1. Moisture and Oil in the Air Supply
PROBLEM: Water and Oil Clump Abrasive
Water and oil are the worst enemies of abrasive blasting equipment. They cause abrasives to clump, which can clog metering valves, hoses, and nozzles. If moisture reaches the surface being cleaned, it can cause steel to rust. If oil reaches the surface, it can cause coatings to blister and fail.
SOLUTION 1: The Expensive Way—Ignore It Till You Can’t
Wait for clumped abrasive to stop abrasive flow through the metering valve. Then the abrasive must be manually cleaned out through the blast machine’s handhole or an operator must choke the machine to force the clumped abrasive through the metering valve. Although choking the machine likely will get abrasive to flow, this process wears out the metering valve and other components faster than usual, as does letting the problem get this far along. Also, some operators may blast with the choke valve partially closed to force abrasive to flow. This “solution” also wears out the metering valve and other components.
COSTS OF WAITING
• Production Slows and Then Downtime—Operators constantly must fight to keep abrasive flowing, and eventually the flow stops.
• Replacement Parts—The metering valve and other components wear out quicker, and then must be repaired or replaced.
SOLUTION 2: The Smart Way—Compressed Air Treatment
Invest in equipment that treats compressed air. The upfront expense saves time, money, and frustration in the long run. More than one type of air-treatment device may be needed if you work in a high-humidity area. Following are descriptions of equipment for treating compressed air.
- NOTE: Compressed air treatment information and images in this section are courtesy of Kaeser Compressors, Inc. See the Kaeser Compressed Air Treatment web page for more information.
A rule of thumb is that every 20°F drop in temperature condenses about 50% of the moisture vapor in the air, so buying an aftercooler with an air compressor is the first major step for improving air quality and preventing clumping and oil problems. Aftercoolers condense moisture to droplets that can be removed by a centrifugal separator that feeds into an automatic drain valve. These separators usually are standard with an aftercooler. Compressor manufacturers offer integrated aftercooler options, and standalone packages also are available for compressors that do not have a built-in aftercooler.
Air filters have two functions: remove particles (dirt, dust, rust) and trap oily mists. Particles are trapped by direct impingement and oils by coalescing. Some coalescing filters can trap both particles and oily mists. Install a filter close to the blast machine so that it traps debris that could enter from the air-supply hose. This allows the air temperature to drop, which also helps with oil removal. A common misconception is that water can be “filtered” out, possibly because some water-removing devices look like air filters. Centrifugal separators, for example, remove water in liquid form but their filter material does not remove moisture vapor from the air stream.
Aftercoolers and moisture separators installed close to the compressor outlet will remove liquid water, but not the considerable amount of moisture vapor that can condense downstream. Dryers are the tool to remove moisture vapor (and some oil) by lowering the pressure dew point below ambient air temperature. As with air filters, it is best to place air dryers closer to the inlet of a blast machine. Air dryers come in several types:
REFRIGERATED—Similar to an air conditioner, refrigerated dryers remove water by chilling compressed air below ambient temperature and then expelling the condensation. They require electricity, but little maintenance. They are not suitable for environments below 40°F.
DESICCANT/ADSORPTION—These vessels are filled with hydrophilic (“water loving”) desiccant beads that adsorb water vapor molecules as air passes through them. When the bed capacity is saturated, no more moisture can be removed unless the desiccant material is “regenerated.” Desiccant dryers are available in a wide range of sizes but are more expensive to buy and operate than refrigerated dryers. Twin-vessel desiccant dryers use one tower to dry air while the second tower is dried with some of the dry compressed air from the first tower. Single-tower models are much simpler and require no electricity, but are limited to smaller sizes. Desiccant dryers are a good choice for cold weather operations.
Important Note: Desiccant beads will not attract water if oil gets on them, so they must be protected with a coalescing filter. Also, desiccant beads break down over time. To protect downstream equipment, install a particulate filter after the dryer.
MEMBRANE—Water vapor from compressed air passes through the hollow fibers of the membrane while a small amount of the dry air is redirected along the length of the fibers. This redirected air sweeps out the water-vapor laden air in the membrane. These dryers are simple and require no electricity, but they consume a significant portion of the air going through them (approximately 30%) and are limited in size. Like desiccant dryers, membrane dryers absolutely must be protected by a coalescing filter to remove oils that would ruin the membrane fibers.
CHEMICAL/DELIQUESCENT—These devices look like filter vessels. Air passes over beds of chemicals/salts (often calcium chloride and lithium chloride) that absorb water vapor. The chemicals become saturated with the water vapor and dissolve. Install a liquid separator with a drain to remove the chemical solution from the air stream. No electricity is needed, but the device must be refilled and may have corrosive effects downstream (depending on the chemical type).
