When Static Electricity Turns Dust Into Dangerous Combustible Material
As you might already know, pneumatic conveying has its many efficiencies and financial advantages. In fact, most of our clients have been enjoying significant savings thanks to Serby’s unique Wave-Phase© technology.
However, no matter how great the advantage is, there are still dangers factories and engineers constantly have their concerns about. One in particular is the combustion probability of dry bulk materials such as powders.
What Does Static Electricity have to do With it?
Generally put, nearly any pulverized substance can combust, and most of the time the cause is static electricity.
There are three main characteristics to static electricity ignition:
a) formation of a combustible environment;
b) the accumulation of static electricity charges;
c) and the occurrence of ignitable discharge.
Formation of a Combustible Environment
As a rule of thumb, a combustible environment begins to build itself up when oxygen and a combustible substance (organic or inorganic) mix in different surfaces of different forms, such as reactors, containers, pipes, a bucket or even on an open surface.
Those potentially dangerous substances can be in a form of fibers, large random pieces, small chips or dust particles in diameter under 450pm.
A combustible environment can slowly build-up depending on a material’s ignition point, specific form and size, its concentration rate in the air and the temperature of the surrounding environment.
The Accumulation of Static Electricity Charges
Normally, for all particle-like materials such as powder, the accumulation of static electricity charges happen, due to the friction between the particles of the conveyed material themselves, as well as between the particles and the walls of the equipment containing it.
Such friction can result from processes like pulverizing, filtering, spilling from one container to another, silo filling and of course pneumatic conveying.
Nevertheless, during the handling process, the quantity of charges differs from one material to another. The handling process itself, however, has a greater influence on the charge level, rather than the material type.
The Occurrence of an Ignitable Discharge
The physical procedure in which an ignitable discharge might occur begins with a charged matter in a combustible environment. The electrical power field around that matter causes an isolation collapse (ionization) of the air between itself and another conductive matter. This results in the transfer of an electrical charge, and is known as “electric pulse”.
The energy density of this electric pulse in the air causes a flow lane within the combustible environment, to heat-up. This heat can reach up to hundreds and even thousands of degrees Celsius.
When the combustible environment is in a higher temperature point than the material’s autoignition temperature, it has a high probability to result with ignition discharge and cause a combustion.
What about Powders, Specifically?
Now that we understand the big picture, let’s see why ignition triggers during the pneumatic conveying process of pulverized matters.
First, as powder’s particles collide into each other and into multiple pipes’ surfaces, they are charged with static electricity. Meanwhile, the flowing particles are wearing-out and are becoming smaller and smaller.
As a result, the fine particles percentage in the pipe increases, which then generates a growing cloud of airborne fine particles that could potentially ignite. This is known as dust suspension.
Moreover, it is common to a hard layer coat of powder accumulates in the curved sections of the conveying pipes. Such situation increases the probability for a swirling discharge, that contributes to ignition within a pipe.
So, What Can You do About it?
In order to assess if there is a combustion risk in a pneumatic conveying system, it is necessary to compare the dust concentration rate in the pipe while transferred, with the flammability rate of the dust suspension. It is also important to take into account the type of the powder – either organic or inorganic.
We, at Serby AG take combustible environment probability in pneumatic conveying very seriously. Our over three decades of experience gave us an extensive understanding of the many essential variables in play to calculate and thoroughly analyze possible risks in the pneumatic conveyor systems we engineer.
Have you ever thought about such risks? Do you need us to check your pneumatic conveying system ? Fill-in our contact form or write us at firstname.lastname@example.org.
Naama Tran Serby Online Marketing & Business Development