stress corrosion in ammonia storage tank

7/21/2019 Stress Corrosion in Ammonia Storage Tank

http://slidepdf.com/reader/full/stress-corrosion-in-ammonia-storage-tank 1/23

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.

122



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'-

23



(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.

1 2 4



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

125



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

26



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.

27



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.

128



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.

29



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.

.

13



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 .

3



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

~ i ~ i i i ; i i i : i i ~ i ~ i ~ i i ~ ' i ~ i : : i

~ ~ ~ ~ ~

. .. . ~ : ~ i I " " + ' ~ ~ ' '"

~'~:~ :; .~i~,~iI~ ":i.; ~,. " E ~

C 2 V 1 0 N o 3

c

~', '~' '~ ~ ~ ~i

M A G x 1 2 5

~~.,~.i~t~:~i~:~..... . . . . . ....... ... , .... ~ . ~

~ i ~ ~ ~ ~ ~ ~ . ~ . : .. .. .. .~ . .. . .. .. .. . . . ~ . . . . . ~ . . . : . . ~ . : . . ~ . . ~ . ~ .

I~':... i i i" '":~::?~'~::~::' ::~:' :~;~'~=:::::~.............~::~:::"::~:~:::;: 'i i

~ ~ i

,~.

M A G x i 0 0 ~ t~ i~ :,

ii i ~;~ ~~.........~ ...... .....~ ~ ... : ....

P H O T O M t C . J t O G R AP H T A K E N F R O M A R E P L I C A

,~'/'ii~ i~ :~:~

, i .... .

~ : :

F U ~ O I ¢ ~ O U H I ) ~ q l f

H A Z

P A R E N I ' M E T A L

F i g u r e 4 . D e f e c t a re a C 2 V l O N o . 3 : a t r a n s v e r s e d e f e c t in t h e c i r cu m f e r e n t i a l s e a m s h o w i n g t w o

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 .

1 3 2



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 .

a

~ J I ~ r ~ ' ~ = ;' | * ' = ' ~ = '~ f ~ ~ . ~ ~

0" "'" "" "i~ i~ '~ . . . . . . . . : ~ ~ l . . . . :1I~ . .. ..

.-

i ~ " . - ; ~ : . . : " - "

~ , ,; ~ ., ~ : ~ . ~ . . . - . J ~ . . . . .

. . . . . . . . . . " . .. ~ ~ ~ , ~ ~ . " ,~;:~ i

• ~ ; ~

b

C 3 V 5 N o 3

~% ; ~ ~ ~?:~;~: ~

~ ~ ~ ~

~ ~ ~ ~ i i ~ : ::~ ~ i ~ ; :

~ ~ ~ "' ; : ' ~ . . . . . . . . . : i~=,'i '

M A G x 2 0

~ . ~ ~ ~ ~ . . . . . . . ~ ., ~. ; . . . . . . ~ . .. .. . . ~ ~ . . . . , . .. .. .. . ~ . . . .. . ~.. . . . . . . . . . ,~ . . . ..~ ~ . ~ . . . . . . . . . . . . . . . . . . . . . ~ ~ ~ ~ : ~ . . . . ~ . .. .. .~. .. . . . . . . . . . . . . . . .

~ ' : , '~ " ' ~ : ~ .~ ' " : '~ : . ~ , :: ' ~ " ' . . . . . " ' . ". " " ~ ~ . " " ~ ' . ~ " . '" ' ' +' ~ . . . . " ~ " " " ~ . " '~ ' ~ " " ~ ' : ~ ~ ' ' ~ ' ~ "~ ~ " ~ " - "' ~ " ~ :- ~ " ~ ° ". . . . . .. . ~ " ~ . . . . . .

~ ' ~ .- .. X ~

i ~ : ~ : ~ . ~ : ~ : : ; ~ ~ ; ~ ~ . . . .. . : ~ :~ : : ~ - ~ : ~ ~ . . . . ~ ; ~ : ~ ? . . . . . .. .. . .. .. . ~ ~

~ ~ . . . . . . . . . . ~ . .. . .. .. .. .. . .. : . .. , . . . . ~ . . . . . . . . . . . . . . . . . . . ~ . . .. ~ . .~ . . ~ : ~ ,.,.. : , ~ . . . . , , ~ .~ . ~ :. . .

