Accepted Manuscript
Prevention of brittle fracture in pressure vessels: A review of the design rules of EN
13445 annex B and BSI PD 5500 appendix D
Isabel Hadley, Steve Garwood
PII:
S0308-0161(18)30104-2
DOI:
https://doi.org/10.1016/j.ijpvp.2018.11.010
Reference:
IPVP 3794
To appear in:
International Journal of Pressure Vessels and Piping
Received Date: 19 March 2018
Revised Date:
24 October 2018
Accepted Date: 13 November 2018
Please cite this article as: Hadley I, Garwood S, Prevention of brittle fracture in pressure vessels: A
review of the design rules of EN 13445 annex B and BSI PD 5500 appendix D, International Journal of
Pressure Vessels and Piping (2018), doi: https://doi.org/10.1016/j.ijpvp.2018.11.010.
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Prevention of Brittle Fracture in Pressure Vessels:
A Review of the Design Rules of EN 13445 Annex B and BSI PD 5500 Appendix D
Isabel Hadley* and Steve Garwood**
* TWI Ltd, Isabel.hadley@twi.co.uk
** Imperial College, s.garwood@imperial.ac.uk
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ABSTRACT
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The selection of materials with appropriate levels of toughness is critical for the avoidance of
brittle fracture in ferritic steel pressure vessels and is usually achieved by specifying a test
temperature at which the material’s Charpy energy meets a certain energy criterion, typically
27J (T27J) or 40J (T40J). This paper traces the history of the toughness requirements in two
pressure vessel codes – the European pressure vessel standard EN 13445-2 and the UK
legacy code PD 5500 (previously BS 5500), which was one of several national standards that
influenced the development of EN 13445-2. Since publication of Issue 1 of EN 13445-2 in
2002, the toughness requirements for certain categories of European pressure vessels (in
particular, those made from lower-strength steels in the as-welded condition) have been
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relaxed considerably, leaving a substantial disparity (which can be more than 60 C) in
Charpy test temperature requirements (T27J) between PD 5500 and EN 13445:2014.
The reasons for the discrepancy between the codes are examined, and the large-scale tests
used to validate the PD 5500 requirements re-interpreted in terms of both the current EN
13445-2 rules and the fracture mechanics-based flaw assessment procedure BS 7910:2013.
The tests used to validate PD 5500 do not support the EN approach, suggesting an urgent
need for further examination.
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Finally, two historical pressure vessel failures (which preceded both codes) are examined in
the light of whether they could have been avoided by the use of either of the codes. In both
cases, use of the PD 5500 procedure would have led to a sufficiently high level of defecttolerance that failure could have been avoided. Use of the EN procedure for materials
selection, however, would not have avoided failure.
Background
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Whilst there is considerable room for opinion about what constitutes an appropriate level of
defect-tolerance (recognising improvements in fabrication methods, inspection techniques
and Quality Assurance systems since the failures occurred), this work underlines the need for
a re-examination of the basis of Annex B of EN 13445-2, in particular its application to lowerstrength steels.
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Failures of ferritic steel pressure vessels, both in service and under hydrotest conditions, are
well documented (1)-(4) and the causes generally understood. To prevent these occurrences,
pressure vessel design rules such as ASME III, BSI PD 5500 and EN 13445 have
incorporated specifications on material toughness and operational pressures and
temperatures to provide adequate resistance to brittle fracture of a potential defect that could
be missed by non-destructive examination or introduced in service. The current EN 13445-2
Annex B rules have evolved from those originally produced in 2002, which in turn were based
on the procedures given in national codes at the time (5) and developed on the basis of
fracture mechanics principles and correlation between Charpy Energy and fracture
toughness.
Large differences can now be observed between certain Charpy energy requirements for
pressure vessels constructed to EN 13445-2 and those made with reference to PD 5500, and
this subject is currently under discussion by the European committee TC54 WG 52 (subgroup Low-temperature) and its UK mirror committee PVE/1/18. The following enquiry has
been submitted to the Low-temperature sub-group:
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‘PVE/1/18 notes a significant disparity between the Charpy test temperature requirements of
EN 13445-2:2014 (Method 2) and those of our legacy code PD 5500, which was one of the
documents which formed the basis of the EN 13445-2 rules. We request the sub-group to
provide an explanation and update on the technical basis of Method 2.’
This paper, prepared as part of the response to the above question, considers mainly
low/medium-strength steels (grades 265/275 and 355) in the as-welded condition with section
thickness between 10 and 35mm and addresses:
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how the EN 13445 rules for low/medium-strength steels have changed over time,
how the EN rules (current and superseded) compare with the PD 5500 rules,
how the PD 5500 Charpy rules were validated against wide plate data,
the extent to which the original PD 5500 validation data support current EN 13445 Charpy
requirements,
an interpretation of the rules, based on the current UK fracture mechanics assessment
procedure BS 7910:2013,
interpretation of two case histories of pressure vessel failure in terms of the EN 13445
and PD 5500 materials selection rules.
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History of the PD 5500 toughness requirements
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During the 1950s and 60s, a spate of brittle failures occurred in the UK in pressure vessels
under hydrotest conditions (Smith and Hamilton, 1968). Research programmes at what is
now TWI, addressed the issue through the use of wide plate tests (6). These tests determined
the temperature at which wide plates of varying thicknesses with a chevron notch
(approximately 10mm long through-thickness) located in weld, HAZ, or plate material
achieved 0.5% plastic strain over a 20 or 30 inch gauge length. This information, together
with industry practice, led to the publication of BC 1515:part1 Appendix C in 1965. Cotton and
Harrison (7) subsequently improved the approach by drawing a series of reference curves
relating the Minimum Design Temperature to the temperature at which Charpy tests in the
longitudinal orientation for plate, or weld metal tests achieved a specified value dependent on
the yield strength of the material. HAZ testing was not required, although there was concern
over strain age embrittlement at the time, it was considered that the wide plate testing used to
derive the design lines which allowed for the thickness of the component and whether an aswelded or PWHT condition was applicable, together with industrial experience, provided an
adequate margin of safety.
