Innovative Reliability Excellence in Helium Gas Leak Testing by Vacuum Method

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1 ISSN (ONLINE): , ISSN (PRINT): Volume6, Issue1, JanuaryFebruary2016 International Journal of Engineering and Management Research Page Number: Innovative Reliability Excellence in Helium Gas Leak Testing by Vacuum Method T. Gurunathan 1, A.NoorulHaq 2 1 Research Scholar, Department of Production Engineering, National Institute of Technology, Tiruchirappalli, INDIA 2 Professor, Department of Production Engineering, National Institute of Technology, Tiruchirappalli INDIA ABSTRACT In Helium gas leak testing, reliability is an imperative factor and improving this is challenging. One of our innovative experiences is shared in the form of a case study in this paper. Heat exchanger for a process plant shall have to be Helium leak tested by vacuum method for the Quality review of weld joints between ring header and straight tubes. Upon conducting the test, all open ends of other side of tubes were sealed using plugs and vacuum was created. A polythene hood covering all the weld joints of ring header and tubes was sprayed with Helium. Helium from hood passed out and moved up due to its high diffusivity. The diffused Helium could find leak paths in the open ends of the tubes with in the seal plug system and entered into the vacuum side. Being a small tube, that too, many in numbers, with no inner diameter control and close pitched condition, complete Helium tightness of seal plugs could not be thrived. Also, permeation of Helium through rubber seal of sealing plugs could not be avoided. Due to this, substantial amount of Helium went into vacuum system when test joints were sprayed with Helium and these were revealed as spurious signals. In order to resolve this, a dummy polythene envelope of gaseous Nitrogen was provided covering all other ends of tubes, including seal plugs. This nitrogen blanket has avoided spurious signals and improved the test reliability. Thus, Response time aspect of leak tests was suitably deployed to obtain reliable test results in a typical Industrial Application by combating the specious signals effectively. Keywords Helium gas, Reliability, Diffusivity I. INTRODUCTION For critical engineering equipments, to certify the reliability of manufacture and installation leak test is conducted as per customer and code requirements. Though, proof tests hydrostatic [1] or pneumatic tests are conducted for certifying complete structural integrity of the manufactured product while the product is subjected to loads, these are not capable of determining fine levels of light fluid leakages through pressure boundary.the sensitivity of proof testing is restricted to 1X10 5 Pa.m 3.s 1 only. Certifying the pressure vessel for the capability of having the leak tight pressure boundary by using light tracer gases (higher sensitivity tests) is a part of reliability certification for the vessel for its future operation while handling fluids which are normallyheavier than those used in leak testing. Ofthe available leak testing methods [2], Helium leak testing is the most sensitiveone as Heliumis the lightest gas next to hydrogen [3] with a molecular diameter of just 1.x10 10 m. Of the two principal types of Helium leak testing, viz, pressure testing and vacuum testing, the experience sharing discussed in this paper is regarding vacuum testing. Being a functional test meant for the certification of reliability of Engineering product, the complete test methodology shall be totally reliable. The reliability in terms of repeatable test results with accurate leakage rate measurementand leak site identification are critical andon this, number of test variables are to be managed successfully. Varying industrial applications demand for the development of novel and innovative techniques to enhance test reliability. There are situations wherein spurious signals are inevitable and one of our practical experience on successfully combating the same is shared ascase study in this paper. Response time is one of the useful concepts followed up in vacuum testing to practically conclude regarding test results. The evacuated system along with leak detector should react as rapidly as possible (quick response) when leak is probed. If response time is poorer, then, there will be always time delay in locating leak. Because of this, spray probe location will differ with actual leak location. Actual leak location will lag behind spray probe location. If volume of vessel is Vm 3 and effective pumping speed 503 Copyright Vandana Publications. All Rights Reserved.