. . . . . . . . . . . . . . . . . . . ~ . . . . . ~ " ~ ~ ' ~ i '~ '~

. :: .. .. .. ~ .: ;~ . :~ i~ ~ ;, , ~ . . . . . . :~ . . . . • ~ . . . . . . .. .~ ; : ~

' . : " ~ ~ . " . ~ : . : : ~ : ~ : : : : ~ ~ . ~ ; ~ . ~ ' . : - ~ ' - " . ~ : i ~ : . . ~ ~ " : . . : , . : : ' , ' ~ " . . " . " . ~

: ;~ : .~ = = ;~ ~ ~ i ~ , ~ ~ . : ~ ,~ 8 h : , , ~ ~ ~ : ~ i l i l l l l l l l ~

• " : " : = ; : : : ' = ~ ~ ~ : i ~ : ~ ; ~ : : ~ ' . ' , ~ , ~ : ~ " . . . . ~ ' ~ , " . ~ . ; Z ~ ~

. ~ - r , u . ~- . ~z ~ . o . ~ , ' P ~ ~ M A G X 1 0 0

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

re p l ic a ,

1 3 3



.

. . . ,

i , ' ,

~ i '. '. " ' . . . . ' . . . : , . : ~ : '. : i

r ,; ., :, :. ~ . . . . . ~ ~ , ~ : . " . . . . . . . . . . ' . . " ~ : ' ~ , i [ ~ , . '' = ' '" ~ : * ~ " ~ ~ ' ' ' ~ ' ' " ~ ~ ~ . , , ~ , ~ ~ ~ ~ : ' " ~. . . . " " ~ ' . . .. , ~ < " ' . ' . . ~ F • ~ b '= : : : • - - " . - " #

llllllllll~~~,~~~~,,. :~...:~.... ~ ~ i : ~ : ~ - ~ , ~ , ~ : . : ~ . : :~ ~ : . . ' ,~ . ~ : . : ~

~ ' . : " . " . ;; ~ : . ' ~ : . ' ~ " ~ . ~

' ' . : . . ~ ' , T ; . ,' ~ ' : ~ ' ' ' ~ - , : - . i . :' ¢ ' ' ~ ' * ; ' : . . . . . : ~ :

- . , , . - " ; . . ~ , < , ~ ' " " " ' . " ' - ' ~ . . , . : ' . : . - . .. ; . ~ . , .. , . .

: . ' ~ . . t ~

, - . . . . . , , ' , .

• , ~ × ~ : % : ~ . , . . ~ , . . , . , .~ ~ S : : . ~ . ~ . . ; . . . . ~ : . . . . :. : ~ , ~ . . . . ,,

~ ~ ~ . ' " 2 " ~ ' ~ ; ~ J ~ ~ . . . . . ~ ~ . ~ : ~ = : i i "" " ' ~ : ~ : : " ' : " ~ ~ " = ~ " ~ : ~ ' ~ "

,

.~ . . . . . . ~ . . . . . .

< .

. : ; ; ~ . , . . , . . . . . ; . ~ . ,

~ • ~ . . . . . . ; . : . : : . .

~ . . ~ ,~ ~ %

~ ' . . , , ,7 :; . . . . " ~ : " ' " ' : ;: ; ~ ;, ' ,' ~ . ' , ~~ ' ~ : " ~ '

i ~ . : , ; i : , ~ ~ ~

, , , , ~ . . , ~ , ~ . , . ~ ~ , ~ , . ~ , ~ ~ . ` . ~ . ~ * . ? ~ ` } ~ : ~ t . .. ~ . ~ : ~ . ; ~ . ~ . ~ ` ~ } ~ r ~ . ~ . ` .~ . ~ : ~ : ~ : ` ~ ` : : ~ : ~ . .~ ` . ` ` ~ : :. ~ % ~ : ~ : : ~ . ~ z ` ~

