- Email: [email protected]
Some aspects of the work of the European working groups relating to basic pressure vessel design
In/. J. Prrs. Copyright
0
Vrs. & Pipirq
1996 published
Printed
in Northern
Ireland.
All
70 (1YY7) 3-10 Science
Limited
rights
reserved
0308-0161/97/$17.00
0308-0161~95)00068-2
ELSEVIER
by Elsevier
Some aspects of the work of the European working groups relating to basic pressure vessel design Josef L. Zeman Institute for Pressure
Vessel and Plant Technology, Vienna Technical University,
University of Technology, Vienna, Austria
and Approval
Department,
(Received 10 November 1995:accepted 18 November 1995)
by Editor-in-Chief-A recent (October 1995)ICPVT Colloquium held in Paris reviewed the recent development in European standardsand Directive. Some of the paperspresentedthere provide a valuable insight into the way that the various standardsare developing and the reasonsbehind their development. Sincesuchpaperscan be of help in the detailed interpretation of the standardsand also in preparing for their future use, it was thought that some of them could be selected for publication in this journal, with the agreementof the Association FrancaisI’Ingenieur pour Appareils le Pression (AFIA) and the European and African Region of ICPVT who organisedthe Colloquia. This, the first such paper, describes some basic PV design criteria as proposed by a Working Group (WGC) of the CEN Technical Committee TC54. It is based on the status at March 1995 and is being published in its presentform for speed.Copyright 0 1996publishedby Elsevier ScienceLtd. introduction
1 GENERAL-TRACES
OF COMPROMISES
l
The
design-by-analysis
route
(DBA),
with
no
limitations within the scope of the standard. A proposal has been developed
that in many places is the result of many compromises, involving different design philosophies, especially in cases where there are different routes to choose from for the same purposes. Some examples are flange design * nominal design stresses * design by analysis (principles and application rules). But even in cases of different routes, quite often one route is coloured by the design philosophy used in another route.
. The experimental method with limitation non-major hazards vessels, amongst others.
to
3 DEFINITIONS
l
There are only a few definitions specific to this section of the standard, most of them being taken from EN 764, but the philosophy behind them is quite important. 3.1 Design pressure-design
2 DESIGN
(ADEQUACY)
CHECK
ROUTES
To check, prove or verify the adequacy of a design, three different routes are at our disposal: The design-by-formulae route (DBF), with the usual restraints, which are stated explicitly and clearly. l
temperature
The design pressure pd is the pressure at the top of each compartment of the pressure equipment chosen for the derivation of the calculation pressure of each component; the calculation pressure pC is the differential pressure (over a wall of a component) used for the purpose of calculation of a component. The design temperature Td is the temperature (of the fluid) in a specific point of a compartment chosen
4
J. L. Zeman
for the derivation of the calculation temperature of each component; the calculation temperature T, is the temperature used for the purpose of calculation of a component. Design pressure pd and design temperature Td are to be considered as a pair (of design parameters) and there may be more than one pair for one compartment.
accessories, but agreed upon and incorporated in the safety philosophy, is that the capacity of the pressure relief device(s) fitted to the pressure equipment is sufficient to discharge the maximum quantity of fluid that can be generated or supplied (under reasonably foreseeable circumstances), without occurrence of a rise in vessel pressure of more than 10% above the allowable pressure.
3.2 Exceptional design conditions 5 REQUIREMENTS
An exceptional design condition is a design condition, in general non-mandatory and specified by the purchaser, which corresponds to events of very low occurrence probability and requiring safe shut-down and inspection of the vessel or plant; examples are pressure loading of a secondary containment, run-away actions, internal explosion, etc. Progressive plastic deformation, fatigue, etc. need not be considered as failure modes. 3.3 Nominal design stresses
The nominal design stress f is the stress value to be used in the formulae for the calculation of pressure parts. For a specific part, i.e. specific material, specific thickness, there are different values of the nominal design stress for the design conditions and the testing conditions; there may also be different values for different design conditions.
4 REQUIREMENTS
FOR (pd, Td)
To avoid ‘philosophical’ discussions with other working groups, these requirements are not, as usual, stated using the notions of allowable pressure and allowable temperature, but refer directly to pressures (and temperatures) that can occur (under reasonably foreseeable circumstances). The design pressure (corresponding to normal operating conditions) shall be not less than (a) the pressure which will exist in the vessel compartment when the pressure relieving device starts to relieve, or the set pressure of the pressure relieving device, whichever is the higher; (b) the maximum pressure which can be attained in service where this pressure is not limited by a relieving device. The design temperature (for normal operating conditions) shall be not less than the maximum fluid temperature (in the specified point) corresponding to the coincident design pressure; in the case of low temperature service, the minimum fluid temperature shall be considered. Not stated in this section, because it is considered to be within the scope of the section on safety
FOR (p,, T,)
The calculation pressure pc to be used shall be based on the most severe condition of coincident pressure and temperature; it shall include the static and dynamic head where applicable, and shall be based on the maximum possible difference in pressure between the inside and outside of a vessel, or between any two compartments of a pressure vessel. The calculation temperature T, (which is used to determine the appropriate nominal design stress) shall be not less than the actual metal temperature expected in service. The calculation temperature shall include an adequate margin to cover uncertainties in temperature prediction. (The calculation temperature for the purpose of determining the low temperature properties shall be determined in accordance with Section 2 of the proposed standard, where appropriate.)
