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International Journal of Pressure Vessels and Piping 89 (2012) 1e8

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International Journal of Pressure Vessels and Piping

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Design of floating type bolted flange connections with grp flanges

H. Kurz*, E. RoosMaterialpruefungsanstalt (MPA), University of Stuttgart, Pfaffenwaldring 32, D-70569 Stuttgart, Germany

a r t i c l e i n f o

Article history:Received 14 January 2010Received in revised form29 July 2011Accepted 10 August 2011

Keywords:DesignBolted flange connectionGlass-fiber reinforced plasticGasketGrpPTFE

* Corresponding author. Tel.: þ49 (0) 711 68562549E-mail address: hariolf.kurz@mpa.uni-stuttgart.de

0308-0161/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.ijpvp.2011.08.004

a b s t r a c t

The goal of the presented work is to enable plant operators in the chemical industry to design and tooperate glass-reinforced-plastics (specified in the following as grp) piping with grp flanges at temper-atures up to 80 �C, using polytetraflourethylene (PTFE) gaskets that shall replace the mandatory rubbergaskets.

Various gaskets on the basis of rubber (EPDM) and PTFE were investigated in view of the boundaryconditions in bolted flange connections with glass-fiber reinforced plastic flanges in compliance with DINEN 13555. The rubber gasket and the PTFE based gasket with a PTFE-diffusion barrier meet the leakagerate criterion of TA Luft under the conditions of bolted flange connections with grp flanges.

The mechanical behavior of bolted flange connections DN50 with grp flanges was investigated and istaken into consideration in a new calculation procedure, that accounts for the specific material propertiesof grp and thus allows higher bolt forces, what leads to increased tightness and operational reliability ofthe plants. This draft procedure with experimental pre-tests (already demanded in DIN 16966) is nowavailable to design bolted flange connections with grp flanges.

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1. Problem and objective

The motivation to this work were problems of the design,especially for the tightness proof, of bolted flange connections withgrp flanges, shown in Fig. 1, which are applied mainly in thechemical industry. Using advanced gaskets on the basis of PTFEincreases or expands the medium resistance and the temperatureapplication range in comparison to the conventional rubber gasketsused so far [1,2]. However, the gaskets on the basis of PTFE usedcurrently require an assembly gasket stress higher than rubbergaskets so that the loads of the stressed components (flanges, bolts)are much higher. Furthermore the higher creep of PTFE comparedto rubber can be a disadvantage [3,4].

The procedure in designing the grp bolted flange connections incompliance with the valid Code (AD-Merkblatt N1 [5]) correspondsto the usual procedure for bolted flange connections with steelflanges. The diverting material behavior of grp compared to steel istaken into consideration by means of several material reductionfactors that take into account the effects of creep, charging,temperature and inhomogeneities associated with scatter. Sincethe specific properties of a grp bolted flange connection are notconsidered, this leads to over dimensioned bolted flange connec-tions. There is no realistic image of its mechanical behavior shown

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in this calculation. The motivation to this work was initiated fromthe considerations, listed in Table 1.

The presented work shows the most important findings ofa research project, that was performed in cooperation with threeresearch institutes:

- Institute for Information Technologies in Mechanical Engi-neering (LMI), University of Magdeburg: Optimization of theconstruction using evolutionary algorithms

- Institute for Polymer Materials und Plastics Engineering (PuK),University of Clausthal-Zellerfeld: Description and optimiza-tion of material behavior

- Institute for Materials Testing, Materials Science and Strengthof Materials (IMWF) in cooperation with Material TestingInstitute (MPA), University of Stuttgart: Experimental investi-gations and optimization of design procedure

2. Experimental investigation of the mechanical behavior offloating type bolted flange connections with grp flanges

To delineate correctly the mechanical behavior of the flanges isan important prerequisite for designing bolted flange connectionsbecause it determines the stresses and strains which are a functionof the geometry and load, and lead to a realistic picture of themaximum tolerable bolt load [6].