~ ` ~ ` ~ 7 h ~ : : ~ . ~ : . ~ = ~ : i i ~ i ~ i ~ t ~ . ~ ` ` ~ ? ~ : i : b ~ : ` ~ ~ ` 1 ~ % ~ ` ~ i ` ` ` ~ ~ ; . ] i~ i ` ~ : : . ~ . ~ : ~ : ~ . : ~ ` ~ i ~ 2 ~ ` : ~ = : ` : ~ : : : ~ : . : . . . .. . . . . . ' . - ' : , i~ , , . ~ : . . ,: . . ' : ~ ; ~ . ~ . ~ ~ : , . i :: ~ : : ~ . " . :" ~ ; . i ~ , := i ~ i ~ ~ ~

~. ~ , • . , : ~ : " ~ ; : . .~ : ~ : ' . . , , o . . . : : . . : . . : : ' : : ~ : : :. , : o ' :. . ~ . . :. ~ : ' ' . . , t ' , . : ~ . , ~ ' . L ~ : . : : ~ . : : : : ' . . : ; . ' . . . ' . ' . ' ~ ' . . , ; ' . ' : .. ~ . : : . . . . , , : ~ ' . : . . . . : . ' : : . - : : . : . . . , . . ~ . '~ . • . . ' . ' .. . . , . . : : ~~ _ ~ ; , ~

~ ; : ~ ' ~ : ~ : ' ~ . , " , , : , . . , , : " : : " 7 : : : : ;: , ~ . :: = : . . . . . . . . . : , : , ; :~ ~ . , . , : i : ,: , , , : :, : , . ' : , . . .. . i " ~ i ~ " : ~ : . . . . . . . . . .

: . , .~ : . ."~ ;~., ~ . : , . . .~ . . . . . ' ~ . . . :' , . - : ' : . . : ; ,. : . . .; .. . . . . . , :. . . . .. . ' ,~ . .. . :. , , . : , , . . , ~ . , .. . . . . . . . . , .. . . . .. . .. :: . . . . . , . . . . . . . • . . . . . ' c . : : :: : ~ . . . . :' . . . .i ~ i ~ . S ; ' ~ i i . ~

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 .

. . ,

, . : ' '1

, , ? , : : , , . , . . . , , N , :

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 .

~: . . . : • , ~

: ~ . t ~ . .

• . . . , ~ : . ~ ' : . . ~ . . ~ . .. , .

~ N ' ~ ' ~ . ~ , ~ N ~ ; ~

" : . ~ , ;; i ~ : . .; . ~ , ; . ~ : . , ~ : ~

~ ' - " . " : ~ ' i ' ~ . : '~ . : - ;~ '. " . - :~ : , , t : ' ~ "

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 .

,_,% ~;. . . .~ . , . '~ . .~ :-.~ : : ~ . ' - ~ ' : ~

. ~ . , - , * . ' ~ ' . ~ ~ : ~ , . . . . . . .. ~ : ~ .

. , I ~?i~; :

' . . . . . . . ~ ' ~ : ~ i : " : : " ' "

• . . . .i ~ t ~ ~ '

~ r ~ ~ : : i ~ ~ : : ~ i ~ i : = ~

I ~ ; ' . ~ . : i ~ ; : : ~k '< ; ~ i ~ . . : :

~"~'~:~=~=~:~ ' " : ' ' : ' " :~: ~ - ~ :~ . ~ ~ i i~ ' . ~ "' ~'" ~ i ~ : :i i~: ' i i i i: : ' : ~ ' : ~ ~ ~ ~ i~ -~ i ~ ;

; , ~ : . . ~ . , : . . . - : . . ~ . . . . : ~ . o . ~ . . , . :

~ ' . - ~ : .¢ . ' . '; ~ 3 ' ~ 1 - ~ ~ , ~ - ~

~ : ~ . ~ N . .

: : : .: : :. - ~ , . :: ~ : : :. ~ - . : ~ . . . . . , , . . ~ ; ~ . ~ . . .

; ~ ~ : i ~ i i ~ ~ . ~ : ~ ., ,, .; ,. .

i ~ i i ~ : . : ~ : : : : , ~ : . . 3 . ? ~

i - i ~ ~ i' : ' ~

i ~ ~ . . ~

. . . . . ~ : . ; ~ i i ~ i ~ .. .. ~

, : ~. :. . . :- '

• : . . . ; , ~ ¢ . ~ . . .

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

: i " : " ~ . " : : : : . . - 7 . ~ , . ' . " . ., < : ~ '~ . : : . " . i : ¢ : : ? ~ i : ~ : ~ , ~ : : ~ ~ . ~ . .