6 RESTRAINTS
ON DBF
The design-by-formulae route is in principle restrained to pressure loading of predominantly non-cyclic nature, the limit being specified to be y1= 500 full pressure cycles. Of course, not all design details have been actually checked against this limit. but it has been included in all considerations. To ease the problems with this, in most cases unnecessarily low, limit, rules for a simplified fatigue analysis will be incorporated. Whenever the number of full pressure cycles or equivalent full pressure cycles is likely to exceed 500 the calculations required by DBF shall be complemented by a simplified fatigue analysis or, if necessary, by a detailed fatigue analysis; a complete check according to DBA is, in general, not required.
7 CONCEPTS
IN DBF
7.1 Thickness
The thickness determined by the various formulae are minima and therefore the specified nominal thickness shall be increased by the amount of any negative tolerance permitted by the specification to which the material is ordered. Whenever fabrication can lead to a reduction of wall thickness this shall be taken into
Work relating to basic pressure vessel design account and the minimum wall thickness after fabrication shall be given in the drawing and stated as minimum wall thickness. 7.2 Corrosion
allowance
In all cases where reduction of the wall thickness is possible as a result of surface corrosion or erosion. of one or other of the surfaces, a corresponding additional thickness sufficient for the design life of the vessel components shall be provided. If required, the value shall be fixed by the purchaser, but shall not be less than 1 mm, except in special cases by agreement of the interested parties. The value shall be stated on the design drawing of the vessel. A corrosion allowance is not required when the pressure vessel walls can be inspected adequately from both sides, and erosion can be excluded., and the materials used are corrosion resistant relative to the content or are reliably protected. Only completely impervious, sufficiently thick and chemicallly stable layers with an average life not less than that of the vessel are considered to be a reliable protection. For plastic coatings the suitability shall be approved by the notified body, taking into account, among other factors, the risk of diffusion. No corrosion allowance is required for heat exchanger tubes and other parts of similar heat exchange duty, unless a specific corrosive environment requires one. 7.3 Weld joint factor
It is difficult to eliminate old, ‘fossilized’ notions. One of these kept in the proposal is the weld joint factor 2 which is considered to take into account the risk of the manufacturing welding process; unless specified by the purchaser, the manufacturer shall choose for I’ one of the values 1.0 or 0.85 (or 0.7) (within the li.mits given in Section 5). The weld joint factor is one of the main parameters in the selection of the applicable testing category and, thus, in the determination of the required amount of non-destructive and destructive testing of the joints; see Section 5. With some restraints, it is possible to use the weld joint factors 1.0 and 0.85 on one and the same vessel. 7.4 Nominal
design stresses
The nominal design stresses shall be determined by the designer from the material properties as given in the material standard (or material specification) using the appropriate safety factors. For testing category 4, for which a weld joint factor
5
of z = 0.7 applies, the (usual) nominal design stresses shall be multiplied by 0.9. For exceptional design conditions the safety factors shall be agreed upon by the interested parties but shall not be less than those for testing conditions. Special conditions, e.g. risk of stress corrosion cracking, special hazard situations, may require lower values of the nominal design stresses; they shall be agreed upon by the interested parties. The upper limits of the nominal design stresses for pressure parts other than bolts are given in Table 1; presuppositions are materials of sufficient ductility, minimum rupture elongation 114% and appropriate impact energy values (specified in Section 2). Most of the expressions are familiar to many, the resulting values being familiar or close to familiar ones. The greater values for austenitic steels with A, 2 35% are based on French experience; a presupposition of their usage is the incorporation of R m,T in the harmonized material standards-if Rm.7. is not given in the standard, R,,.o,r/l~5 (and R,,,,,,J1.05 for testing conditions) shall be used. A typical example of the ratios in the expressions versus calculation temperature is given in Fig. 1. Usage of these values for the austenitic steels with AS 2 35% may result in large deformations and, thus, possible operational problems, leakages, etc. To account for these cases, a warning is given and a short section with correction factors to limit strain will probably be added. Usage of the alternative route is coupled with requirements with regard to material manufacturers’ qualification, design details (including temperature restrictions and wall thickness limitations to 570 mm), materials (especially higher required impact energy values), manufacturing details (heat treatment) and testing (restriction to conformity assessment modules G and B + Cl, greater amount of testingsee Section 5). The safety factor 1.875 is of course the result of a compromise-it renders a collapse load of a cylinder (using Svensson’s formula) for a typical material (with guaranteed values for R, and R, and reasonable values for the uniform elongation) of approximately 2.0.
8 DESIGN
BY ANALYSIS
This section is intended as a complement to DBF for cases not covered there * as a complement for cases requiring superposition of environmental actions-wind, snow, earthquake, etc. as a complement for fitness-for-purpose cases where (quality related) manufacturing tolerances are exceeded l
l
J. L. Zeman
b
Table
1. Nominal
design stresses
Design conditions Normal route
Testing conditions
Alternative route
Steels, other than austenitic with A, P 30% Austenitic steels with A,r35% Austenitic steels with 305A,<35%
R,, a.,Min 15,Min i .
(
*,y
Max(s,%)
.
