Ultrasonically triggered ignition at liquid surfaces

• The verification of the incendivity of focused ultrasound at liquid surfaces.
• Identification of conditions critical for ignitions triggered by ultrasound at liquid surfaces.
• Identification of a threshold intensity for igniting explosive atmospheres at liquid surfaces.
• Worst-case consideration of MHz-frequency ultrasound in liquids.
• Comparison of the currently valid threshold value and the research conducted.

Ultrasound is considered to be an ignition source according to international standards, setting a threshold value of 1mW/mm2 [1] which is based on theoretical estimations but which lacks experimental verification. Therefore, it is assumed that this threshold includes a large safety margin. At the same time, ultrasound is used in a variety of industrial applications where it can come into contact with explosive atmospheres. However, until now, no explosion accidents have been reported in connection with ultrasound, so it has been unclear if the current threshold value is reasonable. Within this paper, it is shown that focused ultrasound coupled into a liquid can in fact ignite explosive atmospheres if a specific target positioned at a liquid’s surface converts the acoustic energy into a hot spot. Based on ignition tests, conditions could be derived that are necessary for an ultrasonically triggered explosion. These conditions show that the current threshold value can be significantly augmented.