THERMAL ANALYSIS OF A VACUUM STIFFENING RING ON A HIGH TEMPERATURE VACUUM COLUMN DURING START UP WITH TEMPERATURE VARYING WITH TIME

摘要

Vacuum columns are generally specified to operate at temperature in excess of 800℉ (427°C). Vacuum columns are vertical pressure vessels designed and constructed in accordance with the requirements of American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section VIII, Division 1 or other recognized code rules. Design temperatures of vacuum columns range from 650℉ (343°C) at the top to 850℉ (454°C) at the bottom. Because they are in vacuum service, they are designed to withstand an external pressure of 14.7 psig (101.4 kPa g) at the design temperature. Stiffening rings are provided on the cylindrical sections to render a cost effective design. External insulation is provided in order to save energy. During the start-up, the operators and/or the owners of the equipment would like to bring the vacuum column on line within the minimum amount of time. The engineers and designers, however, are faced with specifying a realistic yet safe incremental rate of change of temperature inside the vacuum column in order to avoid excessive temperature gradients along the stiffening rings(s). Higher heating rate(s) applied on the inside surface of the vacuum column will produce a higher temperature gradient along the width of the stiffening ring(s) which will result in higher thermal stresses. If the stresses in the stiffening ring(s) exceed the allowable stresses, the stiffening ring(s) could buckle putting the structural integrity of the vacuum column in jeopardy. The aim of this paper is to provide an analytical expression for a temperature distribution along the width of a perfectly insulated stiffening ring on a high temperature vacuum column while the temperature inside the vacuum column is raised as a function of time. Furthermore, a family of curves is provided to estimate the temperature along the stiffening ring(s) for various heating rates.