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January 17 - January 23, 2016

What causes static electricity during abrasive blasting? What risk does it pose, and how can it be controlled?


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From John McCredie of Franklin Roofing Systems on February 17, 2016:
We are using a vapor blaster, where the abrasive is shot through with water. Does this pose a risk?

From Chuck Pease of MMI Tank on February 16, 2016:
Best response to the question to date from OM PRAKASH JAT….. Good job.!!! The static charges caused by abrasive blasting have been described by others well enough, so I won’t be redundant. The resulting possible explosion from sandblasting with ungrounded equipment or inadequate engineering controls to keep dust levels below acceptable levels is called a “deflagration” event . Organic dusts in high enough concentrations can ignite due to static discharge, and if it occurs in a confined space, the event can be devastating to life, limb and property. This event typically occurs in confined spaces; however, it can also occur in spaces that would not be considered permit-required confined spaces. The danger is REAL. However, many blasting contractors don’t take the necessary precautions to protect workers and property. For more details concerning what constitutes (organic dusts), follow this link to see US OSHA’s chart for inclusions. Be aware there are many more combustible dusts encountered during abrasive blasting than you would think. Here is is the link for FED OSHA on the combustible dust chart. https://www.osha.gov/Publications/combustibledustposter.pdf Keep in mind even epoxy resins used in AWWA potable water tank coatings are listed. Here is an example of what is needed for a deflagration event. Deflagration is a term often used to describe combustible dust explosions. A deflagration is an "ordinary" fire such as a gas stove, burning wood or paper, and even the burning of gasoline vapor inside the cylinder of an automobile. In a deflagration, a burning substance releases heat, hot gases, and energetic particles or sparks that ignite and spread the fire. In a dust explosion, the deflagration processes happens so rapidly that the heated air and gaseous fire products (such as carbon dioxide) produce extreme air pressure that can blow out walls and destroy structures. It is in the safety industry referred to as the "Dust Explosion Pentagon." Any fire needs three elements, known as the "fire triangle": 1. fuel to burn 2. oxygen, and 3. an ignition source (heat, spark, etc. ) A dust explosion needs two additional elements - known as the "dust pentagon": 4. dispersion of dust particles in the right concentration, and 5. confinement of the dust cloud. Dispersion means the dust particles are suspended in air. Confinement means the dust is in an enclosed or limited space. This restriction allows pressure to build up, increasing the likelihood of an explosion. Here is a link for above DEP chart if it does not render well on this site: https://www.ccohs.ca/oshanswers/chemicals/combustible_dust.html

From Antonio Leal of IMMT-Institut Macaé Tecnology and Metrology on February 16, 2016:
Usually,   tanks, piping and other equipment are grounded, but this should be checked on a visual inspection and if possible with a ohmmeter.

From Gary Burke of Carlisle Fluid Technologies on February 12, 2016:
All equipment should be grounded to dissipate the static. The danger is that the static can cause the dust to possibly explode.

From Ajay Sunke of ASSETReifurb Engineerr on January 23, 2016:
High velocity movement of blast media through a hose to the surface to be blasted results in swirling of grit/dust on very large surface and friction causing build-up of static electricity. The risk  is generation of sparks leading to fire/explosion and static shock to an operator working with the blast equipment.. The risk is controlled with grounding of equipment with low resistance wire, less than 5 ohms, and wearing electrical-resistant shoes and gloves by operator. Also, Keep all solvents, flammable material combustible material away from job by practicing good housekeeping.

From OM PRAKASH JAT of TECH INTERNATIONAL SHARJAH HAMRIAH UAE. on January 16, 2016:
What causes static electricity during abrasive blasting? How many of us really read the owner's manual that accompanies our new purchase? Static is generated whenever two dissimilar materials are in relative motion to each other. Movement of electrons from one atom to another constitutes what is referred to as electrical energy, including static electricity. Blast equipment, by definition, brings together elements that as a matter of course have materials flowing through pipes or hoses; the activity and conditions surrounding blasting generate static electricity. Though overlooked by many people, the type of shoes worn by the operator can contribute to the generation of static electricity. Dry air, essential to an efficient blasting operation, contributes to the buildup of static electricity. The movement of blast media particles through hoses to the nozzle / gun, over the part being blasted, and in the recovery hose generates static electricity. Cyclone-type separators and media classifiers, where media and dust swirl over large surface area, normally generate a static charge. What risk does it pose? When static electricity is left unchecked in a manual or automated system, the static can cause media flow and separation problems. Static electricity can shock workers and create an ignition source, with the potential for explosion if there is a combustible atmosphere (for example, an atmosphere containing metal dust, organic abrasive or fine paint particles). And how can it be controlled? Grounding is the key to reducing the risk of static shock and avoiding efficiency-robbing static build up throughout the blast system. Where dry blasting is being conducted, an efficient means for the discharge of static electrical charge from the blast nozzle and the object being blasted should be provided. Hoses should be constructed with anti-static rubber linings or fitted with an earth wire or similar mechanism to prevent electric shock.  Grounding cables must be low-resistance at or below 5 ohms of ground. For permanent installations, check the ground with an ohmmeter at least once a year. For temporary or portable installations, check the static ground at start up and again each time the equipment is moved. This charge must be continuously dissipated to prevent an accumulation of an electrostatic charge that can result in a spark. Blasting machines occasionally cause shocks from static electricity. If the operator stands on a mat grounded to the machine and the gun is grounded to the cabinet, this will be eliminated. The cabinet can also be grounded to any conduit for insurance. At some time in our lives, we have all felt the effect of static electricity build-up.

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Tagged categories: Abrasive blasting; Asia Pacific; EMEA (Europe, Middle East and Africa); Latin America; North America; Surface Preparation


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