Gasket Factors

2000 ASME PVP Conference, Seattle

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THE USE OF GASKET FACTORS IN FLANGE CALCULATIONS

Jaroslav BartonicekGKN Gemeinschaftskernkraftwerk Neckar

NeckarwestheimGermany

Manfred SchaafAMTEC Advanced Measurements

LauffenGermany

Friedrich SchoeckleAMTEC Advanced Measurements

LauffenGermany

ABSTRACT A flange calculation has to fullfill three major tasks. First theprestress value for assembly has to be determined. Then a tightnessproof and a stress analysis have to be performed for every relevantstate of operation. For reliable calculation results it is necessary thatall parts of the assembly (i.e. flanges, bolts and gasket) are regarded. Using the example of a flanged joint of a steam generator of anuclear power plant, the European development in gasket calculationin recent years and the necessary gasket factors are summarized inthis paper. Some general conclusions are drawn. The use of gasket factors plays an essential role in calculations forflanged joints. Only if realistic gasket factors for the selected gasketof the joint (that have to be determined in tests explicitly) are used,above tasks of the calculations can be met. In Europe realistic gasketfactors for use in calculations are standardized in prEN 13555 and inGermany in DIN 28090. Within the European standardization tasks there are two proceduresfor the calculation of flanged joints, actually. One is based on theASME-code (incorporated in prEN 13445), the second one uses alimit load theory (prEN 1591). Whereas the ASME-procedure is onlyuseful for formal stress analysis purposes, the EN 1591 procedureprovides the tools for stress analysis and tightness analysis (includingthe output of a prestress value). The gasket factors that are necessaryfor this calculation are defined in prEN 13555. In Germany there is another procedure incorporated in thestandards for nuclear power plants (KTA 3201.2/KTA 3211.2); thisprocedure is based on the German prestandard DIN E 2505. Stressanalysis and tightness analysis is possible with this procedure, ifappropriate gasket factors (prEN 13555) are used. Of course, FE-calculations are commonly used for more complexdesigns, too. Similar to the analytic calculation procedures, the use ofrealistic gasket factors is essential in these calculations.

INTRODUCTION GKN I is a nuclear power plant that was built in the early 1970s;the plant is in operation since 1976. Using the steam generatorhandhole flanged joints of this PWR power plant as an example, thedevelopment in gasket factors in conjunction with flange calculationmethods are summarized and some "lessons learned" are extracted. In the first years of operation already, there were a few minortroubles with leakage of handhole and manhole junctions of thesteam generators. Fig. 1 gives an overview over the steam generatorand the handhole and manhole locations. Minor troubles in this casemeans small leakages, i.e. corrosion effects or some drops of waternear gasketed joints, especially after pressure tests. But as the leakswere only small, the attempts to solve this problem did not get highpriority. Finally, in 1986 there was a major leakage at a handhole closure ofone steam generator. The leakage caused a forced outage of the plantfor repair purposes, i.e. a time and thus money consuming procedure.Additionally the autorized inspectors and the licencing authoritiesbecame involved, thus it was necessary to provide a solution of theproblem with adequate paperwork. Fig. 2 shows the original version of the handhole construction,Fig. 3 gives details about the gasket. 12 bolts (size M27) are used totighten this flange closure construction of nominal diameterDN 200 mm. The gasket consists of a steel plate (outer diameter244 mm, thickness 7 mm) with kamm-profile; there is a 0.5 mmsilver topping on each side of the kamm-profile. Until that event mounting was done using torque wrenches; thenecessary torque was derived from design data; the calculations andtheir results will be discussed in the next chapter. From the state ofknowledge at that time, inaccurate mounting was assumed to be thereason for the leakage. Therefore the first goal was to optimize the

