- Email: [email protected]
Abrasive blasting systems
by Dave
Hansel
Clemco Industries
Corp., San Francisco
A
brasive blasting is the process of propelling abrasive particles from a blast machine, using the power of compressed air. Three fundamental components constitute a blast equipment setup: air compressor, blast machine, and abrasive. The compressor must produce sufficient air pressure and volume to convey abrasive from the blast machine to the surface being blasted. Cleaning takes place as a direct result of how efficiently air moves from the compressor to the surface. A restriction in just one part reduces the entire system’s production rate. A blast machine’s role is to smoothly meter abrasive into the passing air stream. Sometimes, contractors install restrictive air fittings or hoses that actually cut the airflow in half. Avoid these selfimposed problems by selecting a blast machine with large-diameter piping and by connecting it to largediameter air and blast hoses. The third major component in the blasting system is the abrasive. It produces the finish on the surface. Match the abrasive material and size to the surface being blasted to ensure the best possible finish, cleaning speed, and cost efficiency. Count on your abrasive supplier to recommend the best abrasive for the job. With these three components in place, a trained, skilled blast operator has the tools to produce a flawlessly finished surface. APPLICATIONS
Abrasive blasting applications fall into three broad categories-surface preparation, surface cleaning and finishing, and shot peening. Surface preparation removes unwanted material and leaves a surface ready for coating or bonding. Blast cleaning existing steel structures removes old paint, rust, and other contaminants. Blasting removes the mill scale that forms on new steel. The impact of angular abrasive roughens a surface to produce a profile or etch. Most paint manufacturers specify surface profiles that will ensure their products perform as intended. Contractors blast masonry so it will accept sealers or paints. Blast cleaning stucco and brick removes loose paint, mildew, soot, stains, and graffiti and leaves an ideal surface for coating. July
2000
Contractors blast prestressed concrete panels, poured-in-place walls and columns, and other concrete structures to remove latent cement, form marks, and discoloration; and to expose colored aggregate. Beyond steel and masonry, blast cleaning can strip layers of paint from wood. On fiberglass boats, blasting removes the top layer of gelcoat and exposes air bubbles. Blasting aluminum, titanium, magnesium, and other sophisticated metals removes corrosion and, depending on the abrasive and pressure used, leaves a surface profile. Companies use nonaggressive media to dry-strip airplanes, helicopters, cars, trucks, and boats without using rotary sanding tools that might damage the surface. Switching to dry stripping eliminates worker exposure to toxic chemical strippers and the expense of properly disposing of this hazardous waste. As industries create new materials and new surface preparation problems, blast equipment manufacturers race to develop the technology and tools to meet these challenges. Surface cleaning and finishing differ from surface preparation in that the desired result is to improve a product’s appearance and usefulness rather than condition it for coating or bonding. Surface cleaning includes removing production contaminants and heat scale. Surface finishing includes deflashing and deburring molded parts and enhancing visual features. Low-pressure blasting with glass or ceramic beads produces a blended matte appearance with a precise roughness average (Ra) finish on soft metals. Metal foundries blast cast parts to remove small burrs for functional and aesthetic purposes. In most cases blasting exposes minute cracks and defects in metals. This is especially important to facilities that repair and recondition aircraft wheels and landing gear components. Soft materials, such as rubber and plastic, usually emerge from molds with flashing left by tooling separation. Abrasive blasting trims the flashing, leaving a smooth, uniform finish. Using heat to harden metals can discolor parts. Blast finishing heat-treated parts removes the discoloration and the scale that sometimes forms. 23
Abrasive blasting can improve a product’s appearance by removing stains, manufacturing compound residue, corrosion, and tool marks. Some blast media can blend surface variations, such as scratches and tooling marks, into an overall uniform appearance. High operating temperatures build up carbon and burnt oil on many automotive parts. Electric motors become clogged with overheated insulation and melted stator lamination. In most cases retaining original dimensions of these parts is critical. Blasting with plastic media, glass bead, or agricultural abrasive removes the contaminants and provides an acceptable finish without affecting tolerances. Shot peening increases the strength and durability of high-stress components by bombarding the surface with high-velocity, spherical media such as steel shot, ceramic shot, and glass beads. To make a metal product or component, manufacturers must cast, cut, bend, stamp, roll, or weld metal stock to produce the desired shape. Sometimes these processes leave residual stresses in the metal that can cause parts to fail when stressed. Shot peening produces an effect similar to what is produced by pounding a surface with a ball peen hammer, except that the dimples left by shot peening are much smaller and the impacts more consistent in their distribution and intensity. This bombardment creates a uniformly compressed surface, diffusing the stress forces over a larger area and leaving the surface less likely to crack. Shot peening is a precise technology, requiring adherence to exacting specifications for media hardness, blast duration, nozzle angle, and pressure. Under- or overpeening a part may cause premature failure. The automotive and aircraft industries use peening extensively. Gear manufacturers peen to eliminate burrs and sharp edges, and to strengthen gear teeth. Spring manufacturers peen their products to combat stress. Shot peening metal castings and forgings cleans the surface, exposes defects, and improves appearances. Peening threaded parts removes sharp edges while increasing thread-holding power. It is often used to remove mill scale from new steel. SURFACE
PREPARATION
bonding
surface
while
ensur-
standards for their products. Failure to follow specifications may void a coating’s guarantee. Steel surface preparation standards measure two critical specifications-surface profile and degree of cleanliness. Coating manufacturers and professional organizations test paint systems by applying them over various surface profiles and subjecting them to a wide range of environmental conditions. They have found that coatings require specific profiles to ensure adhesion to and complete protection of the substrate. The profile provides a mechanical method of positive, uniform bonding between the coating and the surface. Abrasive particles cut into the steel to form tiny peaks and valleys. The depth of this profile is controlled by the size, type (see Figs. l-31, and hardness of the abrasive; by the air pressure; and by the distance and angle of the nozzle to the surface. Applying heavier layers of paint to compensate for deep profiles adds substantially to the cost of the job. Profiles are expressed in mils, microns, or millimeters. l l l l
1 mil = l/1,000 in. 25 microns = 1 mil 25.4 millimeters = 1 in. 39 mils = millimeter
SPECIFICATIONS
Paint manufacturers have long recognized the importance of surface preparation to the success of their coatings. Improperly cleaning a steel surface will cause premature coating failure; consequently, coating manufacturers specify surface preparation 24
Figure 1. Correct etch maximizes ing complete coating coverage.
Figure 2. Too little etch reduces the material and the substrate.
the
bonding
surface
between
Metal
Finishing
\
/
PAINT
dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter, except for some staining. This is similar to White Metal but allows slight staining on up to 5% of the metal. This degree of cleaning is required where high-performance coatings will be applied to steel exposed to harsh elements and heavy usage. Commercial
Figure creating
3. Too much etch allows surface to show througha starting point for corrosion and coating failure.
In the U.S., mil describes paint thickness as well as surface profile. Typically, specifications state a profile height average. For example, an average profile of 2 mils (50 microns) may include profiles as small as 1 mil(25 microns) and as large as 3 mils (75 microns). This range is acceptable because there is no practical method to produce identical abrasive particle sizes. Deviations in air pressure or in the distance or angle of the nozzle to the surface also affect profile. Reduced air pressure or increased nozzle distance causes smaller profiles. Severe nozzle angles produce a skimmed blast pattern rather than definite peaks and valleys. For blasting structural steel, nozzles should be held at 80 to 90” to the surface. Use a profile-measuring device to document profile conformance. Careful monitoring of the profiles can prevent expensive rework. DEGREES
OF CLEANLINESS
SSPC: The Society for Protective Coatings has established four degrees of cleanliness for blasting, ranging from removal of all contaminants to removal of loose materials only. The four degrees are White Metal Blast, Near-White Metal Blast, Commercial Blast, and Brush-Off Blast. For detailed descriptions of each, refer to the SSPC’s Visual Standards for Abrasive Blast Cleaned Steel. White
Metal
Blast
Viewed without magnification, a white metal blast cleaned surface is free of visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter. This degree of cleanliness is usually required where sophisticated paints, such as zinc-rich coatings, will be applied to surfaces exposed to highly corrosive environmentschemical plants, offshore drilling rigs, and bridges over salt water are typical applications. Near-White
Metal
Blast
Viewed without magnification, a near-white metal blast cleaned surface is free of visible oil, grease, July
2000
Blast
Viewed without magnification, a commercial blast cleaned surface is free of visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter, except for more extensive staining-up to 33% of the metal. For most applications where standard coatings will be applied, commercial blast is specified. Brush-Off
Blast
