Low-Temperature Sealing with Elastomers in Sour Gas

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Elastomer types are generically termed as NBR, HNBR, FKM or FFKM, to name a few, but this is not specific enough to properly assess the performance of the material as there are significant differences in polymer chemistry and compounding that are found within each polymer type. Examples of sour gas resistance will be shown and information provided to assist in elastomer materials selection. Additionally specifications such as API 6A, NORSOK M710 annex A and B, which pertain to pressure and temperature cycling and sour gas service, will be reviewed.

Use of O-rings and Elastomer Seals in Valves

Ever increasing demands for improved reliability, safety and wider service conditions have been ongoing trends in the oil and gas valve industry. Whether it is government oversight, environmental regulations or industry specifications driving the need for improved performance, changes in elastomer materials for those seals will be needed to meet the challenge.

Elastomeric material, particularly O-rings or chevron packings, are common configurations used to seal valves in both the body and stem. Elastomers conform and fill in the gaps between the mating surfaces of metal components, and the resiliency of the material ensures there are no leak paths. O-rings are typically energized by pressure to create an effective seal on a valve, they also require the resiliency of the elastomer to react to pressure and temperature cycles that are part of a valve’s operational life. Changes to the elasticity of the material due to time or physical environment can impact the reliability of the seal. It has been shown that chemical degradation of an elastomer will change its low-temperature performance (Tripathy & Smith, 1998). Therefore, it is reasonable that a valve with a long maintenance interval will need sealing materials that have little change in properties whether by thermal or chemical processes throughout the service life of the seal.

High-temperature resistance is well known and established as a key property of materials for oil and gas valve applications. However, two properties in particular, namely low-temperature resilience and sour gas resistance, are not normally found in the same elastomer. Both properties are important for valves used in the oil and gas industry as exploration and production of hydrocarbons moves to more inhospitable regions such as the arctic, deep-water or sour fields.

Types of Materials Used

A select few types of elastomers normally are used as seals in valves for the oil and gas industry. These elastomer types are NBR (nitrile butadiene rubber), HNBR (hydrogenated NBR), FKM (fluoroelastomer), FEPM (tetrafluoroethylene/propylene) and FFKM (perfluoroelastomer). Of these individual types, general properties exist that are fundamental to that group such as temperature range, chemical resistance and glass transition temperature. However, within each of these elastomer families there also are variations differentiated by monomer ratios, cure chemistries and other factors. Each of these materials will have trade-offs in performance in sour gas and low-temperature resilience as shown in Table 1. There are three materials that are fully resistant to sour gas (FEPM, FFKM, & LT FFKM) and among the elastomer types shown, only low-temperature perfluoroelastomers have low-temperature capabilities below 0°C. Further discussion of sour gas compatibility will be shown in Figures 1- 2 while low-temperature performance is presented in Figures 3- 4.

Specifications

First let us review some the major oil and gas industry specifications for sour gas resistance as shown in Table 2. Each of the test specifications uses a mixture of a hydrocarbon liquid phase and the test media, which is a gas phase containing a known percentage of H2S, all contained within a pressure vessel. The differences and relevance of the test methods for any particular application are beyond the scope of this article, but an awareness of these specifications is important for valve engineers when they begin development of new products for sour service applications.  

Low-temperature performance of an elastomer valve seal is not easily defined. While there are a number of test methods for determining various low-temperature properties of an elastomer as shown in Table 3, the results from the test may not correlate with the performance of an actual seal in application.

There are a number of factors that can affect an O-ring’s low-temperature performance beyond the polymer’s glass transition and a definitive relationship between lab results and service performance is not well established. Additionally, the fluid within an application may permeate the polymer and improve low-temperature properties (Warren, 2007), (Thoman, 1989), (Stevens & Thomas, 1990). In recent years the determination of the glass transition via differential scanning calorimetry (DSC) has become more prevalent in industry as an analytical technique used to compare the relative low-temperature performance of different elastomeric materials in the laboratory. The inflection point temperature of the 1st derivative curve is used as the glass transition temperature per ASTM D7426. However, an elastomer may be flexible below the cited glass transition temperature.

While the lab tests described in Table 3 are useful for comparisons among elastomer compounds, the oil and gas industry has the PR2 level test, as described in API 6A or ISO 10423, and is one of the standards by which valves are rated for temperature and pressure cycles. The test procedure is not a direct test of the elastomer seal but of the entire valve and its components. Meeting the PR2 performance level is critical to the specification and safe operation of valves for both the manufacturer and operator and is a challenge for the seal designer. ISO 10423 4.2.2.3 (Table 2 of the specification) provides temperature class ratings for valves for both low and high temperatures. While the low-temperature callout needs to be low enough so the valve can function both in cold climates or subsea with some safety margin, the elastomer will often need a greater low-temperature rating than the valve. Factors such as cold set, response time to high pressure and temperature changes will require low-temperature performance to be lower than the valve’s rating. Experience has shown that a temperature rating on a valve from ISO 10423 of -18°C would need an elastomer with a glass transition closer to -30°C to pass the PR2 test. The materials tested in the figures are representative of sealing materials for high pressure valves found in the trade.

Conclusion

There are many individual testing methods and specifications that can provide information about an elastomer, but they are not designed to tell the engineer about real-world functionality. Rigorous testing and experience bridges the knowledge gaps and reveals more about performance and can inform the valve engineer and guide the seal selection for successful PR2 qualification and functional testing.

Using NORSOK M-710 annex A methodology, it is shown that exposure of elastomer specimens to sour fluid conditions at elevated temperature will accelerate aging and provide information about long term material stability. FFKMs and FEPMs show no change in properties after aging suggesting there is no chemical attack from the sour gas while FKMs and HNBRs will show a loss of elongation in sour gas. Using DSC, the glass transition can be determined. Supported with low-temperature hardness readings it can be verified that there is a loss of elasticity below the Tg. The glass transition of LT FFKMs is at -32°C meaning it will maintain flexibility and resilience during pressure cycling tests such as the API PR2 test. This has been confirmed by valve manufacturers running real-life PR2 qualifications. Among the elastomers that show no deterioration in sour conditions, only low-temperature FFKM will have resilience at a temperature viable for subsea or arctic conditions.

Works Cited

• Stevens, R. D., & Thomas, E. W. (1990). Low-temperature Sealing Capabilites of Fluorelastomers. SAE International Congress and Expositon. 900194, pp. 7-8. Detroit, Mi: SAE International.
• Thoman, R. A. (1989). Low-temperature Performance Characteristics of Elastomers. 40th Annual Earthmoving Conference. 890988, p. 4. Peroria, IL: SAE International.
• Tripathy, B., & Smith, K. (1998). The Myth About Low-temperature Performance of Fluoroelastomers in Oil Seal Applications. International Congress and Exhibition. 980850, p. 5. Detroit MI: SAE International.
• Warren, P. D. (2007). Low-temperature Sealing Capability of O-rings: The Relationship Between Laboratory Tests and Service Performance Under Varying Conditions. High Performance and Specialty Elastomers Fourth International Conference Procedings . Smithers Rapra Technology.

The author gratefully acknowledges the assistance in preparation of the manuscript by Eric Crawford and Mike Nelson of PPE LLC in Houston.

Steven Jagels is global market manager of oil and gas at Precision Polymer Engineering. Contact him at sjagels@idexcorp.com.