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Qualification of line pipes for pressurised hydrogen applications?

Piping in acc. to ISO 3183 and API SPEC 5L must be evaluated with regard to their suitability for the use of hydrogen. Additional test procedures at qualifications of line pipes are required.

Specifications and standards

Specifications and standards for special hydrogen applications are currently only available to a limited extent and will become increasingly important in the future. Important specifications already available are the

  • ASME B31.12 “Hydrogen Piping and Pipelines”
  • EIGA Doc 121/14 “HYDROGEN PIPELINE SYSTEMS”
  • DVGW G 464 ” Fracture-Mechanical Assessment Concept for Steel Pipelines with a Design Pressure of more than 16 bar for the Transport of Hydrogen”
  • EN 1594 Gas infrastructure – Pipelines for maximum operating pressure over 16 bar – Functional requirements

Specification EIGA Doc 121/14

This specification only allows pipes manufactured according to API Spec 5L. The following additional requirements must be taken into account:

  • Material X52 (not ERW welded), X46, X42 and Grade B (all PSL 2), max. carbon equivalent CE ≤ 0,45%;
  • Material X52 (ERW welded), micro alloyed condition with specified sulfur ≤ 0,01 %, phosphor ≤ 0,025 %, max. carbon equivalent CE ≤ 0,35%

ASME B31.12

This code is applicable for systems that contain 10% or more hydrogen by volume with pressures below 3000 psig. The code covers steel pipelines (e.g., the conversion of steel pipelines from ASME B31.8 to ASME B31.12). This code also includes information on material performance and pressure derating to accommodate hydrogen embrittlement and contains two options for the qualification of hydrogen pipelines:

Option A is prescriptive and similar to design processes contained in ASME B31.8 Natural Gas Pipeline Code. It considers the use of lower basic design factors (F) and a material performance derating factor (Hf) derived from pressure and tensile strength relationships.

Option B is performance based, using a fracture mechanics approach (on the basis of ASME BPVC Section VIII, Div. 3 – Alternative Rules for Construction of High Pressure Vessels). The qualification of the pipeline materials is performed by use of fracture mechanics and crack propagation testing that empowers the use of enhanced design factors and withdraws the limitations on pressure due to the use of the Hf derating factor.
In regards to the second design method, the code introduces additional requirements for pipe material, related to lower Phosphorus content (<=0.015%) and consideration of API 5L Annex G for CVN testing (Enhanced Ductile Fracture Propagation Properties). More specifically, the ASME B31.12 code requires that the threshold stress intensity factor for hydrogen-assisted cracking (denoted as KIH) should be measured according to ASME BPVC Section VIII Div. 3 and ASTM E-1681.

The fracture toughness qualification testing is required to validate the minimum threshold stress intensity factor (KIH) at the design pressure and 100% H2 concentration. The test on the pipes should be performed at the base metal, weld metal and heat affected zone positions, on three heats of the pipe material. The KIH value that qualifies the material is 50ksi√in (or 55 MPa√m) unless otherwise specified by design analysis.

Test of threshold stress intensity factor BPVC Section VIII Div. 3 and ASTM E-1681

  • Samples will be machined in bolt-load compact configuration in compliance with the prescriptions of ASTM E-1681 for the bolt-Load, Compact Specimen. In any case, the request of having at least 85% of the pipe nominal thickness is always satisfied.
  • The determination of the threshold stress intensity factor involves a specimen containing a machined notch, which is placed in base material and, for HFW pipes, in bond line or, for SAWL pipes in weld metal and Heat Affected Zone (HAZ) crossing the fusion line (Coarse Grain HAZ) at the maximum extent.
  • This notch is extended by fatigue cracking under controlled conditions for maximum loading, especially for the final part of the crack growth. The fatigue precracked specimen is then placed in a glovebox filled with a nitrogen atmosphere, under very low oxygen and moisture levels as required per ASME BPVC Section VIII Div. 3.
  • The specimen is then loaded by means of a bolt to the attainment of the target Crack Mouth Opening Displacement, established on the basis of the target stress intensity KIAPP for plain strain conditions. According to the code, the applied KIAPP should be at least 1.6 times greater than the estimated KIH but not more than 180 ksi·√in (198 MPa·√m). After loading, the samples are put inside the test chamber which is sealed while still inside the glove box, preventing any contact of the loaded samples with atmo- sphere oxygen and moisture. The test chamber is then charged with pure hydrogen gas at the target test pressure and maintained at this pressure for 1000 h. In this way, any fresh crack surface that is possibly generated by ductile tearing during bolt loading has never been exposed to oxygen or moisture and is hence prone to hydrogen permeation from the gaseous hydrogen environment.
  • After the specified test period, the specimen is examined to assess whether the initial fatigue crack did or did not grow. The specimens are heat tinted and broken open in liquid nitrogen. The fracture surface is then examined by optical observation and scanning electron microscope. Measurements of the crack front extent are taken in five positions and the average crack growth in hydrogen is calculated.
  • Evaluation
    According to KD-1047 clause of ASME BPVC Section VIII Div. 3 for the constant displacement method, if the average measured crack growth does not exceed 0.01 in. (0.25mm) KIH is equal to 50% of KIAPP. Taking this clause into consideration, the KIAPP initial stress should be selected to be at least double of the minimum threshold stress intensity value required by the code of 55 MPa·√m.

DVGW G 464

The aim of this code of practice is to define a generally valid concept depending on assumed fault variables and operating pressure curves including safety coefficients (e.g. against the number of load cycles and/or critical fault variables) for the fracture mechanics assessment of hydrogen suitability for the construction or conversion of high-pressure gas pipelines.

The fatigue crack growth under the influence of hydrogen is determined by estimating the crack growth under changing operating pressures and load conditions of the piping sections.

EN 1594

This European standard contains requirements for for new gas infrastructure and also conversion of existing natural gas pipelines for the transportation of hydrogen

  • Requirements for the crack growth of pipes (annex A)
  • Material requirements (section 8)
  • Requirements of welding and testing (Section 9)

Read More

Pipes for pipelines: Comparison of ISO 3138 with API Spec 5L pipes

Standards and specifications for steel materials in hydrogen systems

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