API 6A standards mandate seals for gate valves in wellheads, specifying materials for sour service (H2S >50 ppm). Energized metal seals backup elastomers. Low-temp FKM for arctic fields. Pressure tests to 1.5x rating verify integrity. Delta rings seal stems dynamically. NACE MR0175 certifies hardness https://globalsealstech.com/hydraulic-seals/