THE INFLUENCE OF DIFFERENT BRAIDED PACKING MATERIALS AND NUMBER OF RINGS ON STEM TORQUE AND SEALABILITY

Transcription

1 Proceedngs of the ASME 29 Pressure Vessels and Ppng Dvson Conference PVP29 July 26-3, 29, Prague, Czech Republc Proceedngs of the ASME PVP29 29 ASME Pressure Vessel and Ppng Conference July 26-3, 29, Prague, Czech Republc PVP THE IFLUECE OF DIFFERET BRAIDED PACKIG MATERIALS AD UMBER OF RIGS O STEM TORQUE AD SEALABILITY José C. Vega Carlos D. Grão Carlos F. Cpolatt Teadt Industra e Comerco Ltda Av Martn Luther Kng Ro de Janero, RJ Brazl ABSTRACT Ths paper ntroduces a test devce and a protocol that smulates packng performance n dfferent sze valve stuffng boxes and stems. Ths test devce enables measurement of braded packng compresson, relaxaton, axal force at the bottom of the stuffng box, the torque generated upon stem turnng and the nfluence of the number of packng rngs on stem torque. It also enables comparsons between dfferent bradng yarns materals, mpregnatons and correlatons wth seatng stress, stem torque and sealablty. Test results showng these comparsons and correlatons are reported. ITRODUCTIO The number and sze of braded packng rngs used n valves vares wdely from one valve manufacturer to another. Standards are not very clear regardng ths subject. For nstance API 6 / ISO 1434 [1] states that that the nomnal depth of the packng box shall accommodate a mnmum of fve uncompressed packng rngs wth the packng wdth dependent on the szes of the stem dameter. API 62 / ISO 15761[2] specfes the mnmum uncompressed total heght of the nstalled packng regardless of the packng wdth. And MSS SP 12 [3] shows a packng depth of 4W or 5W, whch s the equvalent of 4 or 5 square packng rngs, wth crosssectons varyng accordng to the stem dameter. Laboratory and subsequent feld tests performed wth dfferent packng styles, valves and meda showed that, as for gaskets, t s possble to have an nstallaton seatng stress procedures for packng valves [1]. However, these studes dd not nvestgate the effect of the nstallaton stress on the stem torque. Some test standards, such as Chevron Texaco s [4], establsh a maxmum turnng torque on the valve hand wheel. Ideally, testng should predct the stem torque, whle avodng the common stuaton where the valve hand wheel s not able to turn, thereby allowng the szng of actuators. It was decded Page - 1 to nvestgate the possblty of correlatng the nstallaton stress wth the frcton force holdng the stem, the nfluence of the number of packng rngs nstalled and the consequence on sealablty. TEST RIG A test rg that smulates a stuffng box and a stem was developed to perform the studes as shown n Fgures 1 and 2. Ths rg has a load cell at the bottom of the stuffng box that measures the resdual force at the bottom of the last packng rng. It s equpped wth two gauge bolts that ndcate the force beng appled on the packng by the gland follower. A torque meter fastened to the stem measures the force necessary to turn t. FIGURE 1 TEST RIG SCHEME Copyrght 29 by ASME

