;

b)For uniform corrosion,if reliable data is available,the corrosion allowance for columns shall be determined by Expression(3)based on the expected design life of columns and the corrosion rate of metals.The design life for columns refers to the expected service life at predefined corrosion allowance,and normally,it shall be indicated in the design drawings;

where　NF—expected design life,a;NF shall not exceed 20a for general distillation columns,and NF shall not exceed 30a for thick-wall or critical columns(e.g.,thick-wall hydro-reactors,converters,etc.);

dC2—annual corrosion rate,mm/a.

c)The corrosion allowances,C2,of cylindrical shell and head are indicated in Table 4;

Table 4　Corrosion allowances(C2)of cylindrical shell and head　(unit:mm)

d)The corrosion allowances,C2,of other components are indicated in Table 5.Where the components are exposed to operating fluid at both sides,the corrosion allowance for each side of the component shall be determined based on the operating fluid at that side,and the summation of corrosion allowance at both sides shall be assumed as the total corrosion allowance of this component.

Table 5　Corrosion allowances(C2)of other components