(76e) The Hazards of Thermal Expansion

We examine the sometimes overlooked hazards posed by thermal expansion of trapped liquids in piping systems. Trapped liquids can generate large pressure increases with only modest temperature changes. This phenomenon is well understood by the process safety community. However, it is also a phenomenon that is easily overlooked or dismissed as being of small consequence. This paper discusses a simple model for estimating the overpressure hazard from thermal expansion and illustrates its use with some typical liquids encountered in heat transfer systems. The control of these overpressure hazards is discussed in terms of applicable safety guidelines and standards. We then illustrate the hazard of heating trapped liquids with two case studies; one was a heating system and the other was a refrigeration system. We show that the basic cause of these catastrophic failures could have been avoided by following applicable safety standards in conjunction with a process hazard analysis during the design phase. Our findings underscore two important design guidelines: 1) install a pressure relief device wherever liquid can be trapped and subjected to heating; and 2) verify that each component of the piping system that can be isolated from pressure relief satisfies the design pressure.