More info about this article: http://www.ndt.net/?id=21225 Helium Leak Detection by Detector Probe Technique : Is it a Leak Detection or Concentration Detection Method? Venkat N. Ramani PLASMA & VACUUM TECHNOLOGIES Plot No 17, Road 1-A, GIDC Kathwada Ahmedabad 382430, Gujarat, India E-mail: vnramani@plasvac.com Abstract 491 Non-Destructive Evaluation 2016
This paper reports the work carried out using a Helium Mass Spectrometric Leak Detector by Detector Probe technique in Pressure-Sniff method on two typical samples. The first sample was a very fine capillary, representing a pore (for e.g. porosity in casting, welds). The second sample was a radial groove in a circular flange joint representing a seal-joint leak (for e.g. machining scratch, grinding finish, seal defect). The two samples were subjected to Helium Leak Detection as per procedures recommended by standard codes. The analysis of the obtained results show that both the Detector Probe Technique and Hood Techniques function largely as Helium concentration detector. However, correlation of the detected leak rate to true leak rate is possible for some select situations. Generally it can be said that the correlation of the detected leak rate to true leak rate is better in the case of Hood Technique than in the case of Detector Probe Technique. Key Words : Leak Detection, Helium Leak Detection,, Detector Probe Technique. 1. Introduction Mass Spectrometric Helium Leak Detection () is considered as an ultimate tool in Leak Detection due to its extensive range of application, precise measurement of leak and its capability to be applied in on-line or off-line conditions [1,2]. The technique has been well proven and extensively applied in industrial and Laboratory vacuum systems for detection of very small leaks [3]. Figure 1 gives the major methods prevailing for Helium Leak Detection. While technique is very reliably exploited in vacuum systems, it is found to have some limitations in its application to pressure systems. The difficulties basically arise due to the testing conditions that prevail and the response time realized in such situations. Very careful considerations to the applied technique mitigate the problems to a very large extent. However, the question whether it is a Leak Detection Method or Concentration Detection Method, always prevails. To address this, it is essential to consider not only the scheme deployed but also the functioning of Mass Spectrometer and the transport of molecules to the detector. This paper reports the work carried out using a Helium Mass Spectrometric Leak Detector by Detector probe and Hood techniques in Pressure-Sniff method on two typical samples to understand whether the Helium concentration or leak rate decides the measured leak rate in. Section 2 describes the experimental arrangement and Section 3 presents the observations. Section 4 discusses the results. The conclusion of the work is given in Section 5. Vacuum Spray Method Vacuum System Vacuum System 492 Non-Destructive Evaluation 2016
Helium Gas Spray Gun Helium Gas Tracer Probe Technique Hood Technique Sniffer Probe Pressure Sniff Method Pressure System Helium Gas Pressure System Helium Gas Detector Probe Technique Hood Technique 2. Experimental set-up Figure 1 : Major methods of Helium Leak Detection Figure 2 presents the two sample components that were specially manufactured for addressing the problem of measuring Helium Leak Rate using Detector Probe Method. Figure 2a is the schematic of the sample component 1. This sample component contained a very fine capillary of diameter 0.η microns and length β0 mm, made of quartz, mounted on a standard 1 SSγ04 pipe fitted with 25KF end flanges. This represents a typical pore, as observed in casting porosity, welds etc. The second sample component (Figure 2b) contained a radial slot of 1 micron made in a circular SS304 25 KF flange using EDM wire-cut method. The metal seal joint leads to a seal-joint leak and represents a leak as observed in machining scratch, grinding finish, seal defect etc. He Quartz Radial He 493 Non-Destructive Evaluation 2016
Sample 1 Sample 2 Figure 2 : Schematic view of the Samples 1 and 2. Alcatel make, ADIXEN ASM340 Helium Leak Detector was used in this experiment throughout. The two samples were subjected to Helium Leak Detection by Pressure-Sniff Method using Detector Probe and Hood Techniques, as per procedures recommended by standard codes [3]. Figures 3a and 3b present the schematic of Pressure-Sniff method using Detector Probe and Hood Techniques for sample component 1. Also indicated in Figure 3a and 3b are the orientation of the sniffer probe with respect to the Sample component 1 (Capillary Leak). The point X is situated at 3 mm from the capillary end and all other indicated points are located at a distance of 12 mm from the end of capillary. In a similar manner, Figures 3c and 3d present the schematics for sample component 2 (Seal-joint Leak). The orientation and locations of measurement are indicated with reference to point of leak and the direction normal to face of the sample components. Hood D B A X C E Q D B A X C E Hood Q (a) (b) Sample 1 Sample 2 (c) (d) Figure 3 : Orientation and location of Sniffer Probes during the observations for sample 1 and 2: 494 Non-Destructive Evaluation 2016
