EN 13445-3:2014/prA20:2019
(Amendment)Unfired pressure vessels - Part 3: Design
Unfired pressure vessels - Part 3: Design
New clause 18
Unbefeuerte Druckbehälter - Teil 3: Konstruktion
Récipients sous pression non soumis à la flamme - Partie 3 : Conception
Nekurjene tlačne posode - 3. del: Konstruiranje - Dopolnilo A20
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-maj-2019
1HRJUHYDQHWODþQHSRVRGHGHO .RQVWUXLUDQMH'RSROQLOR$
Unfired pressure vessels - Part 3: Design
Unbefeuerte Druckbehälter - Teil 3: Konstruktion
Récipients sous pression non soumis à la flamme - Partie 3 : Conception
Ta slovenski standard je istoveten z: EN 13445-3:2014/prA20:2019
ICS:
23.020.32 7ODþQHSRVRGH Pressure vessels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
DRAFT
EUROPEAN STANDARD
EN 13445-3:2014
NORME EUROPÉENNE
EUROPÄISCHE NORM
prA20
May 2019
ICS 23.020.30
English Version
Unfired pressure vessels - Part 3: Design
Récipients sous pression non soumis à la flamme - Unbefeuerte Druckbehälter - Teil 3: Konstruktion
Partie 3 : Conception
This draft amendment is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee CEN/TC 54.
This draft amendment A20, if approved, will modify the European Standard EN 13445-3:2014. If this draft becomes an
amendment, CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for
inclusion of this amendment into the relevant national standard without any alteration.
This draft amendment was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 13445-3:2014/prA20:2019:2019 E
worldwide for CEN national Members.
Contents
European foreword . 5
1 Modification to Clause 18 . 5
18 Detailed assessment of fatigue life . 6
18.1 Purpose . 6
18.2 Specific definitions . 8
18.2.16 Notch . 10
18.2.17 Notch stress . 10
18.2.18 Seam weld . 10
18.2.19 Stress on the weld throat . 10
18.2.20 Stress range (Δσ) . 11
18.2.21 Structural stress . 12
18.2.22 Structural hot spot stress . 12
18.2.23 Theoretical elastic stress concentration factor . 12
18.2.24 Partial usage factor . 12
18.2.25 Cumulative usage factor (cumulative fatigue damage index) . 12
18.2.26 Weld throat thickness . 12
18.3 Specific symbols and abbreviations . 12
18.4 Limitations . 14
18.5 Process for detailed fatigue assessment . 16
18.6 Determination of stresses for fatigue assessment of welded components and zones . 17
18.7 Stresses for fatigue assessment of unwelded components and bolts . 19
18.7.1 Unwelded components . 19
18.7.2 Bolts . 20
18.8 Elastic-plastic conditions . 21
18.8.1 General . 21
18.8.2 Mechanical loading . 21
18.8.3 Thermal loading . 22
18.8.4 Combined mechanical and thermal loading . 23
18.8.5 Elastic plastic analysis . 23
18.9 Cycles of equivalent stress range Δσ . 23
eq
18.10 Fatigue strength of welded components . 23
18.10.1 Classification of weld details . 23
18.10.2 Change of classification . 33
18.10.3 Unclassified details . 35
18.10.4 Deviations from design shape . 36
18.10.5 Correction factors . 40
18.10.6 Fatigue design curves . 42
18.11 Fatigue strength of unwelded components . 49
18.11.1 Correction factors . 49
18.11.2 Overall correction factor for unwelded components . 51
18.11.3 Design data . 51
18.12 Fatigue strength of steel bolts . 53
18.12.1 General . 53
18.12.2 Correction factors . 54
18.12.3 Design data . 54
18.13 Cumulative fatigue damage index . 55
18.13.1 Constant amplitude loading . 55
18.13.2 Variable amplitude loading . 55
18.13.3 Fatigue design criteria . 56
18.13.4 Report of initial operating limits and fatigue damage locations . 56
18.13.5 Welding flaws. 56
18.13.6 In-service monitoring of vessel operating in fatigue . 56
2 Modification to Annex N “Bibliography to Clause 18” . 57
