Research on the Leak Testing Technique of Flexible Accumulation Chamber

Transcription

1 11th Euroean Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Reublic More Info at Oen Access Database Abstract Research on the Leak Testing Technique of Flexible Accumulation Chamber Wang YONG, Dou WEI, Ren GUOHUA, Sun LICHEN Beijing Institute of Sacecraft Environment Engineering, Beijing, China Phone: ; At resent, the non-vacuum accumulation leak testing technique and vacuum chamber technique are widely used for testing the total leakage rate of sacecraft, such as hermetic cabin, roulsion ieline system. The vacuum chamber technique is often used for Russian sacecrafts. But the above two kinds of technique are both used for American sacecrafts. Generally seaking, the accumulation leak testing technique is used for satellites. While the vacuum chamber technique is used for these sacecrafts which have high leakage rate demand, such as Airlock Module and Laboratory Module of ISS. In china, the non-vacuum accumulation leak testing technique is often used in order to save the leak testing cost. The traditional non-vacuum accumulation leak testing technique needs a rigid accumulation chamber which is often manufactured by steel and has a better hermetic character, such as transort container widely used in NASA and ESA, secial accumulation chamber in China. But the rigid accumulation chamber has the following disadvantages, such as high cost, long manufacture time, so it is necessary to study the flexible accumulation chamber which is manufactured by flexible membrane material. In this aer, the theoretical research was firstly studied to estimate whether the flexible accumulation chamber is feasible from these three asects of choice of the flexible membrane, the effects of the volume change and leak of the flexible accumulation chamber on the last testing results. Then a series of tests were made to rove the validity of the theoretical analysis. The research results indicated that the flexible accumulation chamber was feasible to aly to the leak testing of sacecraft and instead of the rigid accumulation chamber. Keywords: flexible accumulation chamber, Helium ermeability, selection of film materials, Validation test 1. Introduction At resent, the non-vacuum accumulation leak testing technique is mostly used for testing the total leakage rate of sacecrafts. Its testing system often includes accumulation chamber, leak detector, atmosheric reference gas, leaking gas samling system and leakage rate calibration system, as shown in Fig.1. Among these equiments, the accumulation chamber is core and its role is to rovide a hermetic sace accumulating the search gas in the rocess of leak testing of sacecrafts. The usual accumulation chamber is often welded by steel lates and rofiled bars. Because its volume cannot be changed, it is called the rigid accumulation chamber. The rigid accumulation chamber has the following disadvantages, such as high cost, long manufacture time, oor versatility etc. Therefore, it is necessary to study the flexible accumulation chamber. The flexible accumulation chamber is often manufactured by the flexible film material and sealed by the heat seal means. It was firstly develoed by JPL in 1976 [1-2]. Its skeleton is a cylinder made of hard lastic and the soft film is coated the cylinder. So its volume cannot be changed. Another flexible accumulation chamber was develoed by NASA in 2002, whose

2 volume could be changed, and it was alied to leak testing of TERRA satellite on the launching site []. At resent the domestic and foreign ublic data show that: the research on flexible accumulation chamber mainly centred at the exerimental method, and lacked the theoretical suort. These factors influencing the leak testing results, such as material ermeability, absortion, volume change, are hardly involved. Therefore, it is necessary to study the influence of these factors on the testing results. In this aer, the theoretical research was firstly studied to estimate whether the flexible accumulation chamber is feasible from these three asects of choice of the flexible membrane, the effects of the volume change and leak of the flexible accumulation chamber on the last testing results. Then a series of tests were made to rove the validity of the theoretical analysis. leak detector switch tie-in atmoshere fan base gas sacecraft charge and discharge console accumulation chamber high ure helium Fig.1 samling system The sketch for non-vacuum accumulation leak testing technique 2. Theoretical research 2.1 Research on selection of film materials used on flexible accumulation chamber Research on ermeability of thin film materials Helium ermeability Generally seaking, the thin film is the lastic film whose thickness is below 0.25mm. Now the thin film often used in the market includes PET, PA, PI, PVA, BOPP, LDPE and aluminum lating thin film etc. He( t) = atm ( t) γ ( t) where is gas ressure; the below table He, atm are on behalf of helium and air resectively; ( t ) refers to the amount related with time. To simlify the roblem, the influence of volume change of flexible accumulation chamber on the leak testing results is not considered at the same time in this section. Therefore, 5 He ( t) = γ ( t)... 2 By literature [4], the formula used to calculate leakage rate by the non-vacuum accumulation leak testing technique is 0V0 ( γ 2 γ1) Q = t( γ γ 2).

