CRITICAL REVIEW OF EUROPEAN STANDARDS FOR FLANGED JOINTS

Jaroslav BartonicekGKN Gemeinschaftskernkraftwerk Neckar

NeckarwestheimGermany

Hans KockelmannStaatliche Materialprfungsanstalt

University of StuttgartStuttgartGermany

Manfred SchaafAMTEC Advanced Measurements

LauffenGermany

Friedrich SchoeckleAMTEC Advanced Measurements

LauffenGermany

ABSTRACT The actual standards and proposals / drafts of standards for boltedflanged joints in Europe can be divided into groups, depending ontheir intended application:

- Determination of design characteristics (type, dimensions,material, etc.) for a given medium and given operation levels(internal pressure, temperature),

- Dimensions of flanges, bolts and gaskets,- Definition and evaluation of gasket factors,- Procedures for tightness analysis and stress analysis.

In a first step, a procedure to guarantee function of flanged joints isdiscussed in this paper; function in the case of flanged joints means:the leakage remains below the limits of the required tightness classand the stresses are limited to the appropriate standards demands. With this procedure in background, the existing standards forflanged joints with floating gaskets are introduced and analysed usinguser-defined criteria. The use of the standards and the difficulties thathave to be expected will be demonstrated within this scope. Therelevant gasket factors for flange calculations, for long term behaviourand for quality control are discussed and the actually existing deficitsare summarized. There are neither standards nor work items for flanged joints withmetal-to-metal contact in Europe, until today. However, in Germany,there is an approach to standardize the analysis of this type of flangedjoints; the actual state of the art is summarized in this paper.

INTRODUCTION Large numbers of flanged joints are used in all parts of the industry.As these types of tightening constructions are used for a long period oftime, already, it could be expected that there is enough knowledge

regarding the choice of the appropriate design (flanges, bolts, gaskets)as well as regarding the necessary proofs (tightness and stressanalysis). However, experience shows that there are still difficulties. These difficulties became increasingly obvious with new demandslike replacement of asbestos containing materials, reduction ofemissions (TA-Luft, Clean Air Act) and improvements of economicalaspects. The reasons for these difficulties could not be found in thenew demands; most of the difficulties arose from the fact that therewere no standards for gaskets and no standards (standardizedprocedures) for the analysis (calculation procedures). After this situation was realized the activities regardingstandardization in this field increased, in national standard projects,first. In recent years, there are a lot of standardizing projects in Europeunder the roof of the pressure equipment directive (PED); therefore,most of the standardizing projects in the field of flanged joints movedfrom national to the European standardizing committees (CEN). This paper reviews the presuppositions to guarantee function offlanged joints and then provides a procedure for a step by stepapproach in design and analysis. The standards and the drafts ofstandards are introduced and the state of the art regardingstandardization in Europe is summarized.

FUNCTION OF FLANGED JOINTS Correct function of a flanged joint is given if it is tight, and itsintegrity is guaranteed for the entire period of operation. Tightnessmeans: the joint remains within its tightness class under all states ofoperation, i.e. the leak rates (emissions) are limited. Integrity isachieved by limiting the stresses in the component (safety againstfailure). Both the demanded tightness class and the stress limitationsare input values for the analysis of the joint; they determine the effortsthat are necessary to prove function.

In order to achieve a correct function of a joint, the followingpresuppositions have to be met:

- There must be a knowledge of all relevant loads during assemblyand in operation.

- The flanges and bolts have to be suitable for the given demands.- The gasket has to be suitable and the necessary gasket factors have

to be known.- The (assembly) prestress value has to be determined and the

boundary conditions must be controlled in a calculation.- A suitable mounting procedure (accurate prestressing) has to be

chosen.

