I would like to ask if the two pieces of flange sealing surface is not parallel, how to repair

First, the use of a thin plate with the same size of the sealing surface of the ring, and then use teflon rope (diameter 3-6) to cover the ring.
The coating process must be careful. It is best to wrap 2 layers. Then in the angle of the targeted multi winding (density slightly larger). This pad can be installed in the flange to make up for the trouble. But suitable for low-pressure pipe (gas). Hope to help you

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