Now I have a problem of welding end valveI donot how to choose the type of SCH No.only have ASME B16.25,ASME B16.34 and ASME B36.10MIs that anyone tell me how to choose,when the valverating is 150LB,300LB,600LB,900LB,1500LB,2500LB.Someone will have determined the "Design Pressure" for the piping system. A Class rating is selected to ensure that the flanges and valves have a pressure rating greater than the design pressure. Using the design code (e.g., B31.3) the required wall thickness is calculated (either using the design pressure or the pressure rating of the flanges/valve class that has been selected).Now you would order a butt weld valve that was the Class you required and that had butt weld ends that were the same schedule as the pipe that had been selected.This would be the normal case when the steel used in the valves had the same allowable stress value (or greater) than the pipe. If the valves had a lower allowable stress value than the pipe then you would have a problem to sort out but this is very unusual. In this case, you might specify a thicker schedule for the valve ends and then you might need transition pieces to allow welding into the line.

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