In 1990, two works won the Helmholtz Prize in the field of "Physical and Chemical Safety Technology". The first one was a study on the "Through-ignition behaviour of immersion-type safety devices" by Dr. Karl-Heinz Overhoff which had evolved from his dissertation at the TU Dortmund University.
An immersion-type safety device prevents a flame from spreading from a potentially explosive gas chamber to the gas feed by guiding the gas through a layer of water. During this process, the gas rises to the water surface in the form of single bubbles, is collected in the chamber above and is routed on. Immersion-type safety devices are used, among other things, to ensure that no flashback to the production facilities occurs when explosive exhaust gases are discharged from chemical plants and subsequently burned. The water walls between the bubbles are intended to prevent the flame from propagating and spreading to the feed line when a flame hits the water layer of the immersion-type safety device from above. In the case of highly reactive gases, however, even low loads on the immersion-type safety device can lead to a through-ignition.
Dr. Karl-Heinz Overhoff investigated experimentally and by mathematical simulation which processes lead to a through-ignition and how the ignition safety can be improved. In a first step, he used a high-speed camera and a pressure sensor to observe the behaviour of individual bubbles of a high-explosive gas mixture under water after an electric ignition had taken place. Less than 0.3 ms after an ignition, the pressure in a bubble rose to about 4 bar and fell below the ambient pressure after 2 ms. After 2.8 ms, however, the pressure shot up to 50 bar and then carried out several damped oscillations. Dr. Overhoff attributed such unexpectedly high pressure peaks to the fact that the bubbles collapsed after their explosion-induced expansion and showed similar behaviour to that known from cavitation. By observing rows of six bubbles that were rising in parallel, Dr. Overhoff was able to study the through-ignition process. During the explosion of the first bubble, a jet developed which cut through the adjacent second bubble and was compressed so violently from the inside that the ignition conditions inside the bubble were exceeded. Then, this bubble exploded too. The jet subsequently also penetrated the other bubbles, which led either to an ignition through compression or through direct bubble contact.
In an analogous manner, a flame hitting the water surface of an immersion-type safety device is able to set off a chain reaction that propagates through the gas bubbles and leads to through-ignition. The likelihood of this occurring could be reduced by increasing the spacings between the bubbles, for example by separating the bubbles by a forced current in the water.
As a further measure, Dr. Overhoff suggested that the energy released by the explosion of the bubbles should be dissipated as quickly as possible, e. g. by installing air-filled hoses. The jets of the bubbles are then guided to the hoses, which reduces the risk of an ignition transmission to adjacent bubbles.