3 where γ 1 γ 2 γ are resectively the initial concentration, final concentration and samle concentration. Combining with the formula 2 and formula, the following formula can be obtained 0V0 ( He2 He 1) 0V0 He2 Q = = t( He He2 ) t He. 4 Where 0 is the samle gas ressure, V0 is the volume of samle equiment, t is the accumulation time, He is the increment helium artial ressure after samling a quantity helium, He 2 is the increment helium artial ressure from starting testing to t hours later. Because the influence of ermeability of flexible material on the testing result of is a slow rocess, among above arameters, He 2 is only related with helium ermeation. From the above formula, it is known that the relative error of the leakage rate is identical with He 2. So He 2 was firstly studied. In the flexible accumulation chamber, dhe Q k( He He0) A/ l = Vb dt 5 where Q is the helium leakage rate of the sacecraft, k is the material helium ermeability, A is the surface area of flexible accumulation chamber, l is the thickness of flexible accumulation chamber, He is helium artial ressure in the flexible accumulation chamber, He0 is the initial helium artial ressure, Vb is the effective volume of the flexible accumulation chamber. Formula 5 is the one order ordinary differential equation, where the initial condition is t = 0, He = He0. Using the initial conditions, formula 5 can be solved Ql ka He( t) = + He0 [1 ex( t)] ka lvb. 6 And the ideal case (i.e. no enetration condition) Qt He( t) = He0 + Vb. 7 With formula 6 and formula 7, the relationshi between relative error and time is obtained Qt Ql ka [1 ex( t )] He ( t) He ( t) Vb ka lvb δ ( t) = = He( t) Qt He0 + Vb... 8 With the Taylor formula, the exonential items of formula 6 can be exanded, Qt 1 ka Qt 1 ka 2 Qt 1 ka Qt He( t) = He0 + t + t - t +... Vb 2 lvb Vb 6 lvb Vb 24 lvb Vb 9 Comaring formula 7 with formula 9, the thin film ermeability effect can be denoted by ka the items containing in formula 9. When t 1, the formula 9 can be only taken the first and lvb second items, and the ermeability of flexible material is not affected the test results. In this case, the ermeability coefficient, surface area, cumulative time are hoed to be as short as ossible. If a flexible accumulation chamber was established according to the size of the rigid accumulation chamber often used in ast, the secific arameters are

4 Q = Pa m s / ; V = m l = m b 4 64 ; ; A 2 = 112 m ; and He0 0.5 = Pa. Selected olytetrafluoroethylene as the film material, that is the ermeability coefficient is m 2 /s [5], the ermeability coefficient k affecting the errors on the test results was showed in Fig.2. error accumulation days d Fig. 2 the relationshi between relative error and time, when the film material is Polytetrafluoroethylene Fig.2 is shown that, when the cumulative time is 1 days, the relative error of the results is less than 4%. When the cumulative time increased to 2 days, the relative error of the testing results will reach about 9%. Therefore, when the sensitivity of leak testing meets the leak testing demand, the cumulative time should be as short as ossible. From the above analysis, even if the helium ermeation coefficient is larger (Polytetrafluoroethylene has a larger ermeation coefficient), its influence on the final testing result is still relatively small (In fact, the leakage rate of the sacecraft is about Pam /s, and the cumulative time only needs 1 days). But considering the testing should have a better accuracy, it is suggested that material helium ermeability should be as low as ossible. By literature [5], under the condition of same thickness, the lowest helium ermeability materials are nylon (PA), olyester (PET) and olyimide (PT). So it is suggested that the film material should be chosen from these three kinds of materials Air ermeability The major comonents of air are N 2, O 2, CO 2 and H 2 O. The erformance of nylon film (PA) and coated high barrier film (PVA) reventing H 2 O is oor, and the erformance of low density olyethylene film (LDPE) reventing O 2 is also oor. So it is suggested that film materials should be chosen from olyester film (PET), olyimide film (PI), biaxial oriented olyroylene film (BOPP) Film materials selected from the ermeability asect As we all know, comared with other non-coating films, the thin film coating with metal materials has better gas barrier. While the aluminum lating film is an only film coated with metal, so it is a kind of thin film having the best gas barrier roerty. When the thickness of aluminum lating film is same, the gas barrier roerty comletely deends on the substrate material. So form the ermeability of air and helium, the following film is suggested, that is VMPET, VMPI, VMBOPP Heat sealing erformance of film materials Biaxial oriented olyroylene film (BOPP) is a kind of oriented thin films. The main

