FEATURE

October 2011 Sealing Technology9

Although the rapid gas decompression (RGD) resistance test regimes generated by major operators – such as Total and Shell – are highly regarded and widely accepted, the independent qualification of choice for many equipment suppliers and fluid seal-ing specialists is Norsok M-710 Annex B.

Developed by the Norwegian petroleum industry as a method for the qualification of non-metallic sealing materials and manufac-turers, Norsok M-710 Rev 2 (October 2001) defines the requirements for critical sealing, seat and back-up materials for permanent sub-sea use, and is also applied to topside valves in critical gas systems.

Annex A of the standard concerns the ageing of elastomeric materials, whereas Annex B covers the RGD testing of elastomeric materials. Annex C concentrates on the ageing of thermoplastics.

UpdatedThe complete Norsok M-710 is currently being updated and converted to ISO 23936-2: Petroleum, petrochemical and natural gas indus-tries – Non-metallic materials in contact with media related to oil and gas production – Part 2: Elastomers. An industry-based committee with representation from James Walker and other seal manufacturers is performing this task. ISO 23936-2 is likely to be published later this year.

Revision 3 of Norsok M-710 will be in line with the new ISO standard and should be published around the same time. Moreover, any qualification under the conditions described under Revision 2 of Norsok M-710 will still be valid under ISO 23936-2.

A basic concern with the 2001 revision is that elastomers can gain a ‘‘pass’’ rating for RGD-resistance when the test samples show significant levels of internal damage that would substantially reduce the safety margin of a seal under service conditions.

Norsok in a nutshellThe Norsok M-710 RGD test regime requires a minimum of three BS 1806 O-rings of 5.33 mm section and 37.47 mm ID to be inserted in housing grooves and subjected to a pressure of either 150 bar, 200 bar or 300 bar (2175 psi, 2900 psi or 4351 psi) at temperatures of 100°C, 150°C or 200°C (212°F, 302°F or 392°F).

For both sweet and sour wells, the fluid media is either 3% CO2 + 97% CH4 (‘‘low’’ CO2), or 10% CO2 + 90% CH4 (‘‘high’’ CO2). For carbon-dioxide injection wells, 100% CO2 is used.

After heating the test rig with samples in situ, pressure is applied for 72 hours to allow the media to permeate into the O-ring. Decompression to ambient pressure is then undertaken at a rate of between 20 bar and 40 bar (290 psi and 580 psi) per minute. The sample is held for one hour at ambient, then re-pressurised and soaked at temperature and pressure for 24 hours before further decompres-sion. This cycle is repeated a total of ten times before the test rig is allowed to cool to ambient temperature and left for 24 hours before the O-rings are removed.

Visual examinationEach O-ring is cut into four equal radial sections and visually examined under magnification of at least ×10. The RGD damage at each section is rated between 0 and 5, according to the follow-ing criteria (see Figures 1–6 on page 10):

• 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 rat-ing 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 (that is, 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.

Perfect pass or compromised safety?At James Walker, we test four samples instead of the minimum of three, and use the 0000 rat-ing (zero damage across four cross-sections on all four samples) as a benchmark when develop-ing our RGD-resistant materials.

Our reasoning is that a 0000 rating repre-sents the maximum achievable operational safe-ty margin for end-users – this is especially valid for long-term applications where fluctuations in media composition and temperature can further compromise RGD performance.

However, it is always possible that a minor surface blister may form during the test and, when the seal is sectioned, this would be recorded as a 1 rating. For this reason, it is imperative that photographs of the seal cross-sections are included in any report, whether for Norsok or other accreditations.

As noted above, it is possible to claim a ‘‘pass’’ against Norsok M-710 Annex B with a rating of 3333. Close study of the illustrations

Achieving RGD resistance to meet current oilfield needsBy John Rogers, Senior Materials Engineer, James Walker, UK

Seal damage and observed gas leaks into the atmosphere caused by rapid gas decompression (RGD) in elastomeric seals have been reported in many types of equipment used in the oil and gas industry. These failures can have costly financial, safety and environmental implications for operators and equipment suppliers. This article looks at RGD in general and the implications of Norsok standards, developed by the Norwegian petroleum industry, for RGD testing and sour gas ageing of elastomers and thermoplastics.

