Once a year, a group of industry engineers gather at Volkmann’s corporate headquarters in Soest, Germany to review and discuss how explosion protection factors into powder and bulk material handling. The aim is to bring their explosion protection technology knowledge up to date and establish its connection to powder and bulk goods transport, particularly as it relates to vacuum conveying and other powder handling solutions, in order to make design recommendations and improve equipment as a useful addition to risk analysis programs and develop proper prevention safeguards. Last year’s meeting, hosted by Dr. Martin Glor (Swiss Process Safety Consulting), the "mysterious" area of electrostatics was dealt with in detail.
Conveying technologies--and vacuum conveying in particular with its differentiation and advantages over mechanical conveying systems--are specifically reviewed in these meetings, along with the stringent ATEX directives 95 and 137, corresponding to the equipment protection level for dusts (EPL) and to explosion zones 20, 21, and 22 (U.S. Class 1 Div 1 Groups E, F, and G). Unlike many aspects of NFPA standards, ATEX standards are established by testing and can be viewed as proven. The individual assemblies of a vacuum conveyor are easily demonstrated to be safely designed without an ignition source by using a combination of a Multijector multiple Venturi vacuum pump and all pneumatic controls. The risks associated with ignition sources, such as hot surfaces or electrical and mechanical sparks generated from the conveying system, can be safely excluded with this conveyor design. This is fundamental to removing one of the three fire triangle elements necessary for an explosion to occur in combustible dust.
The further characterization of bulk materials in connection with lean, medium, and dense conveying conditions lead to numerous applications of vacuum conveyors in very different industries and production environments. By eliminating sources of ignition in these applications, they are able to safely operate. The group gives close attention to the consideration of the areas of potential ignition. For example, we know that smoldering “nests” of product, hot surfaces, and mechanical sparks are all possible ignition sources. However, when determining causation after an incident, the real source of an explosion may not be readily identified, thereby resulting in a conclusion that "it must be electrostatics."
Because of this, it is important to consider charging mechanisms, charge accumulations, or their discharge leading to the basic scheme of electrostatics. If a safe discharge is not possible, and one occurs, the ignition effectiveness of this discharge must be assessed. In addition to the relatively well-known spark discharge shown in practical experiments, brush discharges and their ignition effectiveness must also be considered.
In the case of pneumatic conveying at high velocities (lean phase in particular where velocities typically exceed 4,000 ft/min), another type of discharge can occur, the so-called sliding stem tuft discharge. This energy-intensive discharge can be impressively demonstrated by the loud bang and highly visible sparks that occur. Such discharge only occurs under certain conditions and can ignite dust. It must be avoided in pneumatic conveying systems, and therefore results in the frequently discussed requirement for static dissipating conductivity in conveyor hoses with wire coils.
The last of the forms of discharge, corona discharge, also needs consideration. However, corona discharge only has a certain ignition effectiveness when particular gases are present.
Mitigation of risk is a key topic in the discussion of explosion avoidance and effective grounding is certainly the first general protective measure. However, it should not be forgotten that “good housekeeping” (i.e. clean operation without dust deposits spread over a large area) makes a significant contribution to promoting safety. Likewise, it is critical to not only avoid initial fires or deflagrations such as those due to hot surfaces, but also to prevent secondary explosions due to turbulence.
Well-designed powder processing systems that include vacuum conveyors can make a valuable contribution here, as they not only safely transport dust, but also keep the bulk powders in a closed system through the vacuum used, thus automatically ensuring a clean environment.
Nick Hayes, CMgr FCMI, former president of Volkmann Inc., is now a senior advisor to Volkmann Gmbh. He is a mechanical engineer and a Fellow of the Chartered Management Institute.
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November 03, 2020 at 05:07AM
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Electrostatics and Other Explosion Factors Occurring During Powder Transport - Powder Bulk Solids
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