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S t r e s s C o r r o s io n in a 1 2 k t o n n e
F u l l y R e f r i g e r a t e d A m m o n i a
Storage Tank
S t r e s s c o r r o s i o n c r a c k i n g wa s f o u n d d u r in g t h e f i r s t i ns p e C ti on
of a 12 00 0- te -ca p a c i~ fu ll y -re fr igerated am mo n ia s torage tan k
a f t e r n i n e y e a r s o f c o n t i n u o u s s e r v i c e . T h i s p a p e r d e s c r i b e s t h e
inspect ion and metallurgical asp ect s r i sk evaluat ion and remedial
t a s k s n e c e s s a r y b e f o r e i ts r e tu r n t o s e r v i c e a s we l l a s t h e
recom m iss ion in g p rocedu re .
J R B y r n e a n d F E M o i r
National Vu lcan Engineering Insurance Group Ltd. M anchester England
and
R D W i l liams
BA SF Ch em icals Ltd. Middlesbrough England
INTRODUCTION
in 1978 at its seal Sands, UK si te,
Monsanto Ltd commissioned a single wall,
ful ly refrigerated, atmospheric anhydrous
ammonia storage tank of 12000 te capacity
(30m dia x 26.3m high), construction to BS
4741:71, and contained within a ful l height
pre-stressed concrete bund (Figure 1).
Design pressure was 140 mbar g.
For logistical reasons, Acoustic
Emission testing was used to extend the
f i rst internal inspection beyond the six
years recommended in the C A code of
practice (1). No significant defects were
recorded during AET s i n May 1984 and
October 1985 i.e. at six and seven year
intervals. In December 1985 the site was
acquired by BASF AG. They did not support
the use of AET and so an internal
inspection programme was implemented,
culminating in tank entry in October 1987.
This paper describes the inspection
and discusses the discovery and
consequences of stress corrosion cracking.
The apparent fai lure of AETto detect
stress corrosion cracking is not discussed.
DET ILS OF CONSTRUCTION
The cylindrical shell was bu ilt from
11 courses each containing 10 plates as
detai I ed i n Appendix 1.
The bottom floor plates are 6mmthick
to BS4360 : 43A and the outer annular plates
are 8mmthick to BS3460- 50C. The roof
sheets are 5mmthick to BS4360 : 43A. The
plate containing the shell manway and outlet
nozzle was stress relieved prior to
instal lation.
Welding was carried out using Ferex
E7018 LT which is a basic hydrogen
controlled electrode. No preheat or post
weld heat treatment was carried out.
BS 4741 was amended in 1972 and 1980.
The effects of these changes on the original
design cr iteria are summarised in Appendix
i. For the convenience of operators of
tanks Constructed to API 620 APP-R, a
comparison of the standards is given in
Appendix 2.
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HISTORY OF THE TANK
The tank was commissioned in
accordance with the CIA Code of practice
for the large scale storage of f ul ly
refr igerated ammonia in the UK (1975) and
incorporated a nitrogen purge to eliminate
the risk of an explosive ammonia/air
mixture.
Ammonia is received from various
European sources, mainly as 6000 te
shipments, and enters the tank through a
roof nozzle and a dip-pipe terminating at
floor level . All shipments are analysed
for water and oil. Water has been
typica ] ]y O. 02% w/w wi th a range O. 01
to 0.29% (See Appendix 3 for 1983- 1988
results summary). Oil has been typ ica lly
5 ppm with a range 1 to 13 ppm.
Ammonia vapour is condensed and
returned to the tank through a full height
dip pipe. Non condensables are manually
vented as necessary.
Operational practices have been
identical to those used throughout the
industry and there have been no abnormal
incidents during the service l i fe of the
tank.
Decommissioning for the inspection was
carried out in September/October 1987.
There Is no low point drain and so a
positive displacement pump was used for the
final stages of the transfer of any liquid.
This method left approximately 25 te of
anhydrous ammonia af ter de-inventorying.
Water was sprayed through the cool down
line and into the tank to remove gaseous
ammonia. The result ing solution was
disposed of within the Site.
INSPECTION
: _
Procedure
The inspection programme was based On
the requirements of the CIA code of
practice for ful ly refrigerated tanks and
relied on magnetic particle inspection as
the primary defect detection method.
(Appendix 4 and 5). Additional ly, MPI was
done on the complete floor to shell f i l l et
weld, all vertical welds in the fi rs t
course, the complete circumferential weld
between the f i rs t and Second courses, and
al l internal welds of shell and roof
nozzle.
Access to the f i rs t course was made
from mobile platforms and to the other
courses from 6m x O.5m cradles suspended
from the roof beams. Rust .inhibited slurry
blasting Was used for the floor and f i rs t
course welds; rotary grinding discs were
used for the remainder. Figure 2 shows both
platform and cradle and areas prepared for
MPI,
Results
For the purpose of this report the term
constructional weld is applied to all the
welds carried out in the tank which are not
actual parts of the main seams. These are
generally cleat welds, arc strikes, weld run
ons, plate repairs and uncontrolled weld
repairs. Past experience with Ammonia
Spheres (at ambient temperatures) has shown
such areas to be the most susceptible to
stress corrosion cracking, mainly because of
the high hardness microstructures and
locally high residual stress associated with
these areas. (2)
The overall constructional condition of
the tank was considered to be within the
requirements of the code BS 4741 (1971).
However, the following notes were made
during the visual examinations-- there was
evidence of some misalignment of the plates
and uneven weld prof iles with the weld width
varying noticeably in certain areas (the
varying course to course thickness of the
plates could account for some of these
features), there were many arc str ikes and
wel d run-ons.
Defects ....Revealed by N.D.T;
Using MPI, 1305 indications were
recorded within the tank during the in i t i a l
inspection and could be categorised into
three general groups:
1. Transverse defects in the
circumferential seams
2. Defects within constructional welds
3. Defects associated with the bottom
fillet weld
No signif icant defects were found in the
stress relieved outlet/manway plate.
The remainder of the testing gave the
following results'-
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(a) Ultrasonic testing of the shell and
floor plates showed no evidence of
wastage.
(b) Vacuum box testing of the f loor welds
and the shell to annular plate weld
showed no defects.
(c) Examination of four holding down bolts
by ultrasonics together with magnetic
particle testing of the external shell
to annular ring weld showed no defects.
By careful grinding of selected
defects, i t was found that they were less
than 2.0ram deep.
Metal og ap h c Exami na i 0n of Defec Ty pes
A selection of each type of defect
along with any unusual indications were
examined microscopical ly. It was not
possible to examine the bottom f i l le t weld
regi on microscopical y because of
restricted access.
Microscopical examination was carried
out using a Union Portable microscope after
polishing the surface to a 1 pm finish and
etched with 2~ Ni tal . The surface was
subsequently repolished and etched to
ensure complete removal of any deformed
layers. On si te microscopy was capable of
examinations up to 400x magnifications but
replication of areas using Struers
Transcopy and subsequent surface coating of
the replica enabled examination up to 600x
in the laboratory. Replication also
enabled photmicrographs to be produced.
TRANSVERSE DEFECTS IN THE CIRCUMFERENTIAL
- S - E ~ A M -
There was a large number of transverse
defects in the circumferential seams
(particularly in Circumferential Seam 2).