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In 1979, the first edition of BS550 Appendix D ‘Recommended practice for C and C-Mn steel
vessels required to operate at low temperature” was published. The safety of these rules was
examined by Dawes and Denys (8) in a major International study involving the analysis of
over 1000 large scale tensile tests. By this stage the British standard procedure stated that
the criterion for material selection was that a 10mm through thickness flaw should be able to
withstand a deformation of four times the strain at the yield point of the material
(approximately equivalent to 0.5% plastic strain). This was the criterion adopted by the Dawes
and Denys review (8). They concluded the requirements were generally safe, and in some
cases overly conservative, however concern was expressed on the lack of validation for plate
thicknesses below 13mm.
A further major revision of Appendix D occurred in 1988 when the requirements became
mandatory. Garwood and Denham (9) describe the background to the revisions. In particular
the as-welded and PWHT design lines were adjusted and assessed against the original TWI
wide plate results, the results of the Dawes and Denys review, and further tests on higher
strength Offshore grade steels published by Pisarski (10) in 1985. These latter tests had
sharp cracks of varying sizes and a failure criteria of twice the yield point strain was
considered to be adequately safe. The results of these tests were employed by Garwood and
Denham (9) to support the new design lines. The results from bespoke wide plate tests
carried out at TWI on section thicknesses below 12mm were also used to validate changes to
the design conditions for thin sections.
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In describing these revisions, Garwood and Denham (9) conclude that advances in fracture
mechanics procedures would allow for fitness for purpose approaches to be used for future
rationalisation of the Appendix D rules, and a fracture mechanics based approach (Appendix
U) was subsequently added to BS 5500 to be complementary to Appendix D. The 1988 rules
for avoidance of brittle fracture remain in place (with minor modifications) to the present day,
although the UK standard (BS 5500) has been rebadged as a guidance document (PD 5500)
to recognise the role of EN 13445.
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The current PD 5500 rules for grade 265/275 steel (ie steel with a nominal yield strength, Re,
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of 265 or 275N/mm ) in the as-welded (AW) condition are shown in Figure 1. The user
chooses a reference temperature, Tref (related to the minimum design temperature), from the
y-axis and reads off the appropriate materials requirement on the x-axis. The requirement is
given in terms of T27J, the temperature at which the Charpy energy is at least 27J, for the
appropriate section thickness (t). Alternatively, given the quality of material available (T27J)
and its section thickness (t), it is possible to read off the allowable reference temperature on
the y-axis.
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In the PD 5500 rules, the same set of design lines is applicable to a range of different
materials; the requirement for higher-strength steels to have higher Charpy energy is
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accommodated by requiring steels with tensile strength above 450N/mm to meet a criterion
of 40J rather than 27J. This criterion is applicable to grade 355 steel, which has a specified
Ultimate Tensile Strength (UTS) in the range 470-630MPa, so the equivalent to Figure 1 for a
grade 355 steel would be a similar graph with an x-axis of T40J instead of T27J. However, in
order to compare like with like (as seen later, EN requirements for grade 355 are given in
terms of T27J only), the advice in clause D.6.2 of PD 5500:2015+A1:2015 has been followed.
This involves assuming that the Charpy energy/temperature relationship is a straight line with
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a gradient of 1.5J/ C over the range 18-47J for the purposes of ‘converting’ to a temperature
lower than the actual test temperature (if a conversion in the opposite direction is required,
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the assumed gradient is 1.0J/ C (see Figure 2)). Consequently, T27J=(T40J-8.7) C when
estimating T27J from T40J.
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The resulting PD 5500 design line for grade 355, given in terms of T27J, is shown in Figure 3.
Note that the lines relating to different section thicknesses are not parallel; gradients vary
from 0.37:1 for 35mm thick steel to 0.96:1 for a thickness of 10mm. Note also that, for thicker
sections (t≥25mm), the Charpy tests temperature is always lower than the reference
temperature, ie T27J<Tref.
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PD 5500 was one of several source documents used to produce the earliest version of EN
13445 (the Dutch, French, German and Swedish national codes also influenced
developments), as discussed by Wiesner et al (5).
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Figure 1 PD 5500 Charpy requirements for as-welded grade 265/275 steels
Figure 2 PD 5500 method for deriving T27J from T40J and vice-versa
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Figure 3 PD 5500 Charpy requirements for as-welded grade 355 steels; values in terms of
T27J derived from those given in PD 5500 in terms of T40J
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Justification of the PD 5500 design lines is given by Garwood and Denham (8). A series of
wide plate tests was carried out on 1m square plates containing a through-thickness notch
around 10mm long, machined transverse to a butt weld. Consequently, the crack tip would
have experienced driving forces due to both applied stresses and welding residual stresses.
The plate was loaded perpendicular to the notch and the criterion set for adequate fracture
resistance was that it should withstand 0.5% plastic strain or a total strain equivalent to four
times that experienced at the yield strength (these are approximately equivalent for mediumstrength steels).
History of the EN 13445 requirements
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Figure 4 shows the design lines for Grade 265/275 steel from two editions of EN 13445: up to
the 2002 edition (Issue 1) and the editions from 2009 onwards (Issue 5). Data are taken from
Figure B.4-2 of Issue 1 and Figure B.2-2 of Issue 5. The Issue 1 lines relate to all steels with
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yield strength, Re≤310N/mm , whilst the Issue 5 lines are designated ‘265MPa’ (although the
corresponding lines for post-weld heat-treated (PWHT) material refer to the more common
grade 275MPa steel).
