Unexpected corrosion damage is just as likely to arise from defective process control & lack of recognition of potential damage mechanisms, as incorrect alloy selection.
Operating plant is subject to variability of service condition arising from changes in feedstock, temporary process upsets, shutdowns for inspection and restart all representing a significant challenge to reliable operation. Corrosion damage mechanisms may proceed intermittently or steadily and time to failure may range from seconds to years and consist of initiation and propagation phases.
A management practice for avoiding defective process control will be described which requires identification of integrity operating windows (IOW) that define rates of damage propagation:
- target windows for safe and reliable long-term operation;
- standard windows for a known but manageable rate of damage propagation;
- critical windows: operating outside this critical window will lead to rapid failure of equipment.
Examples from hydrocarbon gas processing will be used to illustrate the IOW management practice which includes defining responses to excursions at each IOW, revealed by instruments and sensors which monitor and control the process.
Peter Wilson (Nuffield Group)