Hoher Steg 1374348 Lauffen / N.Germany


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mounting procedure itself and as a second goal to provide training tothe mounting staff. An optimal mounting can be achieved using hydraulic tensioners incombination with a control of elongation of the bolts; therefore thismethod was selected. It was necessary to modify the bolt design forthis purpose, Fig. 4. The new bolts have an additional tensioningthread at the free end and – for elongation measurement purposes - arod in their center. With a special hydraulic tensioning device it ispossible to prestress all bolts of the junction at the same time andwith the elongation measurement the force in the bolts can bemonitored during the entire prestress process (bolt elongation vs.force is calibrated). Principle schemes of the methods are shown inFig. 5 and Fig. 6 The selected mounting procedure has some significant advantages:(a) All bolts are prestressed at the same time (one integrated ring of12 tensioners). This results in a reliable prestress level and a smallscatterband of the bolt forces. With the verification of the forces byelongation measurements the results are even better.(b) The time necessary for prestressing of a joint with the hydraulictensioners in combination with the elongation mesurement (specialsnap-on design of the transducers) is comparable to the timenecessary with torque wrenches (special tightening sequence inseveral load steps). But it is not necessary for the mounting staff tostay near the joint; especially the loading procedure can besupervised from a distance. Thus the radiation exposure of themounting staff could be reduced significantly.(c) It is no problem to provide a prestressing protocol to theauthorized inspectors. Parallel to the modifications to the tightening process, newcalculations were performed to verify the necessary prestress value ofthe bolts and to demonstrate that the junction design is (still) inaccordance with the standards (KTA 3201.2). The calculation will bediscussed in more detail in the chapter below. The new tightening procedure was successful. In the followingyears there were no more leaks at these joints – until 1998. Corrosioneffects at the outer shell of a handhole near the gasket demonstrated,that there was a (very small) leak, again. This time, mounting couldnot be blamed because the responsible staff was able to prove thecorrect mounting procedure with a protocol. Finally, it was found that the metallic gasket plates were the reasonfor the leakage. The plates showed tolerances in dimensions; thethickness of the plate at the kamm-profile varied up to +/-0.25 mmalong the circumference whereas a tolerance of +/-0.1 mm was thelimit. The silver topping is not able to compensate for these tolerancescompletely. This was verified in full scale leak rate tests that wereperformed similar to the gasket testing procedure as outlined inprEN 13555. The test rig is shown in Fig. 7. Gasket stress wascontrolled using bolts equipped with strain gages, the leak rates weremeasured using the pressure decay method. The resulting leak rates vs. gasket stress are given in Fig. 8. Thegasket factor Qmin(L=0.01) (determined on the loading part of the leakrate curve, see below) has a value of 65 MPa for the gasket with thelow tolerance wheras it is 10 MPa higher with the gasket with highertolerances. The unloading part of the leakage rate curve shows thesame behaviour, i.e. the curve of the gasket with the bad tolerances isshifted to higher gasket stresses. As a consequence of this event, the quality control procedures forthe parts of the flanged joints were modified. Meanwhile onlygaskets with low dimension tolerances are in the warehouse.

Fig. 1 : steam generator of GKN I

Fig. 2 : handhole junction - old version (dimensions in mm)

handhole

feedwaternozzle

steam nozzle

manhole

8547

252

7

R 42

30

43

M27

O

O

244204200

O

O

O

370295

O

O


3

unloaded

∆ l = 0

F = 0

loaded

∆∆

l = 0l F

F = 0

∆∆ l F

Fig. 3 : gasket for the handhole junction (dimensions in mm)

Fig. 4 : handhole junction - new version (dimensions in mm)

Fig. 5 : hydraulic prestressing (schematic plot)

Fig. 6 : elongation measurement with internal rod(schematic plot)

secundary - handhole (DN200)

“X” “Y”

7

190204244

O

O

O

Detail “Y”

1

4°

2041905

O

O

R 1

Detail “X”

7

1x45

°4

4°

2445

1,5

O

R 1,5

0,5

0,5

tension nut

force form hydraulictensioner

nut

gasket

FF

closure

flange

8547

252

7

R 42

30

43

M27

O

O

244204200

O

O

O

370295

O

O


4

Fig. 7 : test rig for leak rate measurements (pressure decay method)

Fig. 8 : measured leak rates

boltswith strange gages

deflection measurementtransducers

gasket

closure

reference volume

difference pressuretransducer

temperaturetransducer

pressuretransducer

gasket plate with kamm-profile (di: 204mm do: 244 mm)- silver topping thickness 0.5mmfull line: tolerance in plate thickness 0.12 mm / dotted line: tolerance in plate thickness 0.24 mm