In operation a tightening joint is loaded by internal pressure andtemperature for example and - more relevant to function / tightness -with transients of these parameters. In piping systems there are"external" forces and moments additionally, resulting from thermalconstraints of the piping system or from mounting conditions (distancebetween flanges, angle between flange faces). These loads can lead toa relevant additional loading / unloading of the gasket. If analysis hasto be reliable, every (additional) load parameter must be known orverified. Medium has to be considered as load, too, because it affects longterm stability and the gasket factors regarding tightening behaviour. The choice of a suitable design becomes complicated, if specialdesigns are used. This is often the case with pumps and valves. Herethe function is often misinterpreted and the gasket choice is notcorrect. In order to find suitable parts for the joint (flanges, bolts,gasket) a systematic procedure is necessary in the design state,including guidelines for the selection and a appropriate calculationprocedure. There are a lot of parameters that affect the selection of theparts, some more general parameters are summarized in Fig. 1 forflanges and bolts and in Fig. 2 for gaskets. The gaskets are the weakest point in a flanged joint, thus thecorrect choice of the gasket is perhaps the most important task. Thefirst parameter that has to be taken into account when selecting agasket, is the long term stability under operating conditions. Medium,temperature, loads and the expected time of operation are major pointsto consider. The necessary gasket factors depend on the construction details: thegasket can be floating between the flange faces or the flange plateshave metal-to-metal contact, Fig. 3. Flanges with metal-to-metalcontact are found in valves mainly, whereas flanges in piping systemsand in pressure vessels are of the first type. Regarding flanged joints with the gasket between the flange plates,tightness or the leak rate are determined by a good seating of thegasket during the mounting procedure and by an application of asufficient minimum gasket stress in every relevant operation state.Additionally, the maximum allowable gasket stress values areimportant; they - together with the elastic recovery - characterize thedeformation behaviour of the gasket. Regarding flanged joints with metal-to-metal contact of the flangeplates it is necessary to know the gasket stress which is necessary toreach contact; this stress determines the maximum internal pressurefor a given leak rate of the joint and can only be reduced by relaxationof the gasket stress under operation temperature. For reliable calculation results it is necessary that all parts of theassembly (i.e. flanges, bolts and gasket) and their interaction areregarded.

The first task of a calculation is to determine the prestress level ofthe joint. A prestress value is necessary for every flanged joint,therefore, this part of the calculation has to be done in each case. The prestress level (assembly state) depends on the tightnesscharacteristics of the gasket (minimum necessary gasket stress aftermounting and in operating state), on the stress limits of the parts of theflanged joint (flanges, bolts, gasket) and on the change of the gasketstress between assembly state and operation. The second task of a calculation is a stress analysis (preventdestruction for static loads), the third is a tightness analysis (to controlemissions, i.e. to maintain a demanded tightness class). The calculations for use in tightness analysis and stress analysis haveto

- use relevant and realistic gasket factors,- regard stiffness of flanges, bolts and gasket,- regard realistic operation loads like internal pressure, external

forces and moments, temperature, temperature distributions,deformations etc.

- determine the necessary gasket prestress level for assembly and- determine gasket stress in each operation state.

Finally, the mounting procedure must match the demands ontightness. If the joint is very important and tightness must beguaranteed, controlled prestressing is necessary.

The procedure to achieve function of a flanged joint starts with acollection of input data, Fig. 4. As said above it is important to knowthe

- medium and- all relevant loads in operation.

Additionally, the demands have to be precisely determined:

- demands regarding tightness (tightness class) and- stress limits (code).

The next steps in the procedure are

- choice of the principle design (type);- selection of the flanges and bolts (type, dimensions, materials)- selection of the gasket (type, dimensions, gasket factors)

Then detailed analysis is necessary for

- tightness analysis (proof that emissions are limited according tothe demanded tightness class) and

- stress analysis (integrity proof, safety against failure).

The procedure is iterative; if one of the proofs fails the user has to goback to the selection of the parts and start again with a modifieddesign.

Following above procedure for the design of a joint and theguarantee of function, standards can make life easier. As there are a lotof flanged joints in industrial plants the standardization of designsmakes sense, especially in terms of cost reduction. Standardizedmodelling of material behaviour and the use of standardizedcalculation procedures reduce the complexity of the analysis.

In the chapters below, European standards regarding

- design (types, dimensions, materials),- gasket characteristics and- calculations (analysis tools)

are discussed.