R,,,.,,. -CT
R,,.,, 1.05 R,,,.,
Steel castings
1.33
* as a complement for cases where local authorities require detailed investigations, e.g. in major hazard situations, for environmental protection reasons as an alternative to DBF. The main concepts are dealt with here, because 0 it is a real alternative to DBF, as stated above, with many advantages many concepts are new in pressure equipment design it may be used as a yard-stick for DBF solutions, to show possible improvements some concepts have already influenced the DBF section, their discussion will shed light onto some DBF details it may lead to an improved design philosophy by l
indicating more clearly the critical failure modes, especially of importance for in-service inspections. For the time being, this route is restricted to sufficiently ductile steels and steel castings with calculation temperatures below the creep range. 8.1 Principles
and application
rules
l
l
l
l
Like in the Eurocode (for steel structures) distinction is made between principles and application rules. Principles comprise general statements, definitions and requirements for which there is no alternative, and requirements and analytical models for which no alternative is permitted (unless specifically stated). Application rules are generally recognised rules
W
L/3
---Rp
l/l.!
-Rp
111.2
-
1 .CEN
-
41
Temperature
(“C)
Fig. 1. Nominal design stress, austenitic steel.
5
0
Work
relatirq
to basic pressure
and satisfy their which follow the principles requirements; alternatives are allowed provided it is shown that they accord with the relevant principle. Typical examples are the primary and the primary and secondary stress criteria of the stress categorisation approach, which are stated here, in slightly modified forms, as application rules. 8.2 Actions
This term, which replaces the old term loadings, denotes all thermomechanical quantities imposed on the structure, like forces (including pressure), temperature changes and imposed displacements, causing stress or strain. Actions are classified by their variation in time: permanent actions (G) variable actions (Q) exceptional actions (E) operating pressures and temperatures (p, T)although these are variable actions, they are considered separately to reflect their special characteristics (variation in time, rand,om properties, etc.). The characteristic values of actions, used in determining their design values, depend on the actions’ statistical properties. The characteristic values of permanent actions are usually their mean values (or credible extreme values). The characteristic values of variable actions are defined as mean values, or p %-percentiles, of extreme values, and values specified in relevant codes for wind, snow, earthquake may be used; usually they are adapted to Eurocode concepts anyway. The upper characteristic value of pressure shall not be smaller than the lesser of the set pressure of the protecting device or the highest credible pressure that can occur under normal and upset conditions (reasonably foreseeable), and the upper characteristic value of the temperature not
Table
2. Partial
safety factors
Design check Actions Permanent y. unfavourable favourable Pressure yp Variable y. Combination factor + (stochastic actions) Resistance yK (Temperature -y.,-)
GPD-OC 1.35
GPD-HT
0.9’
1.35 1.0 I.0 1.0’
1.25 (1.0)
1.05 (1.0)
1.:;.0, 1.5(1.0)
’ If not specified differently in the relevant code of environmental actions.
1
vessel desigrl
smaller than the highest credible temperature the same conditions).
(under
8.3 Partial safety factors
To allow for an easy, straightforward combination of pressure action with environmental ones, and, at the same time, to give the flexibility expected from a modern code to adjust safety margins to differences in action variation, likelihood of action combinations, consequences of failure, differences of structural behaviour and consequences in different failure modes, uncertainties in analyses, a multiple safety factor format was introduced, using different partial safety factors for different actions, different combination of actions, different failure modes and corresponding resistances of the structure. Examples of partial safety factors are given in Table 2. The corresponding combination rules for GPD-OC are: all permanent actions shall be included in each load case each pressure action shall be combined with the most unfavourable variable action each pressure action shall be combined with the corresponding sum of variable actions; stochastic actions may be multiplied by the combination factor favourable actions shall not be considered. The partial safety factors of pressure and resistances are calibrated with respect to the DBF results: no attempt has been made to justify the partial safety factors by probabilistic investigations or decision theory under uncertainty; if pressure is the only action the approach can be transformed to a nominal design stress one. 8.4 Design checks-effects
of actions
Design checks are investigations of the structure’s safety under the influence of specified combinations of actions-the design load cases-with respect to specified limit states (representing one or more failure modes). Characteristic values of the actions are multiplied by the corresponding partial safety factors to obtain their design values and their combined design effect (on the structure) is evaluated: &(YGG,
Y,,P, YOQ, . . . ad,. . .)
In the design checks these design effects are compared with the corresponding design resistances, obtained by dividing the resistance of the structure, corresponding to the action’s combinations, by the relevant partial safety factor of the resistance: E,sR,=R(G,p,Q
,...,
cld,... )/yR
This comparison can, in general, be performed in actions, in stress resultants (generalized stresses) or in stresses. The resistances are related to the limit states-
states beyond which the part no longer satisfies the design performance requirements. 8.5 Design
checks-resistances
Design checks are designated by the failure modes they deal with. The following ones will be incorporated in the first issue of the standard: gross plastic deformation (GPD), with corresponding failure modes ductile rupture and. for ‘normal’ designs, also excessive local strains progressive plastic deformation (PD) instability (I) fatigue (F) static equilibrium (SE). l
l
l l l
8.5.1 Checks against gross plastic deformation The design resistances are given by the lower-bound limit loads for proportional increase of all actions a linear-elastic ideal-plastic material (or a rigid ideal-plastic one) first-order theory Tresca’s yield criterion and associated flow rule specified design strength parameters. Design strength parameters RM and partial safety factors of the resistances yR are chosen such that for the simplest structures and pressure action only DBA and DBF results agree. The only exception is steels, other than austenitic ones with A, L 30%, where the design strength parameter RM is given by RcH.T or ~0.8 and yR = RpO.2.T and yR = 1.25 for RJR, 1.5625/R,H/R, otherwise. If the procedure used to determine the limit load does not give a maximum in the region of principal stresses less than 55% the greatest tangent intersection value shall be used (with one tangent through the origin, the other through a point where the maximum principal strain does not exceed +5%). As an application rule the ‘usual’ primary stress criterion is given, formulated in stresses and-for structures where the concept of stress resultants is applicable-in stress resultants and local (technical) limit loads. l l
! !
t=12
!