3 New Annex NA "Examples of determination of the structural hot-spot stress by finite
element analysis using shell and solid elements” . 58
Annex NA (informative) Examples of determination of the structural hot-spot stress by
finite element analysis using shell or solid elements . 58
NA.1 Purpose . 58
NA.2 Specific definitions . 58
NA.3 Specific symbols and abbreviations . 58
NA.4 Determination of structural hot-spot stresses by using shell elements . 59
NA.4.1 Direct access . 59
NA.4.2 Linear surface extrapolation (structural hot-spot stress) . 60
NA.4.3 Quadratic surface extrapolation (structural hot-spot stress) . 61
NA.5 Determination of hot-spot structural stresses by using solid elements . 61
NA.6 General recommendations [8] . 65
NA.7 References . 66
4 New Annex NB "Cycle counting and determination of equivalent stress range" . 68
Annex NB (informative) Cycle counting and determination of equivalent stress range . 68
NB.1 General approach . 68
NB.1.1 Introduction. 68
NB.1.2 Purpose and basic application cases . 68
NB.1.3 Cycle counting parameter for proportional load-stress histories . 69
NB.1.4 Cycle counting for non-proportional load-stress histories: . 69
NB.1.4.1 General . 69
NB.1.4.2 Cycle counting based on the variation of each stress component . 70
NB.1.4.3 Cycle counting based on the critical plane approach . 70
NB.2 Cycle counting . 70
NB.2.1 Recommended procedures . 70
NB.2.2 Simplified cycle counting method. 71
NB.2.3 Reservoir counting method . 71
NB.2.4 Rain-flow counting method . 72
NB.2.4.1 Introduction . 72
NB.2.4.2 Procedure . 73
NB.2.4.3 Example calculation using rain-flow counting . 78
NB.2.5 Design data evaluation . 80
NB.2.5.1 Definition of loading events . 80
NB.2.5.2 Procedure based on the variation of each stress component . 81
NB.2.5.3 Procedure based on the critical plane approach. 82
NB.2.5.3.1 General remark . 82
NB.2.5.3.2 Summary of the procedure . 82
NB.2.5.3.3 Selection of an analysis plane, stresses on this plane . 83
NB.2.5.3.3.1 Analysis plane . 83
NB.2.5.3.3.2 Stresses of interest . 84
NB.2.5.3.4 Cycles definition . 84
NB.2.5.3.4.1 General. 84
NB.2.5.3.4.2 Characteristics of cycles for a point located in an unwelded region . 86
NB.2.5.3.4.3 Characteristics of the cycles for a point located in a welded region . 87
NB.2.5.3.5 – Extreme value counting for external combinations . 88
NB.2.5.3.5.1 – Procedure . 88
NB.2.5.3.5.2 Modified history. 89
NB.2.5.3.6 Rainflow or reservoir counting for internal combinations . 90
NB.2.5.4 Example calculation using Extreme value (outer combinations) and Rainflow
counting (inner combinations) . 91
NB.3 Determination of equivalent stress range and mean stress . 95
NB.3.1 Constant principal stress directions (proportional load-stress history) . 95
NB.3.2 Varying principal stress directions (non-proportional history) . 96
NB.4 Determination of equivalent stress range based on Tresca criterion for non-
proportional load-stress histories . 99
NB.5 Bibliography . 101
5 New Annex NC “Fatigue assessment of partial penetration welds” . 102
Annex NC (informative) Fatigue assessment of partial penetration welds . 102
NC.1 General . 102
NC.2 Nominal stress range on weld throat . 103
NC.3 General procedure . 105
NC.4 Specific procedure for double-sided fillet . 106
6 New Annex ND “Table of stress concentration factors K “ . 109
t
Annex ND (informative) Table of stress concentration factors K . 109
t
European foreword
This document (EN 13445-3:2014/prA20:2019) has been prepared by Technical Committee CEN/TC 54
“Unfired pressure vessels”, the secretariat of which is held by BSI.
