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NONMANDATORY 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 the

substances 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 known to

be negligible or entirely absent.

E-2

When the rate of corrosion is closely predictable, addi-

tional 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 to

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, asappropriate.

528

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 allowance may, ofcourse, 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 pre-vious 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 estab-

lished. The data thus secured should determine the subse-

quent intervals between service inspections 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, con-

tinuous welds or a suitably sized seal weld between the

strength welds will provide protection for the root side.

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