Viewed without magnification, a brush-off blast cleaned surface shall be free of all visible oil, grease, dirt, dust, loose mill scale, loose rust, and loose coating. Some tightly adhering residues of mill scale, rust, and coatings may remain. Tightly adhering means the contaminant cannot be removed by lifting with a dull putty knife. This degree of cleanliness is acceptable for surfaces not subjected to severe environments or where long-term coating life is not expected. The SSPC offers sets of photographs that show four existing steel surface conditions and the degrees of cleanliness for each. The existing conditions include mill scale, mill scale and rust, total rusting, and rust with pitting. The National Association of Corrosion Engineers (NACE) offers a set of encapsulated steel coupons that simulate the four degrees of cleanliness. The Swedish Standards Institution’s (SIS) book of pictorial comparators is widely used in Europe. COMPRESSED
AIR:
AN
ENERGY
SOURCE
A standard air/abrasive pressure blast system uses compressed air to pressurize blast machines, convey abrasive to nozzles, provide breathing air, and operate valves and accessories. Pressure
and Volume
A compressor’s power output is measured in pressure and volume. Pressure is expressed in pounds per square inch (psi), or pounds per square inch gauge (psig), and by volume in cubic feet per minute (cfm). Most air tools consume compressed air intermittently. Abrasive blast cleaning consumes a steady supply of high-pressure, high-volume air. Blast equipment demands more from a compressor than any other air-powered tool. 25
Both a l-hp and a 100-hp compressor can produce 100 psi, but it takes the efficient, powerful lOO-hp compressor to generate the great volume of air needed for blasting. At 100 psi the 1-hp compressor generates about 4 cfm of air, but the typical lOO-hp compressor generates 400 to 450 cfm. This great volume of air must be sustained to maintain the 100 psi needed for blasting. Increasing the air pressure increases the volume of air flowing out the nozzle. If the compressor does not produce the volume of air required by the nozzle, it will never achieve the required pressure. For example, at 100 psi a G-in. orifice blast nozzle allows 200 cfm of air to pass through it. To maintain 100 psi the compressor must produce at least 200 cfm. A compressor rated at 150 cfm will never reach 100 psi because the V&in. nozzle lets air out faster than the compressor can force it in. Each one-pound drop in pressure reduces production by as much as 11/2%. In the example above the overtaxed compressor may reach just 70 psi, which will cut the blasting production rate by 45%. Most contractors blast clean structural steel at 100 to 140 psi. In the U.S., standard blast machines are rated at 125 psi. High-production machines, built from heavier steel, are rated at 150 psi. The compressor’s pressure to the system should never exceed the blast machine rating. For typical blasting applications, 90 to 100 psi, combined with hard, sharp abrasives in standard mesh sizes, delivers acceptable production and surface cleanliness. Higher pressures increase production rates but require durable, recyclable abrasives. Proper pressure is based on the surface condition, the abrasive used, the required surface finish, and the production rate desired. COMPRESSOR
TYPES
AND
SELECTION
Generally, the high pressure and high volume of air required for blasting dictates the use of a rotary vane or rotary screw compressor. Do not use old-style, piston compressors for blasting because they produce pressure fluctuations that vary blasting speed and affect surface finish. Also, piston compressors require a lot of lubrication, which can send oil through the air lines to contaminate the abrasive and surface being blasted. Some rotary screw compressors inject oil onto the screw to cool it. If the compressor is not functioning properly some of this oil may be carried into the air lines. Oilless compressors have sealed bearings. The 26
screws are not cooled by oil so they produce much hotter air. Choose an air compressor that will generate a steady flow at high pressure and high volume, and that is built to withstand the environmental conditions found at a blast site. For blasting, oil-free rotary vane and screw compressors are best. Select a compressor that will satisfy your current and anticipated demands, plus enough reserve to compensate for nozzle wear. The compressor is the workhorse of a blasting system. Do not run it at maximum power for long periods, as this will accelerate wear. To determine the size compressor needed, add the air requirements of all equipment, plus a 50% reserve. The compressor manufacturer can recommend the size and type of system best suited for your applications. Consider the size and type of the compressor’s air outlets. To regulate airflow, many compressor outlet valves have internal slotted plugs about half the size of the valve opening. A l-in. valve actually has a X-in. air passageway, far too small to supply a blast machine. Do not use restrictive air valves or quick-disconnect couplings. The smallest internal diameter of the compressor air outlet should be at least four times the size of the nozzle orifice or larger. For a ?&in. blast nozzle, the compressor air receiver fitting, air valve, and air couplings should all have IDS of at least 11/2 inches. Remember, the smallest opening in the air supply system controls the amount of air to be delivered to the blasting unit. Operation and Maintenance Before buying or renting a compressor tell the sales representative you intend to use the compressor for abrasive blasting. The sales representative should supply information on installing, operating, and maintaining the compressor. Read the manufacturer’s instructions before operating the equipment and follow the instructions, warnings, and maintenance procedures. Position the compressor upwind of dust generated by blasting. Dust, dirt, or other contaminants entering the compressor air inlets will cause premature wear. Exhaust fumes contain carbon monoxide (CO). If exhaust fumes enter the air inlet, blasters connected to the compressor can be killed from inhaling carbon monoxide. Locate the compressor where vehicle exhaust will not enter the air inlets. Do not allow vehicles to park Metal
Finishing
near the compressor. Direct the compressor’s own exhaust away from its air inlet by attaching metal pipe to its exhaust stack and running the pipe downwind from the inlet. Install safety cables at every coupling to prevent injury if fittings disengage and to support the weight of elevated hose. BLAST
MACHINES
There are two types of blast equipment-suction blast and pressure blast. Suction blasting is less aggressive than pressure blasting. It is used in blast cabinets and for light-duty work such as touch-up blasting. Pressure blasting is sometimes used with blast cabinets but is more often used in blast rooms or outdoors to clean tough surfaces and large areas. Suction
Blast
Suction blasting, sometimes called venturi blasting, draws abrasive from a nonpressurized container into a gun chamber then propels the abrasive particles out a nozzle. A suction system consists of a blast gun, an air hose, a media hose, and an abrasive container. Compressed air flows through an air jet in the blast gun to create suction. This suction brings abrasive up through the media hose into the gun body where it is accelerated out the nozzle with the air. The volume of compressed air required for suction blasting is determined by the ID of the air jet orifice in the back of the blast gun not the ID of the suction gun nozzle. A typical suction gun air jet is half the size of a typical nozzle orifice for pressure blasting. This means it will use about one-fourth the volume of air, and propels abrasive to less than one-fourth the velocity created by pressure blasting. Suction blasting is used on softer metals for mild deburring, light shot peening, and scale removal without penetrating the base metal. Such metals include aluminum, titanium, and magnesium. Pressure
Blast
Blast machines are known by a variety of namespots, pressure generators, tanks, and so on. This article will refer to all systems that hold abrasive under pressure as blast machines. A pressure-blast system feeds abrasive, via a metering valve, into a moving stream of compressed air. Air and abrasive travel through a blast hose at high pressure and speed, exiting the nozzle at more than four times the velocity produced by suction blasting. Pressure blast machines are used in structural steel blasting for their high production rates, and in July
2000
Figure 4. A modern, high-production, 150 psi, delivers 20% more cleaning chines.
lightweight media blasting tion of media flow. BLAST
MACHINE
blast power
machine, rated for than standard ma-
for their precise regula-
CONSTRUCTION
In the U.S., blast machines and other pressurized containers must meet American Society of Mechanical Engineers (ASME) standards. ASME specifies the type of steel and welding methods, and an ASME-Authorized Inspector supervises the hydrostatic testing of each pressure vessel, then issues a National Board certificate of approval. A metal plate bearing the board approval number is permanently affixed to the blast machine. In the U.S., most blast machines are rated for working pressures of 125 to 150 psi. The ASME requires a safety margin 50% greater than the working pressure; so 125-psi machines are tested at 188 psi and 150-psi machines at 225 psi. A well-engineered blast machine (see Fig. 4) allows smooth air and abrasive flow throughout the system and is easy to operate and maintain. The machine’s semielliptical, concave head stores abrasive, which flows into the machine when it depressurizes . Most machines have a 35” (or steeper) conical bottom to allow abrasive to flow freely into the metering valve below. Steel grit and other common abrasives have a 32” angle of repose. This is the natural slope of the abrasive when it is poured to form a mound. Plastic and agricultural media have much steeper angles of repose. Blast machines for use with these lightweight media should have 60” cones to ensure free flow. A modern blast machine seals automatically with a pop-up valve-a cone-shaped aluminum casting coated with wear-resistant urethane. When air enters the machine, the internal piping guides the pop-up to seal against a rubber gasket in the valve opening. Upon depressurization, the pop-up valve drops to 27
Install a pressure gauge on the blast machine’s air inlet. Test for pressure loss (without abrasive in the air stream) by inserting a hypodermic needle gauge into the blast hose near the point where it connects to the machine. If the difference is more than 10 lb, examine each component to find the restriction and replace it with an appropriately sized part. BLAST
Figure 5. This blast machine traps any abrasive entrained
muffler in the
reduces exhaust
exhaust air.