This document is currently submitted to the CEN Enquiry.
This document has been prepared under a standardization request given to CEN by the European
Commission and the European Free Trade Association, and supports essential requirements of
EU Directive(s).
For relationship with EU Directive(s), see informative Annex ZA, which is an integral part of
EN 13445-3:2014.
1 Modification to Clause 18
Replace all of Clause 18 by the following:
18 Detailed assessment of fatigue life
18.1 Purpose
18.1.1 This clause specifies requirements for the detailed fatigue assessment of welded and unwelded
components and zones of pressure vessels that are subjected to repeated fluctuations of mechanical and
thermal loads.
NOTE 1 In fatigue, welds and welded material behave differently from plain parent (unwelded) material.
Therefore, the assessment procedures for welded and parent materials are different.
NOTE 2 The fatigue assessment of welded components and regions is based on fatigue design curves (SN
curves) obtained from welded specimens tested under load control, except for applied strains exceeding yield
(low cycle fatigue), where test data are obtained under strain control.
NOTE 3 The fatigue assessment of unwelded components and regions and bolts is based on SN curves obtained
from plain materials under strain-controlled test data.
18.1.2 The requirements in this clause are only applicable to steels specified in EN 13445-2. This
clause assumes that the vessel has been designed in accordance with all other requirements of this
standard.
18.1.3 These requirements can also be applied to steel castings, but in case of welding or weld repairs
on steel castings, the requirements for welded regions shall apply.
18.1.4 This clause includes the effect on fatigue of weld toe improvement where this is carried out.
NOTE In the case of fillet and partial penetration welds, where failure can occur from the weld root,
improvement of the weld toe cannot be relied upon to give an increase in fatigue strength of the joint.
18.1.5 Plain material can contain flush ground weld repairs, which may reduce in the fatigue life of the
material. Hence, only material which is certain to be free from welding shall be assessed as unwelded.
18.1.6 These requirements are not applicable to testing group 4 pressure vessels. For welded joints in
testing group 3 pressure vessels, see the special provisions in 18.10.2.1.
18.1.7 This fatigue assessment method is not intended for design involving elastic follow-up (definition
below and see reference [1] in Annex N).
18.1.8 The fatigue assessment shall be made at all locations where there is risk of fatigue crack
initiation.
Fatigue assessment shall be made at points of highest stress and stress concentration and at locations
subject to high numbers of stress cycles.
18.1.9 Fatigue assessment using this clause may be performed using either a given history of operating
loads or specified design loads. Recommended methods are given in Annex NB.
18.1.10 In this clause the equivalent stress range is used for fatigue assessment of all welded and
unwelded regions. The determination of cycles and equivalent stress range is given in Annex NB.
NOTE The maximum principal stress range is no longer used for fatigue design in this Clause.
18.1.11 This clause applies primarily for components operating below the creep temperature range. It
may, however, also be applied for components operating in the creep temperature range provided
sufficient allowance is made for the effects of creep and high temperatures.
18.1.12 A typical sequence in the design of a vessel for fatigue is shown in Table 18-1.
Table 18–1 — Summary of fatigue assessment process
Task Comment Relevant clause(s)
1. Design vessel for static Gives layout, details, sizes Part 3
loads
2. Define fatigue loading Based on operating specification,
secondary effects identified by
manufacturer, etc.
3. Identify locations of Structural discontinuities, openings,
vessel to be assessed joints (welded, bolted), corners,
repairs, etc.