? medium- materials

? temperature, internal pressure- materials- dimensions

? (additional) forces and moments- gasket- piping system- stress analysis- tightness analysis

? stiffness- deformation of gasket- additional loads- gasket between flanges- flanges with metal-to-metal contact

? thermal expansion- additional forces and moments

Fig. 1 : parameters affecting the choice of flanges and bolts

? design / construction- gasket between flange plates- flange plates have contact

? stability- long term behaviour

? tightness characteristics- seating- stresses

? deformation characteristics- destruction- creep- recovery

Fig. 2 : parameters affecting the choice of gaskets

Fig. 3 : flange type with floating gasket (left) andflange type with metal-to-metal contact (right)

Fig. 4 : procedure to guarantee function of tightening joints

medium,tightness class,

stress limits,relevant

loads

appropriatedesign

materialcharacteristics

(esp. gasket)

stress andtightness analysis

qualifiedmountingprocedure

Tab. 1: standards (EN) for dimensions 2001/03

Flanges and their Joints - prEN 1092-1 part 1: Steel flangescircular flanges for pipes, valves, fittings EN 1092-2 part 2: Cast iron flanges and accessoires cancelled part 3: Copper alloy flangesPN-designated prEN 1092-4 part 4: Aluminum alloy flangesFlanges and their Joints - prEN 1759-1 part 1: Steel flanges - NPS 1/2 to 24circular flanges for pipes, valves, fittings prEN 1759-2 part 2: Steel flanges with bore sizes DN 650 bis DN 1500 and accessoires cancelled part 3: Copper alloy flangesCLASS-designated prEN 1759-4 part 4: Aluminum alloy flanges

Flanges and their Joints - EN 1515-1 part 1: selection of boltingbolts and nuts prEN 1515-2 part 2: Bolting, classification

Flanges and their Joints - EN1514-1 part 1: Non-metallic flat gaskets with or without insertsdimensions of gaskets for EN 1514-2 part 2: Spiral wound gaskets with steel flangesPN-designated flanges EN 1514-3 part 3: Non-metallic PTFE envelope gaskets

EN 1514-4 part 4: Corrugated, flat or grooved metallic and filled metallic gaskets for use with steel flanges

prEN 1514-5 part 5: Metallic ring-joint gaskets for use with steel flangesprEN 1514-6 part 6: Kammprofile gaskets for use with steel flangesprEN 1514-7 part 7: Covered metal jacketed gaskets for use with steel flanges

Flanges and their Joints - prEN 12560-1 part 1: Non-metallic flat gaskets with or without insertsdimensions of gaskets for prEN 12560-2 part 2: Spiral wound gaskets with steel flangesClass-designated flanges prEN 12560-3 part 3: Non-metallic PTFE envelope gaskets

prEN 12560-4 part 4: Corrugated, flat or grooved metallic and filled metallic gaskets for use with steel flanges

prEN 12560-5 part 5: Metallic ring-joint gaskets for use with steel flangesprEN 12560-6 part 6: Kammprofile gaskets for use with steel flangesprEN 12560-7 part 7: Covered metal jacketed gaskets for use with steel flanges

Tab. 2: standards (EN) for gasket characteristics

Flanges and their Joints - prEN 13555Gasket parameters and test procedures relevant to the design rules for gasketed circular flange connectionsFlanges and their Joints - WI 0074031quality assurance standards for gaskets

Tab. 3: standards (EN) regarding flange calculation

Flanges and their Joints - EN 1591 part 1: Calculation methoddesign rules for gasketed circular flange connections (part 2: gasket factors)

unfired pressure vessels prEN 13445-3 Calculation according ASMEpart 3: design

Calculation according EN 1591

Tab. 4: other standards (EN) for flanged joints

Flanges and their Joints - WI 0074018p/T rating method

STANDARDS FOR THE DESIGN The design state for a flanged joint can be divided into several steps.Step 1 is the choice of a flange type. Step 2 contains the selection ofthe parts (flanges, bolts, gasket) for the selected flange type; in thisstep the standards for the selection of the flanges, bolts and gasket asdescribed below become important (supposed that standardizeddimensions can be applied). In step 3 of the design, calculations haveto be made; using the stress limitations and the safety factorsrecommended by the code the correct dimensions are evaluated orverified. Calculations according to ASME-rules are widely used for design offlanged joints. In this calculation procedure, the gasket behaviour ismodelled only in a formal manner, but this fact is not too important inthis stage. The new European standard EN 1591 provides tools for the designof flanged joints, too. Both the ASME standard and EN 1591 apply only to flanges withthe gasket floating between the flange plates.