2 ISSN (ONLINE): , ISSN (PRINT): for Helium is Sm 3 s 1 then V/S is called Time constant T R for the pumping system. This would mean that Helium would have to be over the leak for this time T R (without undergoing diffusion to other areas) called response time for obtaining a sufficient response on leak detector.the signal response up to time constant is normally 63% of maximum realizable signal and generally, time versus response in real leaks standard leaks and actual test leaks exhibit exponential growth. To detect the spurious signals in Helium leak tests and then further to validate the successful elimination of such signals using an innovative technique, the time versus response plots have been conveniently used in this case study. A special and very important product was successfully leak tested with very good reliability and accuracy to satisfy customers. This experience is shared in this paper. II. OUR EXPERIENCE ON RELIABIL ITY IMPROVEMENT 2.1 Product details and Testing Setup A process plant heat exchanger subassembly as shown in Figure 1 was Helium leak tested by vacuum method. The completely austenitic stainless steel heat exchanger subassembly was made of straight tubes (Outer Diameter.53x10 3 m and 1.65x10 3 m thick) 53 numbers welded to a ring header (of pipe of cross section outer diameter 2.8x10 2 m and thickness 3x10 3 m with header wheel diameter m) at one end and to a tube sheet at other end. In tube sheet end, all 53 tubes would be inserted into holes of tube sheet and seal welded during further course. Ring header to tube joint was a single layer manual Gas Tungsten Arc Welded joint of 2x 10 3 m fillet size and had to be Helium leak tested before making tubes to tube sheet seal welds at other end. Two side nozzle ports were available in ring header which facilitated connection of vacuum pump cum leak detector and a standard leak for system sensitivity check (its location matched with typical test joints) respectively as shown. For leak test, a volume enclosing all tubes and ring headerwas first evacuated. To create vacuum, the free ends of tubes projecting after tube sheet (other ends are inserted into ring header and got welded) neededto be sealed first.joint by joint testing by directly evacuating through the concerned tube was not practicable. This also demanded sealing of remaining tubes free ends and other ports. The perfect way for sealing tubes free ends was fusion welding using caps and removal of caps after completion of test. This warranted additional allowance on tube free ends and this was not there. Hence, special sealing plugs using neoprene rubber ( o ring type) were organized suiting to inner diameter of tubes and all 53 ends were closed using sealing plugs. A desired vacuum was then generated in test volume. 2.2 Problem faced In this condition, the vacuum of 8x10 3 Pa was created in test space and for about 1800 seconds vacuum holding [4] was satisfactory. Then, leak detector was tuned and detector s sensitivity was checked. Then, system sensitivity check was performed by sending Helium from standard leak connected to job. In just seconds standard leak s leakage rate was realized by leak detector as 1) the job was clean, 2) the volume of evacuation was comparable to pumping speed of leak detector and 3) Helium flow from standard leak to detector was straight. Figure 2 gives the leak detector s response VS elapsed time during system sensitivity check which depicts the typical nature of exponentially growing response for flow from standard leak against time. (Value of standard leak =6.2X10 Pa.m 3.s 1 ).In about 300 seconds, 63% signal with respect to the standard leak s value was obtained. In about 1500 seconds, 100% signal was obtained. Therefore, system exhibited 300 seconds as response time (T R ) and so1800 seconds (more than 5T R )was kept as leak test duration during actual Helium spray. After system sensitivity check, after bringing down background level to as low as 1X10 10 Pa.m 3.s 1, Helium was sprayed into the polythene hood provided around test joints. For 600 seconds (2T R ), there was no signal on leak detector. Reading was in the range of 1X10 10 Pa.m 3.s 1. Then, leakylike indications started appearing on detector andit was progressively increasing at steep rate. In next 600 seconds, leakage rate on detector exceeded acceptance criteria of 1X10 7 Pa.m 3.s 1.Further, it was with raising trend as shown in Figure 3. Normal Helium spray situation response curves were expected as parallel curves to response curve obtained in system sensitivity check (Figure2) as alike behaviour would be seen in both real and standard leaks. 504 Copyright Vandana Publications. All Rights Reserved.