4. At each location, establish Calculate the components of the stress Welded: 18.6, 18.8 and
stress range during time to be used (nominal, structural, 18.10.4;
period of operation structural hot-spot, notch or weld-
Unwelded: 18.7 and 18.8
considered throat stress, as appropriate) at the
Bolts: 18.7.2.
location under consideration
5. At each location, establish a) Perform cycle counting operation 18.9
design stress range
b) Apply plasticity correction factors 18.8
spectrum
where relevant
18.7
c) Unwelded material: derive
effective notch stress ranges
6. Note relevant a) Requirements for welds 18.10.1, Tables 18–4
implications and inform
b) Control of or assumptions about 18.10.4
relevant manufacturing
misalignment
18.10.5
and inspection personnel
c) Acceptance levels for weld flaws
7. Identify fatigue strength a) Welded material 18.10, Tables 18–4
data, including allowance
b) Unwelded material 18.11
for overall correction
factor
c) Bolted material 18.12
c) Acceptance levels for weld flaws 18.13.5
8. Extract allowable fatigue a) Welded material 18.10, Table 18–7
lives from fatigue design
b) Unwelded material 18.11, Table 18–10
and perform assessment
c) Bolts 18.12
d) Assessment method 18.5.5, 18.5.6
Task Comment Relevant clause(s)
9. Further action if location a) Re-assess using more refined stress 18.6 (welded), 18.7
fails assessment analysis (unwelded)
Task Comment Relevant clause(s)
b) Reduce stresses by increasing
thickness*
c) Change detail Table 18–4
d) Apply weld toe dressing (if 18.10.2.2
appropriate)
*
- for mechanical loading, this is obtained by increasing the wall thickness in the most cases but in
some cases (connections of parts with different wall thicknesses) a better distribution of the wall
thicknesses can reduce the stresses too.
- for thermal loading, more adjusted modifications are required, e.g. stiffness reduction at
appropriate locations of the structure and/or increase of the fatigue strength of the weak parts.
18.2 Specific definitions
The following terms and definitions apply in this clause.
18 2.1 Critical area
Area where the total cumulative fatigue damage (usage factor) exceeds the value 𝐷𝐷 = 0,5
max
18.2.2 Cut-off limit
Cyclic stress range below which fatigue damage is disregarded
18.2.3 Discontinuity
Change in shape, thickness or material which affects the stress distribution
18.2.4 Effective notch stress
Stress which governs fatigue behaviour at a notch (reduced total notch stress at unwelded points)
18.2.5 Effective stress concentration factor
Ratio of effective notch stress to structural stress at same point
18.2.6 Elastic follow-up
Phenomenon of large inelastic (plastic) strain accumulation in a weaker region of a component due to
elastic strain/stress redistribution of other regions of the component
18.2.7 Endurance limit
Cyclic stress range below which, in the absence of any previous loading, no fatigue damage is assumed
to occur under constant amplitude loading
18.2.8 Equivalent stress range
Scalar stress range which represents the multi-axial stress ranges
NOTE 1 The Tresca criterion and von Mises criterion are permitted in this clause.
NOTE 2 The formulae for calculation of equivalent stress range based on the Tresca and von Mises criteria are
given in Annex NB. These formulae depend on whether the principal stress directions remain constant or not
during the cycle.
18.2.9 Fatigue
Progressive and localised structural damage to the material of a component due to fluctuation of stress
18.2.10 Fatigue design curves
Curves given in this clause of ∆σ against N for welded and unwelded material, and of ∆σ / R
R m
R
against N for bolts
18.2.11 Gross structural discontinuity
Structural discontinuity which affects the stress or strain distribution across the entire wall thickness
18.2.12 Hot spot
Point in a structure where a fatigue crack may initiate due to the combined effect of structural stress
fluctuation and the presence of a weld, notch or other stress concentrating feature
18.2.13 Load cycle
Change of load giving rise to a change in stress with time returning to the initial value and initial
direction (i.e. increasing or decreasing) and passing each level once in increasing direction and once in
decreasing direction
NOTE When there is a sequence of load giving rise to variable amplitudes of stress, a load cycle may comprise
different parts of the stress spectrum. For identification of cycles and cycle counting methods refer to Annex NB.
18.2.14 Local structural discontinuity
Discontinuity which affects the stress or strain distribution locally, across a fraction of the wall
thickness
18.2.15 Nominal stress
Stress which would exist in the absence of a structural discontinuity
NOTE 1 Nominal stress is a reference stress (membrane + bending) which is calculated using elementary thin
shell theory of structures. It excludes the effect of structural discontinuities (e.g. welds, openings, branch
connections, nozzles, significant attachments and thickness changes). See Figure 18.2-1.
