National Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014) Vol.20 No.6 (June 2015) - The e-journal of Nondestructive Testing - ISSN 1435-4934 www.ndt.net/?id=17898 Helium Mass Spectrometric Leak Detection In Large Size Process Plants Venkat N. Ramani Plasma & Vacuum Technologies, Plot No 17, Road 1/A, GIDC Kathwada, Ahmedabad, India vnramani@gmail.com Abstract. Helium Mass Spectrometric Leak Detection has evolved over the years as a very reliable and highly sensitive technique for the detection of leaks in industrial process plants. When the method is applied on-line in a process plant, the process gas, temperature and vacuum levels present serious problems for reliable and sensitive detection of leaks. This paper addresses a specific technique of utilizing a specially designed permeator to solve this problem and apply this sensitive technique for on-line detection of leaks in process plants operating in vacuum atmosphere over a wide range. Keywords: Leak Detection, He-MSLD, Helium Leak detection, on-line leak detection. Introduction Helium Leak Detection using Mass Spectrometric technique is a Leak Testing method in NDT [1]. It is attributed as the ultimate technique for sensitive leak detection. Laboratory experiments conducted all over the world have shown that leak rates as low as 10-12 Pa m 3 /s or even less can be measured using this method [2,3]. The application of this technique to laboratory and small vacuum systems has been amply exploited. In this technique of leak detection, the Helium Leak Detector is connected to the vacuum system and Helium is sprayed at the test points using a gun. The application of this technique to the industrial systems and process plants has grown over the last decade as it offers a highly sensitive and very reliable method for the detection of leak. Helium Leak Detection in a pressurized system is achieved using Helium Sniffing method. The vessel is pressurized with Helium, fully or partially, and the escaping helium from the fault locations is detected by a sniff probe connected to the Helium Mass Spectrometric Leak Detector. The detection sensitivity or more appropriately the limit of detection is lower in sniffing method as compared to spray method, which can be easily understood as due to the ambient pressure condition and the prevailing concentration of helium in atmosphere. The sensitivity of Sniffing method does not depend on the size of plant, provided the helium pressure in the vessel is maintained same. However, the high consumption of helium is a point of consideration and this concern is mitigated to some extent by use of allowed mixture of Nitrogen and helium. Background helium level sensed by the detector is also a point of consideration. This aspect, more often than not, determines the limit of detection. Yet another point of consideration in sniff method is that it can be carried out only offline. Hence sniff method is almost always used to detect leaks in vessels prior to installation or after detaching the vessels from installation. On the other hand, the spray method can be used on-line, provided the sensitivity and response time of detection are adequate for the relevant range of leaks.
The application of Spray method to large sized systems faces a complex problem due to low sensitivity and large response time. Further, when the method is applied in a process plant, the process gas, temperature and vacuum levels compound the problem of reliable and sensitive detection of leaks. In this paper, we present a specific technique of utilizing a specially designed permeator which resolves these problems for the application to on-line detection of leaks in process plants operating in vacuum atmosphere over a wide range. When a Mass Spectrometric Helium Leak Detector is connected to a vacuum system, a background helium signal is observed, which is due to the flow of residual helium from the system through the leak detector. The magnitude of the background signal depends on the vacuum level and generally it is very negligible (about 10-9 Pa m 3 /s or less) in vacuum systems operating at 1 Pa or below. However this value becomes very high (about 10-5 Pa m 3 /s or high) when the vacuum level is about 100 Pa or more. Thus it reduces the sensitivity of detection and limits the application of the technique. When a permeator is placed in the path connecting the Leak Detector to the system, relatively more helium flows to the Detector than the remaining gases. This reduces the background, increases the sensitivity of detection and eases the application of the technique over a large range of vacuum. Experiments were conducted with three specially designed permeators and the observed results on sensitivity and its variation over a wide vacuum range 1000 to 10-3 mbar. The experiments and obtained results are discussed in the paper. Development of Technique The development of technique involved 2 aspects : (i) developing permeators; and (ii) establishing a set-up for the study of the performance of the permeator. The experimental set-up established is given in figure 1.