I t was noted that these defects occurred
predomi nantly in the bottom weld run of
each seam and ran vertically down into the
heat affected zone. In many cases a small
step was observed in the indications
corresponding to the position of the fusion
boundary. A typical defect is shown in
Figure 3 after magnetic particle testing.
These ini t ial observations suggested
that these defects were hydrogen cracks
produced during vessel fabrication. It has
been documented that transverse hydrogen
cracks would require comparatively high
hydrogen levels. (5).
Longitudinal cracks could occur at sl ight ly
lower levels. It could not easily be
explained at this stage why the cracks were
occurring predominantly in the bottom weld
run of the circumferential seams but i t is
intended to discuss this later in this
paper.
Examination of a selection of the
transverse defects showed them to have
similar characteristics. A detailed sample
of this is shown in Figure 4. The defect
di rectly above the 6.5/8" mark on the
measure was pol i shed. It consisted
predominantly of a vertical indication
exhibiting a step at the fusion boundary
which extended a short way along the
boundary.
Microstructural examination revealed
that these small steps associated with many
of the cracks corresponded to original
welding defects predominately hydrogen
cracks and some slag inclusions. In the
composite photomicrograph, Figure 4, the
stepped crack running vert ical ly is clearly
a weld metal hydrogen crack exhibiting the
characteristic appearance of hydrogen
cracking and a pronounced stepped appearance
(4). Weld metal hydrogen cracks are usually
transgranular in respect to the final
transformed microstructure but follow the
prior austenite grain boundaries of the high
temperature microstructure. Some finer
hydrogen cracks are also evident. The
horizontal crack in the composite photograph
is much finer and less stepped and can be
seen to run from the weld metal into the
heat affected zone of the parent metal, this
being below the circumferential weld.
Examination of these transverse cracks
at higher magnification, (Figure 4) revealed
them to be transgranular to both the weld
and heat affected zone microstructures and
unaffected by any obvious microstructural
features. These cracks were very fine,
slightly branching and only slightly
wandering with very sharp and fine crack
tips. In any previous microscopical
examination in ammonia storage vessels such
defects have been characterised as stress
corrosion.
Several other areas exhibiting
transverse cracking were polished and
similar features were revealed.
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Hardness testing of the main seams
using the Krautkramer Microdur Portable
Hardness tester were carried out at the
same time and the average values were as
fol l ows- -
Parent Metal - 170 to 188 Hv
Heat affected zone - 220 to 260 Hv
Weld metal top run - 217 to 230 Hv
Weld metal bottom run - 227 to 237 Hv
These hardnesses, although fair ly
high, were not considered at this stage
suf ficiently high to cause concern where
controlled welding of the main seams was
used
DEFECTS IN CONSTRUCTIONAL WELDS
Areas were selected to categorise the
defect types and Figures 5 and 6 are
typical examples where cracking is evident.
In almost every constructional weld
examined the cracking was transverse to the
major axis of the weld and extremely
branched, exhibit ing no favoured direction.
Visual characterisation based on past
experience (backed up in each case by
microscopical work) would classify the
straighter defects as a mixture of hydrogen
cracking and stress corrosion.
Microscopical examination of such
areas revealed similar features to the
previous group of defects.
Figure 7 shows a typical area in
greater detail . The indications below the
27.8 cm mark on the measure were polished
and etched and were ident ified as being
associated with a small arc strike. The
photomacrograph in this figure at x20
magnification shows a darkly etched patch
surrounded by a lighter etched area. These
were identi fied as weld metal in the centre
and heat affected zone on the outside, the
remaining area corresponding to parent
metal. The dark island of weld metal
contained some fair ly coarse hydrogen
cracks as seen before. The two vertical
cracks were much finer and ran through the
heat affected zone. The crack on the left
hand side of the figure ran on into
unaffected parent metal for approximately
3mm. As before these defects were fai r ly
straight with some very fine side branches
were sharp ended. The defects tended to
branch less in parent metal but were st i l l
very fine and slightly wandering.
This sample was very similar in
appearance to a large proportion of defects
within the tank and would be categorised as
stress corrosion cracking in an ammonia
sphere. I t is not considered that any other
crack mechanism could propagate in this
manner into parent metal.
Hardness tests were carried out on many
of the areas; average values were as
fo ll ows -
Parent Metal - 165 to 180 Hv
Heat affected zone - 22D to 310 Hv
Cleat welds etc - 230 to 385 Hv
DEFECTS ASSOCIATED WITH THE BOTTOMFILLET
@ELD ...........................................................................
A large number of crack- like defects
were observed in the floor to shell f i l let
weld, the majority of which were found to be
transverse to the weld. A typical area of
cracking is shown in Figure 8 after MPI and
Figure 9 after light grinding and etching.
Visually these were very similar to the
transverse defects observed in the
circumferential seams described previously.
Again they were predominantly restricted to
the bottom run of weld and continued into
the heat affected zone.
It was impossible to polish these
defects in situ because of di f f icul ty with
access into the corners. Microscopical
categorisation was therefore not possible
but i t was considered likely that the same
mechanism will have been operating here as
on the circumferential seams.
METALLOGRAPHIC COMPARSONSWITH KNOWNS.C.C.
Subsequent to the in i t ial site visi t
the large number of replicas (some 58 areas
had been examined at this stage and replicas
taken) were examined more carefully after
coating with a thin film of gold.
Comparisons were made between these and
other known examples of ammonia stress
corrosion from spheres and tanks. Two
comparisons are given below.
A small bullet type vessel used for
ambient temperature storage of anhydrous
ammonia was found to be leaking after only a
very short period of service. The vessel
was scrapped after every weld seam was found
to contain stress corrosion cracking. A
photograph of a typical area after magnetic
part icle testing is shown in Figure 10 where
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a large number of transverse defects were
apparent in weld, heat affected zone and
parent metal.
Microscopical examination revealed
typical stress corrosion features as shown
in Figures 11 and 12. The similarity
between these and the Seal Sands tank is
apparent.
The only previous example of ammonia
stress corrosion cracking in a large fu l ly
refrigerated tank that we are aware has
also been used for comparative purposes.
The defects in this tank were extremely
small and only three areas of cracking were
examined. The Figure 13 shows an area of
cracking after MPI, the cracking clearly
emanates from theedge of the weld and was
probably associated with small arch strikes
or weld run-ons. Microscopical examination
revealed similar features to those
previously mentioned in this paper. Figure
14 shows a crack running through parent
metal along the edge of a heat affected
zone of a small weld pool. The crack
extends in the unaffected parent material.
As with the cracks found in the tank at
Seal Sands these defects were sl ightly
wandering, only slightly branched, sharp
ended and transgranular. A similar area
at higher magnification is shown in Figure
15 where the very fine nature of the
cracking is evident. The visual and
microscopical characteristics observed with
ammonia stress corrosion cracking in other
vessels has been found to be very similar
in appearance to the defects in this tank.
Theare generally quite straight and
sl ightly branching and more usually
transgranular with very fine crack tips.
At this stage i t was considered by National
Vulcan that a significant proportion of
defects within the tank were of a stress
corrosion type
CONFIRMATION OF STRESS CORROSION CRACKING.
The probability of stress corrosfon at
-33°C in an ammonia storage tank was
alaming. In order that all parties were
satisfied that stress corrosion was
occurring i t was agreed that two boat
samples be removed for laboratory
examination. The sample shown in Figure 16
after MPI consisted of a small cleat weld
exhibit ing several defect fndicatlons.
