The Importance of Dry Sandblasting Abrasives

Sandblasting abrasives must be free flowing in order to work efficiently. Wet abrasives will clog the abrasive discharge valve, will stick to the inside walls of the pot, will require more energy (air pressure/air volume) and will cause flash rusting of a blasted steel surface. The most obvious sign of a wet abrasive is when the discharge valve on the bottom of the pot plugs and only pressurized air comes out of the nozzle. When this happens the pot man will "choke" the pot cutting off the air flow to the blast hose momentarily while forcing all the of the air into the pot, pushing a large slug of material through the blast hose. If the condition persists the pot man will continue to increase the opening in the discharge valve to allow the damp material to flow. This causes the use of additional material ~ possibly five to fifty times as much, increases fugitive dust production and drastically reduces productivity. The above is true for all blasting abrasives.

Manufacturers of blasting abrasives must produce a dry product, if it were wet or even damp they would not be able to screen or package it. The main causes of wet material are due to improper storage at the job site and/or because of wet air coming into the system from the compressor. All blasting abrasives have the ability to absorb moisture from the surrounding environment, including the air, the ground, nearby equipment and of course rain. Typically the packaging the material is delivered in is not sufficient to keep it dry if stored outside. No matter how the media is packaged, proper inside storage is the only method to assure dry abrasive. If you cannot store the material inside of a building then you must take special precautions to keep it dry before you use it.

Material delivered in paper or woven bags must be stored on a dry, non-earthen surface with a tight covering over it. Bags cannot be set directly on a concrete slab or floor – even inside. Bags should be stacked on wooden pallets to keep the bottom bags from pulling moisture from the ground or floor.

Material delivered in bulk bins must also be stored off the ground and under a cover. These bulk bins were not designed to keep the abrasive dry when stored outside. While the tops get most of the blame the reality is that most cases of wet material comes from rain water running down the side of the hopper and wicking up into the abrasive by way of the unloading valve. Hoppers sitting on the ground can pull moisture up from puddles or even damp earth if the valve is low enough. While placing a plastic cover over the hopper will stop water from coming in the top it will not stop the water from running down the sides and wicking up through the bottom valve. If water has come up through the valve you will see the bottom 6 to 12 inches of material being wet. All that is necessary is to dump this material before it gets into the blast pot. If a hopper has been stored outside or you have any reason to suspect it may have gotten wet for any reason you should test for and remove wet material before setting the hopper over the blasting pot. At least leave the lid on the pot and allow a small amount of material to flow through the valve in order to determine its condition before removing the lid. Wet material should never be allowed into the blast pot.

When air is compressed it heats up, as it cools down going from the compressor to the blast pot it will commonly pass through the "dew" point and moisture will condense out. Unless stopped this moisture will enter the blasting system through the pot, wetting the abrasive which causes metering and plugging problems and wetting the blasted surface causing flash rusting. To achieve the necessary dry air for blasting four steps are necessary. An oil filter to remove compressor oils and an aftercooler at the compressor to rapidly cool the compressed air and remove the majority of the moisture before it enters the air distribution system. At a point closer to the pot you should have a desiccant air dryer to remove the remaining moisture or any picked up in the piping. Finally you should have a particulate filter on the line just before it enters the pot and blast hose.