Oil-Filled Equipment & Chemical Sealing

Rubberized cork (BC351-207, or the technical DC100-312/314 and SP52 grades for heavier seating loads) is the industry staple: the cork phase gives controlled compressibility and recovery per ASTM F36, the elastomer binder carries the oil resistance, and the TDS reports fluid aging per ASTM F146 in ASTM reference oil.

Where a homogeneous elastomer is preferred, specify the transformer-oil-grade NBR, and for heavy bolted covers use cloth-inserted nitrile so the fabric plies resist extrusion. Frame the fluid, temperature, and flange detail on the drawing. [2]

Choose by flange mechanics. Cork-elastomer sheet compresses and recovers at modest bolt loads, follows flange waviness, and holds its squeeze through thermal breathing, which is why it owns distribution-class covers; its compressibility and recovery are reported per ASTM F36 on the TDS. Solid NBR seals by elastic deformation and wants flatter flanges and higher seating stress, but it is homogeneous (no granule structure) and available in specific duty grades (transformer-oil, bio-diesel, AMS 3215).

On large or heavily bolted covers, cloth-inserted nitrile adds fabric reinforcement against extrusion and creep. If you can state bolt pattern and available seating stress, engineering will match the construction to it.

When the gasket runs hot. HNBR's hydrogenated (saturated) backbone gives measurably better heat and oil aging than standard NBR, which is the whole reason the grade exists; H-O carries 60, 70, and 80 durometer with ASTM D2000 line callouts on the TDS. If the joint sees sustained elevated temperature near transformers or hot gear oil, and standard NBR coupons come back hardened from heat-aging review per ASTM D573, HNBR is the next step before committing to FKM pricing. [6]

Yes. Bio-diesel blends are chemically different from petroleum diesel (methyl-ester chemistry), and compounds tuned only for petroleum can swell and soften in them; that immersion behavior is exactly what ASTM D471 testing reveals fluid by fluid. H-O's nitrile line carries a dedicated bio-diesel grade for this duty, alongside the transformer-oil grade and the AMS 3215 fuel-resistant grade. Put the blend percentage (B20, B100) on the drawing so the grade review happens against the right fluid. [1]

Two families: Gore® GR ePTFE gasket sheet (Style 300/800), which is chemically inert expanded PTFE and the corrosive-flange default, and FFKM perfluoroelastomer, which adds elastomeric recovery at near-universal chemical compatibility (general-purpose, steam, USP Class VI, and AMS7257 grades). Between the oil families and these sits FKM/Viton®, which covers most hot, aggressive fluids short of the extreme cases.

Name the chemical, concentration, and temperature; the review is done against the maker's compatibility data for the specific grade. [16]

When the fluid list defeats FKM: hot steam (FFKM's steam grade is compounded for it), strong oxidizers and concentrated process chemistry, or a flange whose fluid exposure is broad and changing. FFKM's fully fluorinated backbone is the most chemically inert elastomer chemistry available, so the honest pattern is surgical use: one FFKM gasket at the aggressive interface, FKM or nitrile everywhere else on the same equipment. If the joint does not need elastomeric recovery, inert ePTFE sheet is often the more economical extreme-chemistry answer.

ASTM D471, "Rubber Property — Effect of Liquids," is the fluid-aging method: elastomer coupons are immersed in a service fluid or ASTM reference oil/fuel at a defined temperature and time, then measured for volume swell, hardness change, and tensile change. It is the test behind every honest fluid-compatibility claim, which is why the compatibility matrix on this page is framed as qualitative classes grounded in TDS immersion data rather than a green-light chart. For cork-composite gasket sheet, the parallel method is ASTM F146. [1]

Specify the 576 NSF-certified EPDM by name. NSF/ANSI 61 is the drinking-water system components standard (the health-effects listing for materials in contact with potable water), and NSF/ANSI 372 covers lead content; the 576 grade carries the NSF designation, with FDA 21 CFR 177.2600 also cited on its TDS. A property-equivalent EPDM without the listing does not satisfy the drawing note, because the designation, not the durometer, is what the inspector verifies. [13]

Almost always one of three slow mechanisms: fluid attack (swell or extraction the day-one test cannot see, revealed by ASTM D471 immersion data), heat aging (hardening per ASTM D573 that costs the gasket its recovery), or sealing-stress decay (bolt relaxation plus compression set walking the squeeze below the leak threshold).

The fix is at spec: match the family to the actual fluid and temperature, pick a construction whose recovery suits the flange (cork-elastomer or sponge where bolt load is modest), and state compressed thickness on the drawing.

Send the failed gasket photo with the fluid name and engineering will usually identify the mechanism from the appearance.

When the joint is big, heavily bolted, or long-lived enough that gasket extrusion and creep become the failure mode. The woven plies in CI nitrile (1-ply at 1/8″, 2-ply at 1/4″) carry bolt load and hold the gasket's dimensions on large covers where plain elastomer would slowly squeeze out of the flange. The elastomer phase is still NBR, so the fluid boundaries are unchanged: oils and fuels yes, weather and aggressive chemistry no.

H-O die-cuts and converts. We take sheet and roll stock from the material makers (cork-elastomer, nitrile, HNBR, FKM, FFKM, ePTFE, foams, NSF EPDM) and die-cut, waterjet-cut, kiss-cut, slit, and laminate it to your drawing; we do not mold or extrude elastomers in-house. Molded or extruded profiles, where a design needs them, are coordinated through our partner network.

What we do in Winsted, Connecticut is precision conversion with material traceability and lot-code TDS records, from prototype counts through production, as an ISO 9001:2015 certified organization.

Usually, yes. Legacy transformer prints often call out cork or nitrile by a long-retired brand or house code. Send the print through the form: engineering matches on the properties that drive the joint (construction, durometer or compressibility class, fluid duty, and any designation like AMS or NSF) and proposes the closest current grade with its TDS. It is an engineering cross-reference, not a guaranteed drop-in: where the legacy callout is ambiguous, we will ask for the fluid and flange detail rather than guess.

Six things: (1) the fluid by name, with concentration or blend percentage; (2) continuous temperature at the gasket; (3) flange detail: bolt pattern, available seating stress, compressed thickness; (4) any compliance designation verbatim (NSF/ANSI 61, FDA 21 CFR 177.2600, AMS 3215/3216, AMS7257, MIL-R-83248); (5) quantity for prototype and production; (6) the material family if you have one, or "recommend" if you don't. With those, the engineering review runs against the TDS data instead of a follow-up email chain.