3 ISSN (ONLINE): , ISSN (PRINT): The phenomena of totally different response curve in actual test was puzzling and has brought out the fact about the appearance of some other signal s response after 600 seconds. After detailed brain storming, it was attributed tospurious signal pickups. The appearance of spurious signals has made the test stopped without reliable completion.relevant signals from job were masked. 2.3 Analysis of the problem A careful observation brought out the fact regarding possible source of spurious signal. This was due to inevitablehelium leak from polythene hood around test joints [5], itsquick rise in open air due to high diffusivity and then its entry through leak paths existed around some of the sealing plugs provided at tubes free ends. Though plugs were air leak tight, they were not Helium leak tight and allowed comparatively lighter Helium molecules to enter in larger quantities. Getting all 53 sealing plugs in Helium leak tight condition and keeping them undisturbed for full test duration could bedifficult. Along with this physical leak paths, the permeation (where in gas flows through a membrane with no holes) through rubber seal of sealing plugs also would have caused leakage. It was then decided to do conceptual validation for this leakage in the seal plugssystem resulting in copious entry of Helium as spurious signals. Sniffer probe passed in the region from hood outside to tubes other end side in air revealed a positive signal. Then, a nitrogen spray was made over sealing plugs zone and a deflection in leak detector reading was observed and this could be due todeflection of Helium temporarily by Nitrogen flow (indicated as fall in reading). Once Nitrogen was stopped, the dilution effect on Helium came down and reading raised. A direct Helium spray on sealing plugs revealed quick increasein reading. All thesehave qualitatively confirmed that the leakage rate beyond the acceptance criteria (due to spurious signals) was because of physical leaks and permeation phenomenon of sealing plugs.to eliminate spurious signals,as first step, it was decided to find out its quantitative leakage rate. The test volume was evacuated again after reducing back ground signal. The pump was isolated with test space vacuum of 8X10 3 Pa.Vacuum readings were recorded for 1800 seconds. Loss of vacuum in 1800 seconds = 2.5x10 2 Pa was seen. The inside volume of job of 53 tubes (each 4 m long)and a ring header = 7.0X10 3 m 3. Global Leakage rate = [(loss of vacuum x vacuum side volume) / time] Substituting the values, we got leakage rate = [(2.5X10 2 X7.0X10 3 )/1800]=.7x10 8 Pa.m 3.s 1 of air = 2.6x 10 7 Pa.m 3.s 1 of Helium. Then, pumps were run for 600 seconds to improve vacuum and then isolated. Helium was sprayed on seal plugs. Immediately, the leak detector recorded2.2x10 7 Pa.m 3.s 1 of Helium and it was also further rising up. Based on the almost close equivalence of leakage rates in both vacuum loss test and Helium spray test,the global leaky condition of sealing plugs with a leakage rate of more than 2.6X10 7 Pa.m 3.s 1 of Helium was clearly confirmed which was more than acceptance criteria. For further calculations, let us take this as 3.0 x 10 7 Pa.m 3 s 1. Basically, hood around test joints could not be made tight and Helium passed out of hood and raised up due to diffusion. The diffusivity of Helium through air =6.7x10 5 m 2 s 1 [6]. From hood, free ends of tubes were at 4 m distance. Assuming that the seal plug existed straight above in the same tube s end (from where Helium pass out occurred) had allowed Helium leakage, then the governing area for diffusion action would be 4 mx0.01 m (4m length of tube in one direction and 0.01 m = 10 mm approximate diameter of tube in other direction). Then, the time required for diffusion after covering 4 m x 0.01 m (0.04 m 2 ) would be the time required for Helium leakage through seal plug leak =0.04/(6.7X10 5 ) = 573 seconds. In actual test also, after 600 seconds only, a sudden increase in reading was observed and the timing matched with diffusivity of Helium through air. 505 Copyright Vandana Publications. All Rights Reserved.