3. Observations (a) and (c) - Detector Probe technique; (b) and (d) - Hood Technique; The two sample components were filled with helium gas up to 1 barg. After calibrating the Leak Detector, the two samples were connnected to the one-by-one. The leak rates measured by the Leak Detector Q10 and Q20 were noted as 5.0x10-8 Pa.m 3 s -1 and 7.0x10-8 Pa.m 3 s -1 respectively. Then the two samples were removed from the Leak Detector. A sniffer probe was connected to the leak detector and placed at location X of sample 1, with the probe tip facing the sample component, along the normal to its phase. The measured value Qx. Sensitivity of detection is Qx/Q10. The experiment was then repeated by moving the sniffer probe and placing it locations A, B, C, D, and E. Sensitivity of detection at each point was derived. The obtained sensitivity values are plotted in Figure 4. The experiment was then conducted for measurement of integrated Helium Leak rate Q1 by hood technique. The sensitivity of detection Q was determined as Q1/Q10. Q is also plotted in the same figure 4. The whole experiment was repeated for the second sample component. It is observed that the variation of the measured leak rate with respect to the leak position and orientation are similar in the case of both the sample components. It is observed that the Leak rate measurement is less sensitive as the orientation of the sniffer probe is moved away from the normal. Thus, the observations clearly indicate that the detector probe technique is not an ideal leak rate measurement but a measurement of flux rate of helium reaching the sniffer probe. It can be a measure of leak rate only if all the helium leaking out is ensured to enter the sniffer probe effectively. The hood technique, generally, is a helium concentration measurement technique but can be a leak rate measurement technique only if the conditions are such that the helium concentration in the hood reaches a constant value and helium coming into the hood is balanced by helium entering into the sniffer probe. 495 Non-Destructive Evaluation 2016
Figure 4 : Variation of Sensitivity of detection of Helium Leak Rate (For details of locations in Legend, see Figure 3) Sensitivity 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Sample 1 : Detector Probe Sample 2 : Detector Probe Sample 1 : Hood Sample 2 : Hood Q D B A X C E 4. Discussion A sniffer probe exposed to ambient environment measures a steady helium leak rate in a Mass Spectrometer. The helium concentration in atmosphere is directly proportional to this measured value and hence it can be unquestionably said that this is a concentration measurement technique, under such conditions and when the probe is inserted into a hood. To discuss if this could be used to measure the leak rate, let us consider the two techniques separately. First let us consider Detector Probe technique. When the gas leaks out from a pressurized vessel at small leak rates, the flow is in the form of a jet stream. If the sniffer probe is placed in the line of the jet stream and at a distance not for away from the point of emanation, the probe would collect almost all of the helium that is leaked and at the rate at which is emanating. Then the detector probe technique can be said to measure the leak rate directly and fully. However, if the probe is far away or oriented at an angle to the normal, the measured value would be lesser than the true leak rate. At any location and orientation, the sniffer probe continues to measure the flux that is reaching. At a far distance and at oblique angles, the detector probe measurement is directly proportional to the concentration. 496 Non-Destructive Evaluation 2016
Let us consider the case of hood technique. Since the probe is introduced in the hood, where the helium has collected due to the leak, the probe measurement at the time of insertion will be proportional to the concentration. It will continue to be so, if the volume is large or the filling rate of helium in the hood is larger than the rate at which helium flows into the leak detector through the sniffer probe. However, if the volume is small enough and the leak detector can deplete the helium in the hood through sniffer probe at a rate larger than the rate at which the hood is filled by helium by the leak, the leak rate measured will be same as the true leak rate. 5. Conclusion The analysis of the obtained results show that both the Detector Probe Technique and Hood Techniques function largely as Helium concentration detector. However, correlation of the detected leak rate to true leak rate is possible under some select situations. Generally, it can be said that, in the case of Hood Technique, the correlation with the true leak rate is better than the detector probe technique. Acknowledgments The author wishes to thank his colleagues at Plasma and Vacuum Technologies for their technical support and useful discussions on this work. References [1] L. N. Rozanov, Vacuum Technique, Edited by M H Hablanian, Taylor & Francis 2002 [2] A. Roth, Vacuum Technology, in: North Holland, pp. 439-455. [3] Non-Destructive Testing Handbook, 3 rd Edition, Vol 1, Leak Testing, Ed by Patrick O. Moore, Published by ASNT, USA 2011.