noise
and
allow abrasive stored in the concave head, or a storage hopper, to flow into the machine. A well-made blast machine has a muffler to reduce the noise of the escaping air when the machine is depressurized and momentarily traps any abrasive carried out with the exhaust. (See Fig. 5.) Air and media flow through pipes, valves, hoses, nozzles, and couplings that are all cylindrical. Any reduction in the diameters of these cylinders dramatically reduces the rate of flow. High-production blast machines use large-diameter pipe to minimize pressure loss. A l-in. ID cylinder has an area of 0.80 in2. A X-in. ID cylinder has an area of 0.20 in2. Reducing the diameter of the cylinder by half reduces its area three-fourths. The nozzle should be the smallest orifice between the compressor and the surface being blasted. From the compressor to the nozzle, the internal diameters of the valves, pipes and fittings, blast hoses, and couplings must all be three to four times the ID of the nozzle. If the operator must work at distances greater than 100 ft from the blast machine the ratio of hose ID to nozzle orifice is even greater. On blast machines with capacities greater than one cubic foot, piping is typically l-in. or l%in. ID. There will always be some pressure loss due to the turns and bends inherent in piping. Follow the above recommendations to minimize those losses. A single-operator blast machine should suffer no more than 8 to 10 lb of pressure loss from the air inlet to the outlet coupling on the bottom of the machine. 28
MACHINE
SELECTION
Choose a blast machine that has the capacity, portability, and convenience features that best fit your needs and type of work. The diameter of the nozzle orifice determines the amount of work done and the amount of air and abrasive used per hour. A %-in. nozzle cleans four times faster than a %-in. nozzle, and it uses four times the volume of air and abrasive. Based on the nozzle and compressor you plan to use, choose a blast machine with an abrasive capacity to supply 20 to 30 minutes of steady blasting. Choose a nozzle size, then refer to the Abrasive Consumption Chart (see Table I). Under the loo-psi column find the amount of abrasive consumed in an hour by the chosen nozzle, then calculate the minimum size machine. The table provides comparison information only. Your actual consumption rate will vary based on metering, compressed air quality, hose layout, and blast machine efficiency. For example, a %-in. nozzle consumes about 11 ft3 of abrasive per hour at 100 psi, so your blast machine should hold at least 5.5 ft3 to last 30 minutes. As the nozzle orifice wears to %6 in., abrasive consumption will increase to more than 15 ft3 per hour, or more than 7 ft3 per half hour, which reduces a 6-ft3 machine to about 20 minutes of blast time. Machines that are too small waste the blast operator’s time refilling or waiting for others to refill the abrasive. Machines that are too big are unnecessary because the operator usually will have to reposition after 30 minutes. Light-duty machines have a capacity of 1/2to 1 ft3, have small piping, short blast hose, and straightbarrel nozzles. Their light weight and portability make them ideal for spot cleaning. Medium-duty machines offer pickup-truck portability with capacities from 2 to 4 ft3. They usually have l-in. piping and blast hose, suitable for use with 3/6-, l/4-, and 5/6-in. venturi nozzles. Mediumduty machines are ideal for jobs that take an hour or two to complete. High-production machines offer capacities from 4 to 20 ft3, with 1%in. or l%-in. piping and blast hose feeding %- to %-in. nozzles. Popular among blasting Metal
Finishing
Table
I. Air and Abrasive
Consumption
with
Compressor
Horsepower
Required
(with
abrasive
weighing
approximately
100 Ib/ft3)
Orifice, Nozzle No. 2
in. l/8
No. 3
3116
No. 4
l/4
No. 5
5116
No. 6
318
No. 7
7116
No. 8
l/2
“Requires
blast
20 psi 4.4 26.8 1 10.4 60 2.4 18.8 107.2 4.4 30.8 187.2 7.2 43.2 267.2 9.6 58.8 358.4 13.2 78 464 17.6
cfm lb/h b cfm lbh hp cfm lbh hp cfm lb/hr b cfm lb/hhp cfm lblhr hp cfm lb/l-u hp
machine
rated
for high
30 psi 6.6 40.2 1.5 15.6 90 3.6 28.2 160.8 6.6 46.2 280.8 10.8 64.8 400.8 14.4 88.2 537.6 19.8 117 696 26.4
40 psi 8.8 53.6 2 20.8 120 4.8 37.6 214.4 8.8 61.6 374.4 14.4 86.4 534.4 19.2 117.6 716.8 26.4 156 928 35.2
50 psi 11 67 2.5 26 150 6 47 268 11 77 468 18 108 668 24 147 896 33 195 1,160 44
2000
70 psi 15 88 3.5 33 196 8 61 354 14 101 604 23 143 864 32 194 1,176 44 252 1,512 56
80 psi 17 101 4.0 38 216 9 68 408 16 113 672 26 161 960 36 217 1,312 49 280 1,680 63
90 psi 19 112 4.5 41 238 10 74 448 17 126 740 28 173 1,052 39 240 1,448 54 309 1,856 69
100 psi 20 123 5.0 45 264 10 81 494 18 137 812 31 196 1,152 44 254 1,584 57 338 2,024 75
125 psi” 25 152 5.5 3:: 12 98 608 22 168 982 37 237 1,393 52 314 1,931 69 409 2,459 90
140 psi” 28 170 6.2 62 357 13 110 681 25 188 1,100 41 265 1,560 58 352 2,163 77 458 2,754 101
pressure.
contractors and industrial installations, high-production machines deliver the abrasive capacity needed for extended blasting. Bulk blast machines offer high-production with abrasive capacities from 60 to 800 ft3. They usually have two to four outlets. Portable bulk blast machines have wheels onlyfor movement within a work site-or with brakes, lights, fenders, and other equipment that allows them to be towed on public highways. Most bulk machines can be towed while full; however, moving one filled with steel grit may exceed the gross vehicle weight (GVW) rating. For large jobs, bulk blast machines can save money. Bulk abrasive costs less than bagged, and the time spent loading bagged abrasive is nonproductive. Blower trucks deliver bulk abrasive to onsite storage hoppers. Bulk equipment is common in shipyards and industrial facilities, and is preferred by large blasting contractors. (See Fig. 6.) Bulk blast machines do not blast harder or clean faster than other high-production machines. The size and number of nozzles used determines the production rate. Often, the versatility of four standard-size machines, in conjunction with storage hoppers and transfer equipment, is preferable because the individual machines can be shut down for maintenance or positioned in different areas to reduce blast hose lengths. July
60 psi 13 77 3.0 30 171 7 54 312 12 89 534 20 126 764 28 170 1,032 38 224 1,336 50
Pressure Regulators and Gauges Install a pressure regulator with a gauge on the blast machine to set and monitor the air pressure. Maintaining the correct operating pressure guarantees optimum performance and ensures the machine is working within its pressure limits, normally 125 to 150 psi. The compressor gauge shows air pressure at the compressor outlets, the beginning of the system. The hypodermic needle gauge indicates pressure at the nozzle, the end of the system. A pressure gauge at the blast machine inlet lets you quickly check for pressure loss. Abrasive Metering Valves A metering valve uses gravity to feed abrasive into a fast-flowing stream of compressed air. Using too little abrasive slows production and leaves un-
Figure 6. Highway-towable and yard-towable machine ators.
bulk blast machines (with fenders) (center), serve multiple blast oper-
29
touched surfaces. Using too much abrasive wastes energy and disperses particles unequally within the blast pattern. Exorbitant abrasive usage also wastes material and labor. Properly adjusting the metering valve ensures that you get the maximum amount of cleaning power from each abrasive particle. True metering valves have wear-resistant internal materials housed in sturdy bodies. The valve should have a simple, yet precise, adjustment feature and a scale or gauge to indicate the relative setting. An inspection plate facilitates removal of foreign materials that clog the valve. Remote Controls The Occupational Safety and Health Administration (See: OSHA 29 CFR 1910.244) requires remote controls on all blast machines. Failure to use remote controls can expose a blasting contractor to substantial fines and liabilities if someone is injured or killed. In addition to their safety features, remote con-
trols save substantial amounts of compressed air and abrasive. If the blast operator must wait for someone to shut off the machine, air and abrasive are squandered. Also, removing the need for a dedicated pot tender saves labor by allowing one person to load abrasive for several machines or do other work between refills. Most remote control valves operate pneumatically, but there is a choice between pneumatic or electric actuation. Pneumatic systems are suitable for most applications and usually cost less than electric systems. Pneumatic remote control handles work well at distances up to 100 feet. Electric remote control handles are recommended for distances of 150 ft or more. BLAST
NOZZLES
The objective of everything up to this point is to convey a steady supply of abrasive at adequate pressure to the nozzle. Performance at the blast nozzle
SIMPLE - Automatic Gage Setup and two-button operation allow you to take basic measurements or perform advanced functions easily. Plus, everything you need comes with the gage - you’ll be measuring in seconds! DURABLE - Tough Probes, Robust Housing, Strong Warranty... the PosiTector 6000 is made to measure in rugged environments. Unaffected by vibration, this heavy-duty gage also resists exposure to solvents, acids, oil, water and dust. ACCURATE - Measuring with High Resolution and Accuracy each quality-tested PosiTector 6000 gage leaves our facility with a Certificate of Calibration providing traceability to NIST.
Call us or check I-800-448-3835
our website for details: www.defelsko.com l
CORPORATION, 802 Proctor Avenue P.O. Box 676, Ogdensburg, New York 13669 l Fax: 315-393-8471 Phone: 315-393-4450
DEFELSKO
Circle 007 on reader information card 30
Metal
Finishing
LAPPING? reveals whether or not all of the previous requirements for air and abrasive flow have been correctly followed. Nozzles accelerate air-driven abrasive into a highly effective cutting force that can tackle the toughest application. The size, type, and shape of the nozzle determine the production speed and the appearance of the end product. Nozzle material primarily affects wear life, which is more than just how long a nozzle will last; it is critical to air usage.
For faster cutting try ElectraAbrasives’ Boron CarbidePowder. ElectraAbrasives’ Boron Carbide Powder cuts faster and cleaner than conventional Silicon Carbide finishing powders. Other Boron Carbide benefits include: Longer Life - Cuts down time, and increases efficiency. Faster Cutting - Maximizesyour per-hour product output. Higher Pressures- Handles any application with ease. l
l
l
ElectraAbrasives has Boron Carbide Powders from #220 to #1200 that has application in loose abrasive machining of: Tungsten Carbide Ceramics, Composites Tool Steel l
Figure 7. Large-diameter blast hose, couplings, allow compressed air and abrasive to flow easily machine to the surface being cleaned.
and nozzles from the blast
As the nozzle wears, it uses more air volume to maintain a given air pressure. A nozzle with a S-in. orifice operating at 100 psi requires about 200 cfm. When the orifice enlarges by Ms inch, the air requirement increases to more than 250 cfm-a 25% increase. Replace a nozzle when its orifice wears to Yt6 in. larger than its original size. In addition to wasting air, a nozzle worn beyond 1/6 in. could cause injury if the liner fails. Common nozzle materials include ceramic, tungsten carbide, silicon carbide, and boron carbide. Use ceramic nozzles with nonaggressive abrasive in light-duty equipment and blast cabinets. Carbide nozzles-tungsten, silicon, and boron-are popular for the majority of blasting applications, due to their long life. The abrasive type and blast pressure will affect nozzle life. Tungsten carbide is a hard, heavy material used in many industries for wear resistance. Tungsten carbide is sintered, a process that uses extreme heat and pressure to produce one-piece liners in a mold. Hardness, however, contributes to the nozzle liner’s brittleness. Tungsten carbide nozzles last about 300 hours when used with expendable abrasive. (See Fig. 7.) Silicon carbide grew from research on lightweight and durable materials for aircraft and aerospace industries. Silicon carbide weighs 42% less than a
l l
Our Boron Carbide is available to ISO, FEPA, and ANSI specifications. Produced at our new powders facilities in Buffalo, New York, we will meet your exact need. CalI today. Learn how our full line of premium abrasive grains & powders satisfy your needs.
