Hydrogen Pipelines: B31.12 Prescriptive and Performance-based Methods
The transportation of hydrogen, a clean and versatile energy carrier, necessitates robust infrastructure to ensure its safe and efficient delivery. Among the crucial standards governing the design of hydrogen pipelines is the ASME B31.12 code, which outlines two primary methods: Prescriptive and Performance-based. Each method offers distinct approaches to pipeline design and safety, both of them taking into account the risk of hydrogen embrittlement in steel pipelines.
Working Pressure as a function of Pipe Thickness. Prescriptive Method
ASME B31.12 presents a mathematical expression to determine the working pressure for a steel pipeline intended for hydrogen service, as a function of its geometry, material, and boundary conditions. For the Prescriptive Method, the formula presented is:
The values to be used in the calculation are obtained from the engineering design, and from different tables included in the code.
The working pressure obtained for hydrogen by this expression is usually lower than the equivalent value for natural gas, determined as per ASME B31.8 code. Depending on the conditions, this difference can be very relevant. This is so, above all, because the material performance factor (Hf) adds a safety margin considering the risk of metal embrittlement in the presence of hydrogen.
Working Pressure as a function of Pipe Thickness. Performance-based Method
If the formula we saw previously corresponds to the Prescriptive method, what is the expression corresponding to the Performance-based Method?
Well…exactly the same. With two important nuances: The design factor (F), which depends on the distance of the pipeline with respect to buildings intended for human occupation, and the material performance factor (Hf) take their values from different tables. They correspond to margins of security much less conservative than those used in the Prescriptive method. The final result is that the operating pressure of the pipeline with hydrogen, following the Performance-based Method, reaches values similar to those of natural gas transportation for an equivalent pipeline.
In exchange, and this is the critical point of the difference between both methods, in order to use the Performance-based method, the code imposes additional restrictions on the materials to be used, and requires very exhaustive destructive tests on the material, following the guidelines of article KD-10 of the ASME Boiler and Pressure Vessel Code (BPVC Section VIII Division 3).
This article KD-10 is called “Special Design Requirements for Vessels in High Pressure Gaseous Hydrogen Transport and Storage Service”, and has an approach based on stress analysis, material fatigue and fracture mechanics.
It is very important to keep in mind that the tests must be carried out as close as possible to the actual construction and operating conditions. This involves carrying out the same post-construction and pre/post-welding heat treatments. As an example, the code requires repeating the tests if the detailed welding procedure to be used in the construction of the pipeline is modified.
New hydrogen pipelines and repurposed natural gas pipelines. Implications
In general, the Prescriptive method will result in lower operating pressures than the Performance-based method. Another way to make the same statement is that for a defined working pressure, the Prescriptive method will require a greater pipe thickness than the Performance-based method.
For a new hydroduct, a greater thickness implies greater weight and higher cost, so in general the Performance-based method will tend to be used. This will be especially true to the extent that tests on standardized materials, treatments and welding methods are carried out and published, allowing the reuse of these tests for new projects.
Only in the case of relatively short sections, could the use of the Prescriptive method for new construction be justified, avoiding the cost and complexity of carrying out the tests, in exchange for using more conservative design factors.
In the case of reconversion of existing natural gas pipelines for hydrogen service, the decision is not so obvious. It will depend on the availability of comprehensive information on the construction, heat treatments and welding procedures used in the original work of the gas pipeline. It will also depend on the practical possibility of obtaining samples of the material, for destructive testing, at regular intervals.
Of course, the conversion of a natural gas pipeline to hydrogen, when the application of the Performance-based method is not possible (for example in an offshore pipeline, where sampling the material is not economically viable) will involve the operation of the pipeline at a lower pressure than the original. Lower pressure also implies a lower amount of energy transported.
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