4 ISSN (ONLINE): , ISSN (PRINT): The nature of gas flow in vacuum tests is molecular type[6] as per mathematical theory on flow through leaks. Equation for this flow is Q = [.637 r 3 / l] [T/M] 0.5 [P 1 P 2 ] (1) Where Q is flow ratethrough leak in Pa.m 3 s 1, r = radius of leak (physical) in m, l = length of leak path in m, T = temperature of gas in ºK, M = molecular weight of gas in kg/mol and P 1 & P 2 are upstream and downstream pressures in Pascals across leak. Using this, for any leakage rate (Q) in molecular region, we can estimate the physical size of leak (r). For the maximum global leakage rate of 3.0x10 7 Pa.m 3 s 1 (Q) obtained as leakage throughsealing plugs, the equivalent leak or equivalentcumulative through pore sradius r is obtained using the equation (1). A leak path length of l of 1 mm 1 x10 3 m is considered. P 1 = Pascals& P 2 = 0 Pascals, T=28 K. M = 0.004kg/mol. Substituting the values and solving for r, we get r = 1.04 micron (1.04x10 6 m) as radius of equivalent leak and therefore diameter of equivalent cumulative leak path causing the flow exceeding the acceptance level is 2.08 microns (2.08 x 10 6 m). 2.4 Scope of development work The following are listed as major issues in this test which resulted in spurious signals. 1. Helium hood was not holding Helium without pass out for the complete testduration. 2. Diffusion rise of passed out Helium and its consequent reach to tubes free ends. 3. Physical leaks and permeation in seal plugs system resulted in Helium entry into vacuum system as spurious signals. 4. The spurious signal level was more than acceptance criteria resulting in excess background signal. 5. The requirement of minimum 1800 seconds as test duration with lower back ground to detect finer leakages was not achieved. 6. Ultimately, this led to total inability to perform test. 2.5 Series of options and experiments for arresting spurious signals and their results % Helium leaktightsealing plugs system Design modification on sealing plug system for controlling leakage due to physical leaks and permeation leaks was not successful Welded end covers at tube free ends orseal welding of tube ends after pinching Such welds were objected by designer and hence this option was dropped Provision of Heliumpass out free spray hood covering the test joints With polythene sheets and cellotapes normally used for making hoods, this option has been found to be an unsuccessful one Helium leak test in header up and tube free ends down condition As tendency of Helium is to rise up always in atmosphericair, being a lighter gas, it was thought that it would be appropriate to try a test in header top and tubes down condition as shown in Figure 4. But results were not encouraging, as there has been uniform rate of diffusion of Helium in all directions including downwards Dilution of diffused Helium by Nitrogen gas flowor forced air from fan Gentle spray of nitrogen continuously around plugs area for the test duration, application of air on entire tubes free end area by turning on heavy duty industrial fan and blasting of nitrogen from multiple jets continuously around plugs zone were tried one by one. In all this, momentary improvements were only seen Provision of enveloping nitrogen hood surrounding the Helium hood As shown in Figure 5, we had made an encircling nitrogenhood surrounding the Helium 506 Copyright Vandana Publications. All Rights Reserved.

