How to understand the truncation and isolation of DBB and DIB valve seats?
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API 6D mentions DDB (double block and bleed - double cutoff and bleed) and DIB (double isolation and bleed - double isolation and bleed) valve seats. I don't understand the difference between "cutoff" and "isolation". I hope everyone will speak out.
The problem is this. It’s clear from the picture, the picture comes from API 6D Cutoff means that the pipelines before and after BB are pressurized. If 2B is not sealed, there may be reverse flow, so it is called cutoff and isolation. It means that the pipelines before 1B are pressurized, and the pipelines after 2B are empty and unpressurized, so it is called isolation. I will give you a third diagram as shown in the figure. The first circuit is DBB, which is cutoff. , because the pipeline behind 2B is actually filled with medium. If you remove the other three and leave only the first loop, it is DIB, because the pipeline behind 2B is empty (it is not easy for a layman to draw a picture. Please do not use it for other purposes. I am just a valve salesman. . . . )