| Hydrogen
to hydrogen and new construction, pipeline manufacturers and operators must prove the measures required for the conversion to hydrogen transport have been expertly completed in compliance with the state of the art. The necessary technical measures are outlined in the standards of the German Association of the Gas and Water Industry (Deutscher Verein des Gas- und Wasserfachs e.V. (DVGW)). DVGW standard G 409 addresses conversion to hydrogen transportation.
Checking the suitability of existing pipelines
For this purpose, all factors affecting the life expectancy or integrity of pipelines must be analysed. Are there any flaws, such as cracking or corrosion? What is the general state of repair of the gas pipelines? The preferred method for identifying the condition of the pipeline is an assessment using pipeline inspection gauges known as “PIGS”. The data collected by PIGS and other sources (if any) are then used for further analyses and evaluations. The PIGS supply the initial fault size, which is used as the basis for a fracture mechanics analysis to assess the integrity and the service life expectancy. In pipeline testing, TÜV SÜD experts proceed systematically. In step one, they review the existing documents related to design, construction, operation, servicing and maintenance. Further aspects, such as the extent and frequency of changes in operating pressure and the pipeline’s exposure to additional loads, are also included in the review. Depending on the steel grades and materials used for pipelines and fittings, the experts clarify whether additional destructive laboratory tests will be necessary. This is the case, for example, if the strength of the material being exposed to hydrogen has not yet been tested and is unknown. Final determination of service life by means of fracture mechanics takes into account the actual dimensions of the component, the loads to be expected and the material properties in a hydrogen atmosphere.
Hydrogen embrittlement of steel Electrochemical processes on metal surfaces can result in the formation of hydrogen atoms from molecular hydrogen. These hydrogen atoms can diffuse into the material, changing the crystal lattices of the steel structure. The hydrogen atoms then accumulate at flaws and grain boundaries or may also occur within the steel microstructure. They change the lattice structure, reducing the ductility and strength of pipeline steels. In other words, they cause the material to become brittle. Possible consequences include tiny material separations which may grow into cracks under cyclic loads. This well-known damage mechanism is evaluated in lifetime analysis by means of fracture mechanics.
DN 600 / DP 75 bar / L360 1
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0 0.1 0.2 0.3 0.4 0.5 Lr 0.6 0.7 0.8 0.9 1
Failure assessment diagram (FAD). Kr is stress intensity, Lr is load intensity
Assessment line Assessment point Critical crack size Critical load
Critical fracture toughness
Evaluating faults with the help of a diagram
Fracture mechanics enables the safety and functioning of a pipeline in the presence of an existing fault or hydrogen embrittlement to be analysed. Analysis is based on the interaction of component geometry, material properties, loads and crack-like flaws.
The results are visualised in a failure assessment diagram (FAD), see above. The two variables analysed are load intensity, Lr, and stress intensity, Kr. They are computed from the actual component and fault geometries and the defined load and material parameters. The ratio of these two variables supplies an evaluation (or assessment) point, which is plotted in the diagram. A limit curve divides the FAD into two zones. If the evaluation/assessment point is above the curve, the result is inadmissible; if it is located below the curve, it is safe to assume that there will be no component failure. The diagram also enables statements to be made on critical crack depth, load carrying capacity (critical load) and needed (critical) fracture toughness. Following this analysis, the permissible crack depth is calculated by considering appropriate safety factors. It serves as a criterion for crack- growth analysis and the remaining service life.
Assessment of service life and definition of inspection intervals Managers and operators need to know the intervals at which periodic testing, inspection, servicing and maintenance must be performed and components replaced. To identify these intervals, the experts must first determine the maximum number of load cycles (lifetime) to critical or inadmissible crack depth. This number, in turn, depends on the crack growth rate – in other words, the amount of growth in crack depth per load cycle. The maximum permissible number of cycles also correlates with the load amplitude. It increases when load is reduced. For a typical pipeline steel grade, experts were able to demonstrate that a 50% reduction in
full load resulted in an eightfold increase in the permissible number of load cycles. Exact analysis of crack growth requires load height, amplitude, sequence and the number of load cycles to be taken into account. To summarise, this means that the first step in the assessment of the remaining service life is to define a future aggregated load profile, which must then be used to evaluate or determine the remaining lifetime. In parallel, a conservative inspection interval must be defined – a point in time at which the actual load will be analysed and the crack growth resulting from this load determined realistically. To ensure the safety of pipeline operations, additional on-site inspections are recommended as backup measures.
Published material parameters Material parameters are paramount for fracture mechanics analysis. The American Society of Mechanical Engineers has collected and published comprehensive fracture-mechanics data for pipeline steels manufactured in accordance with the ASME Code, the leading global standard on pressure equipment design, manufacturing and testing. Although the European counterpart of the ASME Code does not yet include such a list, DVGW is currently working on this issue and hopes to present first results shortly. TÜV SÜD is a member of various DVGW committees and supports sustainable safety-related concepts.
Fracture mechanics is critical Where conversion of Germany’s existing natural gas grid for transporting hydrogen is foreseen, fracture mechanics analyses are critical for establishing reliable estimates of pipeline service life. Fracture mechanics analyses include tests and inspections of the available documentation and the existing equipment. Managers and operators benefit from commissioning a third- party expert organisation, such as TÜV SÜD, to assess the special requirements associated with hydrogen.
* Expert, fracture mechanics analysis ** Team lead, pipelines *** Expert, high pressure gas pipelines Contact: TÜV SÜD Industrie Service GmbH, Westendstraße 199, 80686 Munich, Germany +49 89 5791-2176
jan.sachse@
tuvsud.com
www.tuvsud.com/de-pipelines
www.modernpowersystems.com | January/February 2022 | 33
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