5 roblem of the oriented films lies to shrinking when heat sealing. So BOPP is not suitable as the main material of flexible accumulation chamber. Meanwhile, it is difficult to heat sealing for the double-sided aluminum lating film, so it is suggested the single-sided aluminum lating film is used. In summary, the film materials of the flexible accumulation chamber should be selected between a single-sided aluminized olyester film (VMPET) and single-sided aluminized olyimide film (VMPI) Adsortion of film materials Xue Changli (DFH Satellite Co. Ltd) studied the influence of the helium adsortion roerties of some absorbent materials used on the satellites (such as cloth, songe, and multilayer) on the leak testing results [6]. The exeriment result roved that this influence is very small. Through the above analysis, we can obtain the conclusion, single-sided aluminized olyester film (VMPET), single aluminized olyimide film (VMPI) reviously selected can be used as the flexible accumulation chamber materials Mechanical roerties of film material As the material of the flexible accumulation chamber, it should have good tensile strength, toughness and imact resistance. The following table gives mechanical roerties of the aluminized olyester film (VEPET) and aluminized olyimide film (VMPI). Serial number 1 2 Tensile strength MPa Elongation at break % Tab. 1 VMPET, VMPI erformance comarison (thickness of 25 µ m ) Test items aluminized olyester film VMPET aluminized olyimide film VMPI Longitudinal Transverse Longitudinal Transverse Angle tears strength kn/m Imact strength ( J ) From the table, the mechanical roerty of VMPI is better than VMPET Cost comarison The rice of single-sided aluminized olyester film (VMPET) is about 10RMB/m 2, and that of single-sided aluminized olyimide film (VMPI) is 55RMB/m 2. Therefore, VMPET has a better rice advantage than VMPI Summary Considering the gas ermeability, heat sealing, adsortion, mechanical roerties, rice and other factors, the single-sided aluminized olyester film (VMPET) has been ultimately determined as the main material of flexible accumulation chamber.

6 2.2 Research about the influence of volume change on testing result The volume change may affect the testing result from the following three asects: concentration, total gas quantity, the inlet ressure of leak detector Influence of volume change on concentration Helium concentration in accumulation chamber is γ = He atm. 10 where He is helium artial ressure in the accumulation chamber, while atm is the total ressure in the accumulation chamber. According to the state equation of ideal gas, the formula can be written as m M 29 m γ = = = m M 4 m He He a He a a He a. 11 From the above formula, we can find that helium concentration in the accumulation chamber can be written as the ratio of helium mass and total air mass. In fact, if the flexible accumulation chamber is sealed well, the total air mass and the helium mass are both constant. Therefore, its concentration does not change with the volume change of flexible accumulation chamber Influence of volume change on total gas quantity When the volume of flexible accumulation chamber changes, the internal ressure also changes. But the roduct of the internal ressure and volume of the Flexible accumulation chamber is constant, namely the total gas quantity does not change (Boyle s-mariotte s Law). Therefore, the volume change of the flexible accumulation chamber cannot lead the total gas quantity changing Influence of volume change on the inlet ressure of the leak detector According to the rincile of non-vacuum accumulation leak testing method of helium mass sectrometer, we can abstract a mathematical model to study the influence of volume change on the inlet ressure of the leak detector. atm e u Fig. The mathematical model to study the influence of volume change on the inlet ressure of the leak detector

7 When the gas flow in the samling tube balances, we can gain atmse = es where atm, e are resectively the total ressure in accumulation chamber and the inlet ressure of leak detector, S, S e are resectively the uming seed of leak detector and the effective uming seed of leak detector at the remote ort of hose. S u Se = S + u where u is the hose conductance. Through formula 12 and formula 1, we can obtain u e = atm S + u Solving formula 14, e = e atm 15 The formula shows that: the relative change of total ressure in the accumulation chamber is the same with that of the inlet ressure of leak detector. The change of total ressure in the accumulation chamber is about 1000Pa, because of the change of atmosheric ressure and temerature, so the relative change of total ressure in the accumulation chamber is about ±1%. By formula 15, the relative change of inlet ressure of leak detector is ±1%. So this influence is very small, and can be neglected. Considering the above three factors, the volume influence of flexible accumulation chamber on the testing results is very small and can be ignored. 2. Research about the influence of the leak of flexible accumulation chamber on testing result The leak reasons lies to two asects. The first reason is the ressure difference, and the other reason is the concentration difference. Because of the volume changing of the flexible accumulation chamber, the internal and external surface of the flexible accumulation chamber cannot form the ressure difference. But the concentration difference exists The gas transference caused by the concentration difference is defined as the gas diffusion. The rocess of gas diffusion is very slow and deends on the seed of the molecules colliding [7]. The calculation formula of gas diffusion is m D d ρ s t = dx. 16 Where D is the diffusion coefficient, ρ is the gas density, atm s is the unit area, m is the er t unit mass of er unit time. Take an examle for evaluating the influence of gas diffusion on the leak testing result. 4 The leakage rate of sacecraft is 1 10 Pam /s and the cumulating time is 24 hours, then helium mass leaking to the accumulation chamber is = If the effective volume of flexible accumulation chamber is 44.1m, the maximum helium density that has been accumulated after 24 hours is

