Client Background
The client is a toll atomizer, running a “Prilling” operation, also known as spray congealing, spray chilling, or melt atomization where a slurry or molten solid material is jet sprayed in air, to form uniform spheres or droplets.
Client Problem
The fine fraction of material being produced had undesirable explosion characteristics (1 < MIE < 3 Es = 1,6 mJ) when tested in laboratory conditions, i.e. 21 % v/v atmospheric oxygen, meaning they had issues in controlling potential ignitions sources, in the form of electrostatic spark discharges.
The current basis of safety for the bag filter, collecting extremely fine particulates, was to reduce the atmospheric oxygen concentration to a level below the limiting oxygen concentration (LOC) of the product (9 % v/v). The most affective inerting gas is nitrogen, however, it is extremely expensive, meaning huge quantities of nitrogen are required to maintain sufficiently low oxygen levels, to ensure combustion can’t take place.
The current basis of safety for the bag filter, collecting extremely fine particulates, was to reduce the atmospheric oxygen concentration to a level below the limiting oxygen concentration (LOC) of the product (9 % v/v). The most affective inerting gas is nitrogen, however, it is extremely expensive, meaning huge quantities of nitrogen are required to maintain sufficiently low oxygen levels, to ensure combustion can’t take place.
Client Objectives
The client wanted to know how, in reality, specific to their material, ignition sensitivity was affected by atmospheric oxygen concentration. They wanted to plot a graph of minimum ignition energy versus atmospheric oxygen to visually show any trend that maybe present.
Strategy
Due to there being a direct relationship between oxygen concentration and materials explosion characteristics, Sigma-HSE advised that the materials MIE will be affected by the quantity of oxygen available within the atmosphere.
Therefore, trials were conducted using premixed gases (air / nitrogen) of known, reduced oxygen concentrations to realistically show the magnitude of effect, specific to the clients’ material, on MIE.
Therefore, trials were conducted using premixed gases (air / nitrogen) of known, reduced oxygen concentrations to realistically show the magnitude of effect, specific to the clients’ material, on MIE.
Insights and results
Testing Overview
Adjustments to the test procedure and apparatus were required to sufficiently purge the altered Hartman tube apparatus, displacing the ambient atmospheric air with the target gas composition. Additionally, the target gas was used to disperse the sample, to form a dust cloud.
To ensure that these alterations were producing the target atmosphere, tests were conducted using an oxygen analyser, measuring the oxygen concentration within the Hartman tube, after injection of premixed gas. This was crucial in developing a method to establish the length of time the vessel needed to be purged, prior to ignition trials
To ensure that these alterations were producing the target atmosphere, tests were conducted using an oxygen analyser, measuring the oxygen concentration within the Hartman tube, after injection of premixed gas. This was crucial in developing a method to establish the length of time the vessel needed to be purged, prior to ignition trials
Sigma-HSE found that
Testing showed a dramatic decrease in sensitivity on reduction of oxygen available.
Based on this, the client was able to maintain an oxygen concentration at a level rendering the materials MIE so high, that they could control potential electrostatic spark discharges, dramatically reducing the quantity of nitrogen being used to achieve safe operating conditions.
In fact, they were able to control discharges to 30 mJ, which allowed them to half the quantity of nitrogen used compared to full inertion, 18 vs 9 % O2 v/v.