2 The gauged bolts and the load cell are connected to a graphc regster that records the packng axal relaxaton over the tme. Ths procedure was repeated wth two, four, fve and seven packng rngs. PACKIGS TESTED The tests were performed for each of the packng styles descrbed below, and ther dynamc propertes regstered. Style A - Flexble Graphte Yarn renforced wth an Inconel wre mesh. Style B - Flexble Graphte Yarn renforced wth an Inconel wre. Style C - Carbon and Flexble Graphte Yarn wth Graphte mpregnaton Style D Expanded PTFE flled wth Barum Sulphate. TEST RESULTS Torque x Axal Stress FIGURE 2 TEST RIG The stem has a surface fnsh (R a ) of,8µm (32µn) and the stuffng box a surface fnsh (R a ) of 3,2µm (125µn). These values meet the requrements of surface fnsh for valves under API 6 [1], API 62 [2] and MSS SP 12 [3]. TEST PROCEDURE A test procedure was developed to analyze the dfferent packng behavor when subjected to several loads. Ths procedure enables an evaluaton of the torque needed to turn a stem as a functon of the axal stress and a quck observaton of packng relaxaton. An approxmaton of the axal stress dstrbuton can also be determned followng ths protocol. Packngs were tested as descrbed below: The Rg s cleaned to assure that the stem and the stuffng box are free from any unwanted materal/dust. Random loads are appled on a metal bushng to assure the load cell and the gauged bolts are correctly calbrated. The packng rngs are nstalled and an ntal load of 1MPa (145ps) s appled to seat the packng rngs. The heght of the packng set s recorded. The desred load s appled to the packng and the load on the last rng regstered. The packng s allowed to relax for ten mnutes and the loads on the gauged bolts and on the load cell are recorded. The desred load s re-appled and both readngs are once agan regstered. The heght of the packng set s recorded. The stem torque s then measured usng a torque meter. Four readngs are made wth a half-turn of the stem each. The frst readng records the statc torque whle the three subsequent readngs record the dynamc torque. After the two complete stem turns, the readngs on the gauged bolts and on the load cell are regstered. In order to analyze the packng materals effect on the force to turn a valve hand wheel, eght graphs were plotted. The frcton force necessary to turn the stem as a functon of the gland stress and the number of packng rngs, s shown on Fgures 3 through 1. The torque value used to calculate the frcton force was ether statc or dynamc, whchever had the hgher value. The stem for the 6.4mm (1/4n) cross-secton packngs had a dameter of 25.4mm (1n) and for the 7.9mm (5/16n), 5.8mm (2n). Frcton Force () Frcton Force () FIGURE 3 FRICTIO FORCE X AXIAL STRESS: STYLE A 6.4mm FIGURE 4 FRICTIO FORCE X AXIAL STRESS: STYLE A 7.9mm Page - 2 Copyrght 29 by ASME

3 Frcton Force () Frcton Force () FIGURE 5 FRICTIO FORCE X AXIAL STRESS: STYLE B 6.4mm FIGURE 9 FRICTIO FORCE X AXIAL STRESS: STYLE D 6.4mm Frcton Force () Frcton Force () Frcton Force () Frcton Force () FIGURE 6 FRICTIO FORCE X AXIAL STRESS: STYLE B 7.9mm FIGURE 7 FRICTIO FORCE X AXIAL STRESS: STYLE C 6.4mm FIGURE 8 FRICTIO FORCE X AXIAL STRESS: STYLE C 7.9mm Page - 3 FIGURE 1 FRICTIO FORCE X AXIAL STRESS: STYLE D 7.9mm The graphs show that there s an ncrease n the frcton force (force that opposes movement) as more packng rngs are added. The hgher the frcton force, the harder t wll be to operate the valve. The effect of composton on packng behavor can also be analyzed from these graphs. Both packngs constructed from Flexble Graphte Yarn renforced wth Inconel had smlar results. These packngs generated a much hgher frcton force than the packng made wth Expanded PTFE Yarn flled wth Barum Sulphate. It s also possble to observe, that the packngs behave dfferently under low and hgh gland stresses. On Style C, for nstance, where ths dfference can be seen more clearly, the packng behavor to 2MPa (29ps) s completely dfferent from ts behavor under hgher stresses. Ths dfference n tendences s llustrated n Fgure 11 by the blue and red lnes. Frcton Force () R FIGURE 11 TEDECIES: STYLE C 6.4mm Copyrght 29 by ASME

4 After analyzng these results t s mportant to rentroduce the concept of mnmum seatng stress, S mn(,1). Mnmum seatng stress s the pressure appled by the valve gland, σ, to seat the packng so t flls all the vods between the stem and the stuffng box. These values were determned [1] expermentally usng fve rngs wth 6.4mm (1/4n) crosssecton and the results are shown n Table 1. TABLE 1 S mn(,1) VALUES S mn(,1) Packng Style MPa ps A B C 2 29 D The mnmum seatng stress of each packng style s ndcated on the followng charts by a red vertcal lne. otce that the mnmum seatng stress for Style C s 2MPa. There s a drect correlaton between the mnmum seatng stress and the changes n the packng behavor. Once the mnmum seatng stress s reached, the packng stress starts to show a lnear tendency that can be descrbed by the equaton of a straght lne For σ > S mn(,1) F r mσ + b = where σ s the nstallaton stress, F r s the frcton force, m s the slope of the lne and b, the y-ntercept. Even though ths equaton closely models the packng behavor t s not convenently predct the frcton force as a functon of the gland stress. To use ths equaton t would be necessary to expermentally determne m and b for all packngs cross-secton as well as for the dfferent number of packng rngs used. Throughout ths paper, a more effcent mathematcal model wll be proposed. Resdual Axal Stress at the Bottom of the Stuffng Box The test rg enables the measurement of the resdual axal stress at the bottom of the stuffng box. Ths nformaton s crtcal snce t ndcates how much of the axal stress appled on the top of the frst packng rng s actually reachng the bottom of the last rng. To ensure effectve sealng at the bottom of the stuffng box, the radal stress exerted by the bottom rng must be greater than the meda pressure [13]. The percentages of the appled gland stress that reaches the bottom of the stuffng box, ξ, for the dfferent packngs and for dfferent number of packng rngs are shown n fgures 12 through 19: (1) 4% 2% % FIGURE 12 RESIDUAL AXIAL STRESS O THE BOTTOM RIG: STYLE A 6.4mm 4% 2% % FIGURE 13 RESIDUAL AXIAL STRESS O THE BOTTOM RIG: STYLE A 7.9mm 4% 2% % FIGURE 14 RESIDUAL AXIAL STRESS O THE BOTTOM RIG: STYLE B 6.4mm 4% 2% % Page - 4 FIGURE 15 RESIDUAL AXIAL STRESS O THE BOTTOM RIG: STYLE B 7.9mm Copyrght 29 by ASME