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indicates that the potential difference between a 3 rating (‘‘pass’’) and a 4 rating (‘‘fail’’) can be very fine. Therefore, under true operational conditions, damage at the 3 rating level offers the user virtually no safety margin. In our experience, such a material is unfit for RGD-resistant duties.

ResearchJames Walker’s vast experience in this field dates back to the late 1970s and early 1980s, when we initiated a comprehensive research programme into the phenomenon of RGD (then known as ED or explosive decompression) following reports of seal failures in North Sea fields during exploration and production activities.

In collaboration with the research arms of two offshore operators, and oilfield equipment manu-facturers, we investigated the causes of RGD damage with gas permeation and diffusion into elastomeric components at very high pressures.

We have been continuing this research for over 30 years at the James Walker Technology Centre, where we have some of the world’s most advanced RGD testing facilities that match or surpass those used by most inde-pendent test houses (Figure 7). Currently we test O-rings up to 10 mm cross-section within the ranges of 150 bar (2175 psi) at 200°C (392°F), to 350 bar (5076 psi) at 65°C (149°F), with full data-logging of temperature and pressure.

Understanding RGD processesOur current understanding is that an RGD event occurs as two distinct processes: permea-tion and pressure release – with possible phase changes in the fluid media.

Under pressure, a fluid passes through the elastomer surface and towards the core of a seal, dissolving into the material. Even at the very high service pressures specified by some end users, such as 690 bar to 1276 bar (10 000 psi to 18 500 psi), the fluid is generally in a gase-ous phase. However, at high pressures and low temperature, some chemicals/mixtures can be present as supercritical fluids – where the liquid and gas phases approach each other.

The fluid media very slowly saturates the elastomer (extremely slowly at supercritical levels) to equalise the pressure between the seal surface and its core. Complete saturation can take a considerable time – typically days or weeks, rather than hours.

Interestingly, this lengthy permeation proc-ess led to some unreliable test results in the very early days of developing RGD-resistant elastomers, because the test regimes used at that stage did not allow sufficient time for complete saturation of the material down to core level.

When external pressure is suddenly released – such as a blow down – the media within the seal

expands very rapidly in its gaseous state, accom-panied by a temperature drop caused by adiabatic expansion. The gas expands much faster than it can naturally diffuse through the elastomer. If the elastomer cannot resist crack or blister growth under these rapid decompression conditions, then the seal will suffer structural failure.

The ability of an elastomeric seal to survive during an RGD event is also dependent on the material’s resistance to degradation in oilfield media over long-term service, as the material’s mechanical properties will reduce with satura-tion time.

Elastomer ageing estimatesNorsok M-710 Annex A specifies procedures for conducting accelerated ageing tests in an auto-clave on constrained O-ring samples at 100 bar (1450 psi) or higher, and at three or more differ-ent elevated temperatures – all of which must be above the service temperature. The constituents of test media are defined for sweet and sour service conditions, albeit other test fluids may be used to match specific operating conditions.

The mechanical properties of the O-ring samples are measured before and after the accelerated test, with acceptance criteria based on percentage swell, and changes in mate-rial hardness, tensile strength and E-modulus.

Figures 1–6. Norsok M-710 Annex B, rapid gas decompression ratings from 0 to 5. The photographs show – from left to right – ratings 0, 1 & 2 (top row) and ratings 3, 4 & 5 (bottom row).

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October 2011 Sealing Technology11

The service life of the material is estimated by extrapolation of the tensile properties using the Arrhenius method.

Unrealistic values James Walker’s experience of this method for ageing elastomeric materials shows that some highly unrealistic values can be achieved.

We would like to believe that our FR58/90 RGD-resistant fluorocarbon has a service life in sour gas of 817 years before its tensile strength is halved — as determined by a leading inde-pendent test house. However, we must accept that the results of Norsok’s Annex A ageing procedure can be used only as a comparison between different materials; not as an accurate guide to service life expectancy.

We have discovered two additional factors that potentially cause material degradation dur-ing the Norsok ageing test, in addition to the effects of sour gas, which are the basis of the test procedure. These are RGD events and the effect of solvents, both of which can change the physical properties of the test samples – either independently and/or in concert with the effect of sour gas – thus significantly affecting the estimated service life prediction.

