APPENDIX ESUGGESTED GOOD PRACTICE REGARDING

CORROSION ALLOWANCE 1

E-1

From the standpoint of corrosion, pressure vesselsmay be classified under one of the following groups:

(1) vessels in which corrosion rates may be defi-nitely established from information available to thedesigner regarding the chemical characteristics of thesubstances they are to contain. Such information may,in the case of standard commercial products, be obtainedfrom published sources, or, where special processes areinvolved, from reliable records compiled from resultsof previous observations by the user or others undersimilar conditions of operation.

(2) vessels in which corrosion rates, while knownto be relatively high, are either variable or indeterminatein magnitude;

(3) vessels in which corrosion rates, while indeter-minate, are known to be relatively low;

(4) vessels in which corrosion effects are knownto be negligible or entirely absent.

E-2

When the rate of corrosion is closely predictable,additional metal thickness over and above that requiredfor the initial operating conditions should be provided,which should be at least equal to the expected corrosionloss during the desired life of the vessel.

E-3

When corrosion effects are indeterminate prior todesign of the vessel, although known to be inherent

1 When using high alloys and nonferrous materials either for solidwall or clad or lined vessels, refer to UHA-6, UCL-3, and UNF-4,as appropriate.

515

to some degree in the service for which the vessel isto be used, or when corrosion is incidental, localized,and/or variable in rate and extent, the designer mustexercise his best judgment in establishing a reasonablemaximum excess shell thickness. This minimum allow-ance may, of course, be increased according to thedesigner’s judgment.

E-4

When corrosion effects can be shown to be negligibleor entirely absent, no excess thickness need be provided.

E-5

When a vessel goes into corrosive service withoutprevious service experience, it is recommended thatservice inspections be made at frequent intervals untilthe nature and rate of corrosion in service can bedefinitely established. The data thus secured shoulddetermine the subsequent intervals between service in-spections and the probable safe operating life of thevessel.

E-6

For parts which are essential to vessel strength suchas stiffener rings, the attachment of the part to theshell must provide adequate corrosion allowance orprotection to assure the required strength throughoutthe service life. Some attachments, such as intermittentwelds, require protection on both face and root sides;alternatively, continuous welds or a suitably sized sealweld between the strength welds will provide protectionfor the root side.

COPYRIGHT American Society of Mechanical EngineersLicensed by Information Handling ServicesCOPYRIGHT American Society of Mechanical EngineersLicensed by Information Handling Services