External inspection corrosion limits

Localized pitting or line corrosion will not be detected by the hydrostatic test. These types of corrosion may cause significant localized stresses, and care must be exercised in judging whether or not the conditions are extensive enough to require rejection. Use the criteria on corrosion limits discussed under the external visual inspection section. 

Interior corrosion is best evaluated by a hydrostatic test combined with careful visual inspection. Thickness-measuring and flaw detection devices of the ultrasonic type may be used to evaluate specific conditions. Basic corrosion limits for both low and high pressure steel cylinders were discussed under the External Inspection section of this chapter. 

Careful process hazards analysis may show that a particular vessel need not be designed to withstand a full vacuum (e.g., if the maximum attainable vacuum is limited to the performance characteristics of an exhauster). Whatever the vacuum rating, rated vessels must be periodically inspected to ensure that internal or external corrosion has not diminished the vessel strength. 

Wherever possible, there should be a continuous secondary barrier for the entire pipeline system, in the form of jacket pipes, collecting basins, and channels. In some cases, this is not possible because it would impede cathodic corrosion protection or because differential expansion of the pipe and outer jacket threaten the systan. The secondary barrier can then only consist of intensive organizational and operative backup measures shorter inspection periods, shorter distances between shutoff points to limit possible losses, more elaborate provisions for leak detection, continual ronote monitoring of the pipeline systan, both internally and externally, equipment for tackling anergency events and catastrophes, contingency plans, and so forth. 

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