Changes in pipe flange grades
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The flange before the valve is B1C (300 pounds), while the valve and the flange after the valve are A21C (150 pounds). What grade and pressure class should be used for the flange before the valve?
If the pipeline before the valve is 300 pounds, both the valve and the flange before it must be 300 pounds grade. Looking at your legend, it doesn’t seem quite appropriate to change the grade for the check valve here. You can refer to the picture.