This sample had been removed from course 2.
A section through the cleat weld is shown in
Figure 17 after polishing and etching. The
depth of weld penetration was small, as
would be expected with a cleat weld although
the heat affected zone is fairly deep
indicating a high heat input and equally
rapid cooling. Hardness tests of the
microsection shown in the figure gave values
for the parent meta] of approximately 175
Vickers. The weld was 229 to 262 Hv, and
the heat affected zone 229 to 362 Hv, Both
the we]d and heat affected zone were
therefore considered to have susceptible
microstructures for both hydrogen and stress
Corrosion cracking.
Prescence of Hydrogen cra_cking.
The large number of hydrogen cracks
indicated that the welding procedure used
during construction was susceptible to
hydrogen contamination. In the case of the
Seal Sands tank i t was considered that they
were a source of stress intensity with the
potential to produce stress corrosion
cracking in certain environments.
Several microsections were prepared for
microscopical examination and revealed
cracking as detailed below
W ldMeta] Cracking.
Weld metal cracking was observed in
several sections, a typical example of which
is shown in Figure 18 at 400x magnification.
The crack was fairly branching and
transgranular with sharp crack tips. Some
small islands of corrosion are evident as
dark patches along the crack length. The
weld metal microstructure consisted of
fairly course laths of ferrite with aligned
martensite - austenite - carbides (M.A.C.).
This type of weld metal microstructure is
generally associated with fairly poor
toughness and would correspond to a rapidly
cool ed we d.
Heat Affected Zone C ackfng.
Several examples of heat affected zone
cracking were observed, two examples of
which are given here'-
Figure 19 (magnification 500x) shows an
area with some fine cracking. The
microstructure was consistent with a fairly
rapidly cooled transformation exhibit ing
fer ri te with aligned M.A.C. along prior
austenite grain boundaries and islands of
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martensite/bainite transformation product.
This type of microstructure would be
susceptible to both stress corrosion
cracking and hydrogen cracking. Hardness
values up to 360 Hv were measured in this
area. The cracking is very fine and
transgranu]ar exhibit ing some small islands
of corrosion along the length. This area
was considered to be more characteristic of
stress corrosion than hydrogen cracking.
Figure 20 shows a second area of heat
affected zone cracking at 500x magnification
although this crack propagates from a smal]
lack of fusion/slag defect on the weld
edge.
Parent MetaI Cracking.
Only one area of cracking was found
remote from the weld, occurring solely in
the parent metal of the boat sample. It
should be noted that several of the
replicas taken in other areas in the tank
show cracking running out of weld and heat
affected zone into unaffected parent metal.
In this instance however the cracking
appeared to be entirely limited to parent
metal unaffected by welding. The area was
measured as 2.5mm from the outside edge of
the heat affected zone of the cleat weld.
The mocrostructure, shown in Figure 21 at
500x magnification, was consistent with a
fai rly equiaxed fer rite and unresolved
pearli te microstructure. The hardness in
this region was 172 Vickers which was
considered to be a typical value for the
unaffected parent metal. This type of
microstructure is not generally considered
susceptible to hydrogen cracking and would
only be considered susceptible to stress
corrosion under severe conditions. The
remoteness from the weld suggests the
like] ihood of hydrogen absorption from
welding would be very slight the crack was
apparent. In the figure running from a
plasti call y deformed surface into the
equiaxed microstructure in a transgranu]ar
manner, some fine branches were apparent
along the crack length. I t is probable
that the deformed surface has been produced
during cleat removal. The surface is
usually ground flush and smooth after
removal but in the majority of cleats
examined in this vessel the rough
fractured surface was st i l l apparent. The
defect was considered to be stress
corrosion cracking.
CONSEQUENCESOF STRESS CORROSION CRACKING
As no S.C.C. had been found in courses
6 to 11, i t was decided to take no further
action in this region but to extend the MPI
to all weld areas in courses 1 to 5. This
produced another 1454 indications bringing
the total for this region to 1950.
In addition twelve defect indications
were selected as reference areas for
examination at a future inspection so that
i t could be ascertained i f any crack
propagation had occurred during service.
The following work was carried out and
recorded. Each area was ground to a 400
gr it fin ish, a magnetic particle test
carried out and the area photographed. The
areas were then polished, etched and
rep]icas taken.
The very large number of defects found
within the tank produced a considerable
problem relating to removal of defects. The
usual action where defects within ammonia
spheres are found is to grind out the cracks
and weld repair i f necessary. (5). In this
instance i t was an enormous task and whilst
the possibility of grinding out the cracks
was not ruled out at this time i t was agreed
that more information in respect to cr it ica l
crack size should be obtained by fracture
mechanics. This work is described later.
FACTORS CONDUCTIVE TO S.C.C.
The fact that stress corrosion cracking
has not been experienced in the past in
storage tanks operating at -33°C (apart from
one unpublished incident) has generally been
attributed to the lack of oxygen in the
worki ng environment.
Stress corrosion cracking has been
produced in laboratory experiments at-33°C
although susceptibi lit y appeared to be
dependent on al]oy type. Fractographic
evidence has suggested that the same
mechanism operated at -33°C as experienced
at ambient temperature. (6,7).
Susceptibility to stress corrosion at
ambient temperature has always been
considered to be related to the type of
steel, impurities in the ammonia and
stresses in the steel. {8). The interaction
of these three factors dictates whether or
not stress corrosion cracking will occur.
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The examination of this tank revealed
several details which are considered to
have bearing on the formation of stress
corros i on cracking.
Materials of Construction
in line with present practice of using
higher strength steels to save on costs and
weight, three different plate materials
have been employed in the construction of
the tank, namely
BS4360. 50C UTS 490-620 MPA. Yield 355 MPA.
(Courses I to 5)
BS4360. 43D UTS 430-510 MPA. Yield 280 MPA.
(Courses 6 to 9)
BS4360. 43C UTS 430-510 MPA. Yield 245 MPA.
(Courses I0 to 11)
Both the 50C and 43D materials are
fine grained Niobium and Vanadium treated,
the 50C having a fai r ly high yield strength
(within the requirements of BS4741). The
generaT rule has been that ~ncreasing yield
strength will produce an increasing
susceptibility to stress corrosion cracking
(9). It was found with this tank however
that the cracking was predominantly in weld
and heat affected zones.
The welding electrode used throughout
the tank, Ferex E7018 LT is a basic
hydrogen controlled electrode generally
recommended for this type of use. I t
normally produces a weld metal with a yield
strength about 480 MPAwhich would be
adequate for this purpose. Examination of
the weld procedure adopted on s ite gave no
indication of specific electrode drying
procedures. This is necessary i f low
hydrogen welds are to be produced.
Preheating was not specified for the
welding. I t is not mandatory according to
BS4741 although the lack of pre-heat wil l
have a direct effect on the hardness of
yield strength of all the welds.
Construction Practice
The rapid cooling produced when a
small amount of molten metal is laid down
on a large mass of cold metal wil l give
rise lqo a hard transformed weld and heat
affected zone microstructure. The type of
parent material used will have a bearing on
the hardness.
Cleat welds are often deliberately made
bri t t le so that they can be easily removed
after fabr ication. This wi ll leave an area
of hard and br i t t le weld and heat affected
zone metal, i t was noted that although the
cleat welds in the top six courses showed
some cracks there were fewer than in the
bottom five courses.