The ‘Issue 1’ lines are similar to those originally developed and published in PrEN 13445 (see
(5) for further information), and show the following features:
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Requirements are given in terms of graphs of Tref against T27J (as for PD 5500), with
separate graphs for each class of steel (265/275, 355 etc).
Requirements are given for section thicknesses between 10mm and 35mm in 5mm
intervals (for thicknesses beyond 35mm, PWHT would normally be adopted).
The lines relating to different section thicknesses are parallel to each other.
Charpy requirements become more onerous (ie T27J decreases) with increasing section
thickness, as would be expected.
For thicker sections (30 and 35mm), T27J is below the reference temperature. This is
consistent with the PD approach discussed above, and with Charpy requirements for
many safety-critical components used in the as-welded condition. Effectively, because the
thickness of the structure often exceeds that of the Charpy specimen used to qualify its
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toughness, satisfactory Charpy energy needs to be demonstrated below the design
temperature.
Figure 5 shows the corresponding ‘Issue 1’ design lines for grade 355 steel. The same trends
apply as for the lower-strength steel, except that the curves are offset so that the
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requirements for grade 355 are more onerous, eg for a reference temperature of 0 C and
section thickness of 35mm, the value of T27J for a grade 265/275 materials would be
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around -6 C, whereas for a grade 355 material it would be around -18 C.
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Subsequently, the ECOPRESS project, run under the European Framework 5 programme
(see Langenberg et al (11)) provided the basis for a revision of EN 13445 Part 2 Annex B.
The main focus of ECOPRESS was the use of higher-strength quenched and tempered steel
(up to 690MPa yield strength), and duplex stainless steel, in the manufacture of pressure
vessels. A method based on fracture mechanics, referencing the European SINTAP (12)
fracture assessment procedure (but also using some other ad hoc techniques) was
developed, and a new set of design lines drawn up. These first appeared in Issue 5 of EN
13445. For the steels considered in this paper (low-and medium-strength steels in the AW
condition), the EN rules have remained unchanged since then. Wide plate tests were carried
out on selected high-strength steels (grades 500 and 690MPa) as part of the justification of
the new rules, although these are not considered here, in view of the current emphasis on
lower-strength steels. A later project, IMPEX (13) used a different model for the assumed flaw
size, but the same fracture mechanics model as that used for ECOPRESS.
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The background to the fracture mechanics calculations underlying Issue 5 is given in a series
of publications (eg Sandström et al (14)), and has been discussed at length by the relevant
committees – it is therefore not examined in detail here. The underlying model is that of a
surface-breaking flaw in a plate. The through-wall extent of the flaw is assumed to be a
function of the plate thickness, whilst the applied gross stress, σg, is a function of the yield
strength, Re, taken from part 3 of EN 13445 (σg=Re/1.5). An allowance is made for residual
stress which is covered by a general assumption of adding 100 MPa to the σg stress,
independent of which strength class is used. The fracture toughness required for the plate to
tolerate the flaw is adjusted to a critical value, resulting from the limit state condition where
the value of KI on the FAD is equal to Kmat for given boundary conditions. Kmat, is then
calculated from established correlations between Charpy energy and Kmat. The correlation
used is the Master Curve approach going back to the works of Wallin, verified in several
European projects and included today in Annex J of BS 7910.
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Reference temperatures as low as -100 C are cited.
The lines have a slightly steeper gradient (1:1) relative to those presented in Issue 1
(0.94:1)
Lines are shifted significantly to the right, ie the Charpy requirements are more lenient.
For example, Charpy test temperatures are above the reference temperature even for
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the thickest sections. For a section thickness of 35mm and Tref=-20 C, Issue 1 of EN
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13445 would have required T27J∼-30 C for a grade 265/275 steel and T27J∼-40 C for a
grade 355 steel (note that both values of T27J lie below Tref). By the time of Issue 5, this
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had risen to T27J∼0 C for a grade 265/275 steel and T27J∼-15 C for a grade 355 steel – in
both cases, T27J>Tref.
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As seen from the dotted lines in Figure 4 and Figure 5, the ‘Issue 5’ design lines differ in a
number of ways from those of Issue 1:
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Figure 4 EN 13445 Charpy requirements for grade 265/275 steel in the AW condition; Issue 1
(solid lines) and Issue 5 (dashed-dotted lines)
Figure 5 EN 13445 Charpy requirements for grade 355 steel in the AW condition; Issue 1
(solid lines) and Issue 5 (dashed-dotted lines)
Comparison of PD and EN requirements
When the EN 13445 and PD 5500 design lines are shown side by side (see Figure 6 for
grade 265/275 and Figure 7 for grade 355 steel), it is apparent that the EN and PD
approaches were reasonably similar when Issue 1 of EN 13445 was published (although the
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gradients are different, all lines cross each other, except for the case t=10mm), but that they
have since diverged considerably, because of the changes between Issue 1 and Issue 5 of
EN 13445. This is shown in Table 1 for the case of two grades of steel (265/275 and 355) and
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two reference temperatures (-10 C for a 35mm thick steel and -40 C for a 10mm thick steel).
For all cases, PD 5500 gives the lowest value of T27J and Issue 5 the highest – in other
words, Issue 5 consistently allows lower values of toughness.
There are differences in the details of the EN and PD 5500 Charpy requirements too, as
shown in Table 2.
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The difference in T27J requirement between Issue 1 and Issue 5 lies in the different
approaches used in the beginning (issue 1) and for issue 5. Whilst issue 1 was a mixture of
fracture mechanics approaches and experiences from the member states at that time, issue 5
is based purely on fracture mechanics models which have been described elsewhere (14).