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 .

1 3 4



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 . . . . . .. . .

~ ~ "........ ~:~~~ :~i i:~:"-~;~::~ ~"=: ~"~Y ::i~~;~-"~- ~ i ~ :~ ~ . ~ ~ ~ % ~ . ~ ~ ~ '~= ......~ ~ , ~. .

. . . . .. . ~ 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,

135



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 ) ,

• ,

,.~ ; . ~ ' . . : , ~ . . . . .

~~ . . ~ . . . , . . ~ ~: ' ~ L ~ ' , '~ " ~ . . ;~ , ~ . • ' . " , : : . ~ , : ; , "

. . . . .

maiN ~mm

~ ~ : ~ J ~ : ~

. , . ~~.~ 0% ......~..;.~ iii.,.=~.. ~.,.-~..,..~,~...~=~.~

. , ~ ~ , . . , , . . ~ , . < .; ., = , ,: ... . .

~ ; ~ . ~ . . i ~.

~ , , . . . ~ . , : ~ . . , , . : ~ . , , , . ~ , ; ~ . ~ , ~ . ~ , . ~ . . .. .

i :~i i : i l i~./~.".." ......" ...~..... i ."~~"~.:~.i.~ =.~;';~:~" ~.~.~

i i i";~:;:,.",.".."~:":.,= :' :'";;.,","~:.~ :~: ,~:~ ~.~,.:

~ " , " , ~ ' .. ' .: ~ . , ~ . , . . " , : . . ~ " ~ , ~ " : : = , , , , ~ i ~ , i ~ . , , . . ~ : ~ . . . ~ , . ; ' ~

: ~ , i , , : . , ~ . i ' ~ , i . . .~ . , ~ : ~ , ; ~ , \ . , , . , . . : ~ . , , , : : , ~ . ~ ' . . , ' ~ . . . . . . " . . .

~ , . , , . ~ , , , , . . ~ . ~ , , . . ~ , . ' ~ . ; . , . , . ~ , , . , - , .. " - ~ : ~ . . . ,' - ~ ' ,i - ' . - : . , ~ , ~ , ~ ,~ ~

~ i ~ ? , ~ . , , , . . . , , . ~ ~ . : ~ . , . , , ~ . ~ . , ' .. . . ~ .

~

~ii~ ~..;. =~.~i,...,." , i ~,,f :J,i~.,., '~' "~~ °~'~ .....~ -

. ~ . , . . . . 1 ~ , . : i , ~ ; , . : ~ , .~ : ~ i ~ ~ : ~ . . ~ . . ~ i ~ i ' ~ : , . ~ . . . , i ~ i ~ .

~"~ " ~,~,~"~":""~",~' "~" .":-"., ~:~ i~'~ ,~ =.,. .~. .~.: i~.~ .~,,,=:.~.

i ~.::-~I,., .~ ..~ . .... " ,- ~ ' - ~,~,~ii/.~.,~i .

' : . . . : ~ " . " . . : ' . ~ . : . . ~ . ; , .

, . ; ~ . . . . , , , ~ , - ~ : ~ , . . ~ , : ~ . ~ . ~ . . •

~ . . , ~ . , . ~ . , ~ : ~ . . ~ . . ,. : , = ~ . , . ,, , , ~ . ~ . = ~ . . . . , ~ . , . , . ,. . .. i ~ . : ~ . ~

' .~ . ~ : .. ,, ' • ~ i .~ , .. , . . . .. . , . . . . . " " . : ,i . . . . . : ~

: . ~ . i , " . ' " . " ' : ~ " , " , , , " ~ ' : ' . , = ~ ' ~ ' i " , " ' ~ . ~ . . " . . : , . . ' ~ I ~ o ~ : ~ . - . ' : ~ . '.

S..ii~~ ?.~.-/~'.~ ." '~':::~.,, " i ,,-.~~ ~::".~.:,..",,i.-~ ~....,..,:~,-::.,.