STANDARDS FOR THE SELECTION OF FLANGES ANDBOLTS The dimensions of different types of bolts and flanges arestandardized in many countries; therefore the transfer into Europeanstandards is almost completed, Tab. 1. There are standards for PN-designated (EN 1092) and for class-designated (EN 1759) flanges;bolts and nuts are found in EN 1515. In these standards the dimensions mainly depend on the data ofinternal pressure and temperature (p-T ratings). In a first approach thisis sufficient, because the pressure determines the stresses dominantlyand temperature determines the material (incl. limits). Above dimension criteria have been evaluated without detailedconsideration of the gasket and additional loads. Therefore in anactual case - loads are known, appropriate choice of gasket - even forstandardized flanges detailed analysis (tightness, stresses) isrecommended if the joint is important.

STANDARDS FOR THE SELECTION OF GASKETS Dimensions of gaskets are provided in EN 1514 (PN-designatedflanges) and EN 12560 (Class-designated), tab. 1. Like above, theseare standards for dimensions only, gasket characteristics are notincluded. For the choice of the gasket the following points have to be takeninto account:

- long-term characteristics of the gasket,- relevant gasket factors for calculation purposes and- quality control of the gasket manufacturer during production.

Within the long-term characteristics the ability of the gasket towithstand the given medium and the loads, the corrosion potential ofthe flanges with the given medium and the change of the gasketcharacteristics with time have to be considered. These are questions,that must be answered individually by the gasket manufacturer. Thereare no European standard activities in this field. Gasket factors (for calculation purposes) for flange types withfloating gaskets are different from those for flanges with metal-to-metal contact. Gasket factors for circular flanges with the gasket floating betweenthe flanges are defined prEN 13555, Tab. 2. These gasket factors can

be classified in factors describing the tightening characteristics andfactors describing deformation characteristics, Fig. 5. For every gasket there is a certain minimum gasket stress in the stateof assembly (QMIN(L)), that is necessary to reach the requested leak rate(or tightness class). During service, it is necessary to maintain at leasta sufficient minimum gasket stress in every relevant operating state(QSMIN(L)). This minimum gasket stress depends on the appliedpre-deformation of the gasket during mounting of the joint. Thehighest value of QSMIN(L)? equals QMIN(L). With an increase inpre-deformation of the gasket during assembly the QSMIN(L)?-valuedecreases. Regarding tightening characteristics, QSMIN(L)?and therelated Qprestress?-value are the crucial input data into calculations. To prevent destruction of the gasket or drastic changes in tighteningcapabilities, the upper limits of the gasket stress in the state ofassembly (QSMAX(RT)) and in operation (QSMAX(T)) have to beregarded. To determine the changes of the gasket stress between the state ofassembly and operation, the stiffness of the gasket - described by theelastic recovery (represented by the slope KI and the intersept E0) - is anecessary gasket factor. Finally, creep and relaxation of the gasket under operatingconditions must be known, because this can result in a drasticunloading of the joint. gC is the gasket factor, that takes thischaracteristic into account. There are no standard activities in Europe regarding gasket factorsfor flanges with metal-to-metal contact; however, there is a nationalapproach in preparation in the German nuclear standard (KTA). Thereare only 3 gasket factors necessary for this type, Fig. 5. First, thegasket stress to reach metal-to-metal contact is important. This gasketstress does not change any more after contact (except creep /relaxation), it is the value that has to be taken into account fortightness purposes. With this stress acting on the gasket, a maximuminternal pressure can be applied for a given medium; thus the tightnesslimits are given. The factor for creep and relaxation of the gasket issimilar to the floating type.

STANDARDS FOR CALCULATION In European standards, there are two procedures for the calculationof flanged joints with the gasket floating between the flange plates,Tab. 3. One is based on the ASME-code (incorporated inprEN 13445), the second one uses a limit load theory (EN 1591).Some characteristics of the procedures are shown in Tab. 5. As said above, the ASME procedure as it is incorporated inprEN 13445 (there is a new approach on Design Rules for BoltedJoints in an ASME non-mandatory appendix which is not regardedhere) is more or less a dimensioning guideline only applicable for aformal stress analysis. It is not possible to perform a tightness analysison this base because the gasket characteristics are included by use offormal gasket factors (not explicitly proved in tests). It is not possibleto determine the necessary prestress value. With the EN 1591 procedure it is possible to perform stress analysisand tightness analysis for flanged joints. Additionally, the necessaryprestress values are provided, even the mounting procedure can betaken into account. All relevant loads (operation states) of a flangedjoint are considered; it is possible to include external forces andmoments (except torsion moments). The tightness analysis depends toa high degree on the use of realistic gasket factors. The necessarygasket factors are defined in prEN 13555.

