Leak detection of high pressure vessels

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P4

Leak detection

of high pressure

vessels

I Berecz, Gy Horkay and G Langer, institute of Nuclear Research of the Hungarian Academy of Sciences, Bern t&r 18/c Debrecen, H-4001, Hungary

S BohBtka,

The most sensitive leak detection method for high pressure vessels is based on the use of a mass spectrometer leak detector. Our QtiOU type universal quadrupole mass spectrometer was incorporated into the leak test system’ (type LD-60) designed for the testing of the axial compensators of steam pipings. During manufacture of these items it is possible to detect leak rates ranging from 10mg mbar / s-’ and IO-’ mbar Is-’ with a response time of 5 min and 5 s, respectively, using a combined slow-fast sampling unit and He as the test gas. The leak detector is pumped by an air-cooled, automatically controlled diffusion pump system.

Tbe problem

The vesseIs to be tested were axial compensators, in the form of thick wailed stainless steel bellows, which are used to allow for the dilation of heated pipes. According to the manufacturing technology 10 bar overpressure of 30% He-70% air mixture is kept in the axial compensators during testing. The maximum permissible leak rate was 3 . 10-s mbar 1 s-l, consequently the aim of this work was to construct a leak detector capable of detecting leakages of high pressure vessels in the range of 3 - lo-’ mbar 1S-I and higher.

trically cooled baffle. The backing pump is a Duo 1.5 A (Balzers) rotary pump. An electronic console controls the system: by switching on the mains it automatically operates the pumps, actuates the solenoid valves and fans in their correct sequence. Pirani and high pressure ionization gauges control the preset pressure limits for both the diffusion pump and the quadrupole analyser head. In case of failure the electronics unit protects the pumps and the analyser, and lamps indicate the nature of any fault. Sampling unit

The construction

The LD 60 leak detector consists of a small quadrupole mass spectrometer, an automatically operated vacuum system and a sampling unit. Mass spectrometer

The universal quadrupole mass spectrometer (Q6OU) of our own construction was used because its mass range of l-60 amu was enough for this purpose and it gave the possibility of using not only He but other test gases as well. This small quadrupole was especially advantageous due to its built-in total pressure gauge and leak detector facilities. The meter of the Q6OU was calibrated both for He leak rate and total pressure. Vacuum system

The vacuum system is pumped by an air-cooled oil diffusion pump of 150 1 s-’ pumping speed and it is equipped with a thermoelec-

The sampling unit can be assembled in two ways as illustrated in Figure 1. In the fast sampling unit the gas from the leak test probe is sucked through a 1 m capillary by a small membrane pump, and a portion of the sample passes through the adjustable leak valve into the quadrupole mass filter. The flow rate in the capillary is about 5 . lo-’ cm3 s-l. In this arrangement the response time is quite short: 5 s, and the sensitivity is good enough to detect a 1 . lo-’ mbar 1 s-’ leak rate. In the sensitive sampling unit the gas consumption is much reduced (down to 2.5. 10m4 mbar 1 s-l), the sample gas is not diluted as much as in the former case and, consequently, the sensitivity is 100 times higher. The smallest detectable leak rate is then down to 1 . IO-’ mbar 1 s-l. The cost of this improved sensitivity is the loss of fast testing: the evacuation,of the probe (a short capillary) and the adjustable leak valve takes much time and the response time is increased to about 5 min. The solenoid valve protects the system in case of a sudden leakage in the sampler. The fast sampling unit can be changed over to a sensitive one by closing the valve between the membrane 17

S Boh~tka, I Berecz, Gy Horkay and G Langer. Leak detection of high pressure vessels

SAHPLING UNIT FOR

FAST

ADJUSTABLE

TESTING

FOR

SOLENOID SAFETY VALVE

AOJUSTABLE LEAK VALVE PROBE

(~ Irn )

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SENSITIVE

J~

SOLENOID SAFETY VALVE

FLEXIBLE

L_rJ

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QUADR UPOLE

QUAORUPOLE [

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MANUALLY OPERATED VALVE

MEMBRANE PUMP

Figure I. The sampling unit of the LD 60.

pump and the leak valve, but of course in this case the response time is increased to more than 5 min. The sample gas inlet to the mass spectrometer is controlled by the adjustable leak valve to set the pressure in the ion source at 2 • 10-5 mbar as read on the front panel meter.

Calibration A cylinder of 1 1 was filled with a 20% H e - 8 0 % air mixture at atmospheric pressure. A silicon rubber foil of 1.22 mm thickness and 2.65 mm in dia is placed on the cylinder. The He leak rate through the membrane is 2 • 10- 7 mbar I s - I. The instrument can be checked easily by placing the sampling probe or the tip of the capillary against this He-source.

Results Our leak detector, illustrated in Figure 2, is a sensitive and rugged instrument. The smallest detectable leak rate is I • 10- 9'rnbar 1 s - 1 and by changing the sampler a very short response time (5 s) can be achieved with a sensitivity of 1 • 10- 7 mbar I s-~. This latter is quite sufficient for testing high pressure containers and allows a fast inspection procedure. The electronic control unit makes the L D - 6 0 an easy to use device. The use of the quadruple mass filter tunable over a range of masses as the leak detector gives it an odtstanding feature. Not only He but practically any gases or even fluids can be used as a test gas.

18

TESTING

Figure 2. The LD 60 leak detector.