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Gas Leakage Test Using Helium Leak Detector EquipmentOscar S. Orlando
This condition has made necessary the application of a variety of techniques to verify the efficiency of the design and construction of involved elements. Among these techniques we can mention
The sensitivity of the helium loss method is justified taking into account the established rules to verify the airtightness of glove boxes and combustible bars, among other elements.
IRAM norms, which flux parameters established by DIN norms, require severe airtightness conditions for these equipments. They establish very low losses rate per period of time, in very strict pressure and temperature conditions into the glove boxes test environs.
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In the photograph (Fig. 3) a typical failure in a zircaloy cladding can be observed, that shows itself as a longitudinal cut of the wall.
A helium loss was detected working with the spectrometer in the 10-7 mbar.l/sec; minimum recommended sensitivity in these cases.
It must be taken into account that test is performed at room temperature, being highly possible that loss amounts will increase, given the working conditions of the combustible bar due to dilatation originated in reactor's temperature.
Many airtightness tests performed show that to ensure a good atmosphere confinement, the loss test of the equipment to be examined must be conducted in pressure and vacuum conditions, because several components and accessories may allow the passage of gas in one direction and not in the other.
The detection equipment allows the double function of creating vacuum into the receptacle measuring the gas passage from the exterior, and also of detecting the gas exit under a higher than the atmospheric pressure from the interior through a sampling probe.
There are capsules with known leak capillaries through which gas flows and others bearing helium in its interior. These calibrated losses are not only used to control the detectors' operation but also allow checking the response timing in extended systems and estimating real losses ranges, as opposed to virtual ones.
Resultant conversion factors to calculate helium losses in other gases losses
|Laminar flux (20°C)||0.68||0.93||1.14||0.25|
It can be assumed that there is molecular flux in losses of less than 10-8 mbar.l/sec.
Leaks larger than 10-4 mbar.l/sec should be considered as laminar flux and for intermediate values intermediate flux factors should be applied between both types of flux.
In can be then stated that the airtightness condition of the equipment must be the minimum necessary required by the established work pressure.
The leaks rate is defined as the amount of gas that flows towards the inside of a vacuum system through one or several losses per time unit.
In most cases this rate is specified in mbar.l.sec-1. This value is proportional to the flowing air volume per time unit:
1mbar.l.sec.-1=1cm3.sec-1 (NTP). NTP meaning normal 0°C temperature and normal 1013 mbar pressure.
Taking into account these units, in general the following classification of airtightness can be accepted as valid:
Acceptably airtight equipment: total leaks approximately 10-5mbar.l.sec-1.
Non-airtight equipment: total leak larger than 10-4mbar.l.sec-1.
Acceptably airtight equipment: total leaks approximately 10-3mbar.l.sec-1.
Non-airtight equipment: total leak larger than 10-2mbar.l.sec-1.
Before resorting to the helium leaks proof, which sensitivity is naturally very high, it is possible to detect large losses using ionization or thermal conductivity instruments.
Due to the fact that the choice of instruments depends on the type of gas being used, a fast test can be performed in areas where losses are suspected covering the zone with acetone or alcohol, which will make the instruments vary their readings on detection.
The levels of sensitivity of this method are not very high.
When creating vacuum in a system, lack of airtightness from air penetrating from the exterior is detected by a constant range pressure increase, while the air flux evolving due to other reasons diminishes along time.
In diagram in Fig. 4, it is shown the way pressure increases in presence of a real leak, the way it does due to virtual leak and the resulting effects in both cases.
The kind of synthetic rubber tightening elements are made of (e.g. perbunan, neoprene, viton) has a permeability of 5.10-8 to 20.10-8cm/sec (NTP) for a 1 cm thick and a 1 cm2 surface material. This value is just approximate due to variations related to its chemical composition and the compression the part is subject to.
The silicone rubber of identical dimensions has a permeability of 250.10-8 cm3/sec (NTP).
Metals are practically impermeable to helium.
To perform reliable tests two measurement points in the system must be checked. In Fig. 5 an installation with two manometers placed in the best position is displayed.
It is advisable to measure the pressure loss through a compression manometer (Mc Lead). When using a total pressure manometer (Thermotron or Thermovac) the gauge head must be connected to a nitrogen or liquid air trap in order to avoid .the detection of both, real and virtual leaks.
It must be taken into account that the outgassing effect is due to the presence of condensable gases.
Measurement will be repeated in regular intervals that will depend on contained volume and airtightness of measured system.
The A leaks rate is determined according to the following equation:
q = D
where V = volume in liters.
Dp = p2-p1 = pressure difference in taken period of time.
Dt = time difference in seconds.
Operation will be repeated until results are satisfactory, continuing with the vacuum pump operation until all virtual losses are eliminated.
For the detection of helium leaks, the equipment Leybold-Heraeus shown in Fig.6, has been used.
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The device used to test fuel rods and cobalt capsules is also shown in fig. 7-8 and 9 respectively.
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Fuel rods, for nuclear power plant, are controlled by this method and a great experience exists in this field. However it can be applied in efficient way to another special devices, which have different sizes and shapes.
In glove boxes test, working with lake amounts of around 10-8 mbar.l / sec. and cobalt capsules with lake 0,90-1,0 mPa.m3/sec. the methods ensure fulfillment of required norms.
Experience attained makes possible a more detailed analysis of the use of this method for tests in large volume equipment and in devices made of dissimilar materials.
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