~ ,. .. ~ i ..... ~.~..~,~,~..~,... ? i. .. ... ~..,.:-..,..~"...~ ".i.i,,.-..-~ :~.,~.:,~.~.~~

~ i ~ . ~ ~ , . • . . . . ' / ' ~ . ~ " " , ' : ~ , ~ . , ° , , ' ~ : ' :~ ~ : , " ~ " ~ ' ' . ~ ~ . , ' : ~ ~ , . . ' . ~ ' ~

• ~ , ~ - ~ . . . ' , . ~ ~ , , . , , , ~ ' . ~ , . . . . ~ ~ • . . . . . . , . ., . , ~ , ~ . . . . . . . . ~ . ~ , . ; . . . . . . . :

~ . . , ~ ; " ~ ' ~ , " , ~ " ~ . , ~ i ~ . , " , , " , i ~ , ., : , . ~ - ~ ~ , ~ . ~ - ~ . . ~ ~ i ~ . " - , ~ i i ~ i ~ , i ~ . ~ . . ~ - ~ : ~ . , . "~ ~ , . ~

, , . , ~ . ~ . ~ , , . ~ , , ~ , . ° : . ~ . . ~ . . .c : . . : . . , ~ . . . . ~ , , , . , . , . . : , . ~ . , . ~ , , ~ , ~ ,

, - . . : . , , . . . . .. ~ . , , . . . . . , . ~ . ~ . . . , , . ,, , . ,. , , , . , . : , , . . . . . - : - . . . . . . . . . - , . : : : / , . : . ~ . . . ~ . . : , , . . ~ ~ , ' ~

. . ~ . - . < ~ . , ~ . . , o ° ~ o ~ . - . . . . . . ~ - , , / L ~ , . ~ ~ . - . . , - . , , - , : , , , ~ . - . . . . : ~ . ~ , ~

. . . . . . . ~ . . . .. . . . , . ~ ~ . ~ . . . . . . . . . . . ~. , ~ . ~ . . . . . ~ . ~

~ . ' . , ~ . . . . : ~ = ~ " . : : ' . . " , ~ . , ' " ~ ' ; . . . . ' ~ ' i ~ , ~ : . , ' . ~ ~ . i , ~ , , ~ : ' ' ' : " , ~ ' . " ? ~ . ~ o

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 ,

1 3 6



C O M P A R I S O N O F DE S I G N S

.. .. .. .. 7.-_- ':.._" .± ±- 7".~_~':_.'::_'~=~-- '- --------"~=;.--

A P P E N D I X

I

._

. . . . . . . ~ ,.............................. . . . . . O r i g i n a l ~ D e s i g u . . . . . . .. . . . . . . . . .. . . . . . . . . .. . R e v i s e d - D e s l g n . . . . . . . . . . . . .

. C ou r se • W i d t h B S 4 7 4 1 : 9 7 1 B S 4 7 4 1 : 9 7 1 + A M D T 2

M )

b

i T h l e k n e s s M a t e r i a l T h i c k n e s s M a t e r i a l

i ( r a m ) B S 4 3 6 0 ( ra m) B S 4 3 6 0

_ _ . .. .. - . - _ . - . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . .

I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

i

i D

@

4

1

@

e

8@

1

i0

i I

2 . 4 3 0

2 . 3 3 8

2 . 1 1 6

11.4 50C (lep)

1 0 . 3 5 0 C ( b a t c h )

9 • 3 1

II

8 o

3

I ¢ I

. 0 I I¢ I

43D

¢ I t # ~ t

4 3 D ( i e p )

4 3 D ( h a c c h)

4 3 C ( l e p )

t $ i t

I t

1 2 . 9 5 0 E ( : [e p )

I I . 7 wm 11¢

l O , m 6 I T I V

9 . 4

I C t

8 • 2 11 l

. 0 • 111 i

4 3 E ( b a t c h )

11

I¢ U

I f I Y , f T

I

1

N O T E : E G r a d e S t e e l s t o B S 4 3 6 0 a t e p r o d u c e d i n e a c h p l a t e ( i ep ) o n l y

( p a r a 1 . 2 3 . 1 o f BS 4 3 6 0 : 1 9 7 3

p a r a 2 9 . 1 o f B S 4 3 6 0 : 1 9 7 9 )

1 3 7



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 .

a t d e s i g n t e m p . d e s i g n t e m p .

mpac t Lon gi t ud lna i Lon gi t ud ina i T r ansv e rse

T e s t i n g

. . . . . . . . . - L . . _ . _ _ . . . . . . . . . . . • - . _ _ . . . . . : . . . . . - _ _ - - . . . . . . . . . . . . . . . . - . . . . . . . . . .