Similarly, arc strikes and weld run-on
areas produce transformed microstructures
susceptible to stress corrosion cracking.
Influence of Stress
The presence of hard welds and heat
affected zones gives rise to areas with high
yieTd strengths. I t is commonTy agreed that
residual stresses due to welding can reach
yie ld point of the steel. I t is therefore
apparent that there were many areas in the
tank where the local stress levels were much
higher than or iginally intended. The
hardness of the seam weld in parts also gave
some concern where hardnesses up to 300
Vickers were recorded in seam weld heat
affected zones. This value of hardness
equates approximately to a tensile strength
of 990 MPA which could give a yield strength
approaching 700 MPA. The general hardness
of the main seam weld metal was 230 Vickers
which equates to a tensile strength of 745
MPA, much higher than or igi na lly indicated
for the weld metal by the electrode
manufacturers. This suggests that al l the
welding has undergone some rapid cooling and
wil l have a higher residual stress than can
be cons dered des rabl e.
The influence of in-service load
stresses is difficult to quantify but it is
worth noting that the larges accumulation
of cracks were found between the f i rst and
third inclusive circumferential seams.
Twice as many cracks were recorded on the
second circumferential seam than any other.
The distribution of defects has some
correlation with the bending stresses in the
shell.
Ammon... iaQu_a_li y
I t is well documented that the addition
of water and control of the oxygen content
give a lower probabi li ty of stress corrosion
cracking in the liquid phase (8). The water
content of ammonia stored at Seal Sands
averaged 0.02 w/w and was considered
suf ficient to inhib it SCC at -33°C.
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Oxygen contents of liquid or vapour have
not been determined but i t requires 10,000
ppm in the vapour phase to produce 1 ppm in
the liquid at-33°C (7). I t has been
assumed that the oxygen content will
diminish during service as the vapour is
continuously drawn off, condensed and
returned to the tank with venting of the
non-condensables. This points to the SCC
occurring during tank commissioning and is
supported by the presence of cracks which
appeared to have been widened by corrosion
and which contain corrosion products.
However, this viewpoint is contradicted by
the presence of Fine sharp ended cracks
free of corrosion. I t is postulated
therefore that the crack ini tiat ion and
early propagation require relatively high
oxygen levels but subsequent propagation is
dependent on the crack tip environment, not
the bu I k condi i ons.
Summary
The choice of higher strength steel and the
use of welding practices, current at the
time of construction, produced
microstructures which were susceptible to
stress corrosion cracking in an ammonia
environment which had previously been
considered immune. The influence of
in-service stresses, whilst not quantified,
cannot be ignored in this case.
FRA_CTUREMECHANCS ASSESSMENT
As stated previously, the task of
removing defects was enormous and the
calculation of cr itical crack sized was a
possible method of reducing this activi ty
to an acceptable level.
Construct-ion and pu l shed
documentation gave insufficient material
data for fracture assessment, therefore i t
was considered necessary to carry out a
comprehensive material test program using
actbal tank material. The tests involved
Selective measurement of tensile and yield
properties, Charpy impact energies,
fracture toughness tests for parent plate,
weld and heat affected zones. The results
were ini t ial ly based on through thickness
test specimens and used the CEGB R6-REV 3
assessment procedure. ( I0).
To enable this test program to be
carried out a sample 1.2m long, 0.4m wide
was cut from an area of the f i rst to second
course circumferential weld, considered to
give the most pessimistic results.
It was apparent that the majority of
defects found in the tank, i.e. the
transverse,defects and construction weld
defects remote from the seams, have
significant safety margins compared to the
calculated cr itica] surface breaking defect
sizes, with the added security of a leak
before break situation being present.
However, a leak before break situation could
not be calculated for the few longitudinal
defects found in the main seams, base f i l let
weld and transverse defects at T welds which
could propagate into vertical seams in a
longitudinal orientation. (Appendix 6).
As the maximum depth of defects found
in the critical direction (i.e. parallel to
the weld seams) was only 2m, i t was
considered that the fracture toughness
properties used in the calculations based on
through thickness notch specimens were....
pessimistic. Therefore in order to assess
the significance of these shallow surface
breaking defects new fracture toughness
properties for the weld were obtained using
"surface notch" test specimens. The
fracture mechanics assessments were repeated
at the critical location using lower bound
toughness data and a marked increase in
critical surface breaking defect sizes was
calculated.
It was considered that the details of
the fracture analysis showed that transverse
weld defects have significant margins
against the calculated cr itical defect size
with the added bonus of invoking a leak
before break argument. Whilst a leak
before break argument could not be applied
for the longitudinal defects, the fracture
assessment based on surface notch toughness
data gave adequate safety margins against
fast fracture in relation to the actual
defects found.
As a consequence of this work, i t was
considered prudent to remove al I defects
detected in locations where a leak before
break situation could not be determined.
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This was done by controlled grinding and
without the need for weld repairs as all
crack defects were found to be less than
2.0mm deep. One porosity defect was not
removed at 2.0mm but was left at that
depth. (Appendix7),
RECOMM SS ON NG
. . . . . . . . . . . . . . . . . . . .. . .
The tank was considered to be
satisfactory for operation at the design
conditions and work commenced on the
re-commissioning.
The aim of the recommissioning
procedure was to produce an environment
which did not initiate or. propagate stress
corrosion cracking. Published data on
ammonia sphere storage was used to prepare
tentative procedures and local conditions
were adapted to achieve the desired
results: ~
a) oxygen content of tank atmosphere 0.5
v/v maximum before the ammonia purge.
b) 0.2 to 0.5 w/w water in the ammonia
for tank cool down.
c) 0.2 to 0.5 w/w water in the f i rst
shipment of anhydrous ammonia.
d) restricting the operating level to 50
maximum for at least four weeks to
allow adequate purging of oxygen at low.
imposed shell stresses.
Flammability concerns had dictated the
use of nitrogen purging for the original
commissioning and this concept was
retained.
Recommissioning proceeded along the
fol lowing stages'-
a) tank fi l led to original test height of
21.922 metres with Water (imparted
relaxation and some plastic deformation
of existing crack tips, reduced space to
be purged).
b) nitrogen purging of the ullage space.
c) de-inventory the tank wlth constant
addition of nitrogen.
d) purge the anci llary equipment via the
tank.
e) ammonia vapour purge.
f) addition of I0,000 ]itres of 33 aqueous
ammonia solution to tank (source of
water for inhibition).
g) tank cool down.
h) addition of 4,000 tonnes of anhydrous
ammonia containing 0.1 w/w water in
1,000 and 3,000 tonnes transfers. (Final
water content 0.28 w/w).
i) tank contents circulated for four weeks.
CONCLUSIONS
. . . . . . . . . . . . . . . . . ._
I. Stress corrosion cracking has been
identif ied in the ammonia storage tank
operating at-33°C.
2. The material used and the weld procedures
adopted on site produced microstructures
susceptible to both stress corrosion
cracking and hydrogen cracking.
3. Very high hardness values were recorded
in weld and heat affected zone positions.
