Lebensdauervorhersage von Hochdruckrohren unter Berücksichtigung technologischer Einflüsse und bruchmechanischer Konzepte

BHDT GmbH manufactures, among other things, heat exchangers, reactors, and pipelines for the petrochemical industry under extreme loads, such as pressures up to 4000 bar and temperatures up to 350 °C. These mostly jacketed pipes are often made of high-strength quenched and tempered steels or precipitation-hardened stainless steels and are produced by deep hole drilling from bar stock. Especially during start-up, the highest loads act on the high-pressure components. A measure for the service life is the number of start-ups that a high-pressure pipe can withstand without failure. Accurate estimation of fatigue behavior is essential for a safe design of the equipment.
Over time, it has been established that the hot forming process of the bar material has a significant impact on the service life of the high-pressure pipe. For this reason, a key objective of this master's thesis was to investigate the technological influence from the manufacturing process of the bars on the fatigue behavior of the pipelines. To eliminate other potential influences on fatigue strength, the same high-pressure material, SA-723 Gr.2 Cl.2a mod., was used in three different forming variants, in the form of 'Real-Shape' tube samples, subjected to cyclic internal pressure. The results were plotted on the corresponding fatigue curve in accordance with ASME BPVC VIII-3 [1] and compared to long-term experience data. Contrary to previous assumptions, all three variants met the defined requirements without significant differences. One sample failed in the form of a leak, but the number of cycles achieved was within a very satisfactory range.
As failure is preceded by prolonged crack growth, fracture mechanics aspects were also included. For this, SENB comparison specimens were tested to determine the "Paris-Parameters" and the threshold value for crack growth according to ASTM [3]. The results also showed no notable differences among the various material types.
By adhering to ASME BPVC VIII-3 [1] and API 579 [2] Fitness-For-Service standards and using finite element simulations along with material parameters from the fracture mechanics tests, an analytical prediction concept for the service life was developed. This model includes a leak-before-burst estimate, a fatigue calculation, and a fracture mechanics evaluation. Latter was divided into two variants to predict crack growth in the regime of threshold value. The first was defined according to ASME BPVC VIII-3 [1]. For the second, the crack growth rate was modified using the approach by Kohout. Despite the introduction of a load-cycle safety factor of ten for the fatigue calculation the real test results are much higher, leading to a conservative estimate.