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Figure 6 Comparison of EN 13445 and PD 5500 Charpy requirements for grade 265/275
steels: EN issue 1 lines are shown as solid, EN Issue 5 as dashed-dotted and PD 5500 as
dashed lines
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Figure 7 Comparison of EN 13445 and PD 5500 Charpy requirements for grade 355 steels:
EN issue 1 lines are shown as solid, EN Issue 5 as dashed-dotted and PD 5500 as dashed
lines
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Table 1 Comparison of codes for two values of Tref and section thickness
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Tref, C
t, mm
265/275
355
265/275
355
-10
-10
-40
-40
35
35
10
10
T27J
PD 5500
-50
-59
-8
-16
EN, Issue 1
-18
-28
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-12
EN, Issue 5
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Table 2 Details of Charpy impact requirements of EN 13445 and PD 5500
Aspect
EN 13445
Design temperature
Orientation of Charpy
specimens
Position of extraction
of Charpy specimens
Areas to be sampled
by Charpy specimens
Minimum requirement
≤ +50 C
transverse
PD 5500
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Typically ¼ wall
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≤0 C
Longitudinal*
parent metal (PM), HAZ and weld metal PM and WM**
(WM)
Minimum requirement (eg 27J) applies to the mean of three results –
an individual result may lie below 27J but must reach at least 70% of
this value, eg 19J
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Validation of the PD 5500 rules
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* PD 5500 rules are based on Charpy tests carried out in the longitudinal direction (see PD
5500:2015:A1:2015, Table D.2), but it is noted that ‘BS EN and other European Standards
specify that specimens should be transverse, but such data can be used with this annex, as it
will be safe.’
** HAZ tests are not required when multi-run processes are used with heat inputs between 1
and 5 kJ/mm.
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Part of the evidence used for validation of the PD 5500 fracture prevention rules is shown in
Figure 8. This shows the results of a series of wide plate tests carried out on boiler plate
steels (BS 1501-151 and -161, which have specified minimum tensile strength below
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450N/mm , so would be covered by the design lines shown in Figure 1). The area above and
to the left of each line is ‘safe’, whilst the isolated datapoints represent failure of a wide plate
at values of strain below the agreed criterion of 0.5% plastic strain. Consequently, if a point
corresponding to failure lies to the right of the corresponding design line, the design
conditions have been achieved. From Figure 8, it can be seen that:
Five out of five 40mm thick specimens lie to the right of the 35mm line,
Two out of three 30mm thick specimens lie to the right of the 30mm line,
Ten out of twelve 25mm thick specimens lie to the right of the 25mm line,
Five out of five 12mm thick specimens lie to the right of the 15mm line (four out of five lie
to the right of the 10mm line as well).
In all, 22 out of 25 specimens (88%) lay to the right of the corresponding design curve.
As remarked by Garwood and Denham (9), the design curves were influenced by the wide
plate test results, but not derived from those results alone. It should also be noted that the
points shown in Figure 8 represent just a few of the many tests (around 1000) examined as
part of the validation of PD 5500 – further details are given by Dawes and Denys (8).
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Figure 8 Validation of PD 5500 Charpy requirements via wide plate testing (taken from Fig. 3
of (9))
One out of five 40mm thick specimens lie to the right of the 35mm line,
None of the three 30mm thick specimens lie to the right of the 30mm line,
Two out of twelve 25mm thick specimens lie to the right of the (interpolated) 25mm line,
One out of five 12mm thick specimens lie to the right of the (interpolated) 12mm line
In all, 4 out of 25 specimens (16%) lay to the right of the corresponding design curve.
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It is now possible to compare the current (Issue 5) EN design rules for grade 265/275 material
(from Figure 4) with the results of wide plate tests (from Figure 8) to examine the extent to
which the tests used to validate PD 5500 rules also validate EN 13445 rules. The results of
this analysis are shown in Figure 9. Two additional design lines, for 25mm and 12mm
thickness, have been added to those given in EN 13445 Issue 5 (by interpolation between
existing curves) in order to allow a direction comparison between the test plates and the
corresponding design curve. From Figure 9:
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It is thus apparent that the EN 13445 Issue 5 design rules for these lower-strength steels in
the as-welded condition are not supported by the wide plate test data used to validate the PD
5500 rules.
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Figure 9 Comparison of wide plate test data (generated in support of PD 5500) with current
EN 13445 rules for grade 265/275 steels; lines for 25mm and 12mm thick plates obtained by
interpolation
BS 7910 model of wide plate tests used to validate PD 5500
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Of course, it can be argued that the relationship between the current EN rules and the ‘PD
5500’ wide plate tests merely demonstrates that the PD 5500 wide plate tests represented too
severe a test condition – for example, the existence of a 10mm long through-thickness crack
is inconceivable in a pressure vessel (inspection and/or pressure testing would ensure that no
such defects remain in the vessel), and the use of a 0.5% plastic strain pass criterion is
unusually severe. To gauge the relative margins of safety some additional calculations have
been carried out using the flaw assessment procedure BS 7910:2013, which has effectively
superseded the European SINTAP and FITNET procedures.
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The first step was to model a 1m wide x 35mm thick grade 275 plate containing a 10mm
through-thickness flaw similar to those used to validate the rules given in PD 5500. Reference
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temperature was assumed to be -10 C, as per Table 1. The specified T27J was taken from
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Figure 1 and Table 1 as -50 C. The Master Curve equation (Annex J of BS 7910:2013) at
Pf=0.5 was used to derive Kmat=150MPa√m (lower values of Pf may be appropriate, but at this
stage the objective is to use ‘best estimates’ of properties, rather than introducing safety
factors).
From the specified tensile properties of grade 275 steel (SMYS=275MPa,
380≤UTS≤540MPa), a room temperature UTS of 400MPa was assumed, and an allowance
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made for the likely elevation of tensile properties at -10 C relative to the room temperature
value. Hence room temperature yield strength and UTS were assumed to be 275MPa and
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400MPa, rising to 297 and 432MPa at -10 C. Discontinuous yielding was assumed.