RECOMMEND TIONS
. . . . . _ . .: .. .. . . . . . . . . . . : _ . . . . . . . . . . . . . . . . .
a) The vast majority of defects found in
ammonia spheres and in this tank relate
to cleat positions. It is therefore
recommended that wherever possible cleats
be made on the outside of tanks and
proper weld procedures employed for those
inside the vessel.
b) It is apparent that the benefits gained
by using the fine grained higher strength
steels can be outweighed by the
possibili ty of stress corrosion cracking,
particularly the case where uncontrolled
welding gives rise to locally hardened
microstructures. This could be reduced
by the use of preheat at manufacture.
c) It is recommended that where practicable
consideration should be given to limiting
the parent metal to l ower yield strength
steels when constructing new tanks.
This, together with careful choice of
welding consumables and control of weld
procedures, should reduce the risk of
producing a susceptible microstructure.
d) Standardise the method of inspection of
refrigerated tanks.
e) Carry out a construction/operation survey
of fully refrigerated tanks in ammonia
service similar to that compiled by
Mr.J.Van Blanken for pressure storage.
.
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LITERATURE CITED
. . . . . . . . . . . . .
. . . . . . . . ~ . w
I. Chemical Industries Association Code of
Practice for the Large Scale Storage of
Fully Refrigerated Anhydrous Ammonia in
the United Kingdom. (May 1975).
2. The Storage of Liquid Ammonia -
Vigilance, Winter (1983) Vol. 4, No. 4,
Page 43.
3. Hart, P.M. Weld Metal Hydrogen
Cracking . Welding Institu te Research
Bullet in . (Nov. 1978) Vol. 19, No. 11,
Page 320.
4. Hart, P.M. Hydrogen Cracking in Weld
Metals - The Effect of Manganese .
Welding Inst itute Research Bulletin.
(Nov. 1980) Vo]. 21, No. 11, Page 327.
5. Chemical Industries Association, Code
of Practice for the storage of
Anhydrous Ammonia under pressure in the
United Kingdom .
6. Lunde.L. Stress Corrosion Cracking of
Steels in Ammonia specially Vapour
Phase Cracking . AI ChE. Vo]. 24.
Ammonia Plant Safety.
7. Lunde & Nyborg. Stress Corrosion
Cracking of Different Steels in Liquid
and Vapourous Ammonla . Nace Corrosion
87. Pape~ 174.
8. Lunde & Nyborg. Effect of Oxygen &
Water on Stress Corrosion Cracking of
M ] d Steel i n L qu d and Vapou rou s
Ammonia . Plant/Operations Progress.
Vol. 6, No. I, Journal 1987.
9. Clark & Cracknell. Avoidance of
Stress Corrosion Cracking in Ammonia
A I ChE Symposium Atlantic City 1976.
Paper 45C.
10. Milne, I. Ainsworth, R.A, Dow]ing, A.R,
Stewart, A.T., R/H/R6 - Rev 3
Assessment of the Integrity of
Structures Containing Defects (1986).
J.R. Byrne F.E. Moir
R.D. Williams
. .. . i i i U i i i i i i i i i i j f i i i l i i i
i i i i i i i i i i i i i i i i i i i i i i
F i g u r e 1 V i e w o f t a n k I M F 1 0 0 8 .
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F i g u r e 3 . P h o t o g r a p h s h o w i n g t r a n s v e r s e d e fe c t s in
c i r c u m f e r e n t i a l s e a m C 2 af t e r M a g n e t i c P a r t ic l e
T e s t i n g ,
F i g u r e 2 . V i e w o f i n te r n al a r r a n g e m e n t o f p l a te s a n d
m e t h o d o f i n s p e c t i o n .
a
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c r a c k m e c t l a n i s m s , a : M a g n e t i c P a r t i c le I n s p e c t io n ; b : p h o t o m a c r o g r a p h a t 1 2 . 5 x m a g n i f i c a t io n ;
c " p h o t o m i c r o g r a p h f r o m a p l a s ti c r e p li c a .
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F i g u r e 5 , T y p i c a l co n s t r u c t io n d e f e c t a f te r M a g n e t i c
P a r t i c l e I n s p e c t i o n r e v e a l e d c r a c k i n g a s s o c i a t e d w i t h
a c l e a t w e l d .
F i g u r e 6 , A r e a C 6 V l 0 a f te r M a g n e t i c P a r t ic l e
I n s p e c t i o n r e v e a l e d a s m a l l r u n o f w e l d w i t h
t r a n s v e r s e c r a c k s .
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F i g u r e 7 , D e f e c t a r e a C 3 V 5 N o . 3: p r o b a b l y a n a r c s t ri k e f r o m w e l d i n to p a r e n t m e t a l, a : M a g n e t i c
P a r t i c le I n s p e c t i o n ; b: p h o t o m a c r o g r a p h a t 2 0 x m a g n i f ic a t io n ; c : p h o t o m i c r o g r a p h t a k e n f r o m a p l a s t ic
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F i g u r e 8 . T y p i c a l a r e a o f V a n s v e r s e c r a c k i n g i n
f loor -she l l f i ll e t we ld .
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F i g u r e 9 . A r e a i n F i g u r e 8 , a f t e r l i g h t g r in d i n g i n t h e
f loor -she l l f i ll e t we ld .
~ : .
l .
F i g u r e 1 0 . A m m o n i a s t o r a g e b u l le t a f te r v e r y s h o r t
s e r v ic e . M a g n e t i c P a r ti c le I n s p e c t io n r e v e a l e d m a n y
t r a n s v e r s e d e f e c t s i n p a r e n t m e t a l a n d i n t o t h e w e l d .
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F i g u r e 1 1 . P h o t o m i c r o g ra p h o f F i g u r e 1 0 s t o ra g e
b u l l e t f r o m a r e p l i c a , s h o w i n g s t r e s s c o r r o s i o n
c r a c k i n g i n w e l d m e t a l . N o t e s l i g h t b r a n c h i n g a n d
s i m i la r it y to F i g u r e s 4 a n d 7 .
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F i g u r e 1 2 . A s i n F ig u r e 1 1 b u t s h o w i n g c r a c k i n g
i n p a r e n t m e t a l 1 0 0 x ) .
• . : ~ ' . . , , .: . . .: . : : : ~ i ~ ` ` ~ : ; :~ : 7 ~ ~ ` ~ i : ~ : ~ : ~ i ~ : ~ : ~ i ~ : ~ ; : ~ : ~ ` : ; : ; : ~ i :: ~ : ~ 1 ~ r
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F i g u r e 1 3 . S t r e s s c o r r o s i o n c r a c k i n r e fr i g e r a te d
tank near f loor -she l l f i l l e t we ld .
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Figure 14. Photomicro-
graph of Figure 13 I OOx)
showing typ ica l s t ress
corrosion character ist ics.
. . . . . . . , . . . . . . . . . . . . . . . . . . . . .
. . . . ~ . - . , - ~ ' : i : ~ - . = x ' : ~ , o - . - . - : - - : ~ : ~ - : - . . < < ~ > i - . . : . ~ - - ' ~ i . ' ~ . : i : - : ~ . :: - : : . : :~ - i . . . . . .. . .
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. . . . .. . ~ t . . . . . ~"- " ~ i~"~"~" . "
Figu re 16. Boat sample af ter M agnet ic Par t ic le
Inspection.
F igu r e 15 Similar area
to F igure 14, at 400x.
