Agenda Item: 620-269 Title: Radiography of Compression bar on API 620 Tanks Contact: Rob Annett, Alyeska Pipeline Service Company 907.787.8167 AnnettRC@alyesk-pipeline.com

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Agenda Item: 620
-
269


Title:
Radiography of Compression bar on API 620 Tanks


Contact:

Rob Annett, Alyeska Pipeline Service Company



907.787.8167



AnnettRC@alyesk
-
pipeline.com


Source: Hiner email of 9
/12/2003


Purpose:

Provide additional guidance for radiographic requirements for API 620
thick compression bars


Background:

API 620 paragraph 5.26.2 "Wall Thickness"
-
states, "Complete

radiographic examination is required for all double
-
welded butt joints

wherever the thinner of the plates or the tank wall thickness at the

joint
exceed 1 1/4" and the joint is subjected to tension stresses

greater than 0.1
times the specified minimum tensile strength of the material."

API 62
0 paragraph 5.26.4 "Exemptions"

-

states, "Spot or full radiography
examination is not mandatory on tank bottoms that are

uniformly
supported throughout (for example, concrete slab or compacted sand) or
on components that have a design thickness controlled by compressive
stress only.

Tabl
e 5
-
2 which allows "roofs above liq
uid level" to be single
-
welded
butt
joints with backing strip.


Discussion:

Compression bar design thickness is almost always controlled by the

compression stress, however, under dead load or dead load plus external

press
ure the bar will go into tension and this stress could be more than

0.1
times the specified minimum tensile strength of the material.

A statement should be made clearly addressing which paragraph 5.26.2 or

5.26.4 controls in this case.

Table 5
-
2 allows “ro
ofs above liquid level” to be single
-
welded butt joints
with a backing strip. There is no thickness limitation provided. A
compression bar could possibly be considered “Roof”, not double
-
welded
as noted in 5.26.2
,

and radiography would not be required.

J
ustification:

The compression bar can be subject to tension u
nder certain loading
conditions. A
s such
complete radiography is required per paragraph
5.26.2.

(See following proposed wording change)


A compression

bar is not considered “Roof”. Clarificati
on of
what is and
is not included in the “roof” is prohibitve and unnecessary. (No proposed
wording change)



Current Wording

Proposed Wording

5.26.4 Exemptions

Spot or full radiographic examinations is
not mandatory on tank bottoms that are
uniformly s
upported throughout (for
example, concrete slab or compacted
sand) or on components that have a design
thickness controlled by compressive
stresses only.

5.26.4 Exemptions

Spot or full radiographic examinations is
not mandatory on tank bottoms that are
uni
formly supported throughout (for
example, concrete slab or compacted sand)
or on components that
have a design
thickness controlled by compressive stresses

only

are
not subject to
tension

stresses

greater than

0
.1 times the minimum tensile
strength of the
material
.