Elastomer O-rings in API 6A valve bonnets use 90-durometer NORSOK-approved HNBR. Compounds like Parker V8985-90 and Technoflon PL958 survive 15% H2S and 225°C continuous. Helium mass spectrometry testing confirms leak rates below 10⁻⁸ atm·cc/s across full temperature range. https://globalsealstech.com/