F igure 17 . Sec t ion th rough boat sample showing
depth o f heat affected zone,
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F i g u r e 1 8 . P h o t o m i c r o g r a p h o f s t r e s s c o r r o s i o n
c r a c k i n g i n w e l d m e t a l f r o m b o a t s a m p l e ( 4 0 0 x ) ,
• ,
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~ . . , ~ ; " ~ ' ~ , " , ~ " ~ . , ~ i ~ . , " , , " , i ~ , ., : , . ~ - ~ ~ , ~ . ~ - ~ . . ~ ~ i ~ . " - , ~ i i ~ i ~ , i ~ . ~ . . ~ - ~ : ~ . , . "~ ~ , . ~
, , . , ~ . ~ . ~ , , . ~ , , ~ , . ° : . ~ . . ~ . . .c : . . : . . , ~ . . . . ~ , , , . , . , . . : , . ~ . , . ~ , , ~ , ~ ,
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F " ' " ' . , . ' . .. . . " ~ , ~ , , ' . ~ ' , , i ~ " ' , ~ " ~ . . ~ . . . . .. . . ~ : - ' L , ' . , • ~ ~ . ~ . " ~ ' , : ~ . . ~
= . , . . . . . ~ ' , . . . . . , , , . . , ~ . , ~ ' , . . . , . ~ , : . . , , , , : , $ ~ , , , . , : . . ~ , ; . . , ~ . . . . . . ~ . ~ , ~ ' ~ , ~ , . .~
• ~ . . ~ . . . . . , , : : i " , , ~ ' . " ~ - " ' : , ~ ; . T : ' I ' , " . ~ I : ~ ' T " . ~ " . " . , " ' ' ~ i " . ~ ~
. . . . . ,
F i g u r e 2 0 . S t r e s s c o r r o s i o n c r a c k f r o m l a c k o f
f u s i o n / s l a g d e f e c t i n b o a t s a m p l e ( 5 0 0 x ).
F i g u r e 1 9 . H e a t a f f e c t e d z o n e s t r e s s c o r r o s i o n
c r a c k f r o m b o a t s a m p l e ( 5 0 0 x ) .
F i g u r e 2 1 . S t r e s s c o r r o s i o n c r a c k i n p a r e n t m e t a l ,
2 . 5 r a m f r o m o u t s i d e e d g e o f h e a t a f f e c te d z o n e
{ 5 o o x ,
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A P P E N D I X 2
_
B R I E F O U T L I N E O F D I F F E R E N C E S B E T W E E N VARIOUS D E S I G N C O D E S .
. . . . . . . : :: .~ . . . . . . . : . . = . _ =~ .. . _. . . _ . . . . . . . _. .. __ _~ __ _ . .. _ . . _ . .. . .= . ~ . . . = _ .. .. ._ . _= .
C O D E B S 4 7 4 1 : 1 9 7 1 B S 4 7 4 1 : 1 9 7 1 + -
A M D T 2 ( 1 9 8 0 )
A P I 6 2 0 A P P R
M a t e r i a l B a s e d o n F I G 6 B a s e d o n T A B L E 3 G e n e r a l l y r e q u i r e s
S e l e c t i o n n o t n e c e s s a r y i m p a c t t e s t i n g i m p a c t t e s t i n g a t
t o i m p a c t t e s t a t o r b e l o w d e s i g n t e m p .
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mpac t Lon gi t ud lna i Lon gi t ud ina i T r ansv e rse
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0 . 6 2 5 x U T S 0 . 5 0 x U T S
* A P I 6 2 0 5 t h E d i t i o n + S u p p l e m e n t 3 in f o r c e i n 1 9 7 6
7 t h E d i t i o n i s c u r r en t v e r s i o n .
N B
A P I 6 2 0 g i v e s a 2 0 ° F b o n u s f o r s o m e s t e e l s i f p r o d u c e d i n th e n o r m a l i s e d
c ond l t i on a s s e c onda r y c om pone n t s .
( s ee T a b l e R 2 - f o o t n o t e ) .
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APPENDIX
o 3
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A P P E N D I X 4
I N S P E C T I ON N D M I N T E N N C E
- - : _ - - - - : _ T - . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . .
1 2. 1 M A J O R I N S P E C T I O N O F T A N K
. . . . . . . . ~ . . . .. . .. . . .. . .. . . .. . . .. . .. . . .. . .. . . .. . .. . . .. . ..
1 2. I. I A l l t a n k s s h o u l d b e t h o r o u g h l y i n s p e c t e d n o t m o r e t h a n
s i x y e a r s f r o m t h e d a t e o f i n i t i a l c o m m i s s i o n i n g .
T h e r e a f t e r , t he i n t e r v a l b e t w e e n m a j o r i n s p e c t i o n s s h o u l d
b e d e t e r m i n e d b y t he t a n k o w n e r , d e p e n d i n g o n p a s t
e x p e r i e n c e ~ b u t i n a n y c as e s h a l l n o t e x c e e d t w e l v e y e a r s .
1 2 . 1 . 2 It i s r e c o m m e n d e d t h a t th e f i r s t i n s p e c t i o n s h o u l d
i n c l u d e t h e f o l l o w i n g :
1 2 . 1 . 2 . 1 M a g n a - f l u x c r a c k d e t e c t i o n o f a l l t e e - w e l d s i n
f l o o r p l a t e s , f o r a l e n g t h o f 2 3 0 m m a l o n g e a c h
a r m o f t h e w e l d .
1 2 .1 . 2 . 2 M a g n a - f l t 4 x c r a c k d e t e c t i o n o f a t l e a s t 5 0 % o f
t e e - w e l d s i n s h e ll p l a t e s , f o r a l e n g t h o f
2 3 0 m m a l o n g e a c h a r m o f t h e w el d. , T h i s
o p e r a t i o n i s s i m p l i f i e d b y t he u s e o f a c r a d l e
( s ee C l a u s e 4 . 6 . 4 ) .
1 2 . 1 . 2 . 3 C a r e f u l v i s u a l i n s p e c t i o n o f :
( a) a l l o t h e r w e l d s i n f l o o r a n d S h e l l p l a t e s ;
( b) s h e l l t o f l o o r w e l d ;
( c) a l l i n t e r n a l b r a c k e t s a n d a t t a c h m e n % s .
1 2 . 1 . ~ T h e m a g n e t i c f i e l d u s e d i n m a g n a - f l u x t e s t s s ll ou ld h a v e
a m i n i m u m s t r e n g t h o f 8 0 o e r s t e d .
1 2 . 1 . 4 Th e d e g r e e o f m a g n a - f l u x t e s t i n g m a y b e r e d u c e d i n
s u b s e q u e n t t a n k i n s p e c t i o n s , b a s e d o n t he o w n e r ' s p a s t
e x p e r i e n c e .
1 2 . 1 .5 T h e e x t e n t o f e x t e r n a l i n s p e c t i o n w i l l v a r y a c c o r d i n g
t o t h e t y p e o f
t a n k
1 2 . 1 . 5 . 1 Where possible. at least four equally spaced
h o l d i n g d o w n b o l t s s h o u l d b e r e m o v e d a n d v i s u a l l y
i n s p e c t e d . T h e w e l d s o f t h e a s s o c i a t e d b o l t i n ~
b o x e s s h o u l d b e m a g n a - f l u x t e s t e ~ .