Circle July
2000
012 on reader
information
card 31
comparable tungsten carbide nozzle, making it easier to hold for a long time. With expendable abrasive, silicon carbide lasts up to 500 hours, or 50 to 65% longer than tungsten carbide. Boron carbide is the longest wearing nozzle liner material. It is especially effective when using extremely sharp abrasive such as aluminum oxide and silicon carbide. With expendable abrasive, boron carbide lasts up to 1,000 hours. The purchase price for boron carbide is two to three times that for silicon and tungsten; however, the cost per operating hour may be less compared to tungsten, or more compared to silicon. Some liner materials are better suited for specific abrasives. Choose a boron carbide nozzle when using aluminum oxide or silicon carbide abrasive. Boron tolerates extremely sharp particles better. For steel grit, steel shot, or any iron abrasive, use a tungsten carbide nozzle. The high density of steel abrasive causes chipping on other carbide liner materials.
Circle July
2000
023 on reader
Carbide nozzle liners are brittle. They come encased in a protective jacket of metal, urethane, or both. Nozzle Shape Most contractors use blast nozzles with wide coneshaped entrances and gradually tapered exits, which together form a venturi. The venturi’s length, angles of entrance and exit, and orifice size are precisely calculated to maximize acceleration of the abrasive and air. Abrasive enters the converging end of the nozzle, funnels through the orifice, then rapidly expands into a high-powered stream through the diverging exit end. At 100 psi the velocity at the end of the nozzle reaches 660 feet per second. By comparison, abrasive exits a straight barrel nozzle at 318 feet per second. As a venturi nozzle wears beyond YE in. over its original size, it loses its venturi shape and much of the accelerating force that shape provided. An extremely worn venturi nozzle wastes air and delivers
information
card 33
a blast velocity and pattern comparable straight-barrel nozzle. A well-designed venturi nozzle has precise orifice, and outlet dimensions to accelerate the sive and disperse it uniformly within the blast tern-with no scatter and no hot spots. This vides consistent cleaning results.
to
a
inlet, abrapatpro-
Nozzle Pressure Maintaining adequate nozzle pressure is essential to high-production blasting. The gauge on the compressor shows the air pressure at the compressor only. It does not indicate blasting pressure. Hoses, air filters, blast machines, and other components between the compressor and the nozzle all contribute to friction and pressure losses. To accurately determine nozzle pressure, use a hypodermic needle gauge. This simple tool consists of a needle mounted on a pressure gauge. Insert the needle into the blast hose at a 45” angle, about 6 in. behind the nozzle holder, with the tip of the needle pointing toward the nozzle. The
needle should penetrate enough to position the tip into the center of the air stream. The gauge will register the actual pressure at the nozzle. If you are blasting on structural steel at 100 to 110 psi, you would probably accept anything between 90 and 105 psi. In a blast system with 50 ft each of blast hose and air hose, a pressure drop of 10 to 15 psi is expected. Nozzle pressure below 85 psi indicates something wrong. Check the air compressor setting, then check for restrictions at all hoses and fittings, moisture separators, and any system components. Also check the nozzle orifice for excessive wear. Nozzle Care Nozzles are expensive; nozzle gaskets are cheap. Using a worn gasket or worse yet, no gasket, allows the air-driven abrasive to destroy the nozzle jacket and the nozzle holder. Check all gaskets daily and replace with gaskets of the proper diameter to maintain a smooth flow from the hose to the nozzle.
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Water use by 6 to 10 gallons per minute Contaminated Effluent by 6 to 10 gallons a minute Concentration of cleaning solutions being used
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Noxious fumes, dangerous flames, or unsafe fuel storage that are present with “fuel fired” cleaning systems Mixing of chemicals
l
Circle 013 on reader information card Metal Finishing
Most nozzles are destroyed before they reach the end of their useful life, primarily due to careless handling. The same extreme hardness that helps nozzle liners withstand abrasive flow makes them susceptible to cracking from impacts. Never drop, throw, or strike other objects with a nozzle. OPERATOR
SAFETY
EQUIPMENT
Blasting can be dangerous for a poorly trained, poorly equipped operator. A blast machine produces a powerful stream of sharp particles that, in addition to cleaning a surface, creates clouds of potentially toxic dust. To prevent a variety of injuries and illness, personal safety equipment is essential for blast operators and anyone in the work area. Potential hazards of abrasive blasting include high-pressure compressed air; air-propelled abrasive from the nozzle; impurities in the breathing air; toxic dust from pulverized abrasive and coatings; loud noises from blast nozzles and compressor mo-
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tors; potential head impact injuries from beams and equipment; plus, all the other hazards normally associated with construction sites. Air-Fed Helmets Employers are required to establish a comprehensive respirator program. [See: OSHA 29 CFR 1910.134 (a) (b) and 29 CFR 1926.103.1 OSHA defines an air-fed helmet for abrasive blasting as a continuous-flow, supplied-air respirator. A helmet should furnish the operator with breathing air, protect his or her head and face from impacts, muffle noise, and allow an unobstructed field of vision. (See Fig. 8.1 The helmet lens protects the operator’s face from rebounding abrasive. Most helmets have a frame to hold several thin, sacrificial lens covers, which protect a thick inner lens. The thin lenses usually have tabs and perforated borders that permit the operator to tear away the frosted outermost lens to expose
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July
2000
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tor and equipment needs. They usually have filters to remove particulate matter and moisture. Some contractors use one compressor to supply breathing air and blast air. Take extra precautions when lubricated compressors supply your breathing air. Oil-lubricated air compressors sometimes produce CO, a colorless, odorless, deadly gas. To help prevent operator exposure to CO, service the compressor at the manufacturer’s recommended intervals, and install overheating shut-off devices and/or CO alarms. SAFETY
Figure while
8. A blast operator checks wearing the high-pressure,
the settings supplied-air
at his in-line respirator.
filter
another lens. Air furnished to helmets must be clean, dry, contamination-free, and at NIOSH-prescribed pressure and volume. Pay special attention to the source, filtration, and composition of the air. Read all instructions for the equipment used to produce and convey breathing air. Breathing air must meet the minimum requirements, Grade D or higher, described in the Compressed Gas Association Commodity specifications G 7.1. Never attach a breathing air hose to any source without first testing the quality of air. Oil-free air pumps supply breathing air to lowpressure (up to 20 psi) helmets. Air pumps do not compress air; they merely draw in ambient air and push it through the hose. Air pumps will not produce carbon monoxide. Their insides are coated with friction-resistant materials, so they require no lubricants. Because they have built-in filters, air pumps do not need overheating shutdown devices or in-line filters. These pumps are available for one to four operators. Most are hand portable. The best compressors for high-pressure breathing air work without oil lubricants, and, therefore, do not create CO or contaminate the air with oil. Most can sustain the pressure required for highpressure helmets. They cost more than other compressors but their advantages far outweigh the added cost. Oilless compressors come in portable and stationary models, in capacities to support different opera36
PRACTICES
Block off the blasting site and surrounding area to prevent unprotected personnel from entering. Safety personnel should monitor atmospheric dust to determine the size of the blasting zone. Weather, humidity, wind direction and velocity, abrasive composition, type of material being removed, and other factors determine the extent of the zone. Periodically test the atmosphere and adjust the size of the zone. In enclosures, such as tanks and blast rooms, all personnel must wear approved respirators at all times. Enclosures must be ventilated to bring in fresh air and extract dust in sufficient volume to maintain a low concentration of dust. (See: OSHA 29 CFR 1910.1000.) Do not use silica sand in enclosed areas because it generates harmful dust. The Occupational Safety and Health Administration (OSHA) enforces stringent regulations on supplied-air respirators and pressure-demand respirators for abrasive blasting. All manufacturers must submit their respirators to NIOSH, OSHA’s agency for testing, to obtain approval. Manufacturers must display a copy of the approval certificate in their instruction manual. Using, a nonapproved respirator (or using a respirator with nonapproved replacement parts) violates OSHA regulations, and subjects the respirator’s owner to stiff penalties. Employers must supply all required safety clothing and decontamination equipment and enforce their use. Also, employers must train their employees-even when that means reading the instructions to the illiterate and translating instructions into foreign languages. TRAINING
THE
ABRASIVE
BLAST
OPERATOR
Since 1971, OSHA regulations have made it clear that employers are solely responsible for providing safe working environments for their employees. Employers must provide training, supply all personal Metal
Finishing
protection equipment, and enforce effective safety programs. Employers take on a tremendous amount of work to train and equip employees, but in reality, everyone wins. The properly trained employees become craftsmen, taking pride in their work, and become valuable assets to their employer. Employers benefit from the increased productivity, which increases profit. SSPC: The Society for Protective Coatings markets a set of four training videotapes on surface preparation and coatings application. The first is devoted to surface preparation equipment, which includes hand and power tools, and high-pressure water blasting, in addition to wet and dry abrasive blast equipment. For effective in-house training programs, display the actual components and allow the attendees to handle them as they are being discussed. Make cutaways of couplings and blast hose to show how
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2000
001 on reader
information
card
proper installation prevents turbulence and leakage. Supervised assembly and disassembly of components gives hands-on training on what to do in the field. Hands-on demonstrations of equipment teach proficiency. CONCLUSION