Primary loading was represented by a primary stress of 275MPa and a stress concentration
factor of 3 (to replicate the conditions specified for the wide plates), secondary stresses by
assuming full yield magnitude residual stress (275MPa), but allowing for relaxation when high
primary loads are applied, as per the rules of BS 7910. [In practice, the secondary stress
parallel to the weld (acting perpendicular to the flaw) might be a little higher because of the
convention of using overmatched weld consumables, but the effect of this on the final output
is likely to be negligible.]
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Treating the above as a known flaw, and calculating its acceptability using BS 7910:2013,
produces a point just outside the Failure Assessment Line (FAL), as seen in Figure 10. In
other words, a 10mm long through-thickness flaw, in a material just meeting the requirements
of the standard, and subjected to the high stresses described, is just beyond the point of
failure, and this is consistent with the results of the wide plate tests.
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Figure 10 BS 7910 model of 35mm thick grade 275 steel plates at -10 C. A 10mm throughthickness notch and Charpy properties as per PD 5500 are assumed, and loading represents
that used in the PD 5500 validation tests
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The next step was to assume exactly the same materials properties, but to use the loading
and flaw size assumption that were used to derive the EN 13445 (Issue 5) rules, but within
the framework of BS 7910, ie assuming that stress intensity factor, reference stress, welding
residual stress and primary/secondary stress interaction are all calculated in accordance with
BS 7910. This second calculation assumed a surface flaw size of 8.9x27mm (consistent with
the assumptions described by Sandström et al (14), a primary stress of 183.3MPa (67% of
room temperature yield strength) and a secondary stress of 275MPa (100% of room
temperature yield strength), with stress relaxation permitted as per BS 7910. Stress
concentration factors (SCF) of 1, 2 and 3 were assumed – the first is consistent with the
assumption made in the derivation of the EN rules, whilst the last helps to account for stress
concentrations due to features such as nozzles, and to account for misalignment during
manufacture.
As seen in Figure 11, the analysis point lies within the FAL, even when SCF=3. The
conditions envisaged in the development of the EN rules are therefore somewhat more
benign than those associated with the wide plate tests, mainly because the latter assumed a
primary stress of 100% yield (to represent the hydrotest condition, even though hydrotesting
is not usually carried out at low temperatures). The former assumed an applied stress of
67%Re, to represent operating conditions. In other words, the stress intensity magnification
factors (Mm) associated with the 10mm through-thickness flaw (PD 5500 model) and the
8.9mm high surface-breaking flaw (CEN 13445 model) are fairly similar, but the higher global
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stresses assumed for the PD 5500 model make it a more ‘onerous’ model than is the EN
13445 model, so that the values for both Lr and Kr are higher for the case of the PD 5500
model.
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Figure 11 BS 7910 model of 35mm thick grade 275 steel plate at -10 C. A surface notch and
Charpy properties as per PD 5500 are assumed
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A third calculation started from the assumption that the fracture toughness was as specified
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by EN 13445 Issue 5, ie T27J=8 C. Kmat=69.4MPa√m (for Pf=0.5 and T=-10 C) in this case, all
points lie outside the FAL, even when SCF=1 (see Figure 12).
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Given that there is no safety factor in the derivation of Kmat (although there are safety factors
elsewhere in the derivation of the EN 13445 rules, eg in the definition of the reference flaw) in
Figure 12, and also bearing in mind the results shown in Figure 9, there is a legitimate cause
for concern that the current EN 13445 fracture prevention rules could be insufficient, at least
for the case of lower-strength steels used in the as-welded condition. A thorough review of
the entire range of grades and section thicknesses is recommended, using analysis methods
based on BS 7910.
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Figure 12 BS 7910 model of 35mm thick grade 275 steel plate at -10 C. A surface notch and
Charpy properties as per EN 13445 are assumed
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Conclusions and Recommendations
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1. The Charpy energy requirements for low/medium-strength steels given in PD 5500
Appendix D have been reviewed against the original validation wide plate data, the latest
revision of BS 7910 and two historical pressure vessel failures (see Appendix) and are
shown to have a reasonable level of safety.
2. The equivalent requirements in the current edition of EN 13445 Annex B were derived in
a different way, resulting in higher Charpy test temperature requirements, ie a lower
margin of safety.
3. It is recommended that a thorough review of the entire range of steels covered by EN
13445 Annex B is carried out using analysis methods based on BS 7910 to ensure that
these rules are safe.
Acknowledgement
This work was funded by Industrial Members of TWI, as part of the Core Research
Programme.
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Appendix: Analysis of historical pressure vessel failures in terms of
current pressure vessel standards
Introduction
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As part of a programme to validate the rules in PD 6493:1991 (a forerunner of BS
7910:2013), Challenger et al (2),(3) analysed the failure of several pressure vessels, either
during hydrotest or in service. The compendium was later updated by Hadley and Moore
(15),(16), using the 1999/2005 editions of BS 7910 and is in the process of undergoing a
second major revision to take account of BS 7910:2013. These detailed fracture mechanicsbased analyses will not be further considered here – instead, the cases are used as a means
of ‘calibrating’ the suitability of the materials selection requirements of PD 5500 and EN
13445 against real engineering failures rather than laboratory-based tests and theoretical
models. Details of the incident (such as temperature, grade, thickness, condition) are used to
determine what requirement would have been placed on the steel used, were the same
vessel to be manufactured today.
1. Robert Jenkins pressure vessel
One of the failures analysed by Challenger et al (2),(3) and by Hayes and Phaal (17)
concerns a pressure vessel manufactured by Robert Jenkins & Co. Ltd, which failed in 1970
during factory hydrotest. It was actually designed and constructed to ASME VIII requirements;
the conditions at failure (in terms of materials properties and hydrotest pressure) are well
documented in a report by Banks (18).