Nomenclature

b, mm flange widthda, mm outer diameter of the loose flangedga, mm outer diameter of the compressed gasket areadgi, mm inner diameter of the compressed gasket areadi, mm inner diameter of the loose flangedk, mm pitch circle diameterdm, mm gasket force initiation circle diameterdpi, N/mm2 virtual pressuredw, mm diameter of resulting line forced4, mm outer diameter of collarEu, N/mm2 circumferential Young’s modulusFges, N total bolt-forceh, mm blade thickness

lS, mm lever arm for rotational momentlF, mm lever arm for bending momentM, Nm momentn, - number of boltsq, N/m resulting line force at contact area of loose flanger, mm median radius loose flangeWSt, mm3 section modulus against rotationWb, mm3 section modulus against bendingx, mm radial positiony, mm axial positionεu, m/m circumferential strain4, rad flange rotation anglel, mbar$l/(s$m) specific leak ratesu, N/mm2 circumferential design stresssSt, N/mm2 circumferential rotation stress

H. Kurz, E. Roos / International Journal of Pressure Vessels and Piping 89 (2012) 1e82

The mechanical behavior determines the global deformations ofthe bolted flange connections which are a measure of their stiff-ness. After assembly, the bolted flange connections act according tothe overall stiffness, hence changes occur in the condition of thestressed system, consisting of flanges (or loose flanges and collars),bolts and gasket. This is due to the internal pressure, additionalexternal loads (forces and moments), temperature changes (heatexpansion and changes in the modulus of elasticity) and relaxationof the materials.

Within the scope of the research project experimental investi-gations were performed on floating type bolted flange connectionsDN50 of a particular manufacturer. In the next research project,which has already been approved, bolted flange connections ofother nominal widths and manufacturers and standard flangeswith hub and collar will be included in the investigations.

2.1. Compression test

The maximum load bearing capacity and the stiffness of theloose flange and collar were determined in an upsetting test usinga hydraulic test press. The test set up is shown in Fig. 2. The forceinitiation into the loose flange and collar has to imitate the realsituation in bolted flange connections.

Thus, the load initiation into the loose flange is realized bymeans of separated bolt heads, which do not follow the flangerotation. Therefore, the load application point of the bolt force canmove towards the flange axis. Because of the low modulus ofelasticity of 11000 N/mm2, this effect is more distinctive in grp

Fig. 1. Floating type bolted flange connection with grp flanges.

flanges than in steel flanges. The result is that the rotationalmoment is lower in the loose flange as well as bending stress in thebolts.

The load initiation into the collar takes place via a steel flange.The gasket force initiation circle diameter dm corresponds to thegasket diameter, which is determined by equating the gasket areaoutside and inside the gasket force initiation circle according toequation (1):

dm ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffid2ga þ d2gi

2

s(1)

This is realized by means of a special tipping device, shown inFig. 2 (right).

The results of the compression tests are demonstrated in Fig. 3.The loose flanges DN50 showmaximum tolerable loads of 130 kN atRT and 100 kN at 80 �C and are the limiting component in view ofstrength. The component stiffness at RT is for the loose flange at140 kN/mm and for the collar at 320 kN/mm. The compression testsat 80 �C provide important information about the drop in bolt forceduring heating up. Because of the temperature dependence of themodulus of elasticity, the stiffness is reduced to approximately 2/3compared to RT. This effect also occurs in the flange connection andthrows light on the considerable decrease of the total bolt force inFig. 5 during heating up. The strength and so the maximum toler-able load of the collar of 240 kN at RT reduces to approximately200 kN at 80 �C.

2.2. Creep-relaxation of the bolted flange connection

In order to analyze the mechanical behavior, especially thedeformation behavior and creep-relaxation, of a floating type bol-ted flange connection DN50 with grp flanges, the test rig in Fig. 4was developed and set up. It enables a realistic loading situationfor bolted flange connections and simultaneously an extensivemetrological investigation.

During the experimental investigation, the bolt-force, the flangerotation, the strains at selected locations of the flange surface, theinternal pressure and the temperature are determined continu-ously. The test rig is completely vacuum sealed to determine thetightness of the bolted flange connection using helium massspectrometry.

