Determinationn of gasket load drop at welding neck flange joints operating at medium high temperature

Transcription

1 Determnatonn of gasket load drop at large sze weldng neck flange onts operatng at medum hgh temperature A. agy * and B. Dudnszky * * Department of Buldng Servces and Process Engneerng, aculty of Mechancal Engneerng, Budapest Unversty of Technology and Economcs, Budapest, Hungary drnagyandras@freemal.hu, dudnszky@vegyelgep.bme.hu Abstract ths paper nvestgates the development of loosenng at flange onts owng to creep and relaxaton of gaskets at medum hgh temperature. The model presented here s able to descrbe the nteracton of gasket and flange addton to the vscoelastc behavor of non-metallc gasket. The tme-dependent materal characterstcs of the nvestgated gasket [5] were determnedd by standard tests whlst the relablty and precsenesss of the derved equatons are proved by test results. The paper deals wth the dmensonng of flange onts on leak tghtness operatng at medum hgh temperatures. I. ITRODUCTIO gure. Sketch of the nvestgated flange ont The reducton of gasket load at flange onts operatng at medum-hgh temperature s manly due to loosenng effects that arse from nternal pressure and addtonal external moments and forces [3] as well as from creep and relaxaton of the non-metallc gasket materals. Ths drop n gasket load arsng from the nternal pressure and external loads s dscussed n the lterature by lnear and non-lnear gasket model [4]. The only dffculty n determnng the resdual gasket load under pressurzed condton s the tme-dependent creep and relaxaton of non-metallc gasket. There was sgnfcant work done already n the 95 s to determne, by means of test, the tme-dependent characterstcs of gasket [], []. At that tme a test method sutable to determne the long term load carryng capacty of gasket had been nvented and standardzed, and ths method s stll n use [7], [8]. The strength characterstcs determned by ths way cannot be regarded as pure materal characterstcs s of the gasket, because ths s nfluenced by the nteractonn between the bolted ont and the gasket. Thus the prevous mentoned test method s sutable to compare dfferent gasket materal. II. AALITICAL MODEL O LAE JOIT The model presented heree enables the mentoned standardzed method to determned the true characterstcs of the gasket materals. Usng the newly developed model and applyng the mentoned test method, the tme dependent characterstcs (that s creep and relaxaton of gasket materal can be predcted atany arbtrary ont. The basc assumpton of the model presentng the correlaton of actng loads and caused dsplacements are the follows: Heat actng of the flange onts comes only from the medum to be sealed, characterzed by ts pressure p and temperature T U. The effect of the yeld strength reducton and the change of elastcty modulus owng to the temperature ncrement both at flange and bolts can be neglected. Creep and relaxaton at flange and at bolts can also be neglected, so the rotatonal sprng stffness of flange as well as sprng stffness of bolts are ndependent from tme and from dsplacements. Another underlyng assumpton of the model s that durng equalzaton of temperatures among dfferent elements of the flange ont nternal forces and stresses are generated by the dfferent heat expanson not nfluencng the creep and relaxaton of gasket, therefore the two phenomena can be nvestgated ndependently from each other. At flange ont presented n ths g., the dsplacements caused by smultaneous heat and mechancal loads and the changes of nternal forces [4], [6] wll happen accordng to the g., shows.

2 gure. Dsplacements and changes of nternal forces owng to the smultaneous heat and mechancal loads In ths fgure the dsplacements W and angular dsplacements κ can be seen whch are together responsble for the reducton of gasket load. The flanges themselves are replaced by corner rgd skeletons, havng and rotatonal sprng stffness. As the fgure shows both the dsplacements and angular dsplacements and gasket load drop can be dvded nto parts and they can be nvestgated separately from each other. In ths fgure mean the gasket load drop arsng V P from the pressure nduced header end load P and s the unknown gasket load drop caused by creep and relaxaton of the gasket. In order to nvestgate the loosenng effect caused by gasket relaxaton, the gasket should be modelled and the nteracton of gasket and flange should be analysed. To create ts vscoelastc model, the typcal non-lnear characterstcs of non-metallc gasket materals should be taken nto consderaton, as well as the fact that durng the standardzed relaxaton test of frequently appled gasket materals non-zero resdual compresson stress appears. The developed rheologcal analogue model whch s the bass of further nvestgatons can be seen n the next g. 3. As the fgure shows the model fulfls both requrements. The non-lnear characterstcs of the gasket are produced by the sprngs of the elementary Maxwell-model connected n parallel, and the nteracton of gasket and flange s symbolzed by the sprng connected to the gasket model havng the modulus calculated from ths equaton ( where A s the surface area whle s the thckness of the gasket, s the sprng stffness of bolts. ε V A U = = kc + h V [ ( U U + z Ths fgure presents the state belongng to the startng compresson stress. The determnaton of the total decrease of gasket load orgnated from the relaxaton of the gasket s possble only n several steps usng the equatons belongng to the dfferent lnearzed ranges of the compresson characterstc curve. The g. 4, shows the range. of the loosenng process. In ths range the gasket compresson stress can be expressed as a functon of tme wth the followng dfferental equaton (. + E d ( + + E V + E d V = dt dt ( E gure 3. Rheologcal analogue model of flange ont gure 4. The range. of the relaxaton process = = ] h (