5 ISSN (ONLINE): , ISSN (PRINT): hood to dilute raised up Helium. There was only a delay of another 5 minutes to see spurious signals. Significant and stronger effect of diffusion of Helium surmounting compressing effect of nitrogenwas evident Local Helium spray on individual joints one by one without any holding hood It was planned to complete testing with little spray of Helium using local spray guns only on ring header to tubes joints one by one. It had lowered relevant signals also. But spurious signals did not show any improvements. Further, local Helium spray was modified by adding an encircling nitrogen spray simultaneously with Helium. This also resulted in reduction of relevant signal due to dilution.marginal improvementsonly were seen regarding spurious signals Conducting Helium leak test with pumps not isolated The relevant signal has reached as low as 1% in pumps running condition compared to isolated condition and hence this was dropped Useof a suction manifold of vacuum cleaner for diffused Helium collection Collection of diffusion raised Heliumfrom tube free end areas using suction hose of vacuum cleaner was tried. In this trial also.marginaland momentary improvements only were seen Useof a polythene hood around tube free ends to minimize reach of Helium into seal plug leaks As shown in Figure6, a polythene envelope around the tube free ends to restrict the raising Helium reaching the seal plugs area was tried. This has increased background rise time from 10 minutes to 20 minutes only. 53 sealing plug ends as shown in Figure 7. It was improved version to Figure 6.Into this envelope covering tube ends, nitrogen was filled. The physical barrier of dummy envelope of nitrogen would prevent Helium entry into this envelope. Nitrogen presence in the envelope would also dilute traces of Helium entering. Also, nitrogen being relatively heavier, when it comes out of inevitable envelope leaks, it would have a tendency to flow in the downward direction (against rising Helium) and this would also reduce the quantity of Helium entering into this envelope. Also, from the nitrogen blanketing, we expected considerable delay for the Helium to enter into vacuum side Final solution arrived by using a dummy polythene envelope of Nitrogen around tube free ends At this juncture, a dummy polythene envelope of Nitrogen was thought of and it was provided covering all The diffusion coefficient of nitrogen is nearly one fourth of that of Helium. (Diffusion coefficient of Helium = 6.7x10 5 m 2 s 1 & that of nitrogen = 1.75x10 5 m 2 s 1 ). Therefore, in the filled up envelope, the presence of nitrogen would be maintained for longer durations without diffusion spread to other areas through envelope leaks. This presence would give a suppression effect to entering diffusion raised Helium and therefore, it would take longer time for Helium to reach seal plug areas. The lower diffusivity gas would suppress (by its physical presence as barrier) higher diffusivity gas from penetrating through. So, a combined effect of 4 factors, viz, 1)physical barrier of nitrogen provided by envelope, 2) dilution of Helium by nitrogen in envelope, 3) delaying entry of raised Helium (in small quantities only) into envelope by downward flow of nitrogen and 4)suppression effect of lower diffusivity nitrogen on higher diffusivity Helium were expected to render much improved results. Further, to maintain nitrogen indummy envelope throughout the testing period, for every 5 minutes, nitrogen was planned to be filled. This would ensure the availability of nitrogen blanket always around the vicinity of seal plugs. Adoption of this concept of having two hoodsone 507 Copyright Vandana Publications. All Rights Reserved.

6 ISSN (ONLINE): , ISSN (PRINT): Helium hood around test joints and another dummy hood of Nitrogen around seal plugs area has avoided spurious signals effectively. With two hoods in place as explained above, we repeated the test for 1800 seconds(test duration). There has been very slight increase in Helium signal (leakage rate in the detector) at the end of 1800 seconds (in the scale of 1x Pa.m 3.s 1 whereas1x10 7 Pa.m 3.s 1 is acceptance criteria).to confirm leak tightness of the product, for another 1800 seconds, leak detector readings were observed and the rise was restricted to 1x10 10 Pa.m 3.s 1 scale only.thus, spurious signals elimination was demonstrated for more than 1 hour effectively.the improvements obtained based on this novel concept are indicated below in Figure8 in the form of very slow rise of leak detector reading. III. VALIDATION OF NEW CONCEPT The objective for this was fixed as consistent data generation on almost no back ground signal status in the leak detector during a testing period of 