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Helium leak testing on long gas lines

Helium leak testing on long gas lines

Introduction

Helium leakage testing is a non-destructive test method that offers both high sensitivity and a wide dynamic range. However, in some applications the geometry of the tested part introduces limitations in gas dynamics and conductance resulting in a significant performance loss for this technique. This is especially true for long and thin gas lines as used for gas supply lines or spiral wound heat exchangers.

Classical helium spray test

A helium spray test involves evacuation of the part to be tested, connection to the helium leak detector and spraying of helium around the outer surfaces of the part. Helium will penetrate through leaks in the wall and be transported to the analyser (see figure 1). Modern leak detectors combine vacuum pumps with a gas analyser in a compact unit.


Figure 1: Helium spray test

The response time of the analyser is dependent on the volume of the part to be tested and the effective helium pumping speed of the test set-up. The effective helium pumping speed depends upon the helium pumping speed of the leak detector and the conductance of the part and the pipe work connecting it to the leak detector. A typical commercially available leak detector has its maximum sensitivity at comparatively low pressures of less than 10-2 mbar.

When long thin tubes are pumped down to the ultimate pressure of the leak detector, gas flow is in the molecular flow regime. In this flow regime the conductance of the tube is much smaller than for laminar flow which results in extended measuring times. Signal intensity can be drastically reduced by a very broad gas velocity distribution.


Spray test with carrier gas

In order to achieve fast response time and high sensitivity the leak test is best performed in the laminar flow regime. Introduction of a carrier gas provides a fast and active gas transport through the gas line to the leak detector. A typical test set-up is shown in figure 2.


Figure 2: Helium spray test using carrier gas

 


Spray test with and without carrier gas – a comparison

Figure 3 shows the response time differences between a free volume of 1 litre and two gas lines with an internal diameter of 4 mm an a length of 7 m or 20 m respectively. In all measurements the shut-off valve of a calibrated leak was opened when the background of the leak detector has reached a value of 10-9 mbarl/s. Transport of the calibration gas is very fast through the open volume and shows almost instantaneous signal response. In contradiction the signal response time with the gas line comparetively long.


Figure 3: Signal response time and signal decay time of a 1 l volume compared to two gaslines

Figure 4 shows the same gas lines but measured with carrier. It can be clearly seen that the response time is reduced compared to the measurements without a carrier gas and that the signal is visible as a sharp peak. The difference in disappearance times is even more drastically. The sensitivity of the test depends mainly on the helium concentration of the carrier gas. For low-level measurements this concentration must be as low as possible. This can be achieved with high-purity Nitrogen. Use of an unreactive carrier gas also allows the test of gas-supply lines for toxic, reactive, explosive, or flammable gases during installation or after maintenance.


Figure 4: Signal response time of a 1 l volume and two gas lines with carrier gas.

For standard industrial applications of this test method, adixen offers the ASMGraph with powerful integrated roughing pump. The high-pressure threshold for fine vacuum test offers optimum flexibility for selection of test pressure and sensitivity. For high-end applications where oil-free pumping systems are demanded the dedicated ASM 182TD+ is the optimal solution.

The powerful software package ASMView can be used to record data. This software package controls all current adixen leak detector models and is available for free download on adixen’s web site.

Authors

Tim DOUST, Adixen UK, Livingston, UNITED KINGDOM
Rudolf KONWITSCHNY, Alcatel Hochvakuum Technik GmbH, Wertheim, GERMANY



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