"Sour" means a fluid containing hydrogen sulfide (H2S). In oil and gas production it usually also contains carbon dioxide, water and hydrocarbon liquids at high temperature and pressure. Elastomer seals face a combination of chemical attack, swelling and gas permeation that a simple air-oven test does not reproduce.
What goes wrong
- Chemical attack on the polymer. H2S and the amines used as corrosion inhibitors can attack some elastomers. Standard bisphenol-cured FKM is vulnerable to amines and can harden and crack. HNBR can harden in H2S at high temperature, as sulfur reacts with residual unsaturation in the polymer. The effect depends on the cure system and temperature, and is one reason sour-service HNBR grades are tested and not assumed. FEPM is known for good resistance to H2S and amines.
- Swelling and extraction. Hydrocarbons and aromatics swell the rubber, and extraction of plasticisers changes properties.
- Gas permeation and decompression damage. Dissolved CO2 and methane cause RGD damage on pressure release.
- Hydrolysis and steam effects. Water at temperature attacks some cure systems.
A common note: NACE MR0175 / ISO 15156 is for metals
ISO 15156 (NACE MR0175) sets requirements for metallic materials in H2S-containing environments. It does not qualify elastomers. When a specification asks for "NACE" seals, ask what the requirement actually is. It may be the elastomer ageing test in ISO 23936-2, NACE/AMPP TM0187 or TM0296, or an operator document.
How qualification works
Seal materials are aged in test cells filled with a defined medium at several temperatures above the service temperature, for fixed times. Properties are measured before and after: volume and mass, hardness, tensile strength, elongation and modulus, with visual inspection. The results can be extrapolated with the Arrhenius relationship to estimate life at service temperature (see ISO 23936-2, Annex D).
As summarised in supplier documents, a typical test medium is a three-phase mix: a gas phase containing a small percentage of H2S and CO2 in methane, a hydrocarbon liquid phase of heptane and cyclohexane (sometimes with toluene) and distilled water. The pressure, temperature and duration differ between standards and editions, so any claim must name the standard and conditions. The Sour service report on this site compares them, with differences flagged.
Acceptance limits
Supplier documents describe acceptance limits for sour ageing such as volume swell between a small negative value and about +25%, hardness change within about +10 and -20 points, tensile, elongation and modulus changes within about 50%, and no visible damage. These limits may be a manufacturer's own criteria and not text from the standard, so check the standard or the customer specification.
Practical advice
- Be clear about the fluid, temperature and pressure, and about amines and inhibitors.
- Choose a polymer and cure system on evidence. Peroxide cure is generally better than bisphenol in aggressive chemicals.
- Test the specific compound. A polymer family does not qualify a recipe.
Sources and further reading
- ISO 23936-2; NACE/AMPP TM0187 and TM0296.
- James Walker: testing of elastomers for use in sour environments.