8 mhe 7 ρhe = = Where ρ He is the biggest density in the flexible accumulation chamber after 24 hours. If its film thickness is 0.1mm, then d ρhe = dx If there is a leak of 4mm 2 in thin film of flexible accumulation chamber, then the leakage leak caused by gas diffusion is m d ρ 1 Q D = = s = t dx 20 Dimension is transformed into P m /s a 7 Q =.1 10 Pa m /s The above formula is calculated under the condition that the helium concentration in flexible accumulation chamber is maximum. In fact, because the helium concentration of flexible accumulation chamber gradually increases and leak section of the flexible accumulation chamber is not greater than 4 mm 2, the above calculation value is greater. Even if, comaring with the leakage rate of sacecraft, it is very small, just only 0.1%. So it can be negligible. In summary, gas diffusion of helium is not the main reason resulting in leaking of flexible accumulation.. Validation test of the flexible accumulation chamber Based on the theoretical analysis above, a flexible accumulation chamber as shown in Fig. 4 has been designed by Beijing Institute of Sacecraft Environment Engineering, and the following validation tests have been done. Fig. 4 icture of the flexible accumulation chamber A channel leak connecting to a gas cylinder as the testing object was carried out leak

9 5 detection with the rigid accumulation chamber, and the total leakage rate is Pa m /s. The same channel leak was also tested with the flexible accumulation chamber, and the results were shown on tab. 2. Tab. 2 The testing results of the flexible accumulation chamber Initial Final Samle cumulati relative Serial Leakage value value value ng time mean error numbe date rate u 1 / u 2 / u / t ( P a m /s ) r ( P a m /s ) ( Pa m /s ) ( Pa m /s ) ( Pa m /s ) s % % % % -.97% % % 022 Note: the negative value indicates that the concentration of the samle gas value is lower than the concentration of standard gas. The actual calculation formula is as follows u2 - u1 Q =.. 22 t u - u2 The test rocess was shown from Fig. 5 to Fig. 6. Fig. 5 The test rocess of rigid accumulation chamber

10 Fig. 6 The test rocess of flexible accumulation chamber From Tab. 2, we can find that the relative error of testing results between the flexible and the rigid accumulation chamber are better than ±10%, and the mean relative error between the rigid accumulation chamber and the flexible accumulation chamber are even better than ±5%. These results directly indicated that the leak testing technique of flexible accumulation chamber is feasible. In summary, the leak testing technique of flexible accumulation chamber has a well stability, accurate testing results, and its sensitivity can also meet the need of leak testing of various sacecrafts. 4. Conclusion Flexible accumulation chamber has a well sealing erformance, and its sensitivity can meet the requirements of various sacecrafts. At the same time, the testing results of flexible accumulation chamber has good consistency with the rigid accumulation chamber, therefore, it can be alied to leak testing of kinds of sacecraft. References 1. J.L.Manganaro, D.L.Hollinger, Quantitative Leak Test Design Guide, Jet Proulsion Laboratory, NASA , J.L.Manganaro, D.L.Hollinger, Quantitative Leak Test Design Guide Final Reort, Jet Proulsion Laboratory, NASA , htt://terra.nasa.gov/events/terrahotos5.h 4. Wang Yong,Shao Ronging,Yan Rongxin,Sun Lichen,Li Zheng,Sun Wei, Imrovements of Non-Vacuum Accumulation Leak Detection by helium Mass Sectroscoy, Chinese Journal of Vacuum Science and Technology, VOL 2 NO 2, , Zhang Yishen,Huang Ying, Vacuum Materials, Beijing:Metallurgical Industry Press, Xue Changli,Liu Weiqing, Exeriments on helium adsortion caability of materials used in satellite assembly, Sacecraft Environment Engineering, VOL28 NO, , Da Daoan. Handbook of Vacuum Design The third edition, Beijing:National Defense Industry Press,2004.