5 4% 2% % The relatonshp between the number of packng rngs and the amount of the appled gland stress transferred to the bottom of the stuffng box decreases as the gland stress ncreases. Under hgh stresses (above the mnmum seatng stress) all four packngs studed showed very lttle dfference on the amount of the gland stress that reached the bottom of the stuffng box for two and seven rngs. It s usual to fnd n tradtonal lterature [13, 14] the pcture on Fgure 2, where the frst few packng rngs apply most of the radal force. FIGURE 16 RESIDUAL AXIAL STRESS O THE BOTTOM RIG: STYLE C 6.4mm 4% 2% % FIGURE 17 RESIDUAL AXIAL STRESS O THE BOTTOM RIG: STYLE C 7.9mm FIGURE 2 RADIAL STRESS DISTRIBUTIO 4% 2% % FIGURE 18 RESIDUAL AXIAL STRESS O THE BOTTOM RIG: STYLE D 6.4mm The radal stress dstrbuton s drectly proportonal to the axal stress dstrbuton [11]. The tests performed show that the amount of axal stress reachng the bottom of the stuffng box s only slghtly less than that at the gland. Therefore, the radal dstrbuton on the pcture above s not true for valve applcatons, where for stresses above the mnmum seatng stress, more than 5% of the appled gland stress reached the bottom of the stuffng box. In some cases, more than 9% of the gland stress reached the bottom. The behavor shown on fgure 2 mght be true under low stresses, however the study of packngs under gland pressures below the mnmum seatng stress s not wthn the scope of ths project. Packng Relaxaton 4% 2% % The packng relaxaton as a percentage of the ntal stress, ε, was also measured over a perod of ten mnutes. The rg enables a contnuous measurement of the gland stress and the stress on the bottom of the stuffng box. The results for the four packngs studed, usng fve rngs wth a cross-secton of 6.4mm (1/4n) are shown below (Fgures 21 and 22). FIGURE 19 RESIDUAL AXIAL STRESS O THE BOTTOM RIG: STYLE D 7.9mm Page - 5 Copyrght 29 by ASME