3600 seconds by suppression of raised up Helium by nitrogen hood. 3.1 Preparation for Validation Experiments This was planned on stainless steel tube of outer diameter.53mm and thickness 1.65 mm. welded to a ring header segment. Initially, one of the tube end was pinched, seal welded and this joint was first certified for Helium tightness against 1x10 10 Pa.m 3.s 1. Afterwards, one of the open end of header segment was welded with cover and this joint was also certified to same level ofhelium tightness. Then, tube hole was drilled on header segment and tube was welded. Other free end of the header segment was amenable for connection to leak detector as shown in Figure through a flexible hose and adopter. The flexible hose with adopter to header segment free end was also certified for same level of Helium tightness. Possibly, some nitrogen was expected to enter into vacuum system from the nitrogen envelope through the sealing plug system. To seewhether there is any change taking place to response time because of this, system sensitivity check was repeated. Results are given below. 1. System sensitivity check 100% realisation of signal from standard leak was obtained in 1500 seconds. 2. Response time =300 seconds (63% signal) 3. Sensitivity of leak test =1.6X Pa.m 3 1.s 4. Back ground reading before Helium spray =1 X Pa.m 3.s Leak detectorreading after Helium Spray in Helium hood = 1 X Pa.m 3.s Leak detector reading after 1800 seconds = 1.0 X Pa.m 3.s Leak detector reading after 3600 seconds = 1.15 X Pa.m 3.s 1. Thus, we concluded that there were no effects due to spurious signals and accepted all 53 tubes to ring header weld joints in the global test against acceptance criteria of 7 1x10 Pa.m 3.s 1. Even though successful results were obtained by complete elimination of spurious Helium signals, to adopt this new concept as an institutionalized process in multiple jobs, it was decided to validate the concept based on series of planned experiments as listed below. The header segment to tube weld joint (simulating test joint) was subjected to Helium leak test by spraying Helium in hood and a Helium tightness of better than 1x10 10 Pa.m 3.s 1 was obtained. In this test, there was no possibility for spurious signals issue as we had properly seal welded tube ends on top side of tube. Physical leak paths were totally eliminated. Also, as there were no joints with rubber o rings, permeation was also eliminated. After this successful test for the simulated test joint, we had cut the top end of tube to remove the seal weld so that a open tube top end amenable for closing it with seal plug system similar to our actual test condition was obtained. 3.2Details of ValidationExperiments Then, the tube s free end was fitted with seal plug. The test joint was header to tube joint and was covered in the Helium spray hood as shown in Figure10. We have chosen this joint for the validation of diffusion rise of Helium and its suppression by dummy nitrogen envelope made around the other end (seal plug end). 508 Copyright Vandana Publications. All Rights Reserved.

7 ISSN (ONLINE): , ISSN (PRINT): The adopter had provisions for connecting standard leak (for system sensitivity check) and simulated leaks. Simulated leaks of known leakage rate were prepared from pinched injection needles [7]. These leaks were provided closer to test joint in the validation experiments in order to study the effectiveness of dummy nitrogen envelope to contain spurious signals while revealing relevant signals of the simulated leaks in a reliable, exactly quantifiable and stable manner under same conditions applicable for actual test. The standard leak of value 6.2 x10 Pa.m 3.s 1 and one simulated leak of 4 x 10 Pa.m 3.s 1 were connected to adopter in the first trial. X Axis sensitivity check with standard leak of 6.2 x 10 Pa.m 3.s 1 ) Y Axis Upper curve spray in the test hood having simulated leak of 4.0 x 10 Pa.m 3.s 1 ) Y Axis Lower curve ( T R ) 3.0 (63%) 1.88 (63%) (T) 6.20(100%) (100%) ** ** Dummy nitrogen envelope was removed after 1 hour. In just 10 minutes detector reading shot up to much higher levels System sensitivity check was conducted with presence of dummy nitrogen envelope in which response time was obtained as TR=300 seconds and full test time T was obtained as seconds. Then, when back ground reading was at 1x10 10 Pa.m 3.s 1 after filling dummy envelope with nitrogen, Helium was sprayed into test hood. Through simulated leak, Helium went to vacuum side and leakage rate was recorded for every 300 seconds up to 1 hour. Helium concentration was estimated [8] to be 75% in test hood. Figure11 gives the summary of both responsesleak detector readings during system sensitivity check and readings during the Helium