The Role of Calcite Dissolution and Halite Thermal Expansion as Secondary Salt Weathering Mechanisms of Calcite-Bearing Rocks in Marine Environments

This research focuses on the analysis of the influence of two secondary salt weathering processes on the durability of rocks exposed to marine environments: chemical dissolution of rock forming minerals and differential thermal expansion between halite and the hosting rock. These processes are scarcely treated in research compared to salt crystallisation. The methodology followed in this paper includes both in situ rock weathering monitoring and laboratory simulations. Four different calcite-bearing rocks (a marble, a microcrystalline limestone and two different calcarenites) were exposed during a year to a marine semiarid environment. Exposed samples show grain detachment, crystal edge corrosion, halite efflorescences and microfissuring. Crystal edge corrosion was also observed after the laboratory simulation during a brine immersion test. Calcite chemical dissolution causes a negligible porosity increase in all the studied rocks, but a significant modification of their pore size distribution. Laboratory simulations also demonstrate the deterioration of salt-saturated rocks during thermal cycles in climatic cabinet. Sharp differences between the linear thermal expansion of both a pure halite crystal and the different studied rocks justify the registered weight loss during the thermal cycles. The feedback between the chemical dissolution and differential thermal expansion, and the salt crystallisation of halite, contribute actively to the rock decay in marine environments.

Behaviour of flange joints in Steam Generator under thermal loads

Pressure vessels such as steam generators are subjected to high temperature, in addition to high pressure during the operating condition. Flanges and bolts are made up of different materials whose coefficient of thermal expansion varies. Usually, thermal expansion in bolts is greater than that of flanges. At elevated temperatures bolts expand more than that of flanges, resulting in decrease of compression in connected members achieved during assembly stage, which in turn decreases the contact stress in gasket. This can lead to leakage of internal fluid. The loss in gasket contact stress due to differential thermal expansion can be nullified by using sleeves of higher thermal expansion between the flange-nut and flange-bolt head interfaces. At higher temperatures sleeves expand more than bolts and flanges, pushing the flanges closer towards each other, thus decreasing gap created due to differential thermal expansion. The behaviour of gasketed blind flange joint with and without sleeves is analysed and the performances are compared under thermal loads. The non-linear behaviour of gaskets is included by specifying the loading and unloading characteristics with hysteresis.

Elevating the Pressure and Temperature

This article discusses the modernization of the rules of Section I of the ASME Boiler and Pressure Vessel Code to better accommodate the challenges of increasing temperature. At very high pressures and high temperatures, the current Section I rules require components to be comparatively thick, but making things thicker is not always better. In thick components, temperature gradients and consequent differential thermal expansion produce large secondary stresses. When pressure and temperature drive a component’s thickness to be very large compared to the size of the component, it can compromise that component’s ability to endure thermal transients that occur in service. One of the biggest challenges in addressing elevated temperature service is understanding creep and fatigue interaction and developing appropriate design rules to manage that. Another challenge is that corrosion mechanisms change with increasing temperature. The push to higher temperatures will spawn development of new materials to meet all the design goals. The BPVI standards committee on Power Boilers will also need to evaluate whether some of the construction details traditionally used will be appropriate at higher temperatures.

Influence of Applied Misalignment on the Balanced High Speed Flexible Coupling of Fighter Aircraft

The fighter aircraft transmission system consists of a light weight, High Speed Flexible Coupling (HSFC), used to transmit power from engine gear box to accessory gear box at speed ranging from 10,000 to 18,000 rpm. The HSFC accommodates larger parallel and axial misalignment resulting from differential thermal expansion of the aircraft engine and mounting arrangement. As the HSFC operates at higher rotational speeds close to critical velocities, it is important to analyze, the unbalance exciting forces considering the misalignment. In the present work, prediction of critical speed by camp bell diagram and unbalance response of the HSFC has been carried out using FEA. An experimental investigation also been carried out to study the influence of applied misalignment on a bi-plane dynamically balanced HSFC. The study shows that lower reaction forces are transmitted to HSFC end supports with the applied misalignments, as they are accommodated by the elastic material flexure of flexible plates.