Stem back-seat sealing provides a secondary metal-to-metal seal when the valve is fully open, allowing safe replacement of stem packing under pressure. The back-seat bushing is typically Inconel 625 with a mirror finish of 4 µin Ra or better. API 6A requires the back-seat to be tested at 1.1 times rated working pressure during factory acceptance testing to ensure integrity before fiel... https://globalsealstech.com/product-category/oil-gas-seals/api-6a-gate-valve-seals/