The Challenges of Meeting International Oil & Gas Valve Sealing Requirements

Figure 1: Comparison of industry and super major sour gas aging testing standards for evaluating and qualifying sour gas resistance of elastomer seal materials.

NORSOK M710, NACE TM0297, ISO 10423 (API 6A) and similar testing standards provide general methods for qualifying manufacturers of elastomeric valve seals – as well as their sealing materials – for oil and gas service applications requiring resistance to sour gas (H2S) and rapid gas decompression (RGD). Unfortunately, there are sizeable challenges in that these testing standards are often misinterpreted or underutilised

One such challenge is that seal testing standards are not material specifications. Requesting a valve sealing material “to NORSOK M710” without further information around the application requirements is unlikely to provide a sealing material which is fit for purpose. Also, many engineers have only a cursory understanding of these testing standards and their relevancy to field conditions, with the latest revision of a standard seemingly never far away. Consulting a valve seal manufacturer with a comprehensive knowledge of the latest standards can prove helpful when navigating this minefield. Expert guidance can also be sought on how laboratory testing correlates to field conditions, as well as areas where tests are excessive or deficient in predicting field service performance.

Valve sealing materials need to satisfy testing standards from three key body categorisations, namely industries, OEMs and super majors – the six largest non-state-owned oil and gas suppliers. Qualifying elastomer materials to these standards is challenging due to the sheer number of different standards, and the width of test parameters within each of these standards. Disparity between these testing procedures demonstrates the varying levels of sealing or elastomer knowledge across industries, OEMs and super majors. Qualifying valve sealing materials to conform to all of these standards can prove an impossible task, leading to inevitable compromises in performance. For example, formulations optimised to enhance properties for one test standard might result in poorer performance when measured against another standard. Furthermore, certain criteria across test standards are discretionary or vague, which can result in different interpretations and test results between laboratories.

Variations in Sour Gas Aging Test Standards
Sour gas (H2S), or hydrogen sulphide, is a poisonous, flammable and odorous gas found in wells at concentrations from parts per million to in excess of 20%. Elastomeric valve seals which are not resistant to sour gas become hard and brittle, resulting in poor sealing, cracking and eventual seal failure. Sour gas aging tests immerse a material in a heated, pressurized liquid (hydrocarbon-water) and gas (H2S/CO2/CH4) mixture.

Dimensions, hardness and tensile properties are measured before and after sour gas exposure. The table in Figure 1 shows the complexity and variation across several common sour gas aging test standards. The NORSOK M710 Annex A, ISO 23936-2, Shell and Saudi Aramco sour gas aging test standards specify 2% H2S concentrations.

The ISO 10423 (API 6A) DD/EE and ISO 10423 (API 6A) FF/HH standards require much higher H2S levels of 5% and 10%, respectively. The NACE TM0187 testing standard allows gas phase compositions of 5% or 20% H2S. Manufacturers are likely to test using the lower 5% concentration unless their customers specifically request testing at the higher H2S levels. These percentages are the percentage of the gas phase composition. For example, in NORSOK M710 Annex A, the 2% H2S represents 2% of the 30% gas phase.

ISO 10423 standard testing is done at a single test temperature of 177°C. All the other sour gas aging standards require testing at three different temperatures for acceleration purposes. The test temperatures in the NORSOK and Saudi Aramco sour gas testing standards allow manufacturers to select three test temperatures at their discretion. The ISO 23936-2 standard provides nine temperature range options ranging from a low combination of 36°C, 51°C and 66°C to a high set of 195°C, 210°C and 225°C. The Shell standard has different test temperatures depending on the material (FKM versus HNBR).

Variations in Rapid Gas Decompression Resistance Test Standards Rapid gas decompression (RGD), or explosive decompression (ED), is a failure mechanism of elastomer seals and O-rings caused by a rapid reduction in pressure of a gaseous media. Gas that has permeated into the elastomer seal expands violently when the pressure is released rapidly, causing fissuring and seal failure. RGD testing requires specialised high pressure test rigs capable of pressurising various seals at different depressurisation cycles and temperatures.

Between RGD testing standards, the number of test cycles varies from one to 10 across the different RGD testing standards. Soak periods range from six hours to 48 hours. Test temperatures vary from 50°C to 230°C or to “process temperature.” The NACE TM0297 standard has the widest range of allowable test pressures (7 MPa to 38 MPa) and temperatures (50°C to 230°C). While NACE TM0297 could be considered a more aggressive test standard, it could also be considered the least aggressive; the severity of an NACE RGD test depends on the parameters selected. If an engineer requests an elastomeric seal material qualified to “NACE TM0297” without specifying pressures and temperatures, a manufacturer could test using the lowest temperatures and pressure (50°C and 7 MPa). This might not truly qualify a material for the actual field conditions with higher temperatures and pressures, as 80% of oil and gas applications see field temperatures in the 120°C to 150°C range.

Valve seal manufacturers are likely to test materials using the minimal requirements unless their customers provide the field conditions or request specific test parameters within the limits of the standards. Regardless of the standard, the test pressures, temperatures and fluid media should be representative of the field application.

Engineering and Selection of Oil and Gas Elastomeric Seal Materials While having a single valve seal material capable of meeting all of the divergent testing standards might reduce part counts, such a material cannot realistically meet all of the standards. An “over specified” seal material with too many requirements can compromise seal cost and availability.

In field applications with very aggressive conditions, engineers may need requirements beyond typical standards to achieve optimum seal performance. Some valve seal manufacturers choose to undertake material testing at elevated temperatures and sour gas concentrations of 25%, exceeding the standard NORSOK test levels. These aggressive tests find the limits of the elastomer's capability providing the design engineer with a sufficient “safety factor” for extreme sealing situations. Highly engineered perfluoroelastomer (FFKM) materials can provide sour gas resistance, with specialised grades able to combine this resistance with exceptional low temperature performance, which was unheard of a few years ago.

During oil and gas seal selection, design engineers need to balance performance, life, availability and cost when selecting seal materials. Design engineers should consult with experts in elastomer seal materials to meet the diversity of requirements. Specialised valve seal engineers can help select the appropriate test parameters within a testing standard based on an understanding of your field application.

A reputable sealing specialist can rapidly evaluate and qualify seals to mechanical and thermal properties and validate different engineered variations for specific requirements. These requirements may include pressure extrusion resistance, thermal cycling resistance and other mechanical properties.

Contact a specialist application engineer today to discuss your valve sealing application, and evaluate the most suitable high performance elastomer material to improve the performance, reliability and safety of your oil and gas operations.

Tel: 01254 295 400

Published in Valve User Magazine Issue 45

Summer 2018 // Issue 45
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