Damaging hydraulic shock events are typically the condensation-induced type and frequently occur in low-temperature ammonia systems.10 Condensation-induced shock events are often associated with the transition from high temperature and pressure to low temperature and pressure, during and after the defrosting of evaporators with hot gaseous refrigerant. Failed system components typically include evaporators, associated piping components (e.g., suction stop valves), evaporator suction piping, and suction header mains downstream of the evaporators. Based on an examination of the failed components and a review of the evaporator defrost system at the time of the incident, the ammonia released at Millard was likely a result of condensation-induced shock, vapor-propelled liquid, or a combination of both, that ruptured the evaporator piping manifold and suction header. Vapor-propelled liquid and condensation-induced shock are two forms of undesirable transient events that can occur in piping conveying both liquid and vapor. These two mechanisms often occur together during a hydraulic shock event.11
TYPES