Posted By David Bannister
Hi Simon, the concentration of flour dust in air required to create an explosive mixture is a variable, depending on several factors including relative humidity, particle sizes, type and variety of grain.

Your flour supplier or the manufacturer should be able to provide some indication. I don't have a definitive answer for you but you may be interested in the HSE publication: http://www.hse.gov.uk/pubns/fis02.pdf

I have seen a publication which suggests that between 40 and 4000 gm-3 is the explosive limits but I claim no accuracy in those figures.

You may be better advised to speak to a specialist in the subject but beware the explosion suppression equipment salesmen!

Posted By David Bannister
A further thought: Big dust explosions in flour mills and the like have often been secondary explosions caused by a small "pop" lifting a much larger quantity of dust from its resting places creating a large cloud that reaches a source of ignition, with the consequent big bang.

So when considering whether your flour dust cloud is within the limits of flammability, also consider whether a small quantity can explode and lead to the big 'un.