0.0001

0.001

0.01

0.1

1

10

100

0 20 40 60 80 100

gasket stress in MPa

leak

rat

e in

mg/

m/s p = 80 bar


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COMMENTS TO THE HANDHOLE PROBLEMS The steam generators of GKN I were designed in the late 60s/ early70s. For the secondary side of the shell non-nuclear codes as TRD,AD and DIN were used. As shown in detailled analysis in 1986 (afterthe leakage event) and 1996 (for a proof of component integrity) theformulas actually used for dimensioning are almost the same as thoseused during the design state. As a summary of these analysis it can bestated, that the design of the steam generators in general and thus thehandholes still meets the actual standard requirements, even if the(stress) limits have changed (to lower values) with the years. Focussing on the handhole junction, the procedures to analizeflanged joints have been modified with the years. Starting with forcebalances and very generalized gasket factors like k0,KD and k1 (asdefined in the 1960s in DIN V 2505, see Tab. 1), the understandingespecially of the gasket behaviour and the necessary gasket factorshas changed. Additionaly, the calculation procedures were improvedwith the years, the actual state is discussed below. Nevertheless, even with the "simple" calculation procedures usedduring the design state combined with a lot of experience choosingthe appropriate gasket factors (the gasket factors were not explicitlydetermined in tests at that time), it was possible to analize the jointand to evaluate a prestress value for the mounting of the handholejoints (bolt forcedesign: 70 kN). This force (transformed to torque) wasapplied to the bolts via torque wrenches during mounting until 1986. After the leakage in 1986 a new analysis was demanded. As therewas a leakage, the old prestress value was doubted, additionally tothe quality of the mounting procedure. Meanwhile more realisticgasket factors were defined (similar to the actual definitions, seeTab. 1), and the calculation procedure was almost the same as it is inthe actual KTA-standard. However, there were no standards for the determination of thegasket factors and thus – from todays point of view – the gasketfactors still were not reliable because they resulted from different testprocedures (if tested). For the gasket used in the handhole junctionfor example, σVU=125 MPa was given in standard tables (note thatthis value did not depend on a leak rate class). As Fig. 8 shows, thisvalue is high compared to realistic values, determined in moderntests. With the gasket factors of 1986 (Tab. 1) a new prestress valuewas determined: bolt force1986: 110 kN. This force was (and still is)the resulting prestress value after tightening with the hydraulictensioners. In conjunction with the latest event, the handhole joint wascalculated once again, using the EN 1591 procedure and the realisticgasket factors determined in above tests (Fig. 8). For a leak rate classof L=0.01 (which is regarded to be "very good" for the givenmedium) QSMIN(0,01)=40 MPa was determined (note that the actualgasket factors that characterize the tightening behaviour depend on atightness class). This value can only be achieved if a relatedQprestress=90 MPa is applied during prestressing. Therefore, QSMIN(0,01)is the value that determines the tightness proof and Qprestress has to beconsidered in the stress analysis (the gasket factor QMIN(0,01) is lessimportant within the actual calculation procedure, it can be used a astart point for an iterative calculation). The calculation with thesegasket factors provided a bolt force of 80 kN. The actual calculation results in a lower necessary prestress valuemainly due to the lower values of the gasket factors used. But as thejunction design is able to bear the higher prestress value (from astress analysis point of view), it was decided to stay with the oldvalue. The actual demands on a reliable calculation and the author´sexperience are summarized in the following chapters.

FLANGE CALCULATION For reliable calculation results it is necessary that all parts of theassembly (i.e. flanges, bolts and gasket) and their interaction areregarded.

Tasks of a Flange Calculation 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 depends on the gasket used and on the changeof the loads between mounting and operation state. This change ofthe loads depends on the external loads and on the stiffness of theparts of the joint. Therefore a calculation must be performed, even ifstandardized flanges and bolts are used. Within this task theboundary conditions (respectation of the limits of the gasket in use)have to be considered. 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 ofthe flanged joint (flanges, bolts, gasket) and on the change of thegasket stress 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 (tocontrol emissions, i.e. to maintain a demanded tightness class). The calculations for use in tightness analysis and stress analysishave to- use relevant and realistic gasket factors,- regard stiffness of flanges, bolts and gasket,- regard realistic operation loads like intenal pressure, external

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

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

State of the Art regarding Calculation In European standards, there are two procedures for the calculationof flanged joints with the gasket floating between the flange plates,Fig. 9. One is based on the ASME-code (incorporated inprEN 13445), the second one uses a limit load theory (EN 1591). InGermany there is another procedure incorporated in the standards fornuclear power plants (KTA 3201.2/KTA 3211.2); this procedure isbased on the German prestandard DIN E 2505. The ASME procedure is more or less a dimensioning guidelineonly applicable for a simplified stress analysis; it is not possible toperform a tightness analysis on this base. The gasket characteristicsare included by use of formal gasket factors (not explicitly proved intests). It is not possible to determine the necessary prestress value. Itis not possible to perform a tightness analysis. With the EN 1591 procedure and with the procedure according toKTA it is possible to perform stress analysis and tightness analysisfor flanged joints. Additionally, the necessary prestress values areprovided, even the mounting procedure can be taken into account. Allrelevant loads (operation states) of a flanged joint are considered; it ispossible to include external forces and moments (torsion momentsonly in KTA). The tightness analysis depends to a high degree on theuse of realistic gasket factors. The necessary gasket factors aredefined in prEN 13555 (most definitions are similar to DIN 28090).