D=lOOOO
b-
I 1,
I !
xa
;
i j I I i i
i
l l
l
8.5.2 Checks against progressive plastic deformation On repeated application of specified action cycles PD shall not occur for a linear-elastic ideal-plastic material first-order theory von Mises’ yield criterion and associated flow rule specified design strength parameters RM. A slight modification of the ‘usual’ 3S, criterion is rule: it is noted that this given as application application rule, which is derived from shakedown considerations, is only a necessary condition for the fulfilment of the principle, but is considered, together
Fig. 2. Model of cylinder-cone connection.
with all the other checks, to be sufficient to achieve the principle’s goal-avoidance of ratcheting in the structure. 8.5.3 Check agaimt ,fatigue failure Reference is made to the Fatigue
Assessment
Section of the proposed standard. I
I
I
III
I
I I I
I III I I I
I
4
---k-Y
I
I I I
r,=o
,I
I
__I .
‘i
I
l l
P
l l
Fig. 3. Model of flat end-cylinder connection.
Work
relating to basic pressure
vessel design
Stutzen Fig. 4. Model of branch connection.
9 EXAMPLES DBA concepts have already been discus:sed in the literature by means of examples, their usefulness, proven strengths and the weaknesses of existing FE-software shown. A simple but quite important and instructive example is discussed in detail in Seibert and Zeman’-a cylinder-cone intersection (see Fig. 2) for which a design formula is given in the DBF section-which is based on the 3fcriterion and which may be used as a yard-stick. Taking the allowable pressure for a specific material according to this formula as lOO%, the GPD check gives an allowable pressure of 92%, i.e. slightly lower!
The application rule to the PD check, for pressure cycles from zero to maximum value and back to zero, renders slightly more than lOO%-the difference results from the approximation used to obtain the formula in the DBF section and is negligible. The shakedown theorem gives an allowable pressure of 97%, whereas the PD check-the application of the principle-shows that ratcheting cannot occur, i.e. that global plastic deformation governs. The investigation of flat end to cylinder connections under internal pressure-monotonically increasing and cyclic from zero to a maximum value and back-without opening and with concentric opening in the flat plate with a line-load corresponding to the pressure (force on the opening area) acting addition-
10
J. L. Zernan
ally at the opening,‘” (see also Fig. 3) showed remarkable possible gains of 30% and more. This investigation also showed that ratcheting is possible for pressure loading only, i.e. for a pioportional loading, but that it is governing, with the safety factors required in the draft proposal (and with Tresca’s yield criterion for the GPD check and von Mises’ for the PD check), only for small relative pressure p/f It showed that the rules in Ref. 5 are in most cases of ends without hole even conservative, whereas those in the draft proposal (and in BS 5500h) are overconservative, and it showed that the rules in Ref. 5 for ends with (large) openings may be unconservative; those in the draft proposal are again overconservative. The investigation of a more complicated geometry, the branch to (main) cylinder connection shown in Fig, 4, the ORNL-model 1, indicated the strengths and weaknesses of existing PE-software. To prove that existing software on ‘usual’ computers can be used, a reasonable mesh-size, shown in Fig. 4, and shell elements-Shell 93-have been used. On an up-todate but average workstation the GPD check (bi-linear plasticity) required approximately 6 min CPU time. The result was very close to the one obtained by the formulae of reinforcement of openings in the draft proposal. The result of the linear-elastic calculation corresponded very well with the calculations and measurements in the PVRC programme. Since with the software used the stress distribution across the wall in the (fully) plastic areas could not be
obtained, the usually used approach in the PD check, of using the difference between the linear-elastic solution and the ideal-plastic one for optimisation purposes, could not be followed, the advantage of this principle was not fully gained; the application of the corresponding application rule-a variant of the 3f criterion-resulted in an allowable pressure, for pressure cycles from zero to a maximum and back, of 30% below that of the (optimal) GPD check. A small improvement in the software-the values are there but not accessible-would certainly bring a large gain, a value for the maximum allowable pressure for the PD check above that for the GPD check, such that the latter governs.
REFERENCES 1. Seibert, T. & Zeman, J. L., Ar@ytischer Zuverl%ssigkeitsnachweisvon Druckgersten, TU Bd 35 (1994)222-8. 2. Zeman, J. L., Die Verbindung Mantel-ebener Boden unter Druckeinwirkung, To Bd 35 (1994)450-3. 3. Zeman, J. L., Die Verbindung Mantel-ebener Boden mit unverstgrktem ,_konzentrischen Ausschnitt unter Druckeinwirkung, TU Bd 36 (1995) 153-6. 4. Zeman, J. L., Ratcheting limit of flat end cylindrical shell connectionsunder internal pressure,ht. J. Pres. Ves. & Piping, 1996,68, 293-298. 5. AD-Merkblstter, Serie B: Berechnung uon Druckbehdtern. Beuth Verlag GmbH, Berlin. 6. BS 5500: British Standard Specification for Unfired Fusion Welded Pressure Vessels, British Standards Institution, London.
