What do Sch and Class mean in ASME?

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The meaning of Sch: Sch is an abbreviation for Schedule numbers for pipe size. It represents the ratio of the pipe’s design pressure to the allowable stress of the material at the design temperature, multiplied by 1000, and then rounded off to an integer. For example, if the design pressure is 1 MPa and the allowable stress of the material is 100 MPa, then 1/100*1000 = 10; therefore, a pipe with a value greater than Sch.10 should be used. For ordinary pipes, this can be used as a guide: the allowable stress for common materials is around 100 Mpa; Sch.80 can withstand 8 Mpa, while Sch.160 can withstand 16 Mpa, and so on. This provides a rough reference for choosing the appropriate grade. Meaning of the Class rating for B16 series flanges: The Class rating is determined based on the pressure rating at 454°C for materials in Group 1.1 of A105 and WCB, whose allowable stress is 8750 PSI, which is equivalent to 60.3 MPa. The formula is: P1 = (10S/8750) * Pr ≤ Pc. Here, P1 represents the pressure rating of the flange at the corresponding temperature; S is the allowable stress value of the flange material in MPa; Pr is the Class rating of the flange; and Pc is the maximum pressure rating in bars. For a material with an allowable stress of 100 MPa, a 150# flange can have a pressure rating of approximately (10*100/8750)*150 = 17 bars, which is equal to 1.7 MPa. The allowable working pressure equivalent to that of a 150# flange is 1.7% of the material’s allowable stress. Criteria for determining the bolt area of B16 series flanges: For flange or valve body connections: Ab*7000Psi≥Ag*Pc -> Ab*120MPa≥Ag*Pg. Ab is the cross-sectional area of the bolt; Ag is the area within the inner and outer diameters of the gasket contact surface (including the pressure-bearing area within the gasket’s inner diameter); Pc is the class rating of the flange. The bearing capacity of the bolt should be no less than the allowable stress, which is 120 MPa, under internal pressures acting within the outer diameter range of the gasket seal surface. For flange connections that typically use A193B7 bolts with a yield strength of 105 Ksi (724 MPa), the safety factor for bolt design is ns ≥ 6. Therefore, the bolt area required for flanges per ASME B16.5 provides an even greater margin compared to the bolt area needed when designing custom flanges according to ASME VIII Div. I. Due to the large excess length of the bolts, calculating using the formula often results in flange strength that does not meet the requirements. But in reality, the flanges are safe; this is why the codes specify that flanges selected according to the B16.5 flange standard do not require calculation using formulas.