NOTE 2 The use of nominal stress is permitted for some specific weld details for which determination of the
structural stress would be unnecessarily complex. It is also applied to bolts.
NOTE 3 The nominal stress is the stress commonly used to express the results of fatigue tests performed on
laboratory specimens under simple unidirectional axial or bending loading. Hence, fatigue curves derived from
such data include the effect of any notches or other structural discontinuities (e.g. welds) in the test specimen.
Key
A, B reference points
H hot spot
1 structural stress
2 notch stress
3 surface extrapolated structural hot spot stress
NOTE For calculation of structural stress, see Annex NA.
Figure 18.2-1 — Illustration of how a stress varies towards a structural discontinuity and notch
created at the weld toe
18.2.16 Notch
Discontinuous change in surface profile that produces stress concentration and a potential fatigue crack
initiation site (e.g. weld toe, crevice)
18.2.17 Notch stress
Total stress located at the root of a notch, including the non-linear part of the stress distribution
NOTE See Figure 18.2-1 for the case where the component is welded, but notch stresses may similarly be
found at local discontinuities in unwelded components.
Notch stresses are usually calculated using numerical analysis. The stresses given by such calculations
being theoretical stresses, they should be converted into effective stresses. This is done by multiplying
them by the factor . This multiplication may be omitted, leading to conservative results.
K /K
ft
Alternatively, the nominal or structural stress is used in conjunction with the effective stress
concentration factorK .
f
18.2.18 Seam weld
Longitudinal or circumferential full penetrated weld joint
18.2.19 Stress on the weld throat
Average stress across the throat thickness in a double-sided fillet weld
NOTE 1 In the general case of a non-uniformly loaded weld, it is calculated as the maximum load per unit
length of weld divided by the weld throat thickness and it is assumed that none of the load is carried by bearing
between the components joined.
If there is significant bending across the weld throat, the maximum value of the linearized stress should
be used.
NOTE 2 The stress on the weld throat is used exclusively for assessment of fatigue failure by cracking through
weld metal in fillet or partial penetration welds (see Figure 18.2-2).
Key
1 weld toe crack
2 weld throat crack
3 unfused land
Figure 18.2-2 — Two dominant fatigue crack locations in a double-sided fillet weld
18.2.20 Stress range (Δσ)
Value from maximum to minimum in the cycle (see Figure 18.2-3) of a nominal stress, a principal stress,
a structural stress or a stress component, depending on the rule that is applied
Key
1 one cycle; Δσ Stress range
Figure 18.2-3 — Stress range
18.2.21 Structural stress
Stress determined at each point taking into account the actual geometry of the structure with the
exception of the local structural discontinuities generating stress peaks.
In many cases, the structural stress may be considered to be the same as the linearized stress.
The cases where this is not acceptable are those where the following stresses are involved:
— thermal stresses due to a temperature gradient through the wall thickness, stresses whose
distribution may be highly non-linear in particular during transients,
— stresses due to mechanical loadings, in very thick walls (e.g. pressure stresses in a thickwalled
cylinder, where the stress distribution is not uniform through the thickness).
NOTE 1 Structural stress includes the effects of gross structural discontinuities (e.g. branch connections,
cone/cylinder intersections, vessel/end junctions, thickness change, deviations from design shape, presence of an
attachment). It excludes the notch effects of local structural discontinuities (e.g. weld toe). See Figure 18.2-1.
NOTE 2 For the purpose of fatigue assessment, the structural stress is evaluated at the potential crack initiation
site.
NOTE 3 Structural stresses may be determined by one of the following methods: numerical analysis (e.g. finite
element shell analysis (FEA)), use of thin shell theory, or the application of stress concentration factors to nominal
stresses obtained analytically. Guidance on the use of numerical analysis is given in Annex NA.