Firstly, in some instances when a sample is removed from its housing after a sour ageing test, it pops and crackles as RGD events occur. This action can significantly affect the material’s ten-sile strength. Secondly, the stipulated carrier (at 60% by volume of total test fluid) for the sour oilfield media is a cocktail of heptane, cyclo-hex-ane and toluene. These cause swelling and the reduction of tensile strength and other properties in a number of RGD-resistant elastomers.

Designing for RGD resistanceIntimate knowledge of the characteristics of different elastomers is vital when designing and specifying high-integrity sealing products for service in RGD-related applications.

For example, a hydrogenated nitrile (HNBR) compound, such as Elast-O-Lion 101, has sufficient tensile strength to withstand an RGD event, and can operate constantly at up to 160°C (320°F). A fluorocarbon (FKM) compound, such as FR25/90, has the higher constant service temperature of 200°C (392°F). However, FKM is physically weaker than HNBR at elevated temperatures.

We learned many years ago that at elevated temperatures and pressures, significant engi-neering expertise must be introduced to enable the specific benefits of an RGD-resistant elas-tomer to be fully realised.

One of the major aspects is the design of the housing in which the seal is enclosed. In our experience, the design of a new housing must be undertaken by experienced sealing engineers who are fully conversant with the optimisation of an RGD-resistant system. Simply employing a conventional O-seal engineering approach can severely compromise the effectiveness of an RGD-resistant seal.

A further aspect is that RGD-resistance is influenced considerably by the cross-section of the seal – the thicker the seal, the more difficult it is to resist RGD. At 5.33 mm cross-section, many specialised RGD elastomers will gain close to a 0000 rating.

We have recently achieved success with a 10 mm section O-ring of our new generation FR68/90 fluorocarbon (Figure 8) in tests at 100°C (212°F), and a decompression rate of 35 bar (508 psi) per minute. This high decom-pression rate induces significantly more internal

stress in the O-rings, and more accurately represents an operational RGD event than a decompression rate at the lower end of the Norsok range.

The next challenge is to achieve a perfect Norsok rating with a 12.7 mm cross-section O-ring under realistic service conditions. This means using an elastomer, such as FR68/90 that is optimised for sealing efficiency as well as RGD resistance and does not need an exact-ing surface finish on the counter-face, or split housing construction to overcome compound elasticity limitations, for high-integrity sealing.

Manufacturing for RGD resistanceWe consider that the effective compounding of an RGD-resistant elastomer is the most impor-tant part of a seal’s manufacture.

Figure 7. Rapid gas decompression testing at the James Walker Technology Centre.

Figure 8. Results of rapid gas decompression (RGD) tests on different elastomers: a comparison of James Walker’s FR68/90 fluoroelastomer – designed for high-integrity sealing duties in harsh oil and gas environments – and other elastomer materials, following RGD testing (with FR68/90 on the left).

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12Sealing Technology October 2011

Biography of John RogersJohn Rogers holds a BSc(Hons) in pure chemistry. He joined James Walker’s materi-als laboratory to develop elastomers to meet stringent specifications, and then worked on the development of internal mixing of elastomers, and the refining of extrusion and moulding processes. Rogers now specialises in the selection of elastomers for customers’ applications – particularly in the oil and gas and nuclear sectors.

In-house compounding enables the manufactur-er to exert total control over variables such as raw ingredients, temperature, time and the shear energy imposed on the compound by the mixer’s rotors.

Ingredients and mix cyclesWe optimise our ingredients and mix cycles to achieve precisely the right compound for the RGD and sealing qualities required, then always mix the compound on the same machine under exacting computer control for total repeatability and an homogeneous mix. High levels of dispersion and consistent heat history are critical and necessary to produce the quality required.

James Walker always uses the same materi-als from the same suppliers that were initially used for Norsok or other accreditation testing. To ensure consistency of product, any poten-tial ingredient change (including a change in manufacturing plant) involves revalidating the product prior to acceptance.

Putting out the compounding to a contractor imposes the potential risk of inconsistency in the resulting elastomer. Using different mixers and/or different mix cycles will produce very different compounds.

