Mechanical Analysis Of The Cracking Causes Of The Tubesheet In Slurry Oil Steam Generators

The slurry oil steam generator is an important heat exchanger in the catalytic cracking unit of oil refinery plants. Its main function is saving the energy by cooling the slurry oil in the tube side and increasing the temperature of the steam-water-mixture in the shell side. But cracking failures of the tubesheet often occur even within a short time after the generator being used. Those failures affect the safety of equipment and cost a lot in engineering. Many researchers and engineers spent their efforts in studying for prolonging the service life of the generator. In this thesis, cracking causes of the tubesheet in slurry oil steam generators are studied in the mechanical point of view.(1) Applying HTRI software to calculate fluid temperature distribution of the slurry oil steam generator and convective heat transfer coefficients, temperature field of the slurry oil steam generator is obtained by performing finite element analysis with the software ANSYS. It is found that because of tube bundle being plugged into the tubesheet, the temperature distribution at the tubesheet is quite different from that at a circular plate without tubes inside. The depth affected by the shell-side fluid is clearly decreased.(2) Finite element analysis was performed to calculate stress distributions and values at the tubesheet surfaces of the slurry oil steam generator under the action of temperature difference. It is found that under the action of temperature difference, the stresses at the both sides of the fixed tubesheet are almost all tensile stresses. The causes may be that the surfaces of the tube sheet are divided into several regions for the partition of the tube-side and tube bundle are plugged into the tubesheet. It is also found that as the different tube-side passes have different fluid temperatures, non-uniform temperature distributions are presented at the surfaces of the tubesheet. The higher the medium temperature of the pass, the larger the tensile stress of the region is. Of cause. a larger tensile stress could be easier to induce cracking of the tubesheet of the generator.(3) Effects of the thickness of the tubesheet on the stresses at the surfaces of the tubesheet were investigated under six load cases. Results show under the action of the pressure loading, the maximum tensile stress decreases with the increasing of the tubesheet thickness. But under the action of the temperature differences, the maximum tensile stress increases with the increasing of the tubesheet thickness, especially the tensile stress at the tubesheet surface on the shell-side. When the thickness of the tubesheet increases to a certain value, the stress at the tubesheet caused by thermal differences may be larger than that caused by pressures. So the thickness of the tubesheet is not the bigger the better.(4) Effects of the two passes partition patterns on the stresses at the surfaces of the tubesheet of the slurry oil steam generator were investigated under six load cases. It is found that for the same thickness of the tubesheet, different pass partition patterns give different stress distribution, but the stress magnitudes are not so different. This means that changing the pass partition patterns cannot effectively decrease the stresses at the tubesheet surfaces.(5) Effects of the number of the tube-side passes on the stresses at the surfaces of the tubesheet of the slurry oil steam generator were also investigated under six load cases. It is found that for the same thickness of the tubesheet, different tube-side pass number give different high stress zone although the stress distribution form and magnitude are not so different. For the two tube-side passes, the high stress zone is the largest and the next is four passes and so on.(6) The radial stress components along the tubesheet thickness of the slurry oil steam generator under six conditions were investigated. Results show that under the conditions including temperature difference, the radial stress at a thin layer of the tubesheet on the shell-side is the tensile stress, however the stress at the middle of the tubesheet is a compressive stress. The radial stress along the thickness on the tube-side depends on loading conditions.(7) The results obtained in this thesis indicate that for a float-heat shell-and-tube heat exchanger, if the thermal stress is not considered, the equipment may not be safe when the tubesheet thickness is larger and the temperature difference between the fluids on two sides is bigger. The slurry oil steam generator studied in this thesis is a typical case, or in other words, because the thermal stress is not included, the strength design of the slurry oil steam generator is not safe.