18.2.22 Structural hot spot stress
Structural stress extrapolated to the location of the hot spot at a weld toe or other weld detail according
to 18.6
18.2.23 Theoretical elastic stress concentration factor
Ratio of notch stress, calculated on purely elastic basis, to structural (nominal) stress at same point
18.2.24 Partial usage factor
Fatigue damage produced by cycles of a given amplitude
18.2.25 Cumulative usage factor (cumulative fatigue damage index)
Aggregate of partial usage factors for cycles of different amplitudes according to 18.13
18.2.26 Weld throat thickness
Minimum thickness in the weld cross-section
18.3 Specific symbols and abbreviations
The following symbols and abbreviations apply in addition to those in Clause 4.
C are the constants in equation of fatigue design curves for welded components;
C, and
C
is the cumulative fatigue damage index;
D
f
E
is the modulus of elasticity at maximum operating temperature;
F F are intermediate calculation coefficients;
e s
,
f is the overall correction factor applied to bolts;
b
f is the compressive stress correction factor;
c
is the thickness correction factor in unwelded components;
f
e
f is the thickness correction factor in welded components and bolts;
ew
is the mean stress correction factor for unwelded material;
f
m
f is the mean stress correction factor for fully stress relieved welded material;
m*
f is the surface finish correction factor;
S
f is the temperature correction factor;
T*
is the overall correction factor applied to unwelded components;
f
u
is the overall correction factor applied to welded components;
f
w
g
is the depth of groove produced by weld toe grinding;
is the effective stress concentration factor given in equation (18.7.1-3);
K
f
K is the stress magnification factor due to deviations from design shape;
m
is the theoretical elastic stress concentration factor;
K
t
k is the plasticity correction factor for stress due to mechanical loading;
e
is the plasticity correction factor for stress due to thermal loading;
k
ν
M
is the mean stress sensitivity factor;
m are exponents in equations of fatigue design curves for welded components;
m , and
m
is the allowable number of cycles obtained from the fatigue design curves (suffix i refers
N
i
to allowable number of cycles of the ith stress range);
th
n
is the number of applied stress cycles (suffix i refers to number of cycles of the i stress
i
range);
R
is the mean radius of vessel at point considered;
R is the minimum inside radius of cylindrical vessel, including corrosion allowance;
min
is the maximum inside radius of cylindrical vessel, including corrosion allowance;
R
max
R is the peak to valley height;
z
r is the radius of groove produced by weld toe grinding;
S is the difference between either principal stresses (σ and σ ) or structural principal
ij i j
stresses (σ and σ ) as appropriate;
struc,i struc,j
T is the maximum operating temperature;
max
T is the minimum operating temperature;
min
T* is the assumed mean cycle temperature;
∆ε is the total strain range;
Total
th
∆σ
is the stress range (suffix i refers to i stress range; suffix w refers to weld);
th
∆σ
is the equivalent stress range (suffix refers to stress range);
i i
eq
∆σ is the stress range obtained from fatigue design curve;
R
∆σ is the endurance limit;
D
is the cut-off limit;
∆σ
cut
∆σ is the structural stress range;
struc
∆σ is the effective total equivalent stress range;
f
is the equivalent stress range corresponding to variation of equivalent linear
∆σ
eq,l
distribution;
∆σ is the total (or notch) equivalent stress range;
eq,t
∆σ is the stress range corresponding to variation of non-linear part of the stress
eq,nl
distribution;
δ is the total deviation from mean circle of shell at seam weld;
δ is the offset of centre-lines of abutting plates;
θ is the angle between tangents to abutting plates at a seam;
σ
is the direct stress (suffix w applies to weld);
is the mean equivalent stress;
σ
eq
is the reduced mean equivalent stress for elastic-plastic conditions;
σ
eq,r
σ is a structural principal stress (1, 2, 3 apply to the axes) at a given instant;
struc1
is the total principal stress
σ
t
σ is a principal stress (suffices 1, 2, 3 apply to the axes) at a given instant;
are stress ranges obtained in the example of reservoir cycle counting in Annex NB.2.3;
σ σ
v1 v2
,
τ
is the shear stress or stress range as indicated (suffix applies to weld);
w
18.4 Limitations
18.4.1 Where a vessel is designed for fatigue, the method of manufacture of all components, including
temporary fixtures and repairs, shall be specified by the manufacturer.