The development of permeator involved making a diaphragm or a film, uniformly thick, having a reasonable area, through which adequate helium can permeate while the other gases like oxygen, nitrogen, etc. find it as a barrier. After many trials with thin metallic membranes (made of inconel, aluminium, copper), thin quartz disc, and various elastomer discs, it was decided to use three discs, made of PTFE, with a diameter ~ 20 mm for this study. The thickness of first permeator was ~150 microns, second ~100 microns and third ~50 microns. The discs were mounted inside a stainless steel cylindrical pipe having standard KF25 end flanges. The experimental set-up was designed to provide a typical situation that is envisaged in process systems. The vessel used is a large rectangular vacuum furnace chamber (1.5 m x 0.6 m x 1.2 m height) and it was pumped using a Roots Pump (Hind High Vacuum make, RD500) and Rotary pump (Hind High Vacuum make, CD120) combination. The ultimate vacuum achievable in the chamber was better than 10-3 mbar. A combination of 2 capacitance manometers (Alcatel make ASD2001 and ASD2004) measured the vacuum level over the range 1000 to 0.001 mbar. With the help of variable leak valve V1, any vacuum in the range of 1000 to 10-3 mbar could be steadily maintained in the chamber. The permeator was connected to the pumping line through an isolation
valve V3. The other end of the permeator was connected to a mass spectrometric helium leak detector [MSLD] (Alcatel make, ASM310) through an isolation valve V6. A standard calibrated leak-1 (Hind High Vacuum make, Model HSL 101, 2.6 x 10-8 mbar l s -1 ) was used for the instrument calibration of the Helium Leak Detector. Using vacuum lines with valves V2, V4, V5 and V6 as shown in schematic, and using the Rotary Vacuum Pump 2 (Hind High Vacuum make, Model ED6) along with Pirani Gauge 2 (Hind High Vacuum make, Model DHPG-222), appropriate vacuum levels were created for connecting the Standard Leak and Permeator to MSLD. Another calibrated leak Standard Leak-2, was connected to the chamber. At low vacuum (> above 0.1 mbar) and high helium background values (> 10-4 mbar l s -1 ), the standard leak-2 used was of a higher value (Alcatel make, Model FV3, 1.0 x 10-4 mbar l s -1 ). At high vacuum (below 0.1 mbar) and low helium background values (< 10-4 mbar l s -1 ), the standard leak-2 used was of a lower value (Hind High vacuum make, HSL101, 1.0 x 10-6 mbar l s -1 ). This standard Leak is equivalent to a typical leak in chamber. Using vacuum lines with valves V7, V8 and V9 as shown in schematic, and using the Rotary Vacuum Pump 1 (Hind High Vacuum make, Model ED6) along with Pirani Gauge 1 (Hind High Vacuum make, Model DHPG-222), appropriate vacuum levels were created for connecting the Standard Leak-2 to chamber without affecting the vacuum level in chamber. Experiment and Observations The Instrument calibration was first performed on MSLD using the standard leak-1 and the sensitivity of the instrument was noted to be better than 1.0x10-9 mbar l s -1. Permeator-I (150 micron thick diaphragm) was introduced in the vacuum circuit at the location shown in the schematic. With this permeator, it was found that the MSLD could be operated at around 300 mbar. The background level in MSLD was ~ 8.0x10-8 mbar l s -1. When the standard leak-2 was introduced into the chamber, the observed signal was ~ 8.1x10-8 mbar l s -1. The standard leak value was 1.0x10-4 mbar l s -1, and the sensitivity was evaluated as 1.0x10-5. The experiment was repeated at vacuum levels 100, 10, 1 and 0.1 mbar. On the high vacuum side, below 0.1 mbar no helium signal was detected, when the calibrated standard leak-2 was introduced. The observed background signal and sensitivities are plotted in figure 2. The permeator-i was replaced by Permeator-II and the experiment repeated. The observed background signals and sensitivities are plotted in figure 2. Please note that with Permeator-II, MSLD could be turned on only after 100 mbar vacuum was reached in chamber. Also it was found that the background decreased as the chamber vacuum level improved and the sensitivity factor was more than that for Permeator-I. On the high vacuum side, below 0.01 mbar no signal was detected. It is clearly seen that the detection range of vacuum has got shifted in the pressure scale. The entire experiment was then repeated with Permeator-III, having a diaphragm thickness lower than that of Permeator-II. It was found that this permeator worked well in the range 10 to 0.001 mbar chamber vacuum and the senstitivity was found to be ~ 0.05, which is higher than that of other permeators earlier described.
Discussion The sensitivity of Helium detection is found to be higher in Permeator-III when compared to Permeator-II. Permeator-I is found to have the lowest sensitivity among the three. This is in conjunction with the fact that the permeation of helium would increase with decrease in thickness of diaphragm material and hence the sensitivity also would correspondingly increase. There is an effect of background signal increase, but the sensitivity was found to be nearly constant in the range of operation. The observed upper limit of functioning of a permeator is due to the large background helium level and the lower limit is due to the fact that the fraction of helium going to MSLD as compared to the main vacuum pump has become negligible. Conclusions Experiments conducted with specially designed permeators introduced between the MSLD and vacuum pumping line during the Helium Leak Detection of a large vessel, show that the helium leak detection method can be extended to wider range of operating vacuum. It is easily seen that this technique can be applied to detect leaks of size, 10-6 to 10-4 std cc/s on-line with good sensitivity and reliability in large size process plants.
References [1] L. N. Rozanov, Vacuum Technique, Edited by M H Hablanian, Taylor & Francis 2002 [2] Non-Destructive Testing Handbook, 3 rd Edition, Vol 1, Leak Testing, Ed by Patrick O. Moore, Published by ASNT, USA 2011. [3] A. Roth, Vacuum Technology, in: North Holland, pp. 439-455.