Selection of tower section screws for vacuum distillation towers
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I now want to install two sets of distillation towers; the tower sections will be made of 304 stainless steel, and metal gaskets will be used. There are some difficulties in selecting tower section screws: some people say that screws of grade 4.8 will suffice. However, some people insist on using 8.8 grade screws, on the grounds that the screw grade must match that of the metal gasket. I can’t figure out what the insight is here. I would like to ask for your guidance. . . .
Use 8.8 grade screws, as the screw grade needs to match that of the metal gasket
Grade 4.8 and grade 8.8 refer to the performance levels of bolts, and they are not related to the material of the bolts. Grade 4.8 is defined as follows: the number 4 represents 4*100=400 MPa, indicating that the bolt’s tensile strength is 400 MPa; the digit 8 denotes a yield ratio of 0.8, resulting in a nominal yield strength of 400*0.8=320 MPa. For grade 8.8 bolts, the tensile strength is 800 MPa, while the yield strength is 800*0.8=640 MPa. Regardless of the material used, as long as its properties meet these requirements, it is acceptable. Generally, bolts of grade 4.8 are galvanized, which gives them a shiny silver appearance, whereas grade 8.8 bolts undergo blueing treatment (also known as oxidation treatment), giving their surfaces a black color. Usually, the designation “4.8” or “8.8” is marked on the hexagonal head of the bolts; however, bolts produced by some smaller manufacturers may not have such markings.