Thursday, May 16, 2013

Vacuum Collection Systems Designed for Explosive Dust Atmospheres


When milling, machining, polishing, grinding, or drilling materials such as Titanium, Magnesium, Aluminum, Iron Oxides, Stainless Steel, and Carbon Fiber, manufacturers must address the inherent explosion and fire hazards associated with the combustible dust generated by these materials.

NFPA 484 Annex // A.3.3.6.1 Combustible Metal Dust.*
3.3.6.1* Combustible Metal Dust. “A combustible particulate metal that presents a fire or explosion hazard when suspended in air or the process specific oxidizing medium over a range of concentrations, regardless of particle size or shape.”

An overall system solution must address the process requirements, materials, and the volumes processed. All equipment in contact with hazardous / combustible dusts must be constructed in compliance with HazLoc  Class II disciplines, and applicable N.F.P.A., A.N.S.I.,  and A.S.M.E. standards.  Customized systems are usually application specific and sized appropriately for the collection, conveyance, and control of the materials, ratios, and anticipated volumes of debris to be recovered. 

A basic understanding of the issues and solutions as relate to current developments in systems configurations, and best industry practices requires an explanation as to how catastrophic events occur.  In the interest of enhanced worker and plant safety we will also review what can be done to prevent them.

FIRE:                 
               
Direct:
Combustible dust related fires occur in all cases as a result of combustible materials being exposed to an ignition source.  This can occur either during or subsequent to machining when materials are within ignition sensitivity levels capable of supporting a deflagration.

During Transfer:      
When being transferred for collection, combustible dusts can also impact duct work elbows and other constrained joints and as a result of high speed impact create a spark moving the ignition source toward a collection point.  Non-grounded components accumulating an electrostatic charge, exposure to electrical motors or other spark producing equipment can also provide an ignition source.

Collected:
If an ember or spark is eventually transferred to a collection location and maintains ignition energy it can initiate a further transfer of the ignition to additional materials in the process stream.

Accumulated Residuals:
Layers of dust which have accumulated over time have also been documented as transmitting an initial deflagration to secondary areas. In a worst case scenario if suspended as a dust cloud, explosions are easily generated as the deflagration gains rapid expansion due to the increase of combustibles.

EXPLOSION:

Suspension: 
Dust explosions occur when combustible materials are suspended in an air/fuel concentration consistent with rapid ignition transmission. If the initial deflagration generates even a primary explosion, the associated shock waves will dislodge any dusts which have accumulated over time on overhead beams, walls, duct work, machinery, or collection vents. When these materials are dislodged from their resting place, they become airborne and in the presence of the initial flame front ignition source presented by the initial deflagration, they will contribute to an even much larger secondary catastrophic explosion.  Incidents reported in recent years here in the U.S. have documented multiple deaths, injuries, and significant property destruction.

COMINGLED MATERIALS:

 

S.D.S. (SAFETY DATA SHEETS) seldom refer to the inherent danger of finite dust particles generated during machining processes. They fall short by NOT expressing the M.I.E. (Minimum Ignition Energy) and minimum ignition temperature (MIT) thresholds and seldom address the issue of reactivity with other materials.  In some industries such as aerospace, there are usually several materials in a waste stream.  As an example, drilling and assembling aircraft structures will generate carbon fiber, titanium, aluminum, and stainless steel in varying combinations.  These comingled materials lead to secondary handling issue that S.D.S. specifications don’t address. Using one collection system for recovering different materials from diverse operations should be evaluated by sample testing to determine if a volatile combination of materials with lower MIE than each by themselves may be present.

HOUSEKEEPING:

Housekeeping, provides increased worker and plant safety but usually adds to the overall cost of manufacturing operating expenditures.  Safely removing accumulated combustible dusts requires specialized equipment and in some cases access can only be achieved if entire production areas are shut-down.  As a result, cleaning activities are not performed as frequently as they should be thus increasing the risks associated with accumulated combustible dusts. To minimize personal risks, efforts should include materials safety awareness, safe handling protocols and training. For example, using plant compressed air to “blow-off” debris from recessed areas should be avoided as the resultant dust in suspension could easily propagate an explosion under the right conditions.

