Oil & Gas Wellhead Sealing: Extending Seal Life from 3 Months to 18 Months

Operating environment: The wellheads handled a raw gas stream containing 15% H2S, 8% CO2, saturated water vapor, and C1–C4 hydrocarbons at 150°C and 300 bar. The O-ring glands were in static face-seal service but subject to pressure cycling of ±50 bar during well test operations. Ambient temperature at the surface installation ranged from −10°C in winter to +45°C in summer.

Original failure mode: Standard FKM (Type 1, 65–66% fluorine) seals showed a consistent failure pattern. Within 6–8 weeks, seals became noticeably harder on extraction. By 10–12 weeks, Shore A hardness had increased from the nominal 75 to over 90, and the seals no longer recovered their original cross-section after removal from the gland. Leakage began as micro-seepage visible at the flange face and progressed to detectable flow within days. Metallurgical and elastomer analysis confirmed the mechanism as thio-sulfurization: H2S reacts with residual cure-site monomers and low-fluorine polymer segments, causing additional crosslink formation and embrittlement.

Why standard FKM fails in H2S: FKM resistance to H2S is a function of fluorine content and cure system. Type 1 FKM (65–66% F) contains vinylidene fluoride (VF2) monomer, which is susceptible to H2S attack at temperatures above 120°C. The reaction introduces sulfur crosslinks, increases hardness, and raises the glass transition temperature — the same progression observed in this application. High-fluorine FKM Type 2 (GF-type, 70%+ F) replaces a portion of VF2 with perfluoromethyl vinyl ether (PMVE), which is not reactive with H2S, producing a compound with substantially better resistance to sour service at 150°C. This is consistent with NACE TM0187 and ISO 15156 guidance on elastomer selection for sour service.

Extrusion analysis: At 300 bar, the maximum safe diametral clearance gap for 75 Shore A FKM is approximately 0.10–0.12 mm. The original gland was machined to a nominal clearance of 0.15 mm, but manufacturing tolerance produced some assemblies with gaps up to 0.20 mm. Upgrading to 90 Shore A raised the safe clearance limit to approximately 0.18 mm, but we recommended adding PTFE spiral backup rings as an additional safeguard — both to handle tolerance variation in the gland and to account for any thermal expansion of the housing that could open the gap at operating temperature.

Backup ring selection: We evaluated both PEEK and PTFE spiral backup rings for this application. PEEK offers higher hardness and dimensional stability, but its chemical compatibility with H2S/H2O sour brine at 150°C required additional verification. PTFE spiral rings are chemically inert across the entire fluid spectrum present in this application and provided adequate extrusion resistance when paired with the 90 Shore A FKM primary seal. PTFE's low friction also simplified assembly and reduced the risk of gland scoring during ring insertion.

Validation testing: The upgraded seal assembly was tested per a protocol combining ASTM D395 Method B (compression set at 150°C/70 hours), NACE TM0187 sour gas immersion (15% H2S at 150°C for 168 hours), and a pressure cycling test (100 cycles from 0 to 310 bar). Post-test measurements confirmed compression set below 18% (versus >45% for the original compound), no visible surface cracking, and no dimensional change exceeding 2% — all within the OEM's acceptance criteria.

Procurement and supply details: The OEM required traceability to raw FKM compound batch and cure parameters for every production lot. We supplied a certificate of conformance, compound data sheet, and batch-specific test results (tensile, elongation, hardness, compression set) with each shipment. Standard lead time for this specification was 10 business days; an emergency stock program was negotiated for the three most common sizes (AS568-330, -334, -338) to support planned and unplanned maintenance.

Procurement takeaway: When specifying FKM for sour service above 120°C, require the supplier to confirm fluorine content (minimum 70% for Type 2 or GF-grade) and to provide compression set data at operating temperature per ASTM D395. Standard FKM datasheet temperature ratings do not account for H2S chemical attack — the material must be qualified by immersion test, not just temperature chart.