0
Vrs. & Pipirq
1996 published
Printed
in Northern
Ireland.
All
70 (1YY7) 3-10 Science
Limited
rights
reserved
0308-0161/97/$17.00
0308-0161~95)00068-2
ELSEVIER
by Elsevier
Some aspects of the work of the European working groups relating to basic pressure vessel design Josef L. Zeman Institute for Pressure
Vessel and Plant Technology, Vienna Technical University,
University of Technology, Vienna, Austria
and Approval
Department,
(Received 10 November 1995:accepted 18 November 1995)
by Editor-in-Chief-A recent (October 1995)ICPVT Colloquium held in Paris reviewed the recent development in European standardsand Directive. Some of the paperspresentedthere provide a valuable insight into the way that the various standardsare developing and the reasonsbehind their development. Sincesuchpaperscan be of help in the detailed interpretation of the standardsand also in preparing for their future use, it was thought that some of them could be selected for publication in this journal, with the agreementof the Association FrancaisI’Ingenieur pour Appareils le Pression (AFIA) and the European and African Region of ICPVT who organisedthe Colloquia. This, the first such paper, describes some basic PV design criteria as proposed by a Working Group (WGC) of the CEN Technical Committee TC54. It is based on the status at March 1995 and is being published in its presentform for speed.Copyright 0 1996publishedby Elsevier ScienceLtd. introduction
1 GENERAL-TRACES
OF COMPROMISES
l
The
design-by-analysis
route
(DBA),
with
no
limitations within the scope of the standard. A proposal has been developed
that in many places is the result of many compromises, involving different design philosophies, especially in cases where there are different routes to choose from for the same purposes. Some examples are flange design * nominal design stresses * design by analysis (principles and application rules). But even in cases of different routes, quite often one route is coloured by the design philosophy used in another route.
. The experimental method with limitation non-major hazards vessels, amongst others.
to
3 DEFINITIONS
l
There are only a few definitions specific to this section of the standard, most of them being taken from EN 764, but the philosophy behind them is quite important. 3.1 Design pressure-design
2 DESIGN
(ADEQUACY)
CHECK
ROUTES
To check, prove or verify the adequacy of a design, three different routes are at our disposal: The design-by-formulae route (DBF), with the usual restraints, which are stated explicitly and clearly. l
temperature
The design pressure pd is the pressure at the top of each compartment of the pressure equipment chosen for the derivation of the calculation pressure of each component; the calculation pressure pC is the differential pressure (over a wall of a component) used for the purpose of calculation of a component. The design temperature Td is the temperature (of the fluid) in a specific point of a compartment chosen
4
J. L. Zeman
for the derivation of the calculation temperature of each component; the calculation temperature T, is the temperature used for the purpose of calculation of a component. Design pressure pd and design temperature Td are to be considered as a pair (of design parameters) and there may be more than one pair for one compartment.
accessories, but agreed upon and incorporated in the safety philosophy, is that the capacity of the pressure relief device(s) fitted to the pressure equipment is sufficient to discharge the maximum quantity of fluid that can be generated or supplied (under reasonably foreseeable circumstances), without occurrence of a rise in vessel pressure of more than 10% above the allowable pressure.
3.2 Exceptional design conditions 5 REQUIREMENTS
An exceptional design condition is a design condition, in general non-mandatory and specified by the purchaser, which corresponds to events of very low occurrence probability and requiring safe shut-down and inspection of the vessel or plant; examples are pressure loading of a secondary containment, run-away actions, internal explosion, etc. Progressive plastic deformation, fatigue, etc. need not be considered as failure modes. 3.3 Nominal design stresses
The nominal design stress f is the stress value to be used in the formulae for the calculation of pressure parts. For a specific part, i.e. specific material, specific thickness, there are different values of the nominal design stress for the design conditions and the testing conditions; there may also be different values for different design conditions.
4 REQUIREMENTS
FOR (pd, Td)
To avoid ‘philosophical’ discussions with other working groups, these requirements are not, as usual, stated using the notions of allowable pressure and allowable temperature, but refer directly to pressures (and temperatures) that can occur (under reasonably foreseeable circumstances). The design pressure (corresponding to normal operating conditions) shall be not less than (a) the pressure which will exist in the vessel compartment when the pressure relieving device starts to relieve, or the set pressure of the pressure relieving device, whichever is the higher; (b) the maximum pressure which can be attained in service where this pressure is not limited by a relieving device. The design temperature (for normal operating conditions) shall be not less than the maximum fluid temperature (in the specified point) corresponding to the coincident design pressure; in the case of low temperature service, the minimum fluid temperature shall be considered. Not stated in this section, because it is considered to be within the scope of the section on safety
FOR (p,, T,)
The calculation pressure pc to be used shall be based on the most severe condition of coincident pressure and temperature; it shall include the static and dynamic head where applicable, and shall be based on the maximum possible difference in pressure between the inside and outside of a vessel, or between any two compartments of a pressure vessel. The calculation temperature T, (which is used to determine the appropriate nominal design stress) shall be not less than the actual metal temperature expected in service. The calculation temperature shall include an adequate margin to cover uncertainties in temperature prediction. (The calculation temperature for the purpose of determining the low temperature properties shall be determined in accordance with Section 2 of the proposed standard, where appropriate.)
