Dear Mr. Sanjay
Thank you very much for your PO, you will feel happy with SAFI, I'm sure of that!
To answer your question, The encapsulated metal insert is to help flange part to resist more mechanical strength.
As they were inserted,They have no contact point with acid.
With SAFI valve, Size from 3" above , they will be metel insert like this.
Please find the attached for our order acknowledgement
Best regards