Cold boxes, or more precisely, the brazed aluminum heat exchangers (BAHX) that comprise them operate unseen and do not lend themselves to the inspection methods typical of most other critical process equipment. Furthermore, BAHXs are not immune to process upset conditions which over time can threaten the structural integrity of these vital components in the form of thermal fatigue cracking. The challenge to the chemical processing industry is therefore one of attempting to establish the condition of ageing cold boxes especially with regard to quantifying the cumulative damage resulting from multiple normal and emergency upset events.
NOVA Chemicals is one of a growing number of companies that are using finite element methods to closely analyze the long-term response of BAHXs to such transient conditions and to interpret the results in the broader context of fitness-for-service and remaining life. The same methodology is also being employed to look at newer BAHX designs in order to assess their vulnerability to future thermal fatigue damage.
This paper describes a recent case history undertaken at NOVA Chemicals in which a number of older BAHXs were analyzed using finite element modeling (FEM).