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Materials grade: ASTM A515 Grade 70 (C-Mn steel made to coarse-grained practice and
supplied in the ‘as-rolled’ condition with specified minimum yield strength of 38ksi
(262MPa) and specified tensile strength of 70-85ksi (483-586MPa). There is no
mandatory Charpy requirement for this grade of steel.
Section thickness: 28mm
Condition: as-welded
0
Design reference temperature: 12 C (ie the hydrotest temperature)
Actual Charpy energy (see Figure 13):
0
o Tests by the steelmaker suggested a transition temperature of T27J=21 C for
0
longitudinal specimens and 33 C for transverse specimens, but this was based
on material taken from an area remote from the fracture initiation point.
o Tests by the fabricator, taken on casualty material close to the area of failure,
0
showed a value of T27J~59 C (for longitudinal specimens)
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Of course, avoidance of brittle fracture depends on multiple factors (choice of parent material
and consumables, close control of welding, inspection and hydrotest) that are far better
understood now than when this failure occurred over 45 years ago. Nevertheless, it is worth
considering whether, if this vessel were being designed to current pressure vessel standards,
it would have been able to tolerate the large flaw found during the failure investigation. In
particular, it was hoped that this failure would help in rationalising the large differences in
0
Charpy requirement (more than 60 C in T27J in some cases) between PD 5500 and EN
13445-2 for lower-strength steels in the AW condition. The following parameters were
assumed:
Since only the steelmaker’s results are available in both orientations, allowing direct
comparison between actual and required values of T27J, it will be assumed in the first instance
that the designers had access only to these results,
Turning first to PD 5500:2015 (incorporating Amendment 1), and interpolating between the
30mm and 25mm lines of Figure 1, the required impact test temperature (for 27J impact
0
0
energy in longitudinal specimens) is 20 C. Consequently, the steel available (T27J=21 C)
would not have been accepted, albeit by a small margin, and the failure could have been
avoided.
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Figure B.2-2 of EN 13445-2 (reproduced in Figure 4) provides the equivalent information for a
pressure vessel designed to the European standard. This addresses grade 275 materials in
0
the as-welded condition; for TR=12 C and a plate thickness of 28mm, the required T27J is out
0
of the range of the graph (which is limited to T27J<20 C), but (by extrapolation) is around
0
43 C. This requirement is for specimens taken transverse to the rolling direction, ie the notch
is parallel to the rolling direction. Had the designers had access only to the steelmaker’s
0
results of T27J,T=33 C, with extrapolation outside the bounds of Figure B.2-2 of EN 13445-2,
then this steel would have been considered suitable, since its transition temperature was
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below 43 C.
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If the designers had known of the poor properties of the material in the casualty area
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(T27J,L=59 C, with T27J,T expected to be still higher) then the steel would not have been
accepted by either standard.
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The situation is summarised in Figure 14, in which the ‘safe’ areas are shown for the two
different standards, along with the actual properties derived from the steelmaker’s tests. For
the EN standard, the actual transition temperature (T27J,T) lies within the supposedly ‘safe’
zone, yet the vessel was clearly unsafe.
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This example of a historical pressure vessel failure that could have been prevented by
application of the PD 5500 rules, but not by use of the EN 13445-2 rules is not, of course,
conclusive evidence on its own. First, the flaw that was found during the failure investigation
of the Robert Jenkins vessel was a very large arrested brittle flaw – approximately half-wall in
through-wall height and over 100mm long, exceeding the size that would now usually be
considered reasonable in designing defect-tolerant structures. Second, the Charpy energy of
the material in the casualty area was so low that it would have been rejected by both
standards, had the designers been aware of it. In other words, material inhomogeneity played
a role, with the results generated by the steelmaker (used in this example because they were
determined for both longitudinal and transverse specimens) proving much better (ie implying
higher toughness) than those generated by the fabricator as part of the failure investigation.
0
Third, the criterion of T27J,T≤43 C for the EN 13445-2 material was obtained by extrapolation
of the current design curves; this is consistent with the underlying model assumed, but not
0
with a literal interpretation of the rules, which would require T27J,T≤20 C to stay within the
range of Figure 4. Nevertheless, failure case histories provide a useful ‘calibration’ of the
recommendations given in standards, and it would be prudent to re-examine this case using a
fracture mechanics-based model, perhaps using the EN 13445 reference flaw as the basis of
the calculations, rather than the flaw that was actually found.
It can be concluded that:
1. For the temperature, steel grade, section thickness and condition (as-welded) associated
with the failure of the Robert Jenkins pressure vessel, current PD 5500 design rules
0
0
require T27J=20 C, compared with T27J=43 C (by extrapolation) under current EN 134452 rules.
0
2. The significant difference between the EN and PD rules (a 23 C difference in T27J) is
mitigated somewhat by the fact that PD 5500 rules are based on specimens extracted
parallel to the rolling direction of the steel, whereas EN 13445 rules are based on a
transverse specimen; for the steel considered in this case, the 27J transition temperature
0
was around 12 C higher for the case of transverse specimens.
3. Based on the steelmaker’s impact test results, the application of current PD 5500 rules
would have (just) rejected the steel, whereas current EN 13445 rules would have
accepted it.
4. Based on the impact test results taken on casualty material, which had a much higher
transition temperature than implied by the steelmaker’s results, both standards would
have (correctly) rejected the material.
5. The initiating defect was much larger (around half-wall) than that considered in the
derivation of the current EN 13445-2 rules and far exceeds the size of flaw that would be
considered both detectable and acceptable in modern pressure vessel standards;
consequently, an additional assessment based on the EN 13445-2 ‘reference flaw’ and
on Charpy data alone (ie assuming no fracture toughness data to have been available) is
recommended.