6 ε (%) 9% 7% 5% Style A Style B Style C Style D FIGURE 21 RELAXATIO AT GLAD The results showed that the relaxaton for the packng tested n four days was not much hgher than the ntal relaxaton over the ten mnutes perod. Predcton of Frcton Force on the Stem The torque, τ, measured at the stem on the rg can be descrbed as the frcton force, F r, generated by contact between the packng and the stem, tmes the stem radus, r. τ = F r r (2) The frcton force, by defnton, s the force that opposes moton when the surface of one object comes nto contact wth the surface of another. Ths force s gven as the product between the resultant normal force, F, and the mean coeffcent of frcton, µ. F = µ (3) r F ε (%) 9% 7% The normal force s the resultant of the radal forces. A reasonable approxmaton s to treat ths resultant as the sum of the radal forces appled by each packng rng F = = 1 q A (4) 5% Style A Style B Style C Style D FIGURE 22 RELAXATIO AT THE BOTTOM OF THE STUFFIG BOX Over the ten mnutes perod that the packngs were allowed to relax, the decrease on the stress was hgher close to the gland follower than at bottom of the stuffng box. It was also observed that, over the relaxaton perod, packngs under hgher nstallaton stresses relaxed less than packngs wth lower nstallaton stresses. In order to verfy f the ten mnutes perod was a reasonable relaxaton tme, a four-day relaxaton test was performed wth fve rng of Style D under an ntal gland stress of 6MPa (87ps) as shown n Fgure 23. ε (%) 9% 7% 5% 1m n 4days Gland Bottom Gland Stress = 6MPa FIGURE 23 COMPARISO BETWEE 1 MI AD 4 DAYS RELAXATIO PERIODS Page - 6 where s the total number of rngs, q s the radal stress and A s the contact area between the packng and the stem. Ths area s gven by the crcumference of the stem, tmes the heght of the packng set after t has been compressed. Wth an ncrease n the number of the packng rngs, the area wll ncrease and, as a consequence, the frcton force wll also ncrease as shown on Fgures 3 thorough 1. Knowng that the lateral deformaton factor, Κ, whch s rato of the radal stress over the axal stress, σ, s constant [11] q Κ (5) = σ the normal force can then be expressed as: F =1 = Κ σ A (6) and the formula for the torque on a stem wth radus equals r can be re-wrtten as =1 τ = r Κ µ σ A (7) The test results show a consderable varaton n the coeffcent of frcton wth the appled stress for dfferent packng materals. A more accurate mathematcal descrpton would nclude µ nsde the sum sgn, however, one of the man Copyrght 29 by ASME

7 objectves of ths work s to expermentally determne ths coeffcent as well as the lateral deformaton factor, Κ, allowng a predcton of the stem torque as a functon of the appled gland stress. For practcal purposes equaton (7) can be wrtten as λ = µ Κ = r =1 τ σ A where λ s the frcton-deformaton factor, whch s the combnaton of the coeffcent of frcton and the lateral deformaton factor. The above equaton gves a frcton-deformaton factor that s close to the real value. Ths real value s dffcult to be used n servce snce t s necessary to know the axal stress dstrbuton along the packng rngs, σ. A good approxmaton to equaton (8) was determned expermentally (9) wth the test rg usng the value of the stress on the bottom of the stuffng box, σ. (8) 2τ λ = r A ( σ + σ ) (9) An actual frcton-deformaton factor, λ Α, that could be used to predct the frcton force on the stem as a functon of the appled gland stress, σ, can also be determned expermentally. λ λ A = (1 + ξ ) (1) 2 otce that on the lmt where of the stress appled reaches the bottom of the stuffng box ξ = 1, λ Α = λ. The frcton-deformaton factors for the packngs n study are expressed n the fgures below:,3,2,1, FIGURE 24 FRICTIO-DEFORMATIO FACTOR: STYLE A 6.4mm,3,2,1, FIGURE 25 FRICTIO-DEFORMATIO FACTOR: STYLE A 7.9mm,3,2,1, FIGURE 26 FRICTIO-DEFORMATIO FACTOR: STYLE B 6.4mm,3,2,1, FIGURE 27 FRICTIO-DEFORMATIO FACTOR: STYLE B 7.9mm,3,2,1, FIGURE 28 FRICTIO-DEFORMATIO FACTOR: STYLE C 6.4mm Page - 7 Copyrght 29 by ASME

8 ,3,2,1, FIGURE 29 FRICTIO-DEFORMATIO FACTOR: STYLE C 7.9mm,3,2,1, FIGURE 3 FRICTIO-DEFORMATIO FACTOR: STYLE D 6.4mm,3,2,1, FIGURE 31 FRICTIO-DEFORMATIO FACTOR: STYLE D 7.9mm The fgures above show that after a certan appled stress, the value of the frcton-deformaton factor stablzes, becomng a constant. The torque for the 7.9mm (5/16n) cross-secton packngs under a gland stress above 6MPa (87ps) and, on certan cases, 4MPa (58ps) could not be measured due to equpment lmtatons. The packngs studed here had ther mnmum seatng stress values used as a start up pont on the choce of the frcton-deformaton factor shown on Table 2. TABLE 2 AVERAGE FRICTIO-DEFORMATIO FACTORS Packng Style A B C D The values found for two rngs were not used for the determnaton of λ Α, snce all standards studed [1,2,3] do not consderer ths opton. Packng Style C showed two ranges for the frcton-deformaton factor whch are:.6 to.1 from ts mnmum seatng stress to 4MPa (58ps) and.5 to.6 from 4MPa (58ps) to 9MPa (135ps) when t stablzes. To valdate the λ Α, the maxmum and mnmum values found expermentally were appled to the formula below and the results compared wth the actual values. τ r λ σ A (11) A = A Fgure 32 shows the relatonshp between the predcted torque and the torque measured expermentally for Style A packng, wth cross-secton of 6.4mm. A λ Α of.6 was used for four and fve rngs and,5 for seven rngs. τa / τ (%) 2% 15% 1% 5% % -5% -1% -15% -2% FIGURE 32 VALIDATIO: STYLE A 6.4mm Based on the results obtaned, the expermentally found frcton-deformaton factor proved to be a good tool to predct the frcton force on the stem as a functon of the gland stress. SEALABILITY The nfluence of the number of packng rngs on sealablty was also nvestgated. The mnmum seatng stress calculated on the prevous paper [1] for fve packng rngs was now appled for two and seven rngs. It was found that for the four packngs studed, the leak rate was equal or less than.1mbar-l/sec of He under a pressure of 7bar (12ps). Page - 8 Copyrght 29 by ASME