6 RESTRAINTS
ON DBF
The design-by-formulae route is in principle restrained to pressure loading of predominantly non-cyclic nature, the limit being specified to be y1= 500 full pressure cycles. Of course, not all design details have been actually checked against this limit. but it has been included in all considerations. To ease the problems with this, in most cases unnecessarily low, limit, rules for a simplified fatigue analysis will be incorporated. Whenever the number of full pressure cycles or equivalent full pressure cycles is likely to exceed 500 the calculations required by DBF shall be complemented by a simplified fatigue analysis or, if necessary, by a detailed fatigue analysis; a complete check according to DBA is, in general, not required.
7 CONCEPTS
IN DBF
7.1 Thickness
The thickness determined by the various formulae are minima and therefore the specified nominal thickness shall be increased by the amount of any negative tolerance permitted by the specification to which the material is ordered. Whenever fabrication can lead to a reduction of wall thickness this shall be taken into
Work relating to basic pressure vessel design account and the minimum wall thickness after fabrication shall be given in the drawing and stated as minimum wall thickness. 7.2 Corrosion
allowance
In all cases where reduction of the wall thickness is possible as a result of surface corrosion or erosion. of one or other of the surfaces, a corresponding additional thickness sufficient for the design life of the vessel components shall be provided. If required, the value shall be fixed by the purchaser, but shall not be less than 1 mm, except in special cases by agreement of the interested parties. The value shall be stated on the design drawing of the vessel. A corrosion allowance is not required when the pressure vessel walls can be inspected adequately from both sides, and erosion can be excluded., and the materials used are corrosion resistant relative to the content or are reliably protected. Only completely impervious, sufficiently thick and chemicallly stable layers with an average life not less than that of the vessel are considered to be a reliable protection. For plastic coatings the suitability shall be approved by the notified body, taking into account, among other factors, the risk of diffusion. No corrosion allowance is required for heat exchanger tubes and other parts of similar heat exchange duty, unless a specific corrosive environment requires one. 7.3 Weld joint factor
It is difficult to eliminate old, ‘fossilized’ notions. One of these kept in the proposal is the weld joint factor 2 which is considered to take into account the risk of the manufacturing welding process; unless specified by the purchaser, the manufacturer shall choose for I’ one of the values 1.0 or 0.85 (or 0.7) (within the li.mits given in Section 5). The weld joint factor is one of the main parameters in the selection of the applicable testing category and, thus, in the determination of the required amount of non-destructive and destructive testing of the joints; see Section 5. With some restraints, it is possible to use the weld joint factors 1.0 and 0.85 on one and the same vessel. 7.4 Nominal
design stresses
The nominal design stresses shall be determined by the designer from the material properties as given in the material standard (or material specification) using the appropriate safety factors. For testing category 4, for which a weld joint factor
5
of z = 0.7 applies, the (usual) nominal design stresses shall be multiplied by 0.9. For exceptional design conditions the safety factors shall be agreed upon by the interested parties but shall not be less than those for testing conditions. Special conditions, e.g. risk of stress corrosion cracking, special hazard situations, may require lower values of the nominal design stresses; they shall be agreed upon by the interested parties. The upper limits of the nominal design stresses for pressure parts other than bolts are given in Table 1; presuppositions are materials of sufficient ductility, minimum rupture elongation 114% and appropriate impact energy values (specified in Section 2). Most of the expressions are familiar to many, the resulting values being familiar or close to familiar ones. The greater values for austenitic steels with A, 2 35% are based on French experience; a presupposition of their usage is the incorporation of R m,T in the harmonized material standards-if Rm.7. is not given in the standard, R,,.o,r/l~5 (and R,,,,,,J1.05 for testing conditions) shall be used. A typical example of the ratios in the expressions versus calculation temperature is given in Fig. 1. Usage of these values for the austenitic steels with AS 2 35% may result in large deformations and, thus, possible operational problems, leakages, etc. To account for these cases, a warning is given and a short section with correction factors to limit strain will probably be added. Usage of the alternative route is coupled with requirements with regard to material manufacturers’ qualification, design details (including temperature restrictions and wall thickness limitations to 570 mm), materials (especially higher required impact energy values), manufacturing details (heat treatment) and testing (restriction to conformity assessment modules G and B + Cl, greater amount of testingsee Section 5). The safety factor 1.875 is of course the result of a compromise-it renders a collapse load of a cylinder (using Svensson’s formula) for a typical material (with guaranteed values for R, and R, and reasonable values for the uniform elongation) of approximately 2.0.
8 DESIGN
BY ANALYSIS
This section is intended as a complement to DBF for cases not covered there * as a complement for cases requiring superposition of environmental actions-wind, snow, earthquake, etc. as a complement for fitness-for-purpose cases where (quality related) manufacturing tolerances are exceeded l
l
J. L. Zeman
b
Table
1. Nominal
design stresses
Design conditions Normal route
Testing conditions
Alternative route
Steels, other than austenitic with A, P 30% Austenitic steels with A,r35% Austenitic steels with 305A,<35%
R,, a.,Min 15,Min i .
(
*,y
Max(s,%)
.
