O-rings FAQ
How do you select elastomer grade for RGD, sour service, methanol and high-temperature O-ring duty (FKM vs HNBR vs FEPM/Aflas vs FFKM)?
Match polymer family to the controlling duty: RGD-rated compounds where rapid depressurisation is possible; sour-compatible chemistry where H₂S is present; methanol and high-temperature limits from the named compound datasheet. James Walker’s hub portfolio is framed around FKM, FFKM, HNBR and EPDM, with specialist oilfield ranges that include low-temperature and RGD-resistant grades across more than 100 elastomer grades. RGD-resistant specialist compounds on the RGD introduction include HNBR, FKM, FFKM and Aflas. FEPM (Aflas) for oilfield duty sits alongside FKM and HNBR in James Walker oilfield material framing.
RGD resistance is a tested compound property (specialist HNBR, FKM, FFKM and Aflas compounds can be optimised and scored for RGD). Sour gas introduces both RGD risk and potential chemical attack, so H₂S exposure must be assessed separately from decompression behaviour. Confirm continuous temperature, low-temperature flexibility, swell and any RGD test rating with JW applications engineering for the named fluid package.
When does RGD testing matter for O-ring specification, and what drives RGD risk?
Specify RGD-tested compounds wherever seals see absorbed gas at elevated pressure and can depressurise quickly enough to trap expanding gas in the elastomer (valves, wellheads, trees, manifolds, tubing hangers and related upstream equipment). Risk rises with larger cross-section, higher temperature, higher pressure, CO₂/H₂S, faster decompression and repeated cycles.
Absorbed gas equalises with system pressure; a fast depressurisation can blister or crack the seal. Risk becomes substantial above about 50 bar (725 psi), and decompression risk rises substantially above about 10 bar/minute versus about 1 bar/minute. Groove fill and interference can reduce risk when housing geometry is optimised. A perfect RGD test score is reported as 0000 under the stated test conditions. Confirm grade-specific ratings from the compound datasheet.
How do you specify elastomer compatibility for hydrocarbon fuels and aromatics?
Treat fuel type, aromatics content, any oxygenate/methanol blend, temperature and seal design as selection inputs. Specify polymer family and compound against chemical attack (irreversible polymer change) and physical effects such as swell, then confirm with JW media guidance for the named fluid. Fluoroelastomer (FKM) and other high-performance grades are the usual starting families for hydrocarbon service on the hub portfolio.
Chemical attack alters the polymer chain irreversibly; physical effects include swell and, under gas pressure, RGD. Aromatic content and blend composition change performance within a family, so generic fuel-resistant claims are not a valid specification method. Request compound-level compatibility for the fuel slate and duty temperature.
How should compression-set performance be specified for long-term O-ring sealability under SIP and CIP bioprocess duty?
Specify a low-compression-set compound validated for steam and CIP chemistry, not hardness alone. For James Walker high-purity EPDM, Elast-O-Pure EP75B is formulated for very low compression set and dimensional stability under SIP and CIP exposure, supporting long-term sealability in hygienic clamp and O-ring duties.
EP75B is USP Class VI, ADI-free and low-extractables, with compatibility across CIP acids and alkalis and excellent steam resistance for SIP. Confirm housing fill and interference with applications engineering for the named duty.
How should high-purity USP Class VI O-rings be specified for biopharmaceutical fluid-transfer and hygienic clamp duties?
Specify Elast-O-Pure compounds with the required purity package: EP75B (USP Class VI EPDM) and Sil70 (platinum-cured USP Class VI VMQ silicone), including related tri-clamp / hygienic clamp seals, with ADI-free and extractables documentation as required for product-contact use.
EP75B is independently tested to USP Class VI and compounded for extremely low extractables. Sil70 is platinum-cured translucent silicone with low compression set and high-temperature capability. Typical contact applications include process connections, bioreactors and filtration. Require the purity certificates needed for the regulated duty when ordering.
At what system pressures should back-up (anti-extrusion) rings be specified with O-rings?
Specify back-up rings when system pressure exceeds 10 MPa (1450 psi), when a low-strength elastomer is used, or when adverse mechanical conditions exist. Consult JW Technical Support if system pressure exceeds 42 MPa (6092 psi). James Walker supplies Fluolion PTFE (virgin or filled) and PEEK back-up rings.
For double-acting duties, fit one back-up ring either side of the O-ring. Virgin PTFE is typically a two-turn spiral; PEEK is usually endless single-turn. Back-up rings below 3 mm ID are available only as single turn.
How should O-ring ID and cross-section be specified against groove standards (metric vs inch)?
Specify inside diameter and cross-section (section diameter) to the sizing standard that governs the housing (metric or inch), then design groove dimensions and tolerances to that standard. James Walker’s O-ring product guide covers material/size selection, housing design and tolerances, and metric and inch sizing charts for international, national and industry standards.
ID and section must match the same standard family as the groove. Non-standard diameters or sections can be manufactured to order, including mould-joined extruded profile for large diameters. Confirm the exact chart designation for your drawing from the product guide at order time.
How should continuous and low-temperature limits be specified for O-ring compounds?
Do not assume a single temperature envelope for all James Walker O-rings. Specify continuous operating temperature and low-temperature flexibility limits from the named compound datasheet, alongside media and pressure. The hub emphasises materials for temperature extremes, including low-temperature and RGD-resistant grade ranges within the 100+ elastomer library.
Thermal capability is compound-specific. Elast-O-Pure EP75B is described with an exceptionally wide temperature range on its material page, without a hub-level min/max for every compound. Always specify temperature with media and pressure when requesting a material recommendation.
How are non-standard O-ring sizes and custom profiles (including D seals) covered when stock sizes do not match the housing?
Popular sizes and materials are held for same-day despatch. Non-stocked and bespoke parts, including custom profiles such as D seals, are moulded from a large mould inventory; lean manufacture can often precision-make parts quickly when schedules are urgent.
The hub stocks many thousands of types and sizes in popular materials. Related biopharma supply framing cites over one million O-rings in inventory and more than 10,000 moulds. Ask your local James Walker office for lead time on a named size and material.