spray in test hood due to leakage through simulated leak. The table also gives the sudden shoot up of ultimate stable leak detector readings while removing dummy nitrogen envelope to a level much higher than the value of simulated leak (to the extent of 2x10 7 Pa.m 3.s 1 ). This is clearly due to onset of spurious signals resulting in total loss of test reliability. Figure 11 Summary of Responses Elapsed Leak detector Leak detector Time in reading in 10 reading in 10 seconds Pa.m 3.s 1 (System Pa.m 3.s 1 (Helium If we see the table, it is clearly proved that the response due to sprayed Helium on simulated leak is exactly in line with response for standard leak. Further stable sustenance of reading clearly demonstrates that there are no spurious signal entries and therefore capability of dummy nitrogen envelope in containing spurious signals has been proved satisfactorily. Summarised results of this experiment : 1. Response Time (T R )=300 seconds for 63% signal pick up during system sensitivity check and Helium spray on simulated leak. 2. Testing time (T) = 1800 seconds(for both) % of signal was realized in 1800 seconds. 4. Sensitivity of leak test = 1.6x10 11 Pa.m 3.s Helium concentration of 75% was maintained in hood. 6. Global leakage rate of the simulated leak [] = leak detector reading in 1800 seconds during He 50 Copyright Vandana Publications. All Rights Reserved.

8 ISSN (ONLINE): , ISSN (PRINT): lium spray X factor to account for Helium Concentration in test hood X factor to account for system sensitivity. = 3 X 10 X1.33X1= 4 X10 Pa.m 3.s 1. = Known Leakage rate of the Simulated leak. 3.3Some more Validation Experiments These validation experiments as explained in 3.2 were repeated using different simulated leaks. The leakage rates of these leaks were : 8 1) 5.2 X 10 Pa.m 3 1.s 2).6 X 10 Pa.m 3 1.s 3) In this trial, all three leaks were connected to the adopter. Concentration of Helium estimated in these 3 tests were 80%, 75% and 75% respectively. The summary of leak detector readings obtained in these 3 trials are given below in Table 1. Summarised results on global leakage rates obtained in these three experiments are given in Table2. Response time = 300 seconds and testing time = 1800 seconds for all 3 experiments. In the Table2 above, we can clearly see the equivalence of known leakage rates of simulated leaks and the corresponding calculated leakage rates obtained based on these experiments. This clearly validates the successful deployment of dummy nitrogen envelope concept or nitrogen blanketing concept for the effective elimination of spurious signals in order to enhance the test reliability. Elapse d Time in second s 1) Leak detector reading in 10 Pa.m 3.s 1 (Helium spray in the test hood having leakage of 5.2 x 10 8 Pa.m 3.s 1 ) Table 1 2) Leak detector reading in 10 Pa.m 3.s 1 (Helium spray in the test hood having leakage of.6x 10 Pa.m 3.s 1 ) 3) Leak detector reading in 10 Pa.m 3.s 1 (Helium spray in the test hood having all the three total leakages 6.56 x 10 8 Pa.m 3.s 1 ) (T 26.3(63%) 4.5(63%) 31.0(63%) R) (100%) 7.2(100%) 4.2(100%) (T) Experiment No. Simulated leak s known leakage rate Factor to account for System Sensitivity Factor to account for Helium Concentration Stable leak detector reading at the end of Testing time T Global leakage rate of simulated leak (calculated) IV. Table 2 1) 2) 3) 5.2 x 10 8 Pa.m 3.s 1.6 x10 Pa.m 3.s x 10 8 Pa.m 3.s x10 Pa.m 3.s x 10 8 Pa.m 3.s x10 Pa.m 3.s.6 x10 Pa.m 3.s 1 1 CONCLUSIONS 4.2 x10 Pa.m 3.s x 10 8 Pa.m 3.s 1 Theoretical concepts on response time in Helium leak tests and diffusion behavior of Helium and Nitrogen were practically used for an industrial problem solving to enhance test reliability. The following major issues listed in this article were successfully managed in order to control spurious signals in Helium leak test. 1. As Helium pass outs are inevitable in regular Helium spray hoods and they are accepted with a sound judgement to control the same by other means subsequently. 2. Diffusion [10] rise of passed out Helium and its consequent reach to the tubes free endsare also inevitable (due to the physical property of Helium)andthey are also accepted with a sound judgement to suppress the diffusion raised Helium from entering into vacuum side (through seal plugs system s physical and permeation leaks)by some other means subsequently % leak tightness of all the tubes sealplugs for the entire test duration would be difficult and also permeation through sealing system is unavoidable. They are not matters of concern to us as there is going to be non availability of Helium in the vicinity of seal plugs area due to the suppression by nitrogen envelope. 510 Copyright Vandana Publications. All Rights Reserved.

