The following lesson learned outlines an installation of pipeline heat tracing for a new process unit within an existing chemical/petrochemical facility. The system design consisted of self-limiting cables for process pipeline heating with a 240-volt power distribution supply (three-phase ungrounded system). The tracing system was designed to maintain the temperature of the process streams, not for heat-up service.
During the initial life of the installation, process upsets caused lines to plug. Operating personnel purged the pipelines with process steam to clear the lines and restore process flow. Super heated 450°F steam operating at 150 psig was used because of its availability.
The exposure temperature exceeded the manufacturer's temperature limits, damaging the cable. When re-energized, the severely damaged cables were identified when the process temperature was not maintained. The solution was to replace those tracer circuits.
An incident happened that fires were reported within the process area. Investigation of the fires identified the source to be the energized heat-tracing cables that were damaged but had not been replaced following the steam cleaning of the pipelines. The fires were started by arcing between the phase conductors and the stainless steel braid of the heat cable. The fault currents limited by the high circuit impedance were insufficient to trip the branch circuit breakers. The cable continued to burn toward the power source until the circuit was de-energized. Sufficient heat was generated during the fault that combustible materials in the vicinity were ignited.
This lesson learned demonstrates the cause and effect of exceeding service ratings of materials. The possibility of an occurrence such as this needs to be considered during engineering study and design. It also points out the need for operating personnel to understand limiting criteria for installed systems.