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Opinion about the excess power of the explosion of flour/powdered sugar over the power of the explosion, say, a grenade - pros and cons.
In fact, the power of an explosion can be understood as the amount of energy released during the explosion. I must admit that the combustion energy of blasting substances (TNT and so on) is somewhat lower than that of simply combustible materials. This is explained by the fact that blasting substances contain an oxidizing agent, and flammable substances receive an oxidizing agent from the air.
But there is a small but. The speed of combustion (explosion) propagation and pressure dynamics in the explosion zone. For the explosion (detonation) of high explosives, the reaction propagation speed is equal to the speed of sound in the medium and can exceed 1000 m/s. It is important to understand here that the initiator of the reaction in this case is precisely the shock wave.
In the case of dust-air mixtures, this is actually not an explosion, but combustion. There is a limitation here - the lower concentration limit of flame propagation (that is, if there is not enough dust in the air, then combustion will not spread. There is no upper concentration limit for dust, this is also important point. In this case, the speed of flame propagation (that is, the reaction) is significantly lower than during detonation, since even when turbulent flows occur (significantly accelerating the spread of the flame front compared to the initial moment of reaction, at which the flame propagates smoothly, laminarly), the speed of flame propagation is no more than meters, maximum tens of meters per second.
All this explains the different pressure dynamics. During detonation (grenade explosion), the pressure in the shock wave front is very high, but the exposure time high pressure is small, so the impulse is not very large. In a volumetric explosion, the pressure is much lower, but the exposure time is much longer. And that means a lot of momentum. By impact on building construction this can be compared to the blow of a sledgehammer and a slowly driven bulldozer. In the first case, we get a hole and cracks, in the second, the whole house is slowly being destroyed. (The comparison is very, very arbitrary; in fact, everything is somewhat more complicated). In addition, with a volumetric explosion, the temperature in the volume where the explosion occurred significantly increases.
Another feature of dust explosions is that during the first explosion, the dust accumulated on the surfaces is lifted (transformed into an airgel state) by a shock wave and this leads to a repeated, often more powerful explosion. The time difference is subtle without tools, but it is still there.
Alexandra This is so... I heard before, either during the war or simply because bandits blew up bags of flour)
I don't agree. It is impossible to explode a bag of flour.
I heard in some production premises the ventilation shaft was clogged with poplar fluff. And someone, at an unkind hour, suggested setting it on fire. It exploded so hard that it tore apart two load-bearing walls.
It is extremely doubtful to me that it would be so brutal. May I be curious, where does this information come from? (professional interest, you know).
If you are interested, you can throw a handful of small sawdust into the fire for the sake of experiment - a magical “puff” is guaranteed
Bugoga. There won't be an explosion for sure.

Some restaurant employees, when sifting flour, are seriously afraid that if they light a match, the building will fly into the air and they along with it. Tales that flour can explode have been known since Soviet times. But is this true? Can flour particles floating in the air create an explosion when interacting with fire? What concentration is needed for this? Is this really possible in a restaurant, or are we talking about the scale of a bakery plant? With the help of experts, we find out how things really are.

Vladimir Pigor

chief engineer of the Ust-Labinsk elevator, which is part of the Kuban agricultural holding (“Basic element”)

Flour may explode. However, for an explosion to occur, the presence of oxygen, open fire and a suspension of flour is necessary. In this case, an explosion can occur only with a flour concentration of 50 grams per cubic meter in a confined space. If the concentration of flour exceeds the specified one, the flour will burn.

Official requirements for the storage and processing of grain and flour are set out in Federal law“Safety rules for fire and explosion hazardous production facilities for storing and processing plant raw materials.” Naturally, we strictly comply with them; explosion prevention and protection equipment undergoes weekly inspection.

When storing flour at home, an explosion is excluded.

There are no stories about explosions in production. Everyone understands well how terrible the consequences of an explosion can be. At the holding's mills, special stands display photographs depicting the consequences of explosions at grain storage and processing facilities.

I remember during a laboratory course where we were shown a mini-explosion of flour. Flour was poured into a square plastic box with a heating coil at the bottom. The lid was replaced with a paper sheet, which acted as a discharge valve (otherwise the box would have burst). The spiral was heated until red, then turned off and air was immediately forced through a specially located tube to form a suspension of flour. An explosion immediately occurred and the piece of paper was completely burned.

Boris Bulgakov

Senior Researcher, Department of Chemical Technology and New Materials, Moscow State University named after M.V. Lomonosov

Flour can indeed be explosive if small fractions form a thick suspension in the air (for example, when sifting on a vibrating sieve) and there is a spark.

An explosion is essentially a very rapid combustion. For combustion, an oxidizing agent is needed, which is oxygen, and a reducing agent can be flour, among other things. Since the process is heterophasic, that is, they interact solid and gas, the reaction rate depends on the area of ​​contact. The smaller the particles, the larger area and the reaction is faster, so an explosion occurs.