3 Its soluton regardng the boundary condtons s: V ( t = = + ( E E E + ( E + E E ε = + E + E ( ( [ exp λ t ] where ε = means the total compressons of = the gasket and the coeffcent λ can be calculated from ths equaton (4. λ = = [ + ( ] + E E E = ( + E The actual range of valdty for ths functon s a short tme perod t t where ts upper lmt s determned by equaton (5. t = = = ( + E [ + ( + ] E E E (3 (4 ε ln (5 ε The relaxaton and creep of the gasket occurrng durng ths tme perod can be calculated usng these equaton: V V E = = + E + E V ( t ( V = ε = (7 The resultant relaxaton and creep of the gasket consderng the total loosenng process can be calculated by ths way and fnally the followng equatons (8, (9 can be obtaned. E nf ( = = V = V = = + E + E ( (6 (8 d dt V d = dt t= t V t= ( Substtutng ranges of the relaxaton curve after smplfyng the followng recursve formulae can be found. E + E = = + E ( = = E = The factor of vscous flud element at the begnnng as well as sprng constants can be determned by tests. These tests gve the compresson stress-stran characterstc curve that s to be appled parallel to the standardzed long term load carryng capacty test of the gasket whch can be determned by the earler mentoned test method. nowng these test results the searched characterstcs of the model can be calculated usng equatons (, (3, (4. E E nf ( nf ( ( ˆ ˆ V V ˆ nf ( nf ( ( ˆ ˆ V V E + E = ( + ( ( + ˆ ( ( ( E nf nf + ˆ ˆ ˆ V V nf ˆ V (3 = E E ε = d ˆ V (4 = ˆ + E dt t= where devce and relaxaton. assgn the relatve modulus of the testng ˆ V symbolse the measured resultant III. TEST AD CALCULATED RESULT Several experments were carred out to prove the relablty and applcablty of the vscoelastc model. rst several thcknesses of gaskets from dfferent manufacturers were nvestgated to determne characterstcs of the gasket materals ncludng the tme dependent propertes. nf ( nf ( ε = = V = V The characterstcs of sprngs and vscous flud elements can be determned accordng to the followng consderaton. The values of characterstcs of vscous flud element gves the requred contnuty of the resultant relaxaton curve. V (9

4 gure 7.Measured Measured and calculatedresultantrelaxatonsas a functon of pretghtenngatdfferenttypes of hg = mmthckgasketat5 mm C gure 5. ZWIC-type type unversal tensle machne ZWIC type unversal tensle machne equpped wth programmecontrolled heatng g. 5, was used to determne stress-stran stran characterstcs whle whl the tmedependent propertes were determned by BOSSER-type BOSSER standardzed test devce g. 6. gure 8.Measured and calculated resultant relaxatons at IT3 gaskets of several thcknesses as a functon of pretghtenng at 5 C Durng the experments the tme-dependence tme of the relaxaton axaton process takng place n the test devce was also recorded. The next two fgures g. 9,, shows the calculated and measured relaxaton curves belongng to gaskets skets made of REIZ and IT 3 materals wth. mm thckness. gure 6. BOSSER-type type standardzed test devce Usng the test results, the coeffcent for elastcty and vscous flud can already be numercally calculated. nowng the characterstcs of the model and choosng dfferent values for pretghtenngs the calculaton can ca be carred out. The calculaton refer to a ont havng equal stffness to the standardzed test devce used for determnaton of the tme dependent characterstcs, so the test devce gves a possblty to drectly check the precseness of the model. he measured and calculated values of resultant The relaxaton can be seen n the followng fgures. The g. 7, show the changes of calculated and measured resultant relaxaton at 5 C n the functon of pretghtenngs of dfferent gaskets wth equal. mm thckness. In ths fgure the contnuous curves present the measured results whle the dashed curves show the calculated values.g. 8, show the calculated and measured values of t at 5 C for dfferent fferent thckness of IT-type type gasket materal as a functon of pretghtenng. gure 9. Process of relaxaton at 5 C of REIZ AM34 type gaskets havng hg = mm thckness and pretghtened at dfferent values The test proved that ndependently of the type of gasket, the deal exponental losses characterstc of relaxatons could appear only nthe range of heat-keepng heat perod. d. Increasng pretghtenng n the t range of heatng up lead to the pressure losses on the t gasket growng bgger and bgger and they nclne from exponental one.ths Ths s the reason why at the begnnng of the heat keepng perod the calculated and measured results dffer.

