A Case Study of Pipework Fracture due to Hydraulic Shock in
A Case Study of Pipework Fracture due to Hydraulic Shock in an Ammonia System (2008)-In June 2007 a deflagration occurred at a frozen food factory. The incident investigation identified a ruptured coil located in a spiral freezer as the source of the ammonia leak. Within a 20 minute time frame, the ammonia concentration in the room surrounding the freezer enclosure increased to a flammable level and subsequently a deflagration occurred. The arcing found in a wire inside a drinking fountain was the likely ignition source. This freezer design, including the control valve group and liquid transfer vessel, is very typical for the refrigeration industry. The freezer and its associated refrigeration infrastructure were in operation for more than 10 years and underwent 3 Process Hazard Analyses. The incident investigation concluded that a hydraulic shock caused by a vapor propelled liquid slug generated enough of a transient pressure spike to cause coil rupture. A detailed analysis of the control valve group and liquid transfer vessel design and system dynamics, in conjunction with a metallurgic fracture analysis, was used to develop a mathematical model to describe and reconstruct the mechanism of this incident. The results are quite startling and support the fact that a seemingly insignificant system upset has the potential of leading to a catastrophic event. The incident investigation identified additional safeguards that could have prevented this incident.
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