Incendivity of ultrasound used in potentially explosive atmospheres of gases and vapours

Status:

completed 06/2013

Aims:

Modern applications of ultrasound in hazardous areas get in conflict with the current threshold value from safety related regulations (TRBS 2152 Teil 3, DIN EN 1127-1). The evaluation and verification of compliance with this threshold has been unclear or technically impossible. Thus, there was growing interest in the reexamination of the ignition source ultrasound and a revision of the current threshold value.

Activities/Methods:

For this purpose, at first, fields of application of ultrasound and the history of explosion accidents in relation to this ignition source were analyzed to thoroughly and differentiated assess the ignition hazards threatening from it. Subsequently, on the basis of theoretical considerations worst-case scenarios for an ignition were developed which yield conclusions that are generally valid. In accordance with the worst-case scenarios, experimental setups for ignition tests were designed. The combustible, the mixture concentration, the acoustic intensity and the sound absorbing material i. a. were varied. The results yielded a safety related reassessment of the ignition source ultrasound.

Results:

For the first time, it was possible to verify that ultrasound can effectively ignite explosive atmosphere of dusts as well as of vapors of flammable liquids. An ignition requires very high intensities of the ultrasound source and special boundary conditions. In the range of high sound pressure levels respectively intensities ultrasound fields can induce the heating of acoustically absorbing solids which result in hot surfaces which can cause ignitions. In contrast, cavitation is no ignition mechanism. The results allow one to suggest new threshold values that distinguish between applications in gases and liquids and correspond to an augmentation of the current threshold value by more than two orders of magnitude. In consequence, all currently known applications of ultrasound in hazardous places are either noncritical or can be easily secured by constructive means that already exist for technical reasons.

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