Best Practice for Risk Based Inspection

TYPES OF DETERIORATION AND MODES OF FAILURE OF PRESSURE
SYSTEMS AND CONTAINMENTS
• Failure of the Protective Devices
Many boilers and other pressure vessels have failed through over-pressure as a
result of the pressure relief system valves or bursting discs being ineffective.
Periodic inspection and maintenance procedures should detect problems such as
fouling or incorrect adjustment. The position and potential to isolate protective
devices from pressure systems should be checked as part of the procedures,
particularly after plant modifications or maintenance.
• Corrosion/Erosion (general, local, pitting)
Wall thinning due to corrosion or erosion is common and can occur by a variety of
mechanisms including simple rusting, aqueous chloride, high temperature
sulphidation, vapour liquid impingement etc. Wall thinning may be general over the
surface, localised to certain areas or the presence of pitting. Where a plant item may
be subject to internal or external corrosion or erosion, the current thickness of
material should be determined and reviewed against the original design values and
previous inspection results to ascertain the rate of thinning.
Fig.D1 General corrosion of external surface of pipework
When wall thinning due to corrosion or erosion is detected, the position and rate of
thinning needs to be established to enable the implications to be assessed using an
accepted methodology (e.g. BS 7910 or API 579). A certain amount of thinning
may be tolerable in a plain membrane area whereas the same thinning in areas
subject to direct loading, such as saddle points, may be a cause of concern. The
degradation rates, established from the inspection results, should be used to predict
D1