We examine the 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 is a phenomenon that is often overlooked or dismissed as being of small consequence. This article 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 a case study. We show that the basic cause of this catastrophic failure could have been avoided by following applicable safety standards in conjunction with a process hazard analysis of the entire system 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. (C) 2010 American Institute of Chemical Engineers Process Saf Prog 29: 313-317, 2010