1 2 . 1 . 5 . 2 P r i o r t o d e c o m m i s s i o n i n 6 t h e t a nk , t he e x t e r n a l
s u r f a c e s s h o u l d be i n s p e c t e d
f o r
c o l d s p o t s ~
a n d m a n h o l e d o o r s , p i p o w o r k ~ e t c i n s p o c t e d f o r
a m m o n i a l e a k s . T h i s s h o u l d be d o n e w i t h t he
t a n k a t l e a s t t h r e e - q u a r t e r s f u l l o f l i q u i d
a m m o n i a .
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M a ~ . e . . t _ _ o . . . . .. .
P a _ r t i c l e
i n s p . e c _ % i _ . o _ _ n _ . . _ _ _ o _ _ f
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O f
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3 ~ . . . - a ~ i ~ . . X _ __ _1 2~ 6 .3 _m . t - - ~ - O n _ - i a . _ _s O ~ -~ _a_ _ e ~ - T ~ i
1. I n s p e c t i o n p e r s o n n e l m u s t h o l d c u r r e n t c e r t i f i c a t e s o f
c o m p e t a n c e • w hi ch a r e r e c o g n i s e d w i t h i n t he U . K.
2 .
T h e c o n d i t i o n o f t h e s u r f a c e s t o b e i n s p e c t e d i s n o t k n o w n .
F r o m t h e e x p e r i e n c e o f o t h e r t a n k o w ne r s , t h e r e w i l l p r o b a b l y
b e a r u s t c o a t i n g o n a l l s u r f a c e s a n d p o s s i b l y a n o i l f i l m.
T h e l a t t e r i s t h o u g h t t o b e u n l i k e l y ~ b u t I t is m e n t i o n e d
f o r c o n s i d e r a t i o n i n p l a n n i n g t he i n s p e c t i o n .
3. W i t h t h e e x c e p t i o n o f t h r e e l o c a t i o n s , a c c e s s a b o v e t h e f l o o r
w i l l be p r o v i d e d b y c r a d l e s d e s i g n e d t o a c c o m m o d a t e f o u r p e o p l e
a n d t he i n s p e c t i o n e q u i p m e n t . E a c h c r a d l e w i l l c o n t a i n t wo
p r o ~ e s s i o n a l s t e e p l e j a c k s t o o p e r a t e t he w i n c h e s . T h i s a l l o w s
t h e i n s p e c t i o n p e r s o n n e l t o b e f r e e t o c o n c e n t r a t e o n t h e
e x a m i n a t i o n . T h e e x c e p t i o n s a re t h r e e a r e a s w h e r e i n t e r n a l
p i p e w o r k h a m p e r s a c ce s s . I t i s e n v i s a g e d t h a t s m a l l e r c r a d l e s
o r a B o s u n ' s c h a i r m a y h a v e t o b e u s e d .
4 . W h i t e b a c k g r o u n d t o B S 5 0 4 4 s h a l l b e a p p l i e d t o t h e a r e a s t o
b e e x a m i n e d . F o r a l l m a i n s e a m s i t m u s t b e l O m m w i d e a n d
a t t e e - w e l d s i t m u s t e x t e n d f o r a d i s t a n c e o f 23 0r am a l o n g
e a c h w e l d . A t o t h e r l o c a t i o n s e . g. w e l d e d a t t a c h m e n t s ,
t h e b a c k g r o u n d m u s t b e a w i d t h n o l e s s t h a n t h e w i d t h o f t h e
w e l d p l u s 2 5r am e a c h s i d e .
5 - T h e d e t e c t i n g m e d i a s h a l l b e b l a c k I n k t o B S 4 0 6 9 a n d s h a l l b e
a p p l i e d b y s p r a y i n g p r i o r t o a n d d u r i n g e x c i t a t i o n . T h e
m a g n e t i c f i e l d m u s t b e m a i n t a i n e d f o r 5 t o 10 • se c on ds a f t e r
s p r a y i n g .
6. M P I s ha l l b e c a r r i e d o u t u s i n g A C y o k e e q u i p m e n t o p e r a t i n g a t
a p r i m a r y m a i n s v o l t a g e o f 1 1 0 v o l t s . T h i s c u r r e n t w i l l b e
s u p p l i e d t o c o n v e n i e n t l o c a t i o n s b Y B A S F C h e m i c a l s L i m i t e d
a n d w i l l i n c o r p o r a t e t h e n e c e s s a r y s a f e t y d e v i c e s . T h e
c a b l e s f r o m t h e s u p p l y t e r m i n a l s s h a l l b e t h e r e s p o n s i b i l i t y
o f t h e c o n t r a c t o r .
, . . . , "
6 .1 A f i e l d s t r e n g t h o f 8 0 o e r s t e d s i s r e c o m m e n d e d i n t h e
C I A c o d e of p r a c t i c e , S e c t i o n 1 2 . 1 . 3 ( se e a t t a c h m e n t ) .
E v e r y a t t e m p t m u s t b e m a d e t o , p r o d u c e 5 0 s a t u r a t i o n ,
b u t s h a l l n o t b e l e s s t h a n 3 0 a t a n y t e s t l o c a t i o n .
. . -
T h e
f i e l d s t r e n g t h s h a l l be c h e c k e d b y a c o m p a r a t i v e o r
d i r e c t r e a d i n g me tl lo d a n d t h e v a l u e s r e c o r d e d f o r
i n c l u s i o n i n t h e f i n a l r e p o r t .
.-
6 . 2 E a c h a r e a e x a m i n e d m u s t b e c h e c k e d w i t h t h e p o l e s o f t h e
y o k e i n t w o p o s i t i o n s , e a c h 4 5 ° t o t h e w e l d l i n e . T h i s
p r o c e d u r e i s t o b e r e p e a t e d a t h a l f t h e i n t e r v a l o f th e
p o l e s p a c i n g .
7 . T h e f i n a l r e p o r t s h a l l b e w r i t t e n r e l a t i v e t o t h e l o c a t i o n
i d e n t i f i c a t i o n s g i v e n b y B A S F C h e m i c a l s L i m i t e d .
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t h e l a k © £ r c , s , u . , £ r o m t e m ~ b a _ se )
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D I S U S S I O N
M . A P P L ,
BAS F Akt iengesel ischaft Lu dwigsha fen
West German y: i wou ld l ike to em phas ize tha t we have
no t heard an exo t ic s to ry ; a t BASF in Ludwigsha fen
we exper ienced the same s i tua t ion ve ry recen t ly , in
a tank tha t wa s m ore p rope r ly bu i l t w i th regard to
mate r ia ls and const ruc t ion techn ique . W e took the tank
ou t o f se rv ice a t the beg inn ing o f Apr i l and we found
a n e n o rmo u s n u mb e r o f c ra cks , mo re th a n 3 0 0 c ra cks
and crack g roups. The accum u la t ion o f the cracks w as
on those loca t ions w here p la tes o f d i ffe ren t th icknesses
were we lded toge ther . Most o f the accumu la t ions we
had were on the f loo r , bu t we had a cons iderab le
number , 60 ou t o f those 300 , in the bo t tom f ile t we ld
and a few in the f i rs t course we ld . We have no t qu i te
f in ished our invest igat ion. W e in tend to prese nt a pap er
next year abou t th is p rob lem.
I would l ike to br ing your at tent ion to the fact that
we n o l o n g e r h a ve to sp e cu la te wh e th e r s t re ss
corrosion in a fu l ly-refr igerated tank is possib le . I f i t
is rea l ly possib le , I don ' t wish to confuse you. Another
ammon ia tank, fo r examp le , has been opera t ing in a
jo in t ven tu re be tween DSM and BASF a t Pern is , The
Nether lands s ince 1967 . it has been inspected severa l
t imes, and the last inspect ion done in extreme deta i l
and ve ry tho rough ly , found no s t ress co rros ion a t a l l .