Fig. 5 shows the total bolt-force Fges as a function of time. In thefist step, the flange connection is assembled with 40 kN total bolt-force corresponding to 7.5 N/mm2 gasket stress (crosswise tight-ening in four steps) with subsequent heating up to 80 �C via the

Table 1Aim of the work.

Target

Optimization of the gasket Optimization of the flange Optimization of the design procedure

� Improvement of sealing performance even atlow gasket stress

� Reduction of creep under service conditions

� Increase of load bearing capacity� Reduction of gasket stress relaxation duringservice due to viscous deformation

� Improvement of modeling the mechanicalbehavior of GRP components

� Consideration of anisotropy� Reduction of conservatisms

H. Kurz, E. Roos / International Journal of Pressure Vessels and Piping 89 (2012) 1e8 3

internal heating cartridge. The bolt-force drops to approximately20 kN already during the heating up phase and drops furthermorebelow 15 kN after 500 h. In this state a leakage test is performed at16 bar internal pressure leading to a bolt-force and temperatureincrease. After cooling down and pressure release a bolt-force of8 kN remains corresponding to 1.5 N/mm2 gasket stress which isa critical state in view of an internal pressure of 16 bar. In a secondstep, repeated bolt tightening and a temperature cycle leads to aninitially lower drop of bolt-force. The additional temporal coursehowever indicates more loss in bolt-force followed again by criticalconditions in view of tightness.

2.3. Gasket tests

Thirteen different gaskets for grp bolted flange connectionshave been tested within the scope of this research project:

- Rubber gaskets (EPDM, mandatory for grp piping up to now)- Gaskets based on pure PTFE- Gaskets based on filled PTFE (reduced tendency to creep)- Gaskets with PTFE envelope

The test sequence defined in DIN EN 13555 [7] was adjusted tothe boundary conditions in grp bolted flange connections. Thestress levels for the leakage test and the compression test start at2.5 N/mm2. The leakage tests were performed at 80 �C. The creep-relaxation tests were performed for a stiffness of 50 kN/mm andover a testing time span of 100 h, inclusive the cooling phase.

The following statements can bemade on the basis of the resultsof the gasket tests according to DIN EN 13555:

- The PTFE based gasket with a PTFE-diffusion barrier complieswith the leakage rate criterion of TA Luft, Fig. 6 (dashed line)just as well as the previously used standard rubber gaskets tobe replaced in view the medium resistance - this even at lowgasket stress at assembly of 7.5 N/mm2, in equivalence to 40 kNmounting bolt load for a flange connection DN50. If the gasketstress decreases during service due to the internal pressure andcreep-relaxation, both, the PTFE gasket with a PTFE-diffusionbarrier and the rubber gasket allow a remaining gasketstress-level of 2.5 N/mm2 without significant increase in theleak rate.

Fig. 2. Schematic test arrangement for compression tests on loose flanges (left) andcollars (right).

- In Fig. 7 the results of the creep-relaxation tests according toDIN EN 13555 at 80 �C are represented in form of the gasketstress-level as a function of time, for 7.5 N/mm2 initial gasketstress and a stiffness of 50 kN/mm. The tendency to creep of thePTFE based gasket causes a relatively little loss of 1 N/mm2.Even if the testing time span is prolonged from the mandatory4 h up to 100 h and the cooling phase is considered, a gasketstress-level of 6 N/mm2 remains. Therefore, the optimization inview of creep-relaxation has to be more in the area of the grpcomponents.

3. Simulation with the finite element method

A realistic delineation of the strength behavior of a boltedflange connection provides the basis of an analytical calculationmethod for grp bolted flange connections. This requires knowl-edge about the stresses and strains in highly-loaded and failure-relevant positions as a function of the bolt-force. Therefore,a linear-elastic Finite Element (FE) model of the bolted flangeconnection (Fig. 8) is set up, which can be reduced to 1/16 of thepaired bolted flange connection because of symmetry. Thematerial behavior is assumed to be linear elastic with an isotropicyoung’s modulus of 11000 N/mm2. Specific care has to be takenin the modeling of an exact image of the contact locations(gasket/collar; collar/loose flange; loose flange/washer; washer/bolt head). For this it is important to have a sufficient number ofelements.