5 gure. Process of relaxaton at 5 C of IT3 type gaskets havng hg = mm thckness and pretghtened at dfferent values gure 3. Temperature of flange and bolts along the heatng-up perod As the fgure shows the decrease of gasket surface pressure was recorded by measurng four bolts elongaton around the crcumference. Durng the test to keep the requred 5 C temperature nsde the vessel a 4 mm thck solaton was used along the whole body. The followng fgures(g. 3-7, show the measured and calculated results referrng to three dfferent type. mm thck gaskets. gure. Effect of heatng-up velocty on relaxaton process g., represents the effect of the heatng-up velocty on relaxaton n the range of heat-keepng. There are three dfferent relaxaton curves representng three dfferent heatng-up veloctes. Ths test were carred out on. mm thckness REIZ-type gasket at 5 C and the pretghtenngs was 3 Mpa. As the fgure show there s no sgnfcant effect of heatng-up velocty on the total gasket pressure losses, as sgnfcant changes (three tmes bgger n heatng-up velocty durng the test resulted n pressure losses dfferng slghtly from each other. gure 4. Measuredelongaton of boltsat5 CrefferngtoREIZ AM 34 Ong. 3, the dashed lne shows the program controlled temperature of flange rng near to the gasket whle the contnuous curve presents the temperature of the centre of bolts recorded by contact thermometer. gure. Sketch of the test devce To testfy the effect of geometrcal sze on the precseness of developed rheologcal analogue model, an automatcally heated test devce was manufactured whch can be seen n g.. gure 5. Total gasketsurfacepressuredrop (LIER 43

6 On g. 4, a typcal measured bolts elongaton can be seen n the functon of tme referrng to the REIZ AM type gasket.the thn curves shows the measured values whle the thck presents the calculated average. Consderng ths fgure t can be stated that the dfference among bolt elongatons are not to much less than % of the average.measurng the bolt elongaton we can calculate the changes of gasket surface pressure.nally n the last three fgures (g. 5-7, the comparson of calculated and measured results can be seen. gure 6. Total gasket surface pressure drop (REIZ AM 34 can be seen n the functon of pretghtenngs. The sngle symbols show the expermental, whle the contnuous and dashed curves present the calculated values by analytcal and EM model. IV. COCLUSIO The results of presented theoretcal and expermental nvestgatons allow the followng statements. Results calculated accordng to the developed model and test concernng gasket relaxaton n case of thckness less than 3. mm and pretghtenng less than 4 Mpa show good correlatons. The effect of gasket stress drop durng the heatng- dfferences at the up perod causes sgnfcant begnnng of nvestgatons between the calculated and measured relaxaton curves n case of pretghtenng by more than Mpa. Concernng the future the developed model should be extended consderng by takng the creep and relaxaton of flanges and bolts nto account. REERECES [] S. Schwagerel, and E. rageloch, Prüfung von Wechdchtungen. BW 95, 4, [] A. E. Johnson, Creep and relaxaton of metals at elevated temperatures. Engneerng, 949, 68, [3] H. J. Bernhard, lange theory and the revsed Standard B. S.: 96-flanges and boltng for ppes, valves and fttngs Proc. Int. ech E., 963, 78, 7-3. [4] A. agy: Determnaton of the asket Load Drop at Large Sze Weldng eck lange Jonts n the Case of onlnear asket Model. Int. J. Pres. Ves. & Ppng. 67 ( [5] A. agy: Tme Dependng Characterstcs of asket at lange Jonts. Int. J. Pres. Ves. & Ppng. (997. [6] L. Varga, A. agy: Optmale orm und neue Analyse von lanschkonstruktonen. onstrukton 49, H. (997. [7] DI 593 Druchstandversuch an IT Dchtungsplatten (99. [8] BS 753: 99 Specfcaton for Compressed on-asbestos bre Jontng. Appendx B. gure 7. Total gasket surface pressure drop (LIER 443 In these fgures the total gasket surface pressure drop