Compressed gas cylinders under test
Cylinders used for the inflation of escape chutes, oxygen cylinders, automatic fire extinguishers, pressure containers for door opening systems – all of these are deployed only in an emergency. But when they are needed, it is imperative that they work perfectly.
Depending on their content and operating pressure, the cylinders are constructed from steel, aluminum or titanium. The oxygen and carbonic acid mixture used for the emergency chutes is under a pressure of over 200 bar, the oxygen cylinders are pressurized to 120 bar, and the quenching gas in fire extinguishers is maintained at a pressure of between 20 and 60 bar. This permanent pressure loading weakens the material. Hence the condition of the compressed gas cylinder material is checked at predetermined intervals of between three and 14 years in the "hydrostatic test".
To illustrate the aim of this test, the Component Services experts at Lufthansa Technik in Hamburg like to compare the cylinders with a paperclip. If one bends a paper clip and then releases it, it will jump part of the way back to its original shape. If one keeps bending the wire to and fro, at some point it will break. The amount of force required for the last few deformations is noticeably smaller than for the first.
Like the paper clip, the material of the compressed gas cylinder is weakened every time the container is put under pressure. Naturally the aim of the test is not to establish the actual failure point at which the material will break, but to ascertain how far away from the breaking point it is.
As water is not compressible, the material is tested using water pressure. This entails suspending the pressurized cylinder in a cylindrical container filled with water. The container is sealed so that it is airtight and the last remaining air in the cylinder is forced out by introducing water. During the test only special water, which has not moved for some time and is known in the jargon as "calmed– is used. As a result, temperature differentials are balanced and released gases can be liberated. 4,000 litters of this water is available in tanks.
A small capillary tube leads out of the cylinder lid to a small dish containing a little water which is standing on electronic scales. After the scales have been calibrated to zero, pressure is applied to the pressurised cylinder from within. The test pressure is set by the manufacturer, but is normally five-thirds of the regular filling pressure. The material takes some time to adapt: slowly the cylinder expands and forces water out of the container. It flows into the dish and the scales indicate how much water has been forced out. The pressure is now released from the cylinder, water flows back into the container and the cylinder material contracts – almost – to its original condition. The "almost– is the critical factor, which corresponds to the fatigue of the paper clip. The scales are now indicating several grams more than at the beginning of the test, signifying that the cylinder has grown a little bigger as a result of the test pressure. This corresponds to the remaining deformation of the material. After 15 to 20 minutes the test is terminated and the extent to which the material has weakened is calculated from the two measurements. However, the experts reassure us, this high test pressure is never reached during operation.
The limit values and also the tests are prescribed by the authorities. The standards for compressed gas cylinders in aviation come from the US Department of Transportation (DOT). The standards specify amongst other things the manufacture, transport, manner of testing, service life, end use, types of gas etc. For a new pressure vessel to attain DOT certification, prototypes have to withstand extreme situations such as fire, explosions and drop tests, up to a defined limit.
Before the compressed gas cylinder gets its new inspection stamp, a visual examination is undertaken. This entails not only an external inspection but examination of the container interior for corrosion and other distinctive features as well. If all the results are in the permitted range, the test is documented: the test date and certification number of the test station are entered on the embossing or on an ink stamp. A test log is also created.
Altogether Lufthansa Technik Component Services tests 4,400 cylinders per year. The company has been licensed to conduct the hydrostatic test since 1994 and undergoes a DOT audit every five years.