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Fig. 9 : calculation procedures layed down in European and German standards

AD-Merkblatt B71977

DIN V 25051964

DIN E 25051986 / 1990

prEN 13555 description

k0, KD k0, KD σVU QMIN(L)minimum assembly gasket

stress

k1 k1 σBU = m * p QSMIN(L)minimum operation

gasket stress

σVO QSMAXmaximum assembly gasket

stress

k0, KDδ σBO QSMAXmaximum operation

gasket stress

ED ED E0, KIelastic recoveryintercept, slope

− /∆V gCcreepfactor

Tab. 1 : gasket factors used for calculation

p r E N 1 3 4 4 5( A S M E )

K T A 3 2 0 1 .2K T A 3 2 1 1 .2

E N 1 5 9 1

l o a d s :- in t e r n a l p r e s s u r e- fo r c e s / m o m e n t s

(via fic tive in te r n a l p r e s s u r e )

l o a d s :- in t e r n a l p r e s s u r e- fo r c e s / m o m e n ts- te m p e ra tu re

d iffe r e n c e s

l o a d s :- in te r n a l p r e s s u r e- f o r ces / m o m e n ts- te m p e ra tu re

d iffe r e n c e s- r e la x a tio n

r e s u l t s :- s t r e s s e s

r e s u l t s :- s t r e s s e s- d e fo r m a t i o n s /

r o t a t i ons- tig h tn e s s /le a k r a t e s

r e s u l t s :- s tre s s e s- d e fo rm a tio n s /

r o t a t i ons- tig h t n e s s / l e a k r a t e s


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With none of above calculation procedures, it is possible to performa fatigue analysis, because neither of above calculation methodesallows to calculate local stresses explicitly. This can be done withappropriate FE-calculations, for example. There are no standards for flanged joints with metal-to-metalcontact of the flange plates, neither for gasket factors (that aredifferent to those of the floating gasket types) nor for calculation(analysis) procedures.

Gasket Factors The gasket factors as defined in DIN 28090 and prEN13555 (thatare used in EN 1591) are summarized in Tab. 1. These gasket factorscan be classified in factors describing the tightening characteristicsand factors describing deformation characteristics.

Tightening Characteristics For every gasket there is a certain minimum gasket stress in thestate of assembly (QMIN(L)), that is necessary to reach the requestedleak rate (or tightness class). This minimum gasket stress isdetermined using the loading part of the curve e.g. in Fig. 8. During service, it is necessary to maintain at least a sufficientminimum gasket stress in every relevant operating state (QSMIN(L)).This minimum gasket stress depends on the applied predeformationof the gasket during mounting of the joint. The highest value ofQSMIN(L) equals QMIN(L); with an increase in predeformation of thegasket during assembly the QSMIN(L) -value decreases. Regarding tightening characteristics, QSMIN(L) and the relatedQprestress -value are the crucial input data into calculations.

Deformation Characteristics To prevent destruction of the gasket or drastic changes intightening capabilities, the upper limits of the gasket stress in thestate of assembly (QSMAX(RT)) and in operation (QSMAX(T)) have tobe regarded. To determine the changes of the gasket stress between the state ofassembly and operation, the stiffness of the gasket - described usingthe elastic recovery (representated by the slope KI and the interseptE0) - is a necessary 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.

CONCLUSIONS Not only with the presented example, but also with the experienceand the development within the last 15 years (replacement ofasbestos, more rigid environmental demands etc.) there are a fewlessons learned: A flange leakage demonstrates deficits in preventive action. It ismuch more efficient to prevent the reasons for a failure than to try tocontrol the results of a failure. In other words, it is economicallyhighly reasonable to invest in measures to prevent leakage; thus thecost of repair and of forced outages as well as the effort for control ofthe emissions can be reduced to a minimum. Part of the preventive action is the analysis of the joint. In the analysis of flanged joints it is necessary to take all parts(bolts, flanges, gasket) into account. Every approach that neglects theinteraction between the parts involves uncertainty. The most important tasks of a flange calculation are determinationof the prestress level, stress analysis and tightness analysis. There are tools for the reliable analysis of flanged joints. In Europethere is the flange calculation standard EN 1591, that allows stressanalysis as well as tightness analysis explicitly; the bolt forces for themounting state can also be calculated. In a similar way this can bedone with the calculation procedure provided in the German KTA-standard. The use of realistic gasket factors that are determined instandardized tests is highly important in calculations. Part of the preventive action is a qualified mounting, too. Themounting procedure must meet the demands on tightness. Themounting procedure has to be considered in stress and tightnessanalysis, the necessary prestress value has to be a result of acalculation. The mounting staff is involved in a quality process;therefore qualified personnel is necessary.

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Gasket Factors