Pressure Piling: A Review and Analysis

Predicting the dynamics and magnitude of pressure rise that may be experienced within vessels and piping during an explosion is central to many aspects of the control and mitigation of explosion hazards within the process industry. This is especially true when adequate pressure containment or relief is relied upon as a safeguard. However, connections between vessels and equipment (material conveyance systems, or vent ducting for example) can complicate the dynamics of pressure rise. These connections may result in larger than anticipated overpressures and exceed the thresholds used for safeguard design. This problem can arise in systems with either flammable gases/vapors or combustible dusts.

Pressure piling is a term used to describe the combustion phenomena that occurs when an explosion involves two or more interconnected vessels. When an isolated chamber undergoes a deflagration, the maximum pressure that may be achieved can often be predicted from measured or tabulated information about the material. However, when two or more vessels are interconnected the resulting overpressure and combustion rates that can be generated are typically greater than the those predicted for a single vessel of equivalent volume. The effect is especially pronounced when the volume of the source vessel is larger than the receiving vessel. This phenomenon results from a complex and unsteady interplay between the expanding burnt gases in the source vessel, the propagating flame front, and the flow-field geometry. As a result, systems with installed safeguards may still be vulnerable to catastrophic consequences if pressure piling occurs during an overpressure event.

While attempts to understand, predict, and mitigate pressure piling are not new, it remains an important and elusive subject of inquiry. The influence of physical phenomena across multiple time and length scales leads to a variety of potential solution strategies. Here, analytical models aimed at explaining pressure piling, mainly through thermodynamic relations, and numerical solution strategies, generally semi-empirical in nature, are reviewed and compared to the available experimental data.