Oil REfineries normally run smoothly as compared to the fertilizer units but units have to be shut down owing to some mechanical breakdown, utility failure, and overhauling or cleaning of the machines or equipment.
Shutdowns due to mechanical failure or utility failure are unplanned and the unit remains down until the maintenance job is completed or utility supply has been started.
On the other hand, shutdown or called as turn around normally after 5 years of period (commonly abbreviated TAR, TA or ATA) is a planned event wherein an entire process unit of a plant is taken off-stream for a long duration (more than a month) to perform equipment overhauling, cleaning, inspection, maintenance, replacement, modifications, piping work etc. TAs are non-routine, troublesome, complex, and potentially more hazardous than normal plant operations. For further details https://www.opuskinetic.com/2017/09/shutdown-turnaround-of-refineries/