The Hazards of Thermal Expansion

We examine the sometimes overlooked hazards of thermal expansion of seemingly incompressible fluids ? liquids. The increase in specific volume of a fluid with increasing temperature may present a hazard when liquid is unintentionally trapped in process piping. Trapped liquids pose the potential for large pressure increases during high temperature service, but also during typical diurnal ambient temperature changes. This paper will present a model to estimate the pressure versus temperature increase for several typical process fluids then discuss two case studies of actual incidents where overpressure of trapped fluids lead to catastrophic releases.

In case study #1, an industrial oven for a food processing application uses a heat transfer oil (HTO) as the heat source. The oven is comprised of several zones and each zone is connected to a plant-wide HTO network via supply and return pipes. The user installed manual isolation valves for each zone, but failed to provide means for pressure relief between isolation valves. A routine maintenance issue led to the closing of both isolation valves to one zone leaving HTO trapped within them. The oven was subsequently started by personnel who did not know of the trapped fluid condition. The heat from an adjacent zone induced the thermal expansion of trapped HTO, which caused the rupture of a flexible pipe joint. The release of hot, pressurized HTO resulted in an explosion, which caused significant property damage and business interruption.

In case study #2, a custom-made, self-contained freeze dryer for pharmaceutical applications uses a refrigeration system in which nitrogen vapor cools a stream of silicone oil in a heat exchanger. A process upset resulted in a no-flow condition of the oil which in turn resulted in the oil freezing inside the nitrogen-cooled coils of the heat exchanger. The frozen heat exchanger effectively acted like a closed valve. An electric heater located between the heat exchanger and a check valve was activated, thus heating a trapped amount of silicone oil. The combination of faulty design with a lack of adequate safeguards led to a catastrophic pipe rupture, explosion, fire, and personnel injury.

By considering specific case studies, we show the sequence of events that led to catastrophic failures could have been avoided by following well-established safety standards and thorough hazard analysis during the design phase. Our findings underscore the importance of thermal pressure relief devices and their role in the protection of process piping.