What does Class and Sch mean in ASME Standard

Class PN Class PN
Class150 PN20 Class900 PN150
Class300 PN50 Class1500 PN260
Class600 PN110 Class2500 PN420

The meaning of Class rating of B16 series flange: Class rating is named according to the allowable stress (8750PSI) of A105,WCB 1.1 group material under 454°C and pressure rating based on 60.3MP:

  • P1=(10S/8750)*Pr≤Pc

P1 – pressure rating of flange at corresponding temperature, bar.
S – allowable stress value of flange material, MPa.
Pr – flange Class value.

Pc – maximum pressure rating Bar.

For a flange of material 150# with allowable stress of 100MPa, it is available (10*100/8750)*150=17Bar=1.7MPa. It is equivalent to the allowable stress of 150# flange, whose allowable working pressure is 1.7%.

Criteria for Determining Bolt area of B16 series flange:
For flange or valve body connection:

  • AB *7000Psi≥Ag*Pc-> AB *120MPa≥Ag*Pg

Ab – cross sectional area of bolt
Area of Ag gasket contact surface inside and outside diameter (including bearing area in gasket inner diameter).
Pc – Class grade of flange
The bearing capacity of the bolt shall not be less than the inference that the internal pressure is within the outer diameter of the gasket sealing surface when the allowable stress is equal to 120MPa. For flange connection, a193b7 bolt with yield strength of 105ksi (724mpa) is generally used, and the safety factor of bolt design is ns ≥ 6. Therefore, the bolt area of ASME B16.5 flange is larger than that of ASME VIII I non-standard flange. Due to the large amount of bolt surplus, the flange strength is often unqualified when calculated according to the formula. But in fact, the flange is safe, which is why the specification stipulates that the flange selected according to the flange standard of B16.5 can not be calculated according to the formula.

The meaning of Sch: Sch is an abbreviation of schedule umpers for pipe size. It means that the ratio of piping system design pressure to material allowable stress at design temperature is multiplied by 100 0 and the value after rounding. For example, if the design pressure is 1MPa and the allowable pressure of the pipe is 100Mpa, the pipe with 1/100 100 0=10 and> Sch.10 should be used. For general pipes, the allowable stress of general materials is about 100Mpa, Sch.80 can bear 8MPa,Sch.160 can bear 16MPa, and so on, there is a general reference for choosing the appropriate Sch.

Post time: Apr-09-2021