But not only flour can explode, but also coal or wood dust, powdered sugar. In Soviet times, research was conducted on this topic, I don’t know for what purpose. But the patents that came out in last years, are devoted mainly to safety in coal mines.

Illustration: Nastya Grigorieva

The disadvantage of pneumatic flour transport is the occurrence of static electricity during operation of the installation. Electric charges accumulate and are at rest on bodies. Static electricity potentials reaching several thousand volts arise in the installation units and on the surface of the flour. Spark charges of static electricity in flour dust of a certain critical concentration can cause explosions and fires.
Electrification of dispersed systems occurs under the mechanical influence of solid particles suspended in air flow at high speed of movement of aerosols, when insulating material pipelines from the ground and low relative humidity environment. The quantitative side of the electrification process is influenced by physicochemical characteristics particles of suspended matter, their size, shape, concentration, state of their surface, speed of movement, temperature, air pressure, etc.
The surface of smaller dust particles absorbs oxygen more easily and they ignite more easily. At a certain concentration of dust mixture different substances with air they become explosive. Minimum dangerous concentration for flour dust 20.3 g/m3. This mixture can be ignited by an electric spark, which can occur as a result of shocks, when a grounded object approaches a charged one, or for other reasons. The very phenomenon of electrification of aerosols, material pipelines, the formation of explosive mixtures, and the minimum amount of energy required for an explosion have not yet been sufficiently studied.
The explosive concentration of flour dust in the air, the large potential difference in the flour and on the installation parts pose a danger to personnel servicing equipment for storing and transporting flour.

Rice. 1. Layout of the contacting surface in a flour storage container: 1 - contacting surface; 2 - fastening rods; 3 - container lid.

To prevent explosions and fires during the operation of aerosol transport systems in bulk flour storage warehouses, a number of measures have been developed. Neutralization of static electricity charges can be achieved by ensuring contact of all passing flour with current-conducting planes and reliable grounding of them, which is carried out by connecting them to contour grounding. Jumper resistance between any point of the pneumatic conveying installation and the circuit protective grounding should not exceed 0.1-0.2 Ohm. It is recommended to install inclined metal contacting planes in the flour container (Fig. 1). They must be located so that free entry of flour into the container is ensured, the useful volume of the container is not reduced, and the possibility of sparking when metal impurities enter is excluded. The contacting surfaces must be made of non-ferrous conductive metals (bronze, brass, aluminum, etc.). They are mounted in the top lid of a metal container for storing flour, which must be connected to a protective ground. The cone-shaped structures of the contacting surface are attached to the top cover of the metal container using fastening bolts.
Flour lines must be connected to protective grounding, and the receiving panel must have independent grounding. Supporting metal constructions on which flour storage containers rest must have reliable fastening to protective grounding. For reliable grounding of container bodies, a current-carrying circuit is required between the container and the protective grounding loop. This circuit is achieved by tightly connecting the protective ground loop to supporting structures or other conductive structures that have rigid contact with flour storage containers.
When installing containers on strain gauges, each container must have an independent connection to the protective grounding circuit using an elastic conductor.
To increase the conductivity of the walls of reinforced concrete containers for storing flour, it is recommended to coat them with vinyl acetate emulsions or paint them with current-carrying paints. Each section of the flour pipeline or grounded group of equipment must have at least two connections to the protective grounding circuit.
Fabric threads installed on containers, augers, rotary feeders or other equipment must be stitched copper wire and connected to protective ground.
To connect flour lines on flanges, conductive rubber must be used as rubber gaskets. When using gaskets made of ordinary rubber, there must be jumpers at the flange connections. Jumpers between pipes must be in places where fittings, rubber or other inserts are installed that interrupt the conductive network. By using inserts made of conductive rubber, jumpers can be omitted.
It is necessary to monitor the temperature in sluice and screw feeders during their operation. During normal operation, the temperature of the rubbing parts does not rise to the self-ignition temperature of flour (200°C). When the sluice feeder covers are tightly tightened, increased friction occurs between the covers and the rotating rotors, which contributes to their heating. In screw feeders, an excessive increase in temperature is observed when there is a high pressure of the air mixture in the mixing chamber and the installation of an electric motor with a power exceeding the design one. The pressure in the mixing chamber can be high during the initial period of operation, when the flour line is not sufficiently cleaned, plugs form in the flour lines, or the flour supply line is lengthened compared to the design one.
As oil vapors and oil condensate move, a dielectric crust forms on the walls of the pipes, on which it is possible
accumulation of static electricity. It is necessary to promptly clean and flush the pipelines and take measures to reduce the entry of oil impurities into the air ducts.
It is prohibited to produce welding work in containers until they are completely cleared of flour dust. Welding work must not be carried out in warehouses while the flour aerosol transport system is operating.
Before putting into operation a bulk flour storage warehouse, it is necessary to identify and eliminate defects in the grounding system of the aerosol transport unit and flour storage containers. Measure all transition resistances between the equipment, pipelines and the protective grounding loop; if deficiencies are identified, they must be eliminated.