R,,,.,,. -CT
R,,.,, 1.05 R,,,.,
Steel castings
1.33
* as a complement for cases where local authorities require detailed investigations, e.g. in major hazard situations, for environmental protection reasons as an alternative to DBF. The main concepts are dealt with here, because 0 it is a real alternative to DBF, as stated above, with many advantages many concepts are new in pressure equipment design it may be used as a yard-stick for DBF solutions, to show possible improvements some concepts have already influenced the DBF section, their discussion will shed light onto some DBF details it may lead to an improved design philosophy by l
indicating more clearly the critical failure modes, especially of importance for in-service inspections. For the time being, this route is restricted to sufficiently ductile steels and steel castings with calculation temperatures below the creep range. 8.1 Principles
and application
rules
l
l
l
l
Like in the Eurocode (for steel structures) distinction is made between principles and application rules. Principles comprise general statements, definitions and requirements for which there is no alternative, and requirements and analytical models for which no alternative is permitted (unless specifically stated). Application rules are generally recognised rules
W
L/3
---Rp
l/l.!
-Rp
111.2
-
1 .CEN
-
41
Temperature
(“C)
Fig. 1. Nominal design stress, austenitic steel.
5
0
Work
relatirq
to basic pressure
and satisfy their which follow the principles requirements; alternatives are allowed provided it is shown that they accord with the relevant principle. Typical examples are the primary and the primary and secondary stress criteria of the stress categorisation approach, which are stated here, in slightly modified forms, as application rules. 8.2 Actions
This term, which replaces the old term loadings, denotes all thermomechanical quantities imposed on the structure, like forces (including pressure), temperature changes and imposed displacements, causing stress or strain. Actions are classified by their variation in time: permanent actions (G) variable actions (Q) exceptional actions (E) operating pressures and temperatures (p, T)although these are variable actions, they are considered separately to reflect their special characteristics (variation in time, rand,om properties, etc.). The characteristic values of actions, used in determining their design values, depend on the actions’ statistical properties. The characteristic values of permanent actions are usually their mean values (or credible extreme values). The characteristic values of variable actions are defined as mean values, or p %-percentiles, of extreme values, and values specified in relevant codes for wind, snow, earthquake may be used; usually they are adapted to Eurocode concepts anyway. The upper characteristic value of pressure shall not be smaller than the lesser of the set pressure of the protecting device or the highest credible pressure that can occur under normal and upset conditions (reasonably foreseeable), and the upper characteristic value of the temperature not
Table
2. Partial
safety factors
Design check Actions Permanent y. unfavourable favourable Pressure yp Variable y. Combination factor + (stochastic actions) Resistance yK (Temperature -y.,-)
GPD-OC 1.35
GPD-HT
0.9’
1.35 1.0 I.0 1.0’
1.25 (1.0)
1.05 (1.0)
1.:;.0, 1.5(1.0)
’ If not specified differently in the relevant code of environmental actions.
1
vessel desigrl
smaller than the highest credible temperature the same conditions).
(under
8.3 Partial safety factors
To allow for an easy, straightforward combination of pressure action with environmental ones, and, at the same time, to give the flexibility expected from a modern code to adjust safety margins to differences in action variation, likelihood of action combinations, consequences of failure, differences of structural behaviour and consequences in different failure modes, uncertainties in analyses, a multiple safety factor format was introduced, using different partial safety factors for different actions, different combination of actions, different failure modes and corresponding resistances of the structure. Examples of partial safety factors are given in Table 2. The corresponding combination rules for GPD-OC are: all permanent actions shall be included in each load case each pressure action shall be combined with the most unfavourable variable action each pressure action shall be combined with the corresponding sum of variable actions; stochastic actions may be multiplied by the combination factor favourable actions shall not be considered. The partial safety factors of pressure and resistances are calibrated with respect to the DBF results: no attempt has been made to justify the partial safety factors by probabilistic investigations or decision theory under uncertainty; if pressure is the only action the approach can be transformed to a nominal design stress one. 8.4 Design checks-effects
of actions
Design checks are investigations of the structure’s safety under the influence of specified combinations of actions-the design load cases-with respect to specified limit states (representing one or more failure modes). Characteristic values of the actions are multiplied by the corresponding partial safety factors to obtain their design values and their combined design effect (on the structure) is evaluated: &(YGG,
Y,,P, YOQ, . . . ad,. . .)
In the design checks these design effects are compared with the corresponding design resistances, obtained by dividing the resistance of the structure, corresponding to the action’s combinations, by the relevant partial safety factor of the resistance: E,sR,=R(G,p,Q
,...,
cld,... )/yR
This comparison can, in general, be performed in actions, in stress resultants (generalized stresses) or in stresses. The resistances are related to the limit states-
states beyond which the part no longer satisfies the design performance requirements. 8.5 Design
checks-resistances
Design checks are designated by the failure modes they deal with. The following ones will be incorporated in the first issue of the standard: gross plastic deformation (GPD), with corresponding failure modes ductile rupture and. for ‘normal’ designs, also excessive local strains progressive plastic deformation (PD) instability (I) fatigue (F) static equilibrium (SE). l
l
l l l
8.5.1 Checks against gross plastic deformation The design resistances are given by the lower-bound limit loads for proportional increase of all actions a linear-elastic ideal-plastic material (or a rigid ideal-plastic one) first-order theory Tresca’s yield criterion and associated flow rule specified design strength parameters. Design strength parameters RM and partial safety factors of the resistances yR are chosen such that for the simplest structures and pressure action only DBA and DBF results agree. The only exception is steels, other than austenitic ones with A, L 30%, where the design strength parameter RM is given by RcH.T or ~0.8 and yR = RpO.2.T and yR = 1.25 for RJR, 1.5625/R,H/R, otherwise. If the procedure used to determine the limit load does not give a maximum in the region of principal stresses less than 55% the greatest tangent intersection value shall be used (with one tangent through the origin, the other through a point where the maximum principal strain does not exceed +5%). As an application rule the ‘usual’ primary stress criterion is given, formulated in stresses and-for structures where the concept of stress resultants is applicable-in stress resultants and local (technical) limit loads. l l
! !
t=12
!