The BAS F tank in Ludwigsha fen had a ra ther low wate r
con ten t over the las t few years . Up tO now, we have
found no spec ia l po in ts regard ing p la te mate r ia l o r
w e l d i n g p r o c e d u r e s , w h i c h c o u l d g i v e u s an y
explanat ion. Invest igat ions are st i l l go ing on, and we
wi ll re fe r to them as soon as we can . Thank you .
J . B L A N K E N , DSM Fert i l izers i jmuiden The Nether-
lands: i n 1983 , I p resen ted a paper summar iz ing the
pane l d iscuss ion and the su rvey done a t the 1982
Sym posium. I men t ioned tha t a t least ten a tmospher ic
s to rage tanks had been inspected and no s t ress
corros ion had b een found. I a lso add ed that i t is un l ike ly
to find s t ress co rros ion c rack ing in a tmosp her ic s to rage
tanks, beca use as you ment ioned you need 3V2 a i r
o r 7 ,000 ppm o f oxygen in the vapor to ge t 1 ppm
in the l iqu id, in h indsight, i t was not correct. Being a
bad loser , i checked my ca lcu la t ions and checked
wh ethe r th is 3V2 of a i r in equi l ibr ium wi th 1 ppm in
the l iqu id was co rrect . Compar ison o f my equ i l ib r ium
data aga inst da ta o f Mrs . L iv Lunde s how ed tha t the re
was no th ing wrong wi th my ca lcu la t ion . Tha t means
tha t e i the r you ge t s t ress co rros ion crack ing be low 1
pprn o f oxygen o r the oxyge n co n ten t i s h igher a t a
spec i f i c po in t in the tank than the oxygen con ten t in
equi l ibr ium wi th the vapor above the l iqu id, and that 's
a quest ion tha t has to be so lved . H owever , wh a t l sa id
in 198 3 is not correct. Sorry.
APPL Our meta l lu rg is ts be l ieve tha t the oxygen
prob lem is most impor tan t in the comm iss ion ing ph ase
o f a tank. Acc ord ing to them, i t 's ve ry impor tan t to have
very low oxygen con ten t, espec ia l l y in th is phase , and
tha t a t th is t ime s t ress co rros ion cracks cou ld s ta r t
developing.
M . T ERZ lS ,
EKO Chem icals Co. Thessaloniki G reece:
W e a l so h a ve two a m mo n ia ta n ks . F o r o n e ta n k , wh i ch
had been in opera t ion fo r 12 years , a ve ry tho rough
inspect ion was ca rr ied ou t w i th a magne t ic method ,
l ike you descr ibed . We d id no t f ind an y ev idence o f
cracks. We do , however , have p rest ressed s t ress
corros ion Crack ing coupons in the vapor phase and
in the l iqu id phase of the tank, which so far have not
shown any ev idence o f s t ress co rros ion crack ing , i
wondered whe ther th is method o f tes t ing o r guard ing
against stress corrosion cracking is e f fect ive. In the
case o f the tank you descr ibed where s t ress co rros ion
crack ing was found , d id they have these p rest ressed
s t re ss co rro s io n c ra ck in g co u p o n s? T h e se a re so m e -
th ing that look l ike a peta l or a horseshoe which is
prestressed to a certa in stress level .
RE . MO IR, Nat ional V ulcan Eng ineer ing Co. Man-
chester England: I th ink Mr. Wi ll iams can answ er tha t
quest ion better than I can.
R . D. W I L L I A M S , BASF Chem icals L imi ted Middles-
brough Eng land: For a start , I to ta l ly d isregard the
in fo rmat ion you ge t f rom a h orseshoe shape tes t p iece .
I have tr ied it in cyan ide env i ronmen ts an d m any o ther
environments, and i t is d i f f icu l t to actual ly match a
horseshoe-shaped p iece o f meta l to wha t happens in
a tank. The po in t o f the paper was tha t wha t hu r t us
most was h igh hardness and you do no t no rma l ly pu t
a h igh -hardness horseshoe tes t p iece in a tank. t t i s
not a typ ica l pract ice to take a p iece of p la in meta l ,
bend it , and drop i t in . I t is a fact. We d id learn a m ythol-
ogy o f the ammon ia manu factu r ing bus iness: You do
no t p roduce amm on ia with 0 .2 wa te r. You a ll p roduce
wh a t yo u c l a im yo u h a ve p ro d u ce d , wh i ch i s a n h yd ro u s
a mmo n ia - -ze ro wa te r . An d th a t wa s th e o th e r t h i n g
you f ind d i f f i cu l t to ke~p t rack o f when you do these
test p ieces. Because you have many, many d i f fe ren t
sourc es o f amm on ia perhap s go ing in to you r tank, you
get a l l these d i f ferent resul ts and at the end of the
day, you con fuse yourse l f . I don ' t kno w i f tha t answe rs
your quest ion , bu t tha t has been my e xper ience .
T ER Z IS : T h a n k yo u.
C . S . M c C O Y , Mc Co y Consul tants Inc . Or inda CA :
The magne t ic pa r t i c le inspect ion method is ex t reme ly
sens i t i ve and you ment ioned a num ber o f these cracks
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were very sha l low. How many o f these defects were
dee p enou gh to requi re repai r?. W hat perce ntage of
them w as observed bu t le f t a lone?
MOIR: Considering the depths of defects found, none
required repai r, the de epes t defect being ap proxima tely
2 mm. The defects were, therefore, qui te shal low. I t
was examined in de ta i l , because we were very
concerned about the mechan ism o f p ropagat ion and
we wanted to relate this i f possible to the materials
of construct ion and the problems associated wi th the
high-hardness values recorded at various posi t ions in
the tank such as cleat posi t ions. The ap proach detailed
in the paper was adopted, because we wanted to pu t
forward a reasoned approach in conjunct ion wi th
fracture mechanics calculat ions to return the tank to
service for a reasonable period wi thout any further
inspection.
AP PL. I wou ld l ike to add to tha t . W e haven t been
as lucky as you. We, in Ludwigshafen, had crack s that
run near ly a l l th rough the mater ia l - - th rough the
welding.
C.A. VAN GRIEKIEN, DSM Research i jmuiden The
Nether~ands: First , I wou ld l ike to m ake a rem ark abo ut
an eadier com me nt by R.D . Wi l liams. I believe i t is
very di f f icul t to get stress corros ion c racking in a stress
corros ion crack ing specimen, especia l l y on a horse-
shoe. It is possible to get results by using a fracture
me chan ic specimen, but even then you hav e to stress
i t not any earl ier than about 2 hours before i t is exposed
to the stress corrosion promot ing envi ronment. Other-
wise, no stress corrosion ma y occur.
My second comment concerns my re luctance to
accep t stress corrosion cracking a t these tempe ratures
and to plead for a n ondestruct ive testing of these tanks
before commissioning, as thoroughly as has been
done, up to now on ly a f te r a product ion h is tory , to
di fferentiate cracks that are lef t af ter the manufactur ing
of the tank l ike hydrogen - induced crack ing caused by
welding and cracking that occurred later on.