Hydro T e s t P a r t i a l F u l l

. . . . _ . . . . . . . . . _ .

F u l l ( 7 t h E d i t i o n )

P a r t i a l

( S t h E d i t i o n )

D e s i g n S t r e s s 2 / 3 2 / 3

Y i e l d S t r e s s Y l e l d S t r e s s

0 30 UTS

0 60 YS

( + q u a li t y f a c t o r

o f 0 . 9 2 f o r

s t r u c t u r a l

s t e e l s )

T e s t S t r e s s 0 . 8 5 x Y S 0 . 8 5 x Y S 0 . 8 0 x Y S

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 ) .

SHZ~ZN TS ,/~/gJ -

9 8 87

O.1

W ~ R

0.2

APPENDIX

o 3

3 8



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 .

39



M a ~ . e . . t _ _ o . . . . .. .

P a _ r t i c l e

i n s p . e c _ % i _ . o _ _ n _ . . _ _ _ o _ _ f

_the

I n t _ e _ . r . . i 0 . . r . . . .

O f

......

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 .

1 4 0



r _ . . .. . .. • . . . . . . . . . . . . . . .. . .. . .. , _ _. - _ . . . . _ . - _ - _ . . . . . . . . . . . . . . . . . . _ _ -_ - - - - . . .. _ . _ . . . .. _ ._ . . .. . .

. . . . - . . . . . . . . . . . . : _ ~ - ~ . . . . . : : : : c - _ - : - . . . . . . .

_N~T~_T_ 0 NS :-

C t to C 5 c o r r e s p o n d t o t h e c i t e . s e a m n u m b e r

£ r ~ s t a n k b a s t ( e ~ C I ~ - e p r e s e n t s

t h e l a k © £ r c , s , u . , £ r o m t e m ~ b a _ se )

V1 to Y~ -vor t i ca l s eams in the ¢~s pec t¢~

c o u r s e s (eK V1 ~pres en t the

v e r ~ i c m l ~ e a m s L n c o u r s e 1

F M - S h o 1 2 / A n n u 2 a r p l a t e i n s i d e F i l l u t

we l d

X - A n n u l a r ' p l a t e b u t t w e l d

CD - C i r cumFere n t i a l

d ~ £ e c t

A D - A x i a l d e t e c t

APPEND~

¢ 5

8.0r~M

C4

8 .3~n

C 3

9 .3~n

C 2

10.3r~n

CL

IL4r,.~

~ ~ ~ \ l / / / 1

/

A O - A X I A L O ( F ~ C T I O N

C O - C I R C U M F E R E N T I A L O E F 1 .E C T IO N

LOC T IONS OF CRITIC L D EFE CT SIZE SSESSMENTS

M - 1 1 1 4

. . _ . . . . . .

4



r~

I 1

(./3

I . - -

Z

a

O

0o 7,

O

C : ) I - ~

x - - EL i ,

I - -

LL .

: ~ i - - -

., ,~

U 3

Z ~

< L ~ J

F - - ~ -

L J

Z ~ -

O 0

= E i

0

t ul

• - p o - . . . . . . . . . . . . . . . . . . . c - ~ - - ~ o . . . . . . . . . . . ~ ~ ~ o ~

~ . , ~ + . , ~ ~ ,_ ~

i

. .

I ~ I . ~ L

.... ~ ~ ~ ~ ~ _ ~

~ ._ .~ .~ ~,, ~ .... ,.,,

, i ~ I

_ ~ ~ -. . ....~ ..~

~ ~_ ~ ~ ~ ~

l Y - - ~ ) ()-----~,., ; )

J ~ ~ . . q ~ - ~

.

. . . . . . ~ ~ ~

J

. ~ ~ - ~ ~ ~ ~ -

~ - ~ ) ~ ~---~ >

5: ~ 0

~ ( ) , , ,

l l l) ; ~ ; ) 0

z

~

z

=

3

1 4 2



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

143



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.

stress corrosion in ammonia storage tank

Documents