The expression “No chain is stronger than its weakest link” applies to abrasive blast equipment. Within the major components of a blast system, the quality and performance of each element determines the effectiveness of the entire system. Even with an efficient compressor, high-quality blast machine, and the right abrasive, it takes a trained and skillful operator behind the nozzle to make the system perform to its potential. This article was excerpted from “Blast Off 2-Your Guide to Safe and Efficient Abrasive Blasting,“published by Clemco Industries Corp. NW
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Hansel
Clemco Industries
Corp., San Francisco
A
brasive blasting is the process of propelling abrasive particles from a blast machine, using the power of compressed air. Three fundamental components constitute a blast equipment setup: air compressor, blast machine, and abrasive. The compressor must produce sufficient air pressure and volume to convey abrasive from the blast machine to the surface being blasted. Cleaning takes place as a direct result of how efficiently air moves from the compressor to the surface. A restriction in just one part reduces the entire system’s production rate. A blast machine’s role is to smoothly meter abrasive into the passing air stream. Sometimes, contractors install restrictive air fittings or hoses that actually cut the airflow in half. Avoid these selfimposed problems by selecting a blast machine with large-diameter piping and by connecting it to largediameter air and blast hoses. The third major component in the blasting system is the abrasive. It produces the finish on the surface. Match the abrasive material and size to the surface being blasted to ensure the best possible finish, cleaning speed, and cost efficiency. Count on your abrasive supplier to recommend the best abrasive for the job. With these three components in place, a trained, skilled blast operator has the tools to produce a flawlessly finished surface. APPLICATIONS
Abrasive blasting applications fall into three broad categories-surface preparation, surface cleaning and finishing, and shot peening. Surface preparation removes unwanted material and leaves a surface ready for coating or bonding. Blast cleaning existing steel structures removes old paint, rust, and other contaminants. Blasting removes the mill scale that forms on new steel. The impact of angular abrasive roughens a surface to produce a profile or etch. Most paint manufacturers specify surface profiles that will ensure their products perform as intended. Contractors blast masonry so it will accept sealers or paints. Blast cleaning stucco and brick removes loose paint, mildew, soot, stains, and graffiti and leaves an ideal surface for coating. July
2000
Contractors blast prestressed concrete panels, poured-in-place walls and columns, and other concrete structures to remove latent cement, form marks, and discoloration; and to expose colored aggregate. Beyond steel and masonry, blast cleaning can strip layers of paint from wood. On fiberglass boats, blasting removes the top layer of gelcoat and exposes air bubbles. Blasting aluminum, titanium, magnesium, and other sophisticated metals removes corrosion and, depending on the abrasive and pressure used, leaves a surface profile. Companies use nonaggressive media to dry-strip airplanes, helicopters, cars, trucks, and boats without using rotary sanding tools that might damage the surface. Switching to dry stripping eliminates worker exposure to toxic chemical strippers and the expense of properly disposing of this hazardous waste. As industries create new materials and new surface preparation problems, blast equipment manufacturers race to develop the technology and tools to meet these challenges. Surface cleaning and finishing differ from surface preparation in that the desired result is to improve a product’s appearance and usefulness rather than condition it for coating or bonding. Surface cleaning includes removing production contaminants and heat scale. Surface finishing includes deflashing and deburring molded parts and enhancing visual features. Low-pressure blasting with glass or ceramic beads produces a blended matte appearance with a precise roughness average (Ra) finish on soft metals. Metal foundries blast cast parts to remove small burrs for functional and aesthetic purposes. In most cases blasting exposes minute cracks and defects in metals. This is especially important to facilities that repair and recondition aircraft wheels and landing gear components. Soft materials, such as rubber and plastic, usually emerge from molds with flashing left by tooling separation. Abrasive blasting trims the flashing, leaving a smooth, uniform finish. Using heat to harden metals can discolor parts. Blast finishing heat-treated parts removes the discoloration and the scale that sometimes forms. 23
Abrasive blasting can improve a product’s appearance by removing stains, manufacturing compound residue, corrosion, and tool marks. Some blast media can blend surface variations, such as scratches and tooling marks, into an overall uniform appearance. High operating temperatures build up carbon and burnt oil on many automotive parts. Electric motors become clogged with overheated insulation and melted stator lamination. In most cases retaining original dimensions of these parts is critical. Blasting with plastic media, glass bead, or agricultural abrasive removes the contaminants and provides an acceptable finish without affecting tolerances. Shot peening increases the strength and durability of high-stress components by bombarding the surface with high-velocity, spherical media such as steel shot, ceramic shot, and glass beads. To make a metal product or component, manufacturers must cast, cut, bend, stamp, roll, or weld metal stock to produce the desired shape. Sometimes these processes leave residual stresses in the metal that can cause parts to fail when stressed. Shot peening produces an effect similar to what is produced by pounding a surface with a ball peen hammer, except that the dimples left by shot peening are much smaller and the impacts more consistent in their distribution and intensity. This bombardment creates a uniformly compressed surface, diffusing the stress forces over a larger area and leaving the surface less likely to crack. Shot peening is a precise technology, requiring adherence to exacting specifications for media hardness, blast duration, nozzle angle, and pressure. Under- or overpeening a part may cause premature failure. The automotive and aircraft industries use peening extensively. Gear manufacturers peen to eliminate burrs and sharp edges, and to strengthen gear teeth. Spring manufacturers peen their products to combat stress. Shot peening metal castings and forgings cleans the surface, exposes defects, and improves appearances. Peening threaded parts removes sharp edges while increasing thread-holding power. It is often used to remove mill scale from new steel. SURFACE
PREPARATION
bonding
surface
while
ensur-
standards for their products. Failure to follow specifications may void a coating’s guarantee. Steel surface preparation standards measure two critical specifications-surface profile and degree of cleanliness. Coating manufacturers and professional organizations test paint systems by applying them over various surface profiles and subjecting them to a wide range of environmental conditions. They have found that coatings require specific profiles to ensure adhesion to and complete protection of the substrate. The profile provides a mechanical method of positive, uniform bonding between the coating and the surface. Abrasive particles cut into the steel to form tiny peaks and valleys. The depth of this profile is controlled by the size, type (see Figs. l-31, and hardness of the abrasive; by the air pressure; and by the distance and angle of the nozzle to the surface. Applying heavier layers of paint to compensate for deep profiles adds substantially to the cost of the job. Profiles are expressed in mils, microns, or millimeters. l l l l
1 mil = l/1,000 in. 25 microns = 1 mil 25.4 millimeters = 1 in. 39 mils = millimeter
SPECIFICATIONS
Paint manufacturers have long recognized the importance of surface preparation to the success of their coatings. Improperly cleaning a steel surface will cause premature coating failure; consequently, coating manufacturers specify surface preparation 24
Figure 1. Correct etch maximizes ing complete coating coverage.
Figure 2. Too little etch reduces the material and the substrate.
the
bonding
surface
between
Metal
Finishing
\
/
PAINT
dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter, except for some staining. This is similar to White Metal but allows slight staining on up to 5% of the metal. This degree of cleaning is required where high-performance coatings will be applied to steel exposed to harsh elements and heavy usage. Commercial
Figure creating
3. Too much etch allows surface to show througha starting point for corrosion and coating failure.
In the U.S., mil describes paint thickness as well as surface profile. Typically, specifications state a profile height average. For example, an average profile of 2 mils (50 microns) may include profiles as small as 1 mil(25 microns) and as large as 3 mils (75 microns). This range is acceptable because there is no practical method to produce identical abrasive particle sizes. Deviations in air pressure or in the distance or angle of the nozzle to the surface also affect profile. Reduced air pressure or increased nozzle distance causes smaller profiles. Severe nozzle angles produce a skimmed blast pattern rather than definite peaks and valleys. For blasting structural steel, nozzles should be held at 80 to 90” to the surface. Use a profile-measuring device to document profile conformance. Careful monitoring of the profiles can prevent expensive rework. DEGREES
OF CLEANLINESS
SSPC: The Society for Protective Coatings has established four degrees of cleanliness for blasting, ranging from removal of all contaminants to removal of loose materials only. The four degrees are White Metal Blast, Near-White Metal Blast, Commercial Blast, and Brush-Off Blast. For detailed descriptions of each, refer to the SSPC’s Visual Standards for Abrasive Blast Cleaned Steel. White
Metal
Blast
Viewed without magnification, a white metal blast cleaned surface is free of visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter. This degree of cleanliness is usually required where sophisticated paints, such as zinc-rich coatings, will be applied to surfaces exposed to highly corrosive environmentschemical plants, offshore drilling rigs, and bridges over salt water are typical applications. Near-White
Metal
Blast
Viewed without magnification, a near-white metal blast cleaned surface is free of visible oil, grease, July
2000
Blast