9 COLUSIOS The sealablty tests shows that once the mnmum seatng stress s appled no matter how many rngs are used, two, four, fve or seven, the establshed leakage crteron s met. The tests conducted wth the developed rg shows that there s a reasonable ncrease n the frcton force wth the ncrease on the number of packng rngs. Ths leads to the concluson that the best relatonshp between the number of rngs n a valve and sealablty s the use of the lowest number of rngs that meets the valve manufacturer s standard requrements. API 6 [1], for nstance, states that the stuffng box heght should ft at least fve uncompressed rngs. The test rg enabled the measurement of the stress that reaches the bottom of the stuffng box. Results showed that there are reasonable dfferences on the ablty to transfer forces from one materal to another. However, above the mnmum seatng stress most of the appled stress reaches the bottom of the stuffng box. Laboratory tests performed wth dfferent packng styles showed that t s possble to determne a factor that allows the predcton of the frcton force on a stem as a functon of the appled gland stress and the packng contact area. They presented, as well, the frcton force profle on stems as a functon of the gland stress. The test rg also showed that n a ten-mnute relaxaton process, the decrease n the packng stress s hgher close to the gland follower than at bottom of the stuffng box. Ths relaxaton decreases as the appled gland stress ncreases. These results show that there s a need to re-tghten the gland bolts after the nstallaton stress s appled and before process start-up. [9] DI E Gasket Parameters and Test Procedures Relevant to the Desgn Rules for Gasketed Crcular Flange Connectons [1] VEIGA, J., CIPOLATTI, C. GIRÃO, C. et al. Valve Packngs Seatng Stress. PVP [11] BOUZID, A., DIAY, M. Evaluaton of Contact Stress n Stuffng Box Packngs. PVP26-ICPVT [12] RUAIDHE, M. To make a Mathematcal Model of the Load, Frcton and Sealablty Characterstcs of Graphte Valve Packngs. 2. [13] Compresson Packng Techncal Manual. 2 nd Edton. 28. [14] Champagne, R. Study Sheds ew Lght on Whether Increased Packng Heght Seals a uclear Valve Better. Power REFERECES [1] ASI/API STD 6. Bolted Bonnet Steel Gate Valves for Petroleum and atural Gas Industres. 11 th Edton. 26. (ISO 1434:21 Modfed, Bolted bonnet steel gate valves for the petroleum, petrochemcal and alled ndustres) [2] ASI/API STD 62. Steel Gate, Globe and Check Valves for szes D 1 and Smaller for the Petroleum and atural Gas Industres. 8 th Edton. 25. (ISO 15761:22, Steel gate, globe and check valves for szes D 1 and smaller, for the petroleum and natural gas ndustres) [3] MSS SP 12. Flexble Graphte Packng System for Rsng Stem Steel Valves Desgn Requrements. 22. [4] CHEVRO TEXACO. Valve package Fugtve Emsson Qualfcaton Test Procedure. 23. [5] ISO Industral valves - Measurement, test and qualfcaton procedures for fugtve emssons (Part 1: Classfcaton system and qualfcaton procedures for type testng of valves). 1 st Edton. 26. [6] VDI 244. Emsson Control - Mneral Ol Refneres. 2. [7] API STD Testng of Process Valve Packng for Fugtve Emssons. 1 st Edton. 26. [8] The Flud Sealng Assocaton / European Sealng Assocaton. Pump & Valve Installaton Procedures. 23. Page - 9 Copyrght 29 by ASME