Moulding techniquesThe correct moulding and post-moulding techniques used to convert an elastomer into

an efficient seal is critical to obtain RGD performance, and must be optimised for each material. Again, this needs to be an in-house operation to achieve the consistency of results that are required.

A manufacturer needs to know on which particular presses, and at which computer-controlled settings, a seal has been produced in the past to achieve the required results. Unacceptable inconsistencies can result from shifting the moulding process to another type of press, or even to an identical machine at another plant. We routinely take random sam-ples from production runs and test these in-house under Norsok M-710 Annex B, or other accreditation requirements, to ensure product quality.

Perfect resultsThe lesson here is once you have set a process cycle that achieves perfect results for RGD-resistant seals, do not change any parameters – even by the smallest amount. Otherwise, you will need to re-establish the complete process from point zero to maintain product standards, and that can prove a time-consuming and expensive remedy.

Total in-house control of every variable is vital for RGD-resistance with elastomeric seals. At James Walker, we adhere strictly to this principle to achieve the very best quality RGD-resistant seals for our customers.

Contact:

James Walker Oil & Gas Team, 1 Millennium Gate,

Westmere Drive, Crewe, Cheshire CW1 6AY, UK.

Tel: +44 1270 536140, Fax: +44 1270 536065,

Email: oilandgas@jameswalker.biz,

Web: www.jameswalker.biz

Technical Editor’s comment:From my experience of involvement with RGD projects over some 20 years or so this feature provides a good explanation of the problems involved. The effective testing of seals and materials is a difficult subject, and the broad range of test regimes permitted within the Norsok test can cause a lot of confusion, especially when, as discussed here, a seal can look very second-hand but still be classed as a pass.

PATENTSHeat-fusible gasket

Applicant: Prinsco Inc, USA This invention concerns a heat-fusible gasket for affecting a flexible seal between two adjoining components of a coupling, such as the spigot end of a section of plastic pipe and a bell cou-pler. The gasket has an inner core that is made from an elastomeric material, and outer oppos-ing sealing surfaces which are formed at least in part from a heat-fusible material that is compati-ble with the heat-fusible components with which the gasket is intended to engage. The gasket is co-extruded in continuous lengths, with electri-cally conductive heat-resistant wires embedded in the outer compatibly heat-fusible layers. For annular gaskets, the co-extruded lengths of mate-rial may be cut and spliced to form a gasket of any desired diameter. Upon connection to a power source, a seal is created through electro-fusion along the opposing sealing surfaces of the gasket, providing enhanced sealing.Patent number: WO/2011/044080

Inventor: C. DouglassPublication date: 14 April 2011

Cap assembly for preventing gasket sag

Applicant: LG Chem Ltd, KoreaThis invention relates to a cap assembly for pre-venting gasket sag and to a cylindrical second-ary battery that makes use of this assembly. The main gasket is supported by an auxiliary gasket so as not to form a gap at the safety vent. The cap assembly also employs the auxiliary gasket that encircles the outer periphery of the CID in order to prevent sagging of the main gasket which is seated on a beading portion. This improves resistance to electrolyte leakage.Patent number: WO/2011/046261Inventors: S.J. Kim, J.J. Lee and C.H. Ku Publication date: 21 April 2011

Filter cartridge with seal member

Applicant: Donaldson Co Inc, USA This disclosure concerns filters for cleaning air, for example, for use in dust collectors and

other equipment. In particular, this disclosure concerns z-filters that have a wraparound seal. The perimeter gasket member seals against the frame arrangement while the side gasket mem-ber seals against the tube-sheet.Patent number: WO/2011/046782Inventor: T.D. RaetherPublication date: 21 April 2011

Axial labyrinth seal for filtration systems

Applicant: Toray MembraneUSA Inc, USACertain types of filtration systems, used for removing chemical contaminants and organ-isms from water, consist of one or more filtration elements that are sealed within an enclosure. There may be a series of elements placed end to end. In this patent, an axial face seal is described that may be used to seal a spiral membrane element used in a filtra-tion system. The faces of the first and second seal-plates have complementary profiles that form an ‘‘intermeshed’’ contact when one spiral membrane element is coupled to an adjacent spiral membrane element. This contact creates