Investigation of dust explosion phenomena in interconnected process vessels

It is well known that where-ever combustible dusts are handled, or result as a waste product, unwanted ignitions and explosions can occur. Dust explosions present a high potential risk in the process industries. The past has shown that, in the European Union, more than 2,000 dust / air or gas / air explosions occur each year. In Germany alone the damage of dust explosions costs more than £ 16,000 per day. A lot of fundamental research work on dust explosion has been done for single vessels however, in reality, vessels are normally connected via pipes to other process vessels. To prevent secondary vessel explosions, and to protect interconnected vessels from the effects of explosions, further research into these complex phenomena is required. This research work concerns the investigation of dust explosion phenomena in interconnected vessels. Large scale tests were conducted in which I m' or 4.25 m' process vessels were connected via a pipe to a realistically filled 9.4 m' vessel. Pressure transducers were placed in the vessels and along the pipe, also photo diodes, positioned along the pipe, were used to measure the pressure development and the flame front velocity respectively. In further investigations the process vessels were connected to an open ended pipe. It was possible to estimate, with the aid of a CCD camera, the flame shape and flame length of the emerging flame jet dependent on the explosion course at the process vessel. The results have shown that, under different circumstances, dust explosion propagation from the primary process vessel through the connecting pipe which reaches the secondary silo as a flame jet, may ignite a dust / air mixture and thereby initiate a secondary dust explosion. The expected reduced explosion over-pressure of such a secondary dust explosion may be much higher than in case of a single vessel explosion. For the first time an empirical mathematical model for calculating the flame front propagation time inside a connecting pipe after a dust explosion in a connected explosion vessel has been developed. The model is validated for dust and gas explosions in different vessels, independently of whether the explosion vessels are vented or closed and for different diameters of the connecting pipe. A numerical simulation model for dust explosions in closed spherical vessels was written in a C comparable language which runs under Windows 3.11 and Windows 95 on a standard PC. The validation of this numerical model is in good accordance with measurement results of real dust explosions. The model shows good agreement with data from explosions in closed vessels for maize starch, aluminium and magnesium dusts and propane gas. In vented vessels in some cases for maize starch good agreement is also shown.