Viewed without magnification, a commercial blast cleaned surface is free of visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter, except for more extensive staining-up to 33% of the metal. For most applications where standard coatings will be applied, commercial blast is specified. Brush-Off
Blast
Viewed without magnification, a brush-off blast cleaned surface shall be free of all visible oil, grease, dirt, dust, loose mill scale, loose rust, and loose coating. Some tightly adhering residues of mill scale, rust, and coatings may remain. Tightly adhering means the contaminant cannot be removed by lifting with a dull putty knife. This degree of cleanliness is acceptable for surfaces not subjected to severe environments or where long-term coating life is not expected. The SSPC offers sets of photographs that show four existing steel surface conditions and the degrees of cleanliness for each. The existing conditions include mill scale, mill scale and rust, total rusting, and rust with pitting. The National Association of Corrosion Engineers (NACE) offers a set of encapsulated steel coupons that simulate the four degrees of cleanliness. The Swedish Standards Institution’s (SIS) book of pictorial comparators is widely used in Europe. COMPRESSED
AIR:
AN
ENERGY
SOURCE
A standard air/abrasive pressure blast system uses compressed air to pressurize blast machines, convey abrasive to nozzles, provide breathing air, and operate valves and accessories. Pressure
and Volume
A compressor’s power output is measured in pressure and volume. Pressure is expressed in pounds per square inch (psi), or pounds per square inch gauge (psig), and by volume in cubic feet per minute (cfm). Most air tools consume compressed air intermittently. Abrasive blast cleaning consumes a steady supply of high-pressure, high-volume air. Blast equipment demands more from a compressor than any other air-powered tool. 25
Both a l-hp and a 100-hp compressor can produce 100 psi, but it takes the efficient, powerful lOO-hp compressor to generate the great volume of air needed for blasting. At 100 psi the 1-hp compressor generates about 4 cfm of air, but the typical lOO-hp compressor generates 400 to 450 cfm. This great volume of air must be sustained to maintain the 100 psi needed for blasting. Increasing the air pressure increases the volume of air flowing out the nozzle. If the compressor does not produce the volume of air required by the nozzle, it will never achieve the required pressure. For example, at 100 psi a G-in. orifice blast nozzle allows 200 cfm of air to pass through it. To maintain 100 psi the compressor must produce at least 200 cfm. A compressor rated at 150 cfm will never reach 100 psi because the V&in. nozzle lets air out faster than the compressor can force it in. Each one-pound drop in pressure reduces production by as much as 11/2%. In the example above the overtaxed compressor may reach just 70 psi, which will cut the blasting production rate by 45%. Most contractors blast clean structural steel at 100 to 140 psi. In the U.S., standard blast machines are rated at 125 psi. High-production machines, built from heavier steel, are rated at 150 psi. The compressor’s pressure to the system should never exceed the blast machine rating. For typical blasting applications, 90 to 100 psi, combined with hard, sharp abrasives in standard mesh sizes, delivers acceptable production and surface cleanliness. Higher pressures increase production rates but require durable, recyclable abrasives. Proper pressure is based on the surface condition, the abrasive used, the required surface finish, and the production rate desired. COMPRESSOR
TYPES
AND
SELECTION
Generally, the high pressure and high volume of air required for blasting dictates the use of a rotary vane or rotary screw compressor. Do not use old-style, piston compressors for blasting because they produce pressure fluctuations that vary blasting speed and affect surface finish. Also, piston compressors require a lot of lubrication, which can send oil through the air lines to contaminate the abrasive and surface being blasted. Some rotary screw compressors inject oil onto the screw to cool it. If the compressor is not functioning properly some of this oil may be carried into the air lines. Oilless compressors have sealed bearings. The 26
screws are not cooled by oil so they produce much hotter air. Choose an air compressor that will generate a steady flow at high pressure and high volume, and that is built to withstand the environmental conditions found at a blast site. For blasting, oil-free rotary vane and screw compressors are best. Select a compressor that will satisfy your current and anticipated demands, plus enough reserve to compensate for nozzle wear. The compressor is the workhorse of a blasting system. Do not run it at maximum power for long periods, as this will accelerate wear. To determine the size compressor needed, add the air requirements of all equipment, plus a 50% reserve. The compressor manufacturer can recommend the size and type of system best suited for your applications. Consider the size and type of the compressor’s air outlets. To regulate airflow, many compressor outlet valves have internal slotted plugs about half the size of the valve opening. A l-in. valve actually has a X-in. air passageway, far too small to supply a blast machine. Do not use restrictive air valves or quick-disconnect couplings. The smallest internal diameter of the compressor air outlet should be at least four times the size of the nozzle orifice or larger. For a ?&in. blast nozzle, the compressor air receiver fitting, air valve, and air couplings should all have IDS of at least 11/2 inches. Remember, the smallest opening in the air supply system controls the amount of air to be delivered to the blasting unit. Operation and Maintenance Before buying or renting a compressor tell the sales representative you intend to use the compressor for abrasive blasting. The sales representative should supply information on installing, operating, and maintaining the compressor. Read the manufacturer’s instructions before operating the equipment and follow the instructions, warnings, and maintenance procedures. Position the compressor upwind of dust generated by blasting. Dust, dirt, or other contaminants entering the compressor air inlets will cause premature wear. Exhaust fumes contain carbon monoxide (CO). If exhaust fumes enter the air inlet, blasters connected to the compressor can be killed from inhaling carbon monoxide. Locate the compressor where vehicle exhaust will not enter the air inlets. Do not allow vehicles to park Metal
Finishing
near the compressor. Direct the compressor’s own exhaust away from its air inlet by attaching metal pipe to its exhaust stack and running the pipe downwind from the inlet. Install safety cables at every coupling to prevent injury if fittings disengage and to support the weight of elevated hose. BLAST
MACHINES
There are two types of blast equipment-suction blast and pressure blast. Suction blasting is less aggressive than pressure blasting. It is used in blast cabinets and for light-duty work such as touch-up blasting. Pressure blasting is sometimes used with blast cabinets but is more often used in blast rooms or outdoors to clean tough surfaces and large areas. Suction
Blast
Suction blasting, sometimes called venturi blasting, draws abrasive from a nonpressurized container into a gun chamber then propels the abrasive particles out a nozzle. A suction system consists of a blast gun, an air hose, a media hose, and an abrasive container. Compressed air flows through an air jet in the blast gun to create suction. This suction brings abrasive up through the media hose into the gun body where it is accelerated out the nozzle with the air. The volume of compressed air required for suction blasting is determined by the ID of the air jet orifice in the back of the blast gun not the ID of the suction gun nozzle. A typical suction gun air jet is half the size of a typical nozzle orifice for pressure blasting. This means it will use about one-fourth the volume of air, and propels abrasive to less than one-fourth the velocity created by pressure blasting. Suction blasting is used on softer metals for mild deburring, light shot peening, and scale removal without penetrating the base metal. Such metals include aluminum, titanium, and magnesium. Pressure
Blast
Blast machines are known by a variety of namespots, pressure generators, tanks, and so on. This article will refer to all systems that hold abrasive under pressure as blast machines. A pressure-blast system feeds abrasive, via a metering valve, into a moving stream of compressed air. Air and abrasive travel through a blast hose at high pressure and speed, exiting the nozzle at more than four times the velocity produced by suction blasting. Pressure blast machines are used in structural steel blasting for their high production rates, and in July
2000
Figure 4. A modern, high-production, 150 psi, delivers 20% more cleaning chines.
lightweight media blasting tion of media flow. BLAST
MACHINE
blast power
machine, rated for than standard ma-
for their precise regula-
CONSTRUCTION
In the U.S., blast machines and other pressurized containers must meet American Society of Mechanical Engineers (ASME) standards. ASME specifies the type of steel and welding methods, and an ASME-Authorized Inspector supervises the hydrostatic testing of each pressure vessel, then issues a National Board certificate of approval. A metal plate bearing the board approval number is permanently affixed to the blast machine. In the U.S., most blast machines are rated for working pressures of 125 to 150 psi. The ASME requires a safety margin 50% greater than the working pressure; so 125-psi machines are tested at 188 psi and 150-psi machines at 225 psi. A well-engineered blast machine (see Fig. 4) allows smooth air and abrasive flow throughout the system and is easy to operate and maintain. The machine’s semielliptical, concave head stores abrasive, which flows into the machine when it depressurizes . Most machines have a 35” (or steeper) conical bottom to allow abrasive to flow freely into the metering valve below. Steel grit and other common abrasives have a 32” angle of repose. This is the natural slope of the abrasive when it is poured to form a mound. Plastic and agricultural media have much steeper angles of repose. Blast machines for use with these lightweight media should have 60” cones to ensure free flow. A modern blast machine seals automatically with a pop-up valve-a cone-shaped aluminum casting coated with wear-resistant urethane. When air enters the machine, the internal piping guides the pop-up to seal against a rubber gasket in the valve opening. Upon depressurization, the pop-up valve drops to 27
Install a pressure gauge on the blast machine’s air inlet. Test for pressure loss (without abrasive in the air stream) by inserting a hypodermic needle gauge into the blast hose near the point where it connects to the machine. If the difference is more than 10 lb, examine each component to find the restriction and replace it with an appropriately sized part. BLAST
Figure 5. This blast machine traps any abrasive entrained
muffler in the
reduces exhaust
exhaust air.
noise
and
allow abrasive stored in the concave head, or a storage hopper, to flow into the machine. A well-made blast machine has a muffler to reduce the noise of the escaping air when the machine is depressurized and momentarily traps any abrasive carried out with the exhaust. (See Fig. 5.) Air and media flow through pipes, valves, hoses, nozzles, and couplings that are all cylindrical. Any reduction in the diameters of these cylinders dramatically reduces the rate of flow. High-production blast machines use large-diameter pipe to minimize pressure loss. A l-in. ID cylinder has an area of 0.80 in2. A X-in. ID cylinder has an area of 0.20 in2. Reducing the diameter of the cylinder by half reduces its area three-fourths. The nozzle should be the smallest orifice between the compressor and the surface being blasted. From the compressor to the nozzle, the internal diameters of the valves, pipes and fittings, blast hoses, and couplings must all be three to four times the ID of the nozzle. If the operator must work at distances greater than 100 ft from the blast machine the ratio of hose ID to nozzle orifice is even greater. On blast machines with capacities greater than one cubic foot, piping is typically l-in. or l%in. ID. There will always be some pressure loss due to the turns and bends inherent in piping. Follow the above recommendations to minimize those losses. A single-operator blast machine should suffer no more than 8 to 10 lb of pressure loss from the air inlet to the outlet coupling on the bottom of the machine. 28
MACHINE
SELECTION
Choose a blast machine that has the capacity, portability, and convenience features that best fit your needs and type of work. The diameter of the nozzle orifice determines the amount of work done and the amount of air and abrasive used per hour. A %-in. nozzle cleans four times faster than a %-in. nozzle, and it uses four times the volume of air and abrasive. Based on the nozzle and compressor you plan to use, choose a blast machine with an abrasive capacity to supply 20 to 30 minutes of steady blasting. Choose a nozzle size, then refer to the Abrasive Consumption Chart (see Table I). Under the loo-psi column find the amount of abrasive consumed in an hour by the chosen nozzle, then calculate the minimum size machine. The table provides comparison information only. Your actual consumption rate will vary based on metering, compressed air quality, hose layout, and blast machine efficiency. For example, a %-in. nozzle consumes about 11 ft3 of abrasive per hour at 100 psi, so your blast machine should hold at least 5.5 ft3 to last 30 minutes. As the nozzle orifice wears to %6 in., abrasive consumption will increase to more than 15 ft3 per hour, or more than 7 ft3 per half hour, which reduces a 6-ft3 machine to about 20 minutes of blast time. Machines that are too small waste the blast operator’s time refilling or waiting for others to refill the abrasive. Machines that are too big are unnecessary because the operator usually will have to reposition after 30 minutes. Light-duty machines have a capacity of 1/2to 1 ft3, have small piping, short blast hose, and straightbarrel nozzles. Their light weight and portability make them ideal for spot cleaning. Medium-duty machines offer pickup-truck portability with capacities from 2 to 4 ft3. They usually have l-in. piping and blast hose, suitable for use with 3/6-, l/4-, and 5/6-in. venturi nozzles. Mediumduty machines are ideal for jobs that take an hour or two to complete. High-production machines offer capacities from 4 to 20 ft3, with 1%in. or l%-in. piping and blast hose feeding %- to %-in. nozzles. Popular among blasting Metal
Finishing
Table
I. Air and Abrasive
Consumption
with
Compressor
Horsepower
Required
(with
abrasive
weighing
approximately
100 Ib/ft3)
Orifice, Nozzle No. 2
in. l/8
No. 3
3116
No. 4
l/4
No. 5
5116
No. 6
318
No. 7
7116
No. 8
l/2
“Requires
blast
20 psi 4.4 26.8 1 10.4 60 2.4 18.8 107.2 4.4 30.8 187.2 7.2 43.2 267.2 9.6 58.8 358.4 13.2 78 464 17.6
cfm lb/h b cfm lbh hp cfm lbh hp cfm lb/hr b cfm lb/hhp cfm lblhr hp cfm lb/l-u hp
machine
rated
for high
30 psi 6.6 40.2 1.5 15.6 90 3.6 28.2 160.8 6.6 46.2 280.8 10.8 64.8 400.8 14.4 88.2 537.6 19.8 117 696 26.4
40 psi 8.8 53.6 2 20.8 120 4.8 37.6 214.4 8.8 61.6 374.4 14.4 86.4 534.4 19.2 117.6 716.8 26.4 156 928 35.2
50 psi 11 67 2.5 26 150 6 47 268 11 77 468 18 108 668 24 147 896 33 195 1,160 44
2000
70 psi 15 88 3.5 33 196 8 61 354 14 101 604 23 143 864 32 194 1,176 44 252 1,512 56
80 psi 17 101 4.0 38 216 9 68 408 16 113 672 26 161 960 36 217 1,312 49 280 1,680 63
90 psi 19 112 4.5 41 238 10 74 448 17 126 740 28 173 1,052 39 240 1,448 54 309 1,856 69
100 psi 20 123 5.0 45 264 10 81 494 18 137 812 31 196 1,152 44 254 1,584 57 338 2,024 75
125 psi” 25 152 5.5 3:: 12 98 608 22 168 982 37 237 1,393 52 314 1,931 69 409 2,459 90
140 psi” 28 170 6.2 62 357 13 110 681 25 188 1,100 41 265 1,560 58 352 2,163 77 458 2,754 101
pressure.
contractors and industrial installations, high-production machines deliver the abrasive capacity needed for extended blasting. Bulk blast machines offer high-production with abrasive capacities from 60 to 800 ft3. They usually have two to four outlets. Portable bulk blast machines have wheels onlyfor movement within a work site-or with brakes, lights, fenders, and other equipment that allows them to be towed on public highways. Most bulk machines can be towed while full; however, moving one filled with steel grit may exceed the gross vehicle weight (GVW) rating. For large jobs, bulk blast machines can save money. Bulk abrasive costs less than bagged, and the time spent loading bagged abrasive is nonproductive. Blower trucks deliver bulk abrasive to onsite storage hoppers. Bulk equipment is common in shipyards and industrial facilities, and is preferred by large blasting contractors. (See Fig. 6.) Bulk blast machines do not blast harder or clean faster than other high-production machines. The size and number of nozzles used determines the production rate. Often, the versatility of four standard-size machines, in conjunction with storage hoppers and transfer equipment, is preferable because the individual machines can be shut down for maintenance or positioned in different areas to reduce blast hose lengths. July
60 psi 13 77 3.0 30 171 7 54 312 12 89 534 20 126 764 28 170 1,032 38 224 1,336 50
Pressure Regulators and Gauges Install a pressure regulator with a gauge on the blast machine to set and monitor the air pressure. Maintaining the correct operating pressure guarantees optimum performance and ensures the machine is working within its pressure limits, normally 125 to 150 psi. The compressor gauge shows air pressure at the compressor outlets, the beginning of the system. The hypodermic needle gauge indicates pressure at the nozzle, the end of the system. A pressure gauge at the blast machine inlet lets you quickly check for pressure loss. Abrasive Metering Valves A metering valve uses gravity to feed abrasive into a fast-flowing stream of compressed air. Using too little abrasive slows production and leaves un-
Figure 6. Highway-towable and yard-towable machine ators.
bulk blast machines (with fenders) (center), serve multiple blast oper-
29
touched surfaces. Using too much abrasive wastes energy and disperses particles unequally within the blast pattern. Exorbitant abrasive usage also wastes material and labor. Properly adjusting the metering valve ensures that you get the maximum amount of cleaning power from each abrasive particle. True metering valves have wear-resistant internal materials housed in sturdy bodies. The valve should have a simple, yet precise, adjustment feature and a scale or gauge to indicate the relative setting. An inspection plate facilitates removal of foreign materials that clog the valve. Remote Controls The Occupational Safety and Health Administration (See: OSHA 29 CFR 1910.244) requires remote controls on all blast machines. Failure to use remote controls can expose a blasting contractor to substantial fines and liabilities if someone is injured or killed. In addition to their safety features, remote con-
trols save substantial amounts of compressed air and abrasive. If the blast operator must wait for someone to shut off the machine, air and abrasive are squandered. Also, removing the need for a dedicated pot tender saves labor by allowing one person to load abrasive for several machines or do other work between refills. Most remote control valves operate pneumatically, but there is a choice between pneumatic or electric actuation. Pneumatic systems are suitable for most applications and usually cost less than electric systems. Pneumatic remote control handles work well at distances up to 100 feet. Electric remote control handles are recommended for distances of 150 ft or more. BLAST
NOZZLES
The objective of everything up to this point is to convey a steady supply of abrasive at adequate pressure to the nozzle. Performance at the blast nozzle
SIMPLE - Automatic Gage Setup and two-button operation allow you to take basic measurements or perform advanced functions easily. Plus, everything you need comes with the gage - you’ll be measuring in seconds! DURABLE - Tough Probes, Robust Housing, Strong Warranty... the PosiTector 6000 is made to measure in rugged environments. Unaffected by vibration, this heavy-duty gage also resists exposure to solvents, acids, oil, water and dust. ACCURATE - Measuring with High Resolution and Accuracy each quality-tested PosiTector 6000 gage leaves our facility with a Certificate of Calibration providing traceability to NIST.
Call us or check I-800-448-3835
our website for details: www.defelsko.com l
CORPORATION, 802 Proctor Avenue P.O. Box 676, Ogdensburg, New York 13669 l Fax: 315-393-8471 Phone: 315-393-4450
DEFELSKO
Circle 007 on reader information card 30
Metal
Finishing
LAPPING? reveals whether or not all of the previous requirements for air and abrasive flow have been correctly followed. Nozzles accelerate air-driven abrasive into a highly effective cutting force that can tackle the toughest application. The size, type, and shape of the nozzle determine the production speed and the appearance of the end product. Nozzle material primarily affects wear life, which is more than just how long a nozzle will last; it is critical to air usage.
For faster cutting try ElectraAbrasives’ Boron CarbidePowder. ElectraAbrasives’ Boron Carbide Powder cuts faster and cleaner than conventional Silicon Carbide finishing powders. Other Boron Carbide benefits include: Longer Life - Cuts down time, and increases efficiency. Faster Cutting - Maximizesyour per-hour product output. Higher Pressures- Handles any application with ease. l
l
l
ElectraAbrasives has Boron Carbide Powders from #220 to #1200 that has application in loose abrasive machining of: Tungsten Carbide Ceramics, Composites Tool Steel l
Figure 7. Large-diameter blast hose, couplings, allow compressed air and abrasive to flow easily machine to the surface being cleaned.
and nozzles from the blast
As the nozzle wears, it uses more air volume to maintain a given air pressure. A nozzle with a S-in. orifice operating at 100 psi requires about 200 cfm. When the orifice enlarges by Ms inch, the air requirement increases to more than 250 cfm-a 25% increase. Replace a nozzle when its orifice wears to Yt6 in. larger than its original size. In addition to wasting air, a nozzle worn beyond 1/6 in. could cause injury if the liner fails. Common nozzle materials include ceramic, tungsten carbide, silicon carbide, and boron carbide. Use ceramic nozzles with nonaggressive abrasive in light-duty equipment and blast cabinets. Carbide nozzles-tungsten, silicon, and boron-are popular for the majority of blasting applications, due to their long life. The abrasive type and blast pressure will affect nozzle life. Tungsten carbide is a hard, heavy material used in many industries for wear resistance. Tungsten carbide is sintered, a process that uses extreme heat and pressure to produce one-piece liners in a mold. Hardness, however, contributes to the nozzle liner’s brittleness. Tungsten carbide nozzles last about 300 hours when used with expendable abrasive. (See Fig. 7.) Silicon carbide grew from research on lightweight and durable materials for aircraft and aerospace industries. Silicon carbide weighs 42% less than a
l l
Our Boron Carbide is available to ISO, FEPA, and ANSI specifications. Produced at our new powders facilities in Buffalo, New York, we will meet your exact need. CalI today. Learn how our full line of premium abrasive grains & powders satisfy your needs.
Circle July
2000
012 on reader
information
card 31
comparable tungsten carbide nozzle, making it easier to hold for a long time. With expendable abrasive, silicon carbide lasts up to 500 hours, or 50 to 65% longer than tungsten carbide. Boron carbide is the longest wearing nozzle liner material. It is especially effective when using extremely sharp abrasive such as aluminum oxide and silicon carbide. With expendable abrasive, boron carbide lasts up to 1,000 hours. The purchase price for boron carbide is two to three times that for silicon and tungsten; however, the cost per operating hour may be less compared to tungsten, or more compared to silicon. Some liner materials are better suited for specific abrasives. Choose a boron carbide nozzle when using aluminum oxide or silicon carbide abrasive. Boron tolerates extremely sharp particles better. For steel grit, steel shot, or any iron abrasive, use a tungsten carbide nozzle. The high density of steel abrasive causes chipping on other carbide liner materials.
Circle July
2000
023 on reader
Carbide nozzle liners are brittle. They come encased in a protective jacket of metal, urethane, or both. Nozzle Shape Most contractors use blast nozzles with wide coneshaped entrances and gradually tapered exits, which together form a venturi. The venturi’s length, angles of entrance and exit, and orifice size are precisely calculated to maximize acceleration of the abrasive and air. Abrasive enters the converging end of the nozzle, funnels through the orifice, then rapidly expands into a high-powered stream through the diverging exit end. At 100 psi the velocity at the end of the nozzle reaches 660 feet per second. By comparison, abrasive exits a straight barrel nozzle at 318 feet per second. As a venturi nozzle wears beyond YE in. over its original size, it loses its venturi shape and much of the accelerating force that shape provided. An extremely worn venturi nozzle wastes air and delivers
information
card 33
a blast velocity and pattern comparable straight-barrel nozzle. A well-designed venturi nozzle has precise orifice, and outlet dimensions to accelerate the sive and disperse it uniformly within the blast tern-with no scatter and no hot spots. This vides consistent cleaning results.
to
a
inlet, abrapatpro-
Nozzle Pressure Maintaining adequate nozzle pressure is essential to high-production blasting. The gauge on the compressor shows the air pressure at the compressor only. It does not indicate blasting pressure. Hoses, air filters, blast machines, and other components between the compressor and the nozzle all contribute to friction and pressure losses. To accurately determine nozzle pressure, use a hypodermic needle gauge. This simple tool consists of a needle mounted on a pressure gauge. Insert the needle into the blast hose at a 45” angle, about 6 in. behind the nozzle holder, with the tip of the needle pointing toward the nozzle. The
needle should penetrate enough to position the tip into the center of the air stream. The gauge will register the actual pressure at the nozzle. If you are blasting on structural steel at 100 to 110 psi, you would probably accept anything between 90 and 105 psi. In a blast system with 50 ft each of blast hose and air hose, a pressure drop of 10 to 15 psi is expected. Nozzle pressure below 85 psi indicates something wrong. Check the air compressor setting, then check for restrictions at all hoses and fittings, moisture separators, and any system components. Also check the nozzle orifice for excessive wear. Nozzle Care Nozzles are expensive; nozzle gaskets are cheap. Using a worn gasket or worse yet, no gasket, allows the air-driven abrasive to destroy the nozzle jacket and the nozzle holder. Check all gaskets daily and replace with gaskets of the proper diameter to maintain a smooth flow from the hose to the nozzle.
The
Rectifiers & Plating Equipment Ltd.
ELECTRO-STEAM GENERATOR creates a total cleaning system for cleaning or cleaner/phosphatizing process. Metal surface preparation for coatings to meet several Mil Specs
Our Steam Generators Will:
Specializes in the design and manufacture of electroplating power supplies for the metal finishing industry.
l l l
We supply power
Water use by 6 to 10 gallons per minute Contaminated Effluent by 6 to 10 gallons a minute Concentration of cleaning solutions being used
supplies with:
l Thyristors
(SCR) Manual and motor-driven @Tap switches
l
Powerstat
Controls include automatic voltage control with current limit, automatic current control with voltage limit, average current density. Options that can be supplied els, ramp rise, timers, alarms,
Rectifiers
& Plating
are multiple output levcomputer control etc.
Equipment
Ltd.
31 Leading Road l Rexdale, Ontario Tel: (416) 744-7848 l Fax: (416) 744-l 169 E-mail: [email protected]
www.rpe.zip411
l
l
Exact Continuous Mixture Electra-Steam
Generator Corp. 1000 Bernard Street Alexandria,VA 22314-1299 l-800-634-8177 Fax:
l-888-783-2624
l-703-549-0664 Fax: l-703-836-2581
.net
Circle 038 on reader information card 34
Noxious fumes, dangerous flames, or unsafe fuel storage that are present with “fuel fired” cleaning systems Mixing of chemicals
l
Circle 013 on reader information card Metal Finishing
Most nozzles are destroyed before they reach the end of their useful life, primarily due to careless handling. The same extreme hardness that helps nozzle liners withstand abrasive flow makes them susceptible to cracking from impacts. Never drop, throw, or strike other objects with a nozzle. OPERATOR
SAFETY
EQUIPMENT
Blasting can be dangerous for a poorly trained, poorly equipped operator. A blast machine produces a powerful stream of sharp particles that, in addition to cleaning a surface, creates clouds of potentially toxic dust. To prevent a variety of injuries and illness, personal safety equipment is essential for blast operators and anyone in the work area. Potential hazards of abrasive blasting include high-pressure compressed air; air-propelled abrasive from the nozzle; impurities in the breathing air; toxic dust from pulverized abrasive and coatings; loud noises from blast nozzles and compressor mo-
The Electrodeposition Tin and Its Alloys
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Send Orders
to:
METAL FINISHING 660 White Plains Rd. Tarrytown, NY 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333-2570 All book orders must be prepald.. Please include $5.00 shipping and handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped express to Canada; and $20.00 for each book shipped express to all other countries.
tors; potential head impact injuries from beams and equipment; plus, all the other hazards normally associated with construction sites. Air-Fed Helmets Employers are required to establish a comprehensive respirator program. [See: OSHA 29 CFR 1910.134 (a) (b) and 29 CFR 1926.103.1 OSHA defines an air-fed helmet for abrasive blasting as a continuous-flow, supplied-air respirator. A helmet should furnish the operator with breathing air, protect his or her head and face from impacts, muffle noise, and allow an unobstructed field of vision. (See Fig. 8.1 The helmet lens protects the operator’s face from rebounding abrasive. Most helmets have a frame to hold several thin, sacrificial lens covers, which protect a thick inner lens. The thin lenses usually have tabs and perforated borders that permit the operator to tear away the frosted outermost lens to expose
we?&3qbtmz%u
Pumping
Solution
DO YOU EXPERIENCE: 9 Aeration of your liquid? l
l
High Energy costs in operating your pumps? Noisy operating pumps? (Lipseals - Bungholes) 9 Limited positioning of your discharge-nozzle?
WE CAN ASSIST We offer Semi-Submersible Pumps and Filtration Systems VISIT OUR WEBSITE Semi-Submersible Pumps and Filtration Systems in CPVC, Polypropylene, and PVDF for the Plating and Printed Circuit Board industries, Fume Scrubbing, Sump, Transfer, Recirculating and Water Treatment pumping applications.
WA L RU S Pump
& Filtration
LLC
709 North Dallas Street, Amarillo, TX 79107 Tel. (806) 383-3355 Fax: (806) 383-5577 e-mail: [email protected] Internet: www.walruspump.com Circle 049 on reader information card
July
2000
35
tor and equipment needs. They usually have filters to remove particulate matter and moisture. Some contractors use one compressor to supply breathing air and blast air. Take extra precautions when lubricated compressors supply your breathing air. Oil-lubricated air compressors sometimes produce CO, a colorless, odorless, deadly gas. To help prevent operator exposure to CO, service the compressor at the manufacturer’s recommended intervals, and install overheating shut-off devices and/or CO alarms. SAFETY
Figure while
8. A blast operator checks wearing the high-pressure,
the settings supplied-air
at his in-line respirator.
filter
another lens. Air furnished to helmets must be clean, dry, contamination-free, and at NIOSH-prescribed pressure and volume. Pay special attention to the source, filtration, and composition of the air. Read all instructions for the equipment used to produce and convey breathing air. Breathing air must meet the minimum requirements, Grade D or higher, described in the Compressed Gas Association Commodity specifications G 7.1. Never attach a breathing air hose to any source without first testing the quality of air. Oil-free air pumps supply breathing air to lowpressure (up to 20 psi) helmets. Air pumps do not compress air; they merely draw in ambient air and push it through the hose. Air pumps will not produce carbon monoxide. Their insides are coated with friction-resistant materials, so they require no lubricants. Because they have built-in filters, air pumps do not need overheating shutdown devices or in-line filters. These pumps are available for one to four operators. Most are hand portable. The best compressors for high-pressure breathing air work without oil lubricants, and, therefore, do not create CO or contaminate the air with oil. Most can sustain the pressure required for highpressure helmets. They cost more than other compressors but their advantages far outweigh the added cost. Oilless compressors come in portable and stationary models, in capacities to support different opera36
PRACTICES
Block off the blasting site and surrounding area to prevent unprotected personnel from entering. Safety personnel should monitor atmospheric dust to determine the size of the blasting zone. Weather, humidity, wind direction and velocity, abrasive composition, type of material being removed, and other factors determine the extent of the zone. Periodically test the atmosphere and adjust the size of the zone. In enclosures, such as tanks and blast rooms, all personnel must wear approved