Helium leak test
However, there is no such thing as an absolutely tight technical system. Connections always represent a leak risk. That is why engineers are very intent on achieving a high degree of leak-tightness in the development of sealing points and positive fit connections.
We use a helium leak tester to regularly test the leak-tightness of our metal tube unions for current orders and on all new designs, and also if there is a change in material or supplier. The union to be tested is mounted on a tube closed off on one side and is then tested in a vacuum chamber. The test medium is helium, which distributes quickly and evenly in the connection at a pressure of 10 bar.
Why is helium ideal for this test? Because helium has the second smallest atom size after hydrogen and in contrast to hydrogen it is not dangerous, not poisonous and not flammable. High concentrations can be introduced into the test specimen and results are obtained quickly.
If the helium exits through a leak in the union, it is extracted by the vacuum and measured. A so-called “sniffer test” locates the leak. The industry standard for tightness is 10-6 mbar l/s. We guarantee this value for all our metal products. When the individual components are adapted optimally, when the tube processing, mounting and handling is done cleanly and according to our instructions, SERTO unions achieve tightness values of up to 10-9 mbar l/s. This corresponds to a loss of one drop of water in 44 years.
Vibration test unit
Many of our products are exposed to vibration, pressure, tension and torsion during everyday use. Take, for example, unions in automobiles or trains. The tube systems are not statically fixed but must permit a certain degree of movement and still function reliably.
In co-operation with the Technical College in Brugg, SERTO developed and constructed a vibration test unit last year. Since then we have been testing the leak-tightness of products under vibration, especially unions for transport applications. The swivel-mounted clamp permits translational loads in vertical and horizontal directions. The amplitudes, i.e. the vibration path, are in the millimetre range (0-2.5 mm), the frequency can be set up to max. 50 Hz. We subject the test pieces to this test for up to one hour. Before and after they are checked for a secure fit and they must pass the SERTO leak test which requires industrial gas tightness.
Bending fatigue test unit
DIN 3859-3 defines how to test the reverse bending strength. The number and load of the load changes (107) are given, amplitude and frequency are variable. Eight specimens are filled with oil at 1.3 times the nominal pressure of the union. Each test unit is controlled separately. The top union is clamped tight, while the lower one moves on a rotary disk that generates a rotational load. A PC controls and monitors the test sequence and the results are documented in a test protocol.
This test is so demanding that according to DIN 3859-3 it is passed when at least 5 of the 8 specimens have come through the test successfully. The SERTO compression ferrule connection has proven itself to be extremely resistant to vibration; generally all the test specimens pass.
To establish the leak tightness of a valve, two different aspects are examined: the leak tightness during and after short-term high pressures and the leak tightness under long-term loads.
The static pressure resistance is examined in an extreme test with loads that are seldom realised under normal conditions. Operating parameters, such as temperature, pressure and flow rate, can be adjusted continuously in this test.
What exactly do SERTO engineers look at? Turbulence and noise caused by the inner form, transitions and flow velocities. Resistance and pressure drop determined by the design and fit of the spindle. For this test, water is used because it can be heated, is incompressible and readily available. The same test facility is also used to establish the resistance values. The pressure loss with a defined flow and pressure indicates the resistance value. On request and in complaint cases, the engineers perform tests with pressure surges, i.e. water hammer. In such tests, the pressure can be increased to 240 bar within a matter of a few milliseconds.
Another load test determines the leak tightness of the valves after a specific number of cycles. First, the valve opening and closing is simulated 500 to 15,000 times. Then a leak test is done and the spindle wear is assessed.