A safety audit is almost invariably performed on new plant designs in the chemical industry. This will include at least a Hazop study of the basic design. Many plant owners also require an analysis of the safety integrity level or other methods of the installed safety measures. The concept of acceptable risk changes with time, but many existing plants operate year after year without a systematic assessment of the safety level, even if the plant is revamped or modified. In addition, operating procedures may change without all safety aspects being considered in a formalized way. This sometimes leads to very dangerous and costly incidents, which could have been avoided if regular and systematic safety audits had been performed. This article will discuss the systematic safety assessments that are performed at regular intervals on the 40-and 27years old OCI Nitrogen ammonia plants in The Netherlands. Examples are presented showing learning points that were found after a Hazop and a SIL analysis. (C) 2011 American Institute of Chemical Engineers Process Saf Prog 31: 83-88, 2012