We know of no simply stated rule. Most API 5K flanges fall into the category API refers to as 6B flanges, these flanges seal with R or RX type ring gaskets. 6B weld neck flanges, through 11" nominal size, have their weld necks specified in API Spec 6A, and these weld necks correspond to standard pipe sizes and schedules. Keep in mind that 6B standard weld neck flanges always have restricted bores compared to the full bores specified for equipment with the same size integral flanges.

When API introduced the current 10M, 15M, and 20M flanges that seal with BX ring gaskets, API referred to them as 6BX flanges. These API 6BX weld neck flanges all have full bores. API also specifies the O.D. of the weld neck based on allowable stress levels of the flange material. Weld necks on 6BX weld neck flanges do not correspond to any standard pipe size except by coincidence.

Originally API specified weld neck "transition pieces" to bridge the mismatch between the flange weld neck and available commercial pipe, but API no longer specifies this component.

To see API Flange Weld Neck Dimensions and suggested pipe or tubular material for attachment by welding, Click Here.

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