D=lOOOO
b-
I 1,
I !
xa
;
i j I I i i
i
l l
l
8.5.2 Checks against progressive plastic deformation On repeated application of specified action cycles PD shall not occur for a linear-elastic ideal-plastic material first-order theory von Mises’ yield criterion and associated flow rule specified design strength parameters RM. A slight modification of the ‘usual’ 3S, criterion is rule: it is noted that this given as application application rule, which is derived from shakedown considerations, is only a necessary condition for the fulfilment of the principle, but is considered, together
Fig. 2. Model of cylinder-cone connection.
with all the other checks, to be sufficient to achieve the principle’s goal-avoidance of ratcheting in the structure. 8.5.3 Check agaimt ,fatigue failure Reference is made to the Fatigue
Assessment
Section of the proposed standard. I
I
I
III
I
I I I
I III I I I
I
4
---k-Y
I
I I I
r,=o
,I
I
__I .
‘i
I
l l
P
l l
Fig. 3. Model of flat end-cylinder connection.
Work
relating to basic pressure
vessel design
Stutzen Fig. 4. Model of branch connection.
9 EXAMPLES DBA concepts have already been discus:sed in the literature by means of examples, their usefulness, proven strengths and the weaknesses of existing FE-software shown. A simple but quite important and instructive example is discussed in detail in Seibert and Zeman’-a cylinder-cone intersection (see Fig. 2) for which a design formula is given in the DBF section-which is based on the 3fcriterion and which may be used as a yard-stick. Taking the allowable pressure for a specific material according to this formula as lOO%, the GPD check gives an allowable pressure of 92%, i.e. slightly lower!
The application rule to the PD check, for pressure cycles from zero to maximum value and back to zero, renders slightly more than lOO%-the difference results from the approximation used to obtain the formula in the DBF section and is negligible. The shakedown theorem gives an allowable pressure of 97%, whereas the PD check-the application of the principle-shows that ratcheting cannot occur, i.e. that global plastic deformation governs. The investigation of flat end to cylinder connections under internal pressure-monotonically increasing and cyclic from zero to a maximum value and back-without opening and with concentric opening in the flat plate with a line-load corresponding to the pressure (force on the opening area) acting addition-
10
J. L. Zernan
ally at the opening,‘” (see also Fig. 3) showed remarkable possible gains of 30% and more. This investigation also showed that ratcheting is possible for pressure loading only, i.e. for a pioportional loading, but that it is governing, with the safety factors required in the draft proposal (and with Tresca’s yield criterion for the GPD check and von Mises’ for the PD check), only for small relative pressure p/f It showed that the rules in Ref. 5 are in most cases of ends without hole even conservative, whereas those in the draft proposal (and in BS 5500h) are overconservative, and it showed that the rules in Ref. 5 for ends with (large) openings may be unconservative; those in the draft proposal are again overconservative. The investigation of a more complicated geometry, the branch to (main) cylinder connection shown in Fig, 4, the ORNL-model 1, indicated the strengths and weaknesses of existing PE-software. To prove that existing software on ‘usual’ computers can be used, a reasonable mesh-size, shown in Fig. 4, and shell elements-Shell 93-have been used. On an up-todate but average workstation the GPD check (bi-linear plasticity) required approximately 6 min CPU time. The result was very close to the one obtained by the formulae of reinforcement of openings in the draft proposal. The result of the linear-elastic calculation corresponded very well with the calculations and measurements in the PVRC programme. Since with the software used the stress distribution across the wall in the (fully) plastic areas could not be
obtained, the usually used approach in the PD check, of using the difference between the linear-elastic solution and the ideal-plastic one for optimisation purposes, could not be followed, the advantage of this principle was not fully gained; the application of the corresponding application rule-a variant of the 3f criterion-resulted in an allowable pressure, for pressure cycles from zero to a maximum and back, of 30% below that of the (optimal) GPD check. A small improvement in the software-the values are there but not accessible-would certainly bring a large gain, a value for the maximum allowable pressure for the PD check above that for the GPD check, such that the latter governs.
REFERENCES 1. Seibert, T. & Zeman, J. L., Ar@ytischer Zuverl%ssigkeitsnachweisvon Druckgersten, TU Bd 35 (1994)222-8. 2. Zeman, J. L., Die Verbindung Mantel-ebener Boden unter Druckeinwirkung, To Bd 35 (1994)450-3. 3. Zeman, J. L., Die Verbindung Mantel-ebener Boden mit unverstgrktem ,_konzentrischen Ausschnitt unter Druckeinwirkung, TU Bd 36 (1995) 153-6. 4. Zeman, J. L., Ratcheting limit of flat end cylindrical shell connectionsunder internal pressure,ht. J. Pres. Ves. & Piping, 1996,68, 293-298. 5. AD-Merkblstter, Serie B: Berechnung uon Druckbehdtern. Beuth Verlag GmbH, Berlin. 6. BS 5500: British Standard Specification for Unfired Fusion Welded Pressure Vessels, British Standards Institution, London.