TESTING:

When dealing with any of the debris generated within the production waste stream, one of the first steps must include combustible dust testing of the materials for explosive severity and ignition sensitivity as they would be generated in the work environment.  Sample collection(s) and submittal to an independent laboratory for testing under N.F.P.A. Code 68 is essential. Data received from such test results is mandatory when designing fire prevention and explosion protection equipment and process systems with sufficient capacities to safely accommodate subsequent collection, conveyance, and containment of the materials being recovered.

 A PRO-ACTIVE SOLUTION:  COLLECT – CONTROL – CONTAIN

Central Vacuum System
Current systems have incorporated both high volume air flow AND high vacuum to optimize collection efficiencies and transfer capabilities of heavy combustible dusts.   The ability to collect and control the transfer of any debris or dust is related directly to the volume, size, weight, specific gravity, and surface area of the material to be addressed.  Vacuum in itself provides no means to act upon any material unless there is substantial air volume available to generate the motive force behind transfer.  Dust collectors generally rely upon large volumes of air flow and as a result impart minimal vacuum on materials to maintain their velocities in collection ducts. 

Collect dust as it is being generated.
Hazardous dust migration in many cases has been eliminated by using specialized high volume – high vacuum / dust recovery systems to collect debris simultaneous to generation further containing the recovered materials and minimizing the burden of house-keeping. Within the aerospace industry, complete recovery of drill chips and dust at the work piece is currently in use on several projects and in automotive applications, debris recovery has been accomplished within assembly and machining operations with the same process.

Control dust during transfer:
The recovery of combustible particulate solids must assure that air/material ratios never approach critical M.E.C. (Minimal Explosive Concentrations) which could support ignition resulting in a deflagration which could travel either up or down stream of the event. M.E.C. ratios vary based on materials and process requirements, however the speed at which materials are transferred and the separation of these materials by excessive air volumes provides a means to isolate one particle from the next. High volume rate transfer also maintains the materials in suspension which minimizes their contact with duct work thus reducing the build-up of fines, clogs, static, and sparks

Contain dust for safe disposal:
Many terms apply to the initial separation of the combustible dusts from the recovering air stream such as Vortex, Centrifugal, or Gate type systems. Initial contact between recovered dusts and the receiving receptacle act to slow the materials in the air stream such that heavy materials drop out of the airstream before contact with any filtering media. General dust collectors employ filter “bag-house” configurations with high pressure air jets to back flush the filters on occasion to maintain collection capabilities as the finer materials in these type systems have a tendency to migrate into the filter media. Rotary valves or gates at the bottom of the recovery receptacles allow recovered materials to be collected for disposal.

SYSTEM FEATURES:
Items which may have been considered as “OPTIONAL” in the past should be considered as MINIMAL requirements in systems collecting and conveying combustible dusts in compliance to HazLoc Class II.  Terminology may differ between industries and suppliers however the intent to which these are applied remains consistent with providing safety in the work place.                        

 Specifically best industry practices include:

Grounding – and incorporating non-spark producing elements, materials, motors, switches
Spark / Heat detection – includes multiple high temperature rise sensors
Explosion detection – detect first pressure wave of an initial explosion ( see isolation below )
Explosion venting – pressure relief and / or rupture disk,  vents explosion toward a safe area
Flame / Fire / Deflagration  Suppression – appropriate to the material(s) being encountered
Flameless venting – prevents flame travel beyond location of occurrence
Isolation-Explosion gates – prevents flame travel “up-stream” or to other process areas
Minimized debris contained for safe daily removal
Plant personnel education, awareness and training

Author: J. Byron Walker is Director for Systems Design and Engineering @ TECH TRANS UNLIMITEDCORP.  He has over 35 years of experience in Robotics and automation systems integration working in the automotive, aerospace, and electronics industries.  He holds patents in finite parts cleaning technologies and is responsible for providing total systems solutions for the safe recovery and transfer of Hazardous Materials to which this article is directed.


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