As a result, the elastic equivalent stresses according to von-Mises are shown for the mounting situation in a segment of theloose flange and the collar in Fig. 9. Under the washer, the materialis compressed, resulting in a complex, multi-axial stress state.

Further on bending occurs in the circumferential direction withthe maximum between the bolts on the top surface of the looseflange. The collar is be compressed in contact with the loose flange.A stress concentration results at the transition of the flat surfacetowards the hub.

Fig. 3. Load-deformation behavior of loose flanges and collars DN50 at RT and 80 �C.

Fig. 4. Test rig for experimental investigations on bolted flange connections DN50 (without displacement transducers).

H. Kurz, E. Roos / International Journal of Pressure Vessels and Piping 89 (2012) 1e84

In order to verify the FE simulation, a comparison of thedisplacement of the loose flange surface parallel to the flange axisbetween experiment and simulation is shown in Fig. 10. Thecomparison shows an excellent agreement.

4. Analytical model to calculate the load and deformation ofbolted flange connections with grp loose flanges

4.1. Calculation of the loose flange

The modeling of the bolted flange connection according to AD-Merkblatt N1, shown in Fig. 11, does not agree with the real loadingsituation in a loose flange shown in Fig. 12.

The design is based on a bending beam clamped in the radialdirection. Thus, the design leads to a virtual radial stress whichonly represents the usage level of the flange. In real terms, theloose flange is rotated due to the lever arm caused by thedifference between the diameter of the bolt-force circle and theouter diameter of the collar and the acting bolt-force. Addition-ally, a linear distributed load q in the circumferential direction iscaused by the finite bolt hole pitch with the consequence of anadditional bending stress. The circumferential stresses and therotation can be described analytically as follows showing good

Fig. 5. Bolt-force relaxation in a floating type bolted flange connection DN50 with grpflanges and EPDM-gasket at 80 �C.

agreement with the results of the FE simulation and the experi-mental investigations.

The rotation of the loose flange by the angle 4 (Fig. 12) causes anexpansion dr(4) and a circumferential strain εu of a ring shapedsegment with the cross section dA ¼ dx$dy in the radial directionaccording to dr(4) ¼ y(xy)$4 (valid for small rotation 4). Thisexpansion causes a circumferential stress according to equation (2):

sSt ¼ Eu$εu ¼ Eu$drð4Þxjxy

¼ Eu$yjxy$4xjxy

(2)

It corresponds to the circumferential stress caused by a virtualinternal pressure dpi on the inner surface of the ring shapedsegment according to equation (3) (“long term hydrostatic pressureresistance formula”):

sSt ¼ dpixjxydx

(3)

In equation (4), the circumferential stress is considered asa function of a virtual force on the inner surface of the ring shapedsegment.

dFi ¼ sSt2pdydx (4)

Fig. 6. Leakage rate of various gasket types as a function of bolt-force in a grp boltedflange connection DN50.

Fig. 7. Creep-relaxation of various gasket types at 80 �C and 7.5 N/mm2 initial gasketstress at a total stiffness of 50 kN/mm (testing time span 100 h, with the considerationof the cooling down phase).

H. Kurz, E. Roos / International Journal of Pressure Vessels and Piping 89 (2012) 1e8 5

Due to this, an internal moment (equation (5)) is caused inopposition to the external rotational moment (equation (6)):

dMjxy ¼Eu2p

�yjxy

�2dxdy

xjxy(5)

and

dMjxy ¼ dFi$yjxy (6)

From the balance of moments follows equation (7):

ZA

dMjxydAþ Fges$lS ¼Zh=2

�h=2

Zb=2�b=2

Eup�yjxy

�2dxdy

xjxy

þ Fges$lS ¼ 0 ð7Þ

with the underlying assumptions in equations (8)e(11).

