8/13/2019 ASME Seccin VIII D1 Non Mandatory Appendices E

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APPENDIX E

SUGGESTED GOOD PRACTICE REGARDING

CORROSION ALLOWANCE1

E-1

From the standpoint of corrosion, pressure vessels

may be classified under one of the following groups:

(1) vessels in which corrosion rates may be defi-

nitely established from information available to the

designer regarding the chemical characteristics of thesubstances they are to contain. Such information may,

in the case of standard commercial products, be obtained

from published sources, or, where special processes are

involved, from reliable records compiled from results

of previous observations by the user or others under

similar conditions of operation.

(2) vessels in which corrosion rates, while known

to be relatively high, are either variable or indeterminate

in 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 required

for the initial operating conditions should be provided,

which should be at least equal to the expected corrosion

loss during the desired life of the vessel.

E-3

When corrosion effects are indeterminate prior to

design 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.

504

to some degree in the service for which the vessel is

to be used, or when corrosion is incidental, localized,

and/or variable in rate and extent, the designer must

exercise his best judgment in establishing a reasonable

maximum excess shell thickness. This minimum allow-

ance may, of course, be increased according to the

designers judgment.

E-4

When corrosion effects can be shown to be negligible

or entirely absent, no excess thickness need be provided.

E-5

When a vessel goes into corrosive service without

previous service experience, it is recommended that

service inspections be made at frequent intervals until

the nature and rate of corrosion in service can be

definitely established. The data thus secured should

determine the subsequent intervals between service in-

spections and the probable safe operating life of the

vessel.

E-6

For parts which are essential to vessel strength such

as stiffener rings, the attachment of the part to the

shell must provide adequate corrosion allowance or

protection to assure the required strength throughout

the service life. Some attachments, such as intermittent

welds, require protection on both face and root sides;

alternatively, continuous welds or a suitably sized seal

weld between the strength welds will provide protection

for the root side.

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