There is no single comprehensive code or standard prescribing full requirements for materials and components, design, fabrication, assembly, erection, examination, inspection, and testing of plants exposed to SCC (Stress Corrosion Cracking) caused by H2S, chlorides and CO2 in upstream gas processing plants. Owners and designers need to establish their own Best Practices to manage the risks of SCC and to write these into the design specifications.
The most used code for oil & gas plants is ASME which refers to NACE Standard MR0175 ( ISO 15156) which covers materials to be used in hydrogen sulphide (H2S) bearing hydrocarbon service.
ASME specifies in several clauses that the responsibility to verify detailed requirements for corrosive service is covered within the engineering design by the designer or owner.
NACE standards clarify that the specified material requirements do not provide design specifications and that users shall first assess the conditions to which the materials they wish to select may be exposed and shall consequently assess the need for other requirements.
The design scope needs to include a combination of different codes and additional clauses in order to consider, evaluate and specify a comprehensive set of requirements necessary to assure reliable and safe operations.
Best Practices written into the design specifications should cover: materials (carbon steel and CRA), components (e.g. fittings, valves, pressure vessels, heat exchangers), details and fabrication processes (forming, welding, cladding).
In our experience the Best Practices need to address issues related to the use of Carbon Steel which may be prone to hardening induced by fabrication processes and may not be suitable for services where SCC (H2S, chlorides, CO2) is a concern. Some specific NACEMR0175/ISO15156 approved materials may crack in highly sour service at moderate load if exposed to wet and acid conditions. Design and construction codes do not fully address how to control hardness in WM, FL and HAZ. Standard NDT methods and acceptance criteria that neither fully address weldments resistance to critical sour service nor address in-service inspection needs, must be upgraded by the Best Practices.
Owners should specify design details and materials (e.g. socket joint, weldolet, internal cladding or lining, ring joint) in order to ensure suitability for sour service, for cyclic loads or for specific construction operations phases. Special consideration should be given to NACEMR0175/ISO15156-approved base materials (LTCS, Stainless Steels, Alloys) and the effects of fabrication, construction and installation processes. The affected components and equipment are: pressure vessels, piping, pipe fitting, flanges, valves, pipelines, equipment.
There is a long list of specifics to include in a Best Practices document for each gas processing project exposed to SSC, and this is the best way to manage risks. It takes time to develop at the outset, but pays off during construction and operation. [Article ends]
Do you write a Best Practices document for gas processing projects exposed to Stress Corrosion Cracking (SCC)? How do you manage risks associated with SCC? Let us know your views below.
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