Fig. 5 : gasket factors for use in calculation (top: floating gasket type; bottom: metal-to-metal contact type)

With none of above calculation procedures, it is possible to performa realistic fatigue analysis, because it is not possible to calculate localstresses, explicitly. This can be done with appropriate FE-calculations,for example. There are no standards for flanged joints with metal-to-metal contactof the flange plates, neither for gasket factors as said above - nor forcalculation (analysis) procedures. In the revised version of German standard KTA 3211.2 which is inpreparation, there is a guideline for the analysis of such types of joints.The calculation procedure itself is quite similar to those of flangeswith floating gaskets. The joint acts completely like a joint withfloating gasket, until contact is reached; so the well known procedurecan be used without restriction. From the point of contact on newcantilever arms have to be taken into account. The first task of the design stage is to reduce flange rotation to aminimum (rigid design), in order to get sufficient contact. Stress andtightness analysis have to be performed with the appropriate gasketfactors (metal-to-metal, see above).

CONCLUSIONS Regarding flanges and their joints, the process of standardization isproceeding, in Europe. As standardization always is a political task,too, the resulting papers actually may contain compromises that limittheir field of application, or, due to the lack of detailed descriptions,

the guideline is not clear enough. Some examples are stated in thispaper. Therefore, some of the standards have to be optimised in futurerevisions. In the meantime, the responsibility for the correct use (andinterpretation, if necessary) clearly remains a task of the user. For circular flanges with the gasket floating between the flangeplates, there are standards (existing or under construction in variousstages) regarding the type (design principals) and the dimensions offlanges, bolts and gaskets. Material properties are considered, too.There are definitions of gasket characteristics that can be used for theselection of the appropriate gasket as well as in stress and tightnessanalysis calculations. And there are standards for these calculationprocedures. And, as a result of such calculations, the prestress valuethat has to be applied during mounting is provided, including theallowable scatterband. Thus, implicitly, the necessary effort ofmounting (demanded accuracy of the prestressing procedure) isdetermined. In general, the European standardization projects cover the entirechain that is necessary to prove function of a flanged joint with thegasket floating between the flange plates. There are no European standards (and no projects) regarding flangedjoints with metal-to-metal contact of the flange plates. However, therewere some national research projects, recently, that provided a basisfor future standardization in this field. A national attempt is madeactually, in the KTA standard code for nuclear components inGermany.

gasket factors prEN 13555

description

QMIN(L) minimum assembly gasket stress QSMIN(L) minimum operation

gasket stress QSMAX(RT) maximum assembly gasket stress

QSMAX maximum operation gasket stress

E0, KI elastic recovery intercept, slope

gC creep / relaxation factor

gasket factors

description

? MMC minimum gasket stress to reach contact

pMMC/L maximum pressure for tightness class L

gC creep / relaxation factor

mediuminternal pressuretemperatureforcesmoments

mediuminternal pressuretemperatureforcesmoments

Tab. 5 : characteristics of calculation procedures

REFERENCES European standards: see Tab. 1 to Tab. 4

ASME

- e.g. in ASME-section VIII,division 1, appendix 2 and

- prEN 13445, chapter 11

EN 1591

- also in prEN 13445,appendix G

input parameters- internal pressure- external forces / moments via

equivalent pressure

parameters of calculation(selected items)

- internal pressure- external forces and bending

moments- temperature differences between

bolts and flanges- bolt forces (incl. scatterband for

assembly)- stress limits- entire bolted joint considered

loads- prestress of bolts- operation

loads- assembly- (pressure) test- operation

resultsstresses in flange

stresses in bolts- assembly and operation

results- usage factors- flange deformation / rotation- tightness / leak rate (gasket

stress)