2. Insufficient Air
PROBLEM: Blast Nozzle Does Not Receive Enough Air Volume
Blasting productivity and quality—and therefore profitability—require that sufficient compressed air volume reaches a blast nozzle. This air volume is measured in cubic feet per minute (CFM). Without enough CFM, pressure cannot be maintained at the nozzle to effectively blast.
POSSIBLE CAUSE 1: Compressor Too Small
An air compressor in a blast system needs to produce enough compressed air to pressurize the blast machine, convey abrasive to the nozzle, provide Grade-D breathing air to a supplied-air respirator, power other accessories (such as cooling devices)—and, of course, ensure that the blast nozzle receives enough CFM to maintain high nozzle pressure. So before beginning a blasting job, ask yourself if you have a large enough compressor.
POSSIBLE CAUSE 2: Nozzles Wear; Nozzle Orifices Grow Larger
No matter how well you prepare, one fact of abrasive blasting cannot be avoided—blast nozzles wear out. As a blast nozzle wears, its orifice size increases and it then needs more air volume to maintain maximum pressure at the nozzle. The rule of thumb is that after a blast nozzle orifice has enlarged one standard size (1/16”) beyond its original size, replace it or accept a drop-off in blasting productivity and quality—and potentially safety if abrasive wears through a nozzle’s liner.
POSSIBLE CAUSE 3: Air-Supply Line Inner Diameter (ID) Is Too Small
A correctly sized compressed-air supply line carries the required volume of air to a blast nozzle. But an airline ID that is too small causes a pressure drop throughout the blast system, resulting in low nozzle pressure and decreased production. To avoid a pressure drop, ensure that the air line’s ID is correctly sized for the IDs of all fittings in the blast system. Keep in mind that hose fittings with an internal barb cause additional restrictions that can lower nozzle pressure and decrease production.
POSSIBLE CAUSE 4: Blast Nozzle Orifice Is Too Large for the Air-Supply Line
The CFM and pressure of air passing through a blast nozzle determine how much blasting can be accomplished. An incorrectly sized air-supply line reduces the amount of pressure that can be generated at the nozzle. For example, a 10 PSI drop of pressure at the nozzle leads to a 15% drop in production. As a general rule of thumb, for air lines up to 100 ft the air-line ID should be at least four times the nozzle orifice size. For air lines longer than 100 ft, increase the air-line ID by one standard size. However, no air line is too large. Whenever possible, use air lines larger than the minimum recommended for a blast nozzle. The following chart shows the minimum air-line ID recommended for different nozzle sizes:
Minimum Compressor Air-Line Sizes
POSSIBLE CAUSE 5: Incorrect Hose Connectors
Use air-hose connectors that offer the least resistance and greatest internal area. Do not confuse outer diameter (OD) with inner diameter (ID). An air-hose connector’s size refers to pipe-thread size or to the ID of hose that it fits. For example, a 1-1/2” (38 mm) threaded air-hose connector has an OD of 1-1/2”, but an ID of 1-1/4” (32 mm) or less will limit the volume of air.
Be wary of quick-disconnect connectors and threaded swivel air-hose fittings. While these couplings offer convenience and the swivel reduces kinking, their internal passageways can be much smaller than their external openings.
POSSIBLE CAUSE 6: Undersized Plumbing
Undersized plumbing, just like an undersized air-supply line, restricts air flow to a blast nozzle and causes a pressure drop throughout the blast system. So make sure that a blast machine’s plumbing is correctly sized to support the nozzle; otherwise, the blast system will be unable to supply the blast nozzle with enough CFM. Be especially wary of “hobby” style blast machines—they almost always have small plumbing that restricts air flow.
3. Improperly Trained Operators
PROBLEM: Productivity Dives and Safety Hazards Rise with Unprepared Operators
Blast operators who do not understand their jobs or equipment cannot recognize when their equipment is underperforming or when a breakdown is around the corner. This lack of understanding, regardless of whether it is due to poor training or operator indifference, translates into drops in productivity, more downtime, and profit loss. It also dramatically increases safety risks that can lead to serious injury or even death to an operator and to others in the vicinity.
SOLUTION: Thorough Hands-On Training and Understanding Printed Material
For blast machine performance and safety reasons, operators must be thoroughly trained in how to use their blasting and safety equipment. They must not attempt to assemble or repair equipment until they have completed in-depth training on the blasting process, equipment functions, and safety procedures. Furthermore, they must precisely follow instructions and perform all necessary or recommended maintenance.
Training must include hands-on instruction on proper blast and safety equipment use, in addition to operators reading and understanding all equipment instructional manuals, tags, labels, and other printed materials furnished with their equipment.
An Ounce of Prevention …
Avoiding the abrasive blasting mistakes discussed on this web page is not rocket science. But lack of awareness of these potential mistakes can kill productivity, increase downtime, and lead to safety hazards. But with consistent practice, following the tips on this web page becomes second nature, and then so does safer, more efficient, and more profitable abrasive blasting.