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Figure 13 Charpy transition curves for failed pressure vessel; the fabricator’s tests were
carried out on casualty material, ie extracted from close to the area of initiation
Figure 14 Comparison of PD 5500 and EN 13445-2 requirements with the results reported by
the steelmaker
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Table 3 Summary of Charpy test results and specification requirements
Steelmaker’s tests
Casualty material, fabricator’s tests
BS specification
EN specification
T27J,L
0
21 C
0
59 C
0
≤20 C
0
<~31 C*
T27J,T
0
33 C
0
~71 C*
0
<~32 C*
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<43 C**
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2: Vertical Refinery Vessel Failure
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* assuming T27J,T - T27J,L=12 C
** by extrapolation of EN 13445-2, Figure B.2-2
The relevant data are summarised below:
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Material: ASTM A204 Grade C (C-½Mo steel)
Specified Minimum Yield strength: 320MPa (the material will therefore be treated as
grade S265/275 for the purposes of analysis to EN 13445 Appendix B)
Specified tensile strength: 515-655MPa
Charpy requirements for steel: no mandatory requirement in the ASTM specification
0
Temperature at failure: 8 C
Reference thickness: the flaw was in the outer weld of a detail that was effectively a set-in
nozzle (see Figure 15) with reinforcement provided by a cover plate. The thickness of the
three parent metal components are: e1=13mm (nozzle), e2=21mm (shell) and e3=31mm
(cover plate). According to the tables given in both PD 5500 and EN 13445, the reference
thicknesses for both the AW and PWHT cases are: 13mm for the nozzle, 21mm for the
shell and 31mm for the weld.
Actual Charpy transition temperature (T27J) of parent steels:
0
o 20 C for the 13mm (nozzle) material (transverse specimens)
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o 12 C for the 13mm (nozzle) material (longitudinal specimens)
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o 15 C for the 31mm (cover plate) material (transverse and longitudinal specimens)
0
o 25 C for the 21mm (shell) material (transverse specimens)
0
o 21 C for the 21mm (shell) material (longitudinal specimens)
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Actual Charpy transition temperature (T27J) of weld: +40 C
Condition: notionally post-weld heat-treated (PWHT).
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Another engineering failure considered by Challenger et al (2)-(3) was the failure of a vertical
refinery tower in 1981, originating from a transverse weld metal hydrogen crack that had
extended by creep. Failure occurred during a hydrotest, carried out after the installation of
new nozzles, and was attributed to a number of factors, including the presence of a flaw,
occurrence of in-service creep, poor fracture toughness of the materials of construction and
relatively high levels of welding residual stress (although the weldment was supposed to have
received post-weld heat-treatment). A fracture mechanics-based analysis of the failure is
given in (2)-(3), but the question addressed here is: could this failure have been prevented by
the use of the current editions of PD 5500 or EN 13445?
Because of the uncertainty surrounding the level of post-weld heat-treatment and therefore
the welding residual stress, two bounding cases have been considered: assuming PWHT to
have been carried out correctly, and assuming the as-welded (AW) conditions.
PWHT condition
From Figure D.2 of PD 5500 (reproduced here as Figure 16), the required impact test
0
temperature is beyond the limits given in the graph, but by extrapolation is around +37 C (for
0
an L-T specimen). Since the actual transition temperature of the weld was +40 C, this
material would not have been considered acceptable for this particular component, had the
designers been aware of the high Charpy transition temperature of the weldment. As for the
case of the Robert Jenkins vessel, the margin between acceptable and actual T27J is small,
but suggests that application of PD 5500 could have prevented this failure.
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Had the vessel instead been designed in accordance with EN 13445, then the equivalent
requirement for a 31mm thick grade 265MPa material in the PWHT condition would have
0
been around +60 C (well out of range of Figure B.2-1 of EN 13445-2, reproduced in Figure
17). The requirement would apply to a specimen in the transverse direction, with a minimum
Charpy energy of 27J. A designer using EN 13445-2 might therefore have accepted the poorquality weld metal as meeting the requirements of the code (although this assertion assumes
both extrapolation of the current design lines and correct application of PWHT).
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As-welded condition
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Had the vessel been designed in accordance with current PD 5500 guidance for use in the
as-welded condition, then the required impact test temperature for the weld would have been
based on a reference thickness of 31mm (weld metal is expected to satisfy the same impact
0
energy requirements as those tabulated for the parent materials, ie +6 C (or lower) and a
minimum Charpy energy of 27J for longitudinal specimens (Figure D.1 of PD 5500:2015/
Figure 1).
Had the vessel instead been designed in accordance with EN 13445, then the equivalent
requirement for a 31mm thick grade 265MPa material in the AW condition would have been
0
around +33 C (out of range of Figure B.2-2/Figure 4). The requirement would apply to a
specimen in the transverse direction, with a minimum Charpy energy of 27J. Consequently, if
the designers had treated this vessel as being in the as-welded condition, then both PD 5500
and EN 13445 rules would have rejected the material.
Table 4 summarises the results of this exercise. The inadequate heat-treatment reported
during the failure investigation was probably a contributory factor to the failure, along with the
original flaw, some in-service crack growth by creep and the poor toughness of the material.
Table 4 Summary of PD 5500 and EN 13445 requirements for failed refinery tower
T27J criterion,
0
(PD), C
T27J criterion,
0
(EN), C
PWHT
AW
37
6
60
35
Could PD have
prevented failure?
40
40
Yes (borderline)
yes
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Concluding remarks
Actual
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T27J, C
D
Condition
Could EN have
prevented
failure?
no
yes
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Note that neither of the two case histories considered in the Appendix definitively show the
codes to be ‘safe’, ‘unsafe’ or ‘excessively conservative’, for the reasons given previously.