lS ¼ dk � dw

2¼

dk ��di þ 2dþ 3d4

4

�2

(8)

xjxy ¼ rþ x (9)

Fig. 8. FE model of a bolted flange conne

r ¼ da þ di2

(10)

b ¼ da � di2

(11)

The solution of this equation leads to the section modulusagainst rotation in equation (12):

WSt ¼ph2diln

�dadi

�6

(12)

From this follows the maximum stress in the circumferentialdirection sSt according to equation (13).

sSt ¼ 6FgeslS

ph2diln�dadi

� (13)

and the resulting rotation angle 4 in equation (14):

4 ¼ 6FgeslS

pEuh3ln

�dadi

� (14)

The load from the rotational moment is superimposed by theload from the local initiation of the bolt-force. This leads to bendingin the circumferential direction through the surface load betweencollar and loose flange.

The bending moment between the bolts is calculated on thebasis of equation (15):

MF ¼ ql2

24(15)

The bending stress is calculated from the circumferentialbending moment MF and the section modulus against bending Wbaccording to equation (16). The bending cross section is the flangecross section.

sF ¼ MF

Wb¼ ql2F

4bh2 (16)

ction with loose flanges and collars.

Fig. 9. Results of the FE simulation (elastic equivalent stress) with highly loaded areas (dark) in the loose flange (left) and collar (right).

H. Kurz, E. Roos / International Journal of Pressure Vessels and Piping 89 (2012) 1e86

This leads to equation (17):

sF ¼ Fgespd2k

4bh2n2dw(17)

with the assumptions in equation (18) and (19).

q ¼ Fgespdw

(18)

lF ¼ pdkn

(19)

The two stresses add together according to equation (20) at theposition on the flange surface between the bolts:

su ¼ sSt þ sF ¼ 6FgeslS

ph2diln�dadi

�þ Fgespd2k

4bh2n2dw: (20)

As in this analytical derivation of the design stress, only thecircumferential Young’s modulus is taken into account, thedesign stress is valid if the circumferential Young’s modulus ofthe grp-structure is constant. This applies to the examined flangeand is reasonable for grp-loose flanges. Differing an isotropicproperties in the radial and or axial direction do not affect thedesign stress.

In Fig. 13, a comparison between the circumferential stressesfrom the FE simulation, the nominal stresses according to AD-Merkblatt N1 (without safety margin), the analytically calculated

Fig. 10. Axial deflection measurement at individual positions on the flange top faceand comparison to results of FE simulation during mounting situation.

design stress and the stresses from the experimental strainmeasurements for the flange connection DN50 is shown.

It can be concluded, that the AD-Merkblatt N1 underestimatesthe circumferential stress in the loose flange. Furthermore, thesuggested analytical concept provides a good image of the (simu-lated) stresses for all nominal widths. The circumferential stress,achieved in the experimental investigation of the loose flange DN50 via strain gauge of 75 N/mm2 correlates to the results of thefinite element simulation, as well as to the design concept. Alter-natively, the outcome of the ASME-Code leads to reasonable stress-levels with a slight tendency to overestimate the stresses at highnominal diameters.

The aim of designing a bolted flange connection is in most casesto determine the required thickness of the flange blade for the givenservice conditions. The flange blade thickness is not varied in thedimensional flange standard DIN 16966 for grp flanges. However,operators requiremoreflexibility for largerflangeblade thicknesses.

Therefore in Fig. 14 the stresses according to AD-Merkblatt N1and the stresses delineated from the new analytical concept arecompared to the stresses of the FE simulation for a flange bladethickness 1.5 times according to DIN 16966, leading to much lowerstresses. The stresses according to AD-Merkblatt N1 (again withoutsafety margin) are also in this case too low. The results of the new

Fig. 11. Loading situation defined in AD-Merkblatt N1.

Fig. 12. Real loading situation of the loose flange (superposition of rotation (left) and bending in circumferential direction (right)).

H. Kurz, E. Roos / International Journal of Pressure Vessels and Piping 89 (2012) 1e8 7

analytical concept correspond exactly to the results of the FEsimulation.