9 ISSN (ONLINE): , ISSN (PRINT): Helium entry into the vacuum system as spurious signals through the seal plugs has been completely avoided by suppressing the Helium with the help of Nitrogen envelope. 5. As no Helium could enter through seal plugs due to the presence of dummy nitrogen envelope, amount of spurious signal has been maintained at zero level providing excellent sensitivity and reliability levels achievable for the actual test of the job against the specified acceptance criteria based onrendering very low background leak detector reading. Nitrogen blanketing by means of dummy envelope for masking Helium signals raised up due to inevitable nature of high diffusivity [10] of Helium has resulted in elimination of spurious signals in this novel approach. A challenging technical problem in leak testing for the establishment of effective, reliable and foolproof methodology for global leakagerate Quantification was successfully attended and this opportunity is viewed with a sense of pride amongst our team. Customers of these products haveexpressedhappiness, confidence and satisfaction based on repeatable and reliable test results obtained. leak detector or residual gas analyzer in the hood method; July, [10] ASTM Standard E ; Standard terminology for Non destructive Examinations. REFERENCES [1] ASTM Standard E ; Standard test method for Hydrostatic Leak Testing; December [2] Andrej Pregelj, Martin Drab, IEVT, Teslova 30, Ljubljana MiranMozetic, ITPO, Teslova 30, Ljubljana; Leak detection methods and defining the sizes of leaks; NDT.net, February 1;4(2). [3] TQC; Helium leak testing; Guide to the fundamentals of Helium leak testing. [4] TQC; Leak testing Getting down to the fundamentals of leak detection & measurement; Medical device technology ; May [5] Lycan ; Paul Dale (Brooklyn, MI) &Schellenberg;Aaron Thomas (Canton, MI) ; Patent # Patent Genius ; Helium leak tester for vehicle fuel tanks ; July 1. [6] Charles N. Jackson Jr, Charles N.Sherlock& Patrick O.Moore ;Nondestructive testing hand book, Volume1, Leak testing, Third edition; American Society for Nondestructive Testing ASNT ; 18. [7] T.Gurunathan,R.Thiagarajan&AR.Subramanian Paper on Quantitative evaluation of leak rates in pressure type Helium leak tests a new methodology for system calibration ; 14 th World Conference on NDT, New Delhi, India ; December16. [8] T.Gurunathan, Samuel Wilfred, S.Arunagiri& S.K. Mazumder Paper on Measurment of Helium concentration in hood type Helium leak testsa technique for enhancing the test reliability. ; National NDT Seminar, Trichy, India ; December 12. [] ASTM Standard E 1603/E1603M11; Standard practice for leakage measurement using the mass spectrometer 511 Copyright Vandana Publications. All Rights Reserved.