First, fabrication practice, inspection techniques and oversight of pressure vessel
manufacturing (eg the use of notified bodies) have improved considerably since these failures
occurred, and it is difficult to imagine flaws of the sizes considered in these failure case
histories remaining undetected in modern pressure vessels – indeed, the failures themselves
will have influenced pressure vessel manufacture and testing for the better. Second, some of
the figures derived for both failure cases come from extrapolation of the design curves, and
are therefore based on the spirit, rather than the letter, of the documents. Nevertheless,
engineering failures do tend to occur when several factors go wrong simultaneously, and
‘calibration’ of design rules against real case histories is one of several measures that can be
used to validate them.
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Figure 15 Configuration of failed joint (from Table D.4 of PD 5500:2015); Table B.4-1 of EN
13445 is similar
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Figure 16 Charpy requirements for materials in the PWHT condition, taken from Figure D.2 of
PD 5500
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Figure 17 Charpy requirements for materials in the PWHT condition, taken from Figure B.2-1
of EN 13445-2
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References
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1. TA Smith and RG Warwick: ‘A survey of defects in pressure vessels in the UK for the
period 1962-1978 and its relevance to nuclear primary circuits’, International Journal of
Pressure Vessels and Piping (IJPVP), 11, 127-166, 1983
2. N V Challenger, R Phaal and S J Garwood, ‘Appraisal of PD 6493:1991 Fracture
Assessment Procedures Part III: Assessment of Actual Failures’, TWI Industrial Member
Report 512(III)/1995
3. N V Challenger, R Phaal and S J Garwood, ‘Fracture mechanics assessment of industrial
pressure vessel failure, IJPVP, 61 (1995), 433-456
4. N Smith and IG Hamilton, ‘Failure in heavy pressure vessels during manufacture and
hydraulic testing’, Paper no. 591, Conference on failures in large steel structures, West of
Scotland Iron and Steel Institute, May 1968
5. C S Wiesner, S J Garwood, R Sandström, D M Street and K J Coulson, ‘Background to
requirements for the prevention of brittle fracture in the European standards for unfired
pressure vessels (prEN 13445) and metallic industrial piping (prEN 13480)’, IJPVP, 78/6,
391-399
6. Woodley C.C., Burdekin F.M., and Wells A.A. 1964 ‘Mild steel for Pressure Equipment at
sub zero temperatures’ British Welding Journal March 1964 pp 123-136.
7. HC Cotton and DBJ Thomas, ‘Current requirements in the oil and chemical industries for
the avoidance of brittle fracture in carbon-manganese steels’, Paper no. 589, Conference
on failures in large steel structures, West of Scotland Iron and Steel Institute, May 1968
8. Dawes and Denys, 1984: ‘BS 5500 Appendix D: an assessment based on wide plate
brittle fracture test data’, MG Dawes and R Denys, IJPVP 15 (1984), 161-192
9. SJ Garwood and JB Denham, ‘The fracture toughness requirements of BS 5500’,
presented at ASME PVP Conference, June 19-24 1988, Pittsburgh, USA
10. Pisarski, H.G. 1985 ‘Basis for the Charpy V requirements for parent plate, HAZ and weld
metal in the proposed revisions to the 1977 edition of the Dep’t of Energy Guidance
notes’ TWI report 3866/2 Feb 1985.
11. Langenberg et al, 2003: EPERC Bulletin 9: ECOPRESS: Economical and safe design of
pressure vessels applying new modern steels’, December 2003, ed G Baylac, P Bocquet,
A Diamanoudis, D Porter, R Sandström and P Castello
12. SINTAP, 1999: http://www.eurofitnet.org/sintap_Procedure_version_1a.pdf (note that this
document has effectively been superseded by BS 7910:2013)
13. Langenberg, 2010: ‘Improvement of existing limitations in EN 13445-2, IMPEX2009, Final
Report of a Group Sponsored Project
14. Sandström et al, 2004: ‘New brittle fracture model for the European pressure vessel
standard’, Rolf Sandström, Peter Langenberg and Henrik Sieurin, IJPVP, 81 (2004), 837845
15. Hadley and Moore (2006a): ‘Fracture case studies for validation of fitness-for-service
procedures’, I Hadley and P Moore, TWI Industrial Members’ Report 850/2006.
16. Hadley and Moore (2006b): ‘Validation of fracture assessment procedures through fullscale testing’, Isabel Hadley and Philippa Moore, Paper FITNET 06-018, presented at
FITNET 2006. International Conference on Fitness-for-Service, 17-19 May 2006,
Amsterdam, The Netherlands
17. Hayes and Phaal, 1998: ‘Catastrophic failures of steel structures in industry: Case
histories’, B Hayes and R Phaal, TWI Industrial Member Report 632/1998
18. Banks, 1973: ‘Pressure vessel failure during hydrotest’, B Banks, Welding and Metal
Fabrication, January 1973
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Highlights
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The paper compares the brittle fracture avoidance strategies of two pressure vessel design
procedures: the European standard CEN 13445 and the UK document PD 5500 (formerly BS
5500).
For certain types of steel (especially lower-strength steels in the as-welded condition), there is a
substantial disparity (which can be more than 600C) in Charpy test temperature requirements
(T27J) between PD 5500 and EN 13445. The PD 5500 requirements are considerably more
onerous, ie they cite lower Charpy test temperatures.
The history of the derivation of both the CEN and PD rules is summarised, and the underlying
models and full-scale tests interpreted in terms of a modern flaw assessment procedure, BS
7910.
Two cases of historical pressure vessel failure are examined, which might not have been avoided
had the current CEN materials selection criteria been applied.
Since PD 5500 is at ‘standstill’, a comprehensive re-examination of the brittle fracture avoidance
strategy of CEN 13445 is recommended, and is being implemented via an international research
project which has recently been initiated.
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