RGD testing ‑ assessing an elastomer’s resistance to rapid gas decompression

The industry standard for testing an elastomer’s RGD resistance is Norsok M710 rev 3, now also reproduced and identical in ISO23936-2 (note these standards have a separate section on determining sour ageing resistance of elastomer and plastic materials in addition to RGD assessment).
The test conditions are deliberately chosen to be challenging:

• O-ring cross section standard 5.33 mm.
• Test temperature 100°C, pressure 150 bar.
• Decompression rate; 20-40 bar/min.
• Gas composition, standard 90/10 methane/carbon dioxide
• Eight pressure cycles from 150 bar to ambient pressure with 1 hour dwell between cycles.

There is a rating system to assess any damage and its extent. Each O-ring is cut into four equal radial sections and visually examined under magnification of at least x10. The RGD damage at each section is rated between 0 and 5, according to the following criteria (see figures below):
0 rating: No internal cracks, holes or blisters of any size.
1 rating: Less than four internal cracks, each shorter than 50% of cross section, with a total crack length less than the cross section.
2 rating: Less than six internal cracks, each shorter than 50% of cross section, with a total crack length less than 2.5 times the cross section.

3 rating: Less than nine internal cracks, of which a maximum of two cracks can have a
length between 50% and 80% of the cross section.
4 rating: More than eight internal cracks, or one or more cracks longer than 80% of the
cross section.
5 rating: One or more cracks going through the cross section, or complete separation of
the seal into fragments.

Norsok stipulates that ratings of 4 and 5 are not acceptable (‘fail’), which means that ratings of 0, 1, 2 and 3, can be considered as acceptable (‘pass’) for RGD resistance. Each ring’s performance is defined by its rating on all four of the cut sections. Thus a 0000 rating defines a ring with a perfect ‘pass’ and no visible internal faults, whereas a 3434 rating would be defined as a ‘failure’. The highest numbers (ie, poorest performances) are taken for the three samples tested at the same time to give an overall rating for the elastomer under the stated test conditions.

The diameter of the O-ring tested is also important. A common test size is BS-312 (15.24 mm ID) and the standard indicates that O-rings of this size are examined with four cuts equally spaced around the circumference. Larger diameter O-rings can also be tested, however these require more cuts, for example a BS-329 O-ring (50.17 mm ID) would have 8 equally spaced cuts around the circumference with the four worse scores being used when reporting the results. This can make achieving a higher rating more difficult as more cuts are inspected.

The best possible rating is 0000, where no damage is evident. This offers greatest confidence in an elastomer’s RGD resistance and this rating is the only acceptable performance for any of James Walkers compounds.

Apart from the Norsok/ISO standard, the Total Fina RGD test, GS EP PVV 154/155, is also often referenced in the Industry. The test conditions in this test differ to those in the Norsok/ISO test:

• O-ring cross section standard 5.33 mm or 6.99 mm
• Test temperature 75°C,
• Pressure 190 bar.
• Decompression rate; 127 bar/min.
• Gas composition, standard 80/20 methane/carbon dioxide
• Five pressure cycles with 1 hour dwell between cycles.

Samples are scored using the same rating system described in NORSOK M710.
The Total testing program challenge the elastomer with high decompression rates and a high pressure.
Similarly SHELL MESC 85/301 looks to challenge elastomers by testing for RGD at high temperatures making it extremely difficult to pass.

• O-ring cross section standard 5.33 mm
• Test temperature 150°C,
• Pressure 150 bar.
• Decompression rate; 40 bar/min.
• Gas composition, standard 90/10 methane/carbon dioxide
• 8 pressure cycles from 150 bar to ambient pressure with 1 hour dwell between cycles.

Samples are scored using the same rating system described in NORSOK M710. The following table summarises the various tests used in the assessment of an elastomer material and its resistance to RGD.