Galvanizing kettle life

Abstract

The prediction of life under varying conditions of stress and temperature is a major problem in the design of machines. Available data are usually obtained in tests under constant, non-varying conditions and test data under varying conditions are usually limited. If variable condition data are available it usually does not exactly correspond to the data required. The designer is then faced with either setting up the necessary test equipment to obtain the necessary data, or establishing a theory to relate steady state data to the variable conditions, or some combination of both. In this thesis both test equipment and a life-fraction theory were used to determine the design and ultimate life of a galvanizing kettle. The test equipment was used to obtain actual field condition data. This data was then used to determine the constants in the analytical equations used in the design so as to relate actual conditions to theory. The equations representing temperature, wear rate, static stress, thermal stress and rupture stress fitted with suitable constants to represent the field data are combined by a life fraction theory for a theoretical kettle life. This was done for various kettle design of different plate thicknesses, plate depths, and heat transfer rates to obtain optimum conditions. The complexity of the problem required computer solution. For the case involving variable temperature and stress, the life-fraction theory estimates the life by assuming that during any small interval of time the specimen loses some fraction of its life which is independent of the stress and temperature history. Failure occurs when the sum of these fractions is equal to unity. In the case of gradual varying stress or temperature an analytical solution is possible for simple cases. For complex cases of gradual varying stress or temperature or when both stress and temperature vary under simple or complex conditions a computer solution is necessitated to approximate the analytical approach by a number of finite steps. The basic assumption in this thesis is that once the kettle is loaded and put into service the stress increases gradually and the temperature decreases gradually to failure. Although this assumption is not strictly true, it is on the safe side since any unloading of the kettle will reduce the stress and temperature conditions to safe values and contribute only to the extension of the kettle's life. However, it should be noted that carelessness in the field in the initial start up of the kettle or careless shutdowns or restarts of the kettle can alter the kettle life drastically. Slow start ups and shutdowns are also assumed with no thermal or mechanical shocks. The importance of knowing the serviceable life of a machine to prevent catastrophic failures cannot be underestimated in these days of increased liability; not withstanding the economics of utilizing materials to the maximum efficiency and economy.