18.4.2 There are no restrictions on the use of the fatigue design curves for vessels which operate at
sub-zero temperatures, provided that the material through which a fatigue crack might propagate is
shown to be sufficiently tough to ensure that fracture will not initiate from a fatigue crack.
18.4.3 The fatigue design curves are applicable up to the maximum temperature of negligible creep
(375°C for ferritic steels, 425°C for austenitic steels, see Clause 19). Their use above this temperature
may be non-conservative. The user is advised to confirm the applicability of the curves at the relevant
temperature.
18.4.4 It is a condition of the use of these requirements that all regions which are fatigue-critical (see
18.13.4) are accessible for inspection and non-destructive testing, and that instructions for appropriate
maintenance are established and included in the operating instructions.
NOTE Recommendations on appropriate in-service inspection and maintenance are given in Annex M.
As regards weld defects produced during manufacturing, for application of this clause, the conditions
required by EN 13445-5:2014, Annex G shall be met in addition to the general acceptance criteria for
weld imperfections given in EN 13445-5:2014.
18.4.5 Corrosive conditions are detrimental to the fatigue lives of steels. Environmentally-assisted
fatigue cracks can occur at lower levels of fluctuating stress than in air and the rate at which they
propagate can be higher. The fatigue strengths specified do not include any allowances for corrosive
conditions. Therefore, where corrosion fatigue is anticipated and effective protection from the corrosive
medium cannot be guaranteed, a factor should be chosen, on the basis of experience or testing, by which
the fatigue strengths given in these requirements should be reduced to compensate for the corrosion. If,
because of lack of experience, it is not certain that the chosen fatigue strengths are low enough, the
frequency of inspection should be increased until there is sufficient experience to justify the factor used.
As regards shape deviations:
— manufacturing tolerances shall not exceed those given in EN 13445-4:2014;
— for all seam welds at the design stage, certain tolerances shall be assumed to be used for fatigue
assessment (see 18.10.4). Then the assumed tolerances, within the tolerance field specified in
EN 13445-4, shall be checked and guaranteed after manufacturing;
— for fatigue analysis it is permitted to use 50 % of the corrosion allowance.
18.4.6 For water conducting parts made from non-austenitic steels, operating at temperatures
exceeding 200 C, conservation of the magnetite protective layer shall be ensured. This will be obtained
if the stress at any point on the surface in contact with water always remains within the following
limits:
(σ ) ≤ (σ ) + 200 MPa (18.4-1)
eq,T max eq,T op
(σ ) ≥ (σ ) − 600 MPa (18.4-2)
eq,T min eq,T op
where
is the equivalent total stress due to operating pressure (for specific use in 18.4.6)
(σ )
eq,T op
is the maximum equivalent total stress due to operating pressure and thermal loading
(σ )
eq,T max
(for specific use in 18.4.6);
is the minimum equivalent total stress due to operating pressure and thermal loading
(σ )
eq,T min
(for specific use in 18.4.6)
NOTE It is assumed that under the operating conditions at which the magnetite layer forms, there is no stress
in that layer.
18.4.7 Where vibration (e.g. due to machinery, pressure pulsing or wind) cannot be removed by
suitable strengthening, support or dampening, it shall be assessed using the method in this clause.
18.5 Process for detailed fatigue assessment
18.5.1 This sub-clause aims to guide the user through the process for undertaking detailed fatigue
assessment according to Clause 18. A flow chart of the process is shown in Figure 18.5-1.
18.5.2 Clause 18.6 deals with the determination of stresses for fatigue assessment of welded
components and zones. It specifies the approaches to be taken in the case of aligned seam welds and
simple attachments, double sided fillet welds, and full penetration and similar types of weld.
Approaches based on nominal, structural and struc
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