The calculated circumferential stresses correspond with thehighly-loaded locations between the bolts on the top surface of theloose flange in Fig. 9. This is an uni-axial stress state. If the statis-tically firm strength value of the given grp material is known in thecircumferential direction at this particular location at all operatingtemperatures, appropriate design is possible taking account ofsufficient safety margin. Scattering and inhomogeneity of strengthin the component have not to be taken into account by means ofmaterial reduction factors as in AD-Merkblatt N1.

Fig. 15 shows the loose flange on the left after the compressiontest up to the maximum tolerable load described in section 2. Thisloose flange fails under the washer. Therefore, the previouslydescribed concept cannot be applied. Alternatively it is possible todetermine the maximum load bearing capacity and the stiffness inthe compression test. The loose flange of another manufacturerfails at the position between the bolts, see Fig. 15, right. If the

Fig. 13. Comparison of nominal stresses from FE simulation, AD-Merkblatt N1, newanalytical design concept and experimental investigations (at 7.5 N/mm2 gasketstress).

material strength value is known, it can be designed according tothe presented new analytical concept.

If the maximum strength of the grp material in the circumfer-ential direction is reached at the position between the bolts it canbe assumed, that there exist weak points in the structure of the grp-compound. Optimization of the fiber reinforcement in the areaunder the washer of the examined grp-loose flange leads to failureof the loose flange between the bolts at a 1.5 times increased load.Consequently, the compression test discovers weak spots in thecomponent. These can be eliminated by changing the productiontechnology.

4.2. Calculating the collar

The collar can be designed using the adapted safety factorsaccording to DIN 2505 [8] or KTA [9]. Again in a pre-test (accordingto Fig. 2) possible early failure due to inhomogenities in the fiberreinforcement can be found out and excluded.

Fig. 14. Comparison of nominal stresses from FE simulation, AD-Merkblatt N1, newanalytical design concept and experimental investigations (at 7.5 N/mm2 gasket stress)for 1.5 times flange blade thickness compared to DIN 16966.

Fig. 15. Failure of the loose flange under the washer (left) and between the bolts(right).

H. Kurz, E. Roos / International Journal of Pressure Vessels and Piping 89 (2012) 1e88

5. Conclusions

Altogether, nine PTFE-gaskets have been tested according to EINEN 13555 under the conditions in grp flange connections. The PTFE-gasket with a PTFE-diffusion barrier shows sufficient tightness tofulfill the leak rate criterion of the TA Luft and a tolerable creep-relaxation.

The mechanical behavior of the flanges has been evaluated viaexperimental investigations and finite element simulations. Theresults of an analytical design concept show good agreement withthe experiment and as well with the finite element simulation.Following the design concept, the material reduction factors can beneglected, what leads to higher allowable bolt forces, accompaniedby increased tightness and operational reliability of the plants.

Acknowledgments

The research project “design of floating type bolted flangeconnections with grp flanges for the chemical industry” has been

thankworthy sponsored by the federal ministry of economy andtechnology (BMWi) via joint venture of industrial research orga-nisation “Otto von Guericke” e.V. (AiF).

The authors thank to DECHEMA e. V. and the following indus-trial companies that participated in the project-attendantcommittee: BASF AG; Bayer Industry Services GmbH & Co. OHG;Frenzelit-Werke GmbH & Co. KG; Freudenberg Dichtungs- undSchwingungstechnik KG; Garlock GmbH; IBK Wiesehahn GmbH;IDT Industrietechnik GmbH; IMA Materialforschung und Anwen-dungstechnik; Kempchen Dichtungstechnik GmbH; Kroll & ZillerGmbH & Co. KG.; KWO Dichtungstechnik-GmbH; Nessler GmbH;REINZ-Dichtungs-GmbH; W.L. Gore & Associates GmbH.

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[9] Safety Standards of the Nuclear Safety Standards Commission (KTA), KTA3211.2. Pressure and activity retaining components of systems outside theprimary circuit. Part 2: design and analysis, Draft March 2003; June 1992.