Abrasives Operational Guidelines



2.3 Pressure Pot/Media Delivery System The pressure pot and media delivery system is the heart of the blast process. Properly designed equipment is crucial for achieving satisfactory process results. The components include a single or dual-stage pressure pot system, equipped with a volumetric starch media flow valve, transfer valves, depressurization or exhaust valves, and a compressed air feed to deliver the media under pressurized air. The pressure pots specified must meet the standards in the country of use with a rated 8 bar (125 PSIG) maximum operating pressure. For example U.S. pressure vessels are constructed to standards set by the American Society of Manufacturing Engineers (ASME). The pressure pot system should be equipped with exhaust and transfer valves, which operate problem-free when using light abrasives. Condensation on exhaust valves within the pressure pot system must be avoided. This problem can occur during depressurizations if the exhaust system is not properly designed. The pressure pot must be designed to uniformly feed media into the media flow valve. The media flow valve should be able to deliver a media flow-rate from 5 - 15 pounds per minute (2.3 to 6.8 kilograms per minute) depending upon equipment specifications and process application. 2.4 Compressed Air Supply The compressed air needed to deliver the media under pressure must be dry, oil-free process air. The compressed air system should be properly installed to provide air at an operating pressure dew point not to exceed 40oF. It is recommended that the compressed air unit have an after-cooler to remove moisture prior to the pressure tank. A refrigerant or desiccant air dryer, installed on the compressed air line, should be located as close as possible to the blast equipment. 2.5 Nozzles From the pressure pot outlet media is conveyed through blast hoses to the blasting nozzle. Blast hoses come in various lengths and ply. Typically thick ply hoses are used from the blast equipment to the blast room. A ‘whip hose’ (thin ply, flexible short hose connected to the nozzle) should be considered for the last 10+ feet to maximize the operators ability to carry and move the hose during blasting. Blast hoses can develop wear spots at sharp curves or corners in the line. Due to pressure drop losses and hose wear, excessive coiling or curvatures in blast hose lines should be avoided if possible. Several types of blast nozzles exist today. Conventional round blast nozzles, which have been in existence for decades, are used for manual stripping requirements. Round nozzles are characterized by the throat geometry and include straight-bore, single-venturi and double-venturi varieties. Single-venturi nozzles have convergent-divergent throats, which help to accelerate lightweight media particles to velocities sufficient to effect coating removal. Double-venturi nozzles have the same convergent-divergent throat, however air is entrained at the nozzle exit, which spreads out the media blast trace and reduces the unwanted hot-shot found with straight-bore or single venturi nozzles. Double Venturi nozzles are preferred for most blast applications. The next generation of nozzles recently introduced are categorized as ‘Flat Nozzles’ because of their linear blast trace. These nozzles can offer unparalleled productivity. Studies show that, in certain applications, the flat nozzles achieve a 100% increase in strip rates over conventional round nozzles. The Flat Nozzle uses similar media flow rates as round nozzles while typically removing more coating per minute (significantly reducing media consumption). 3.0 Operational Considerations 3.1 Housekeeping Media should not come into direct contact with water. Envirostrip® XL-CHP and eStrip® GPX are moisture resistant and can be dried and reused if they come into contact with water. Envirostrip® Wheat Starch and eStrip® GP cannot be reused if directly exposed to water. It is a good housekeeping rule to try to keep rain and/or wash water out of the blast area. Compressed air, if properly designed with air dryers, should not create a problem. If cleaning requirements mandate water to be used in the blast area, caution should be exercised. Media floor grates should be temporarily sealed to prevent water ingress into ducting. Water should not be used directly on the blast equipment. Floors should be sweep clean of all media and plastic sheeting should be used to prevent ingress of water into hoppers or blast equipment. 3.3 Waste Disposal Waste material generated by the process is most often found to be non-toxic and can be disposed of as industrial waste. A “leachate” test is normally performed on dust samples to determine the level of heavy metals in the total mix. Operators using ADM’s abrasive products for several years have continuously tested material and, in most cases, have determined the metals (hexavalent chrome – typically found in aircraft primers), to be under the toxic level determined by U.S. State and Federal regulations. 4.0 Engineering Controls and Safety Procedures 4.1 Overview Ignition properties of starch based media and its dust are less hazardous than Type V plastic media. Plastic media has been used throughout the world for nearly 20 years without incident. The safety precautions and engineering design required for starch based media compatible equipment is identical to plastic media. Consequently, at the very minimum, the same equipment precautions and equipment design should be employed for both starch media and plastic media stripping operations. 4.2 Explosibility Data Several independent studies have been produced showing the ignition properties and explosibility data of starch based media. These studies show that ADM abrasive products, when purchased in available mesh sizes, are not explosible. These products cannot be ignited. It is only the very fine dust produced from the breakdown of these products after blasting that is ignitable (material at a size less than 120 mesh size US Std. >0.125 mm), and only when reaching a very dense concentration in the presence of a significant (high) energy source. Properly designed media/dust and electrical controls offered by equipment manufacturers manage dust concentrations and ignition sources to levels far below the upper limit requirements. Secondary safety systems such as blowout panels and ducting can also be utilized. Several hazardous assessment reports are available from ADM/Ogilvie and will be provided upon request. 4.3 Safety Controls All equipment exposed to the blasting media and process environment should be properly grounded. This includes the part being stripped, the operator, the nozzle and hose. Note that when stripping composites, electric (static – cold spark) discharge is more prevalent because the non-conducting composite parts can create more electrostatic build-up. Properly designed ventilation of the blast booth or enclosure, as well as good housekeeping rules in and around the blast facility, helps to ensure worker safety and high productivity. Equipment exposed to the blast process must meet a minimum electrical standard as provided for in the National Fire Protection (NFPA) Code and other applicable government regulations. This includes any lighting, switches, or other electrical equipment directly exposed to the process. Note that standard equipment design provides for the lighting to be usually mounted outside the blast enclosure and sealed off by a dust tight/dust proof enclosure. Dust collectors should follow all applicable NFPA and government regulations concerning dust handling and collection.
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