American Society Of Mechanical Engineers Boiler Research Articles

Strain-Rate Sensitivity Analysis for Ni Alloys to Be Used in Very High Temperature Reactors

Both Alloy 617 and Alloy 230 have been considered the most promising structural materials for the Very High Temperature Reactor (VHTR). In this study, mechanical properties of both alloys were examined by performing tensile tests at three different strain rates and at temperatures up to 1000°C. This range covers time-dependent (plasticity) to time-independent (creep) deformations. Strain-rate sensitivity analysis for each alloy was conducted in order to approximate the long-term flow stresses. The strain-rate sensitivities for the 0.2% flow stress were found to be temperature independent (m [approximate] 0) at temperatures ranging from room temperature to 700°C due to dynamic strain aging. At elevated temperatures (800°C to 1000°C), the strain-rate sensitivity significantly increased (m > 0.1). Compared to Alloy 617, Alloy 230 displayed higher strain-rate sensitivities at high temperatures. This leads to lower estimated long-term flow stresses. Results of this analysis were used to evaluate the current American Society of Mechanical Engineers (ASME) allowable design limits for each alloy. The study showed that the allowable design stresses in the ASME Boiler and Pressure Vessel Code for either alloy do not provide adequate long-term degradation estimation. Nevertheless, rupture stresses for Alloy 617, developed in the ASME code case N-47-28, can generally satisfy the safety margins at 800°C and 1000°C estimated in the study following the strain-rate sensitivity analysis. Furthermore, additional material development studies might be required, since the design parameters for rupture stresses are constrained such that the current VHTR conceptual designs cannot satisfy the material limits.

Design-Guideline Strategies for High-Pressure/High-Temperature Equipment

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 23943, ’Design-Guideline Strategies for High-Pressure/High-Temperature Equipment,’ by H. Brian Skeels, Kwok Lun Lee, and Anand Venkatesh, FMC Technologies, prepared for the 2013 Offshore Technology Conference, Houston, 6-9 May. The paper has not been peer reviewed. As the need for oil and gas equipment working in hotter and higher-pressure environments continues to mount, the effort to develop an adequate set of design, material, and validation practices continues to be challenging. More-rigorous stress analysis and design methods that more closely model fast-fracture burst conditions are needed to achieve safe, reliable, and cost-effective equipment designs. Introduction As oilfield equipment is subjected to more-severe and longer periods of cyclic forces (internally from irregular wellbore flowing conditions or externally from metocean conditions), many agree that fatigue life and monitoring for these conditions has to be a part of the design equation. This fact is mentioned in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, recognizing the two failure modes of a pressure vessel: leak before burst or a fast-fracture burst. Many oilfield design guidelines negated a need for fatigue analysis because of the long-standing practice of keeping maximum design loads and stresses well below two-thirds material yield strength, a practice that also keeps cyclic-loading issues well inside the “infinite” region on S/N curves in most cases. Unfortunately, this quasi-static design philosophy grows less accurate as the pressure vessel’s wall thicknesses grow or geometries become more complex when addressing high-pressure/high-temperature (HP/HT) conditions. As a result, many have turned to the ASME Boiler and Pressure Vessel Code for guidance. Divisions 2 and 3 of the ASME Code present alternative design methodologies to be considered as internal pressure and temperature increase. However, the designer, using the code, is still free to select which division to follow for his application. The American Petroleum Institute (API) has long followed a subset of Division 2 of the ASME Code, but it now recognizes many shortcomings as one considers HP/HT conditions. The published technical report only cites the current state of the art and discusses the issues to be tackled. Many observers agree that leaving the issue to so-called “sound engineering judgment” would result in many variations of opinion between operators and manufacturers, likely resulting in inefficiency and repeated tasks. Current efforts address two options: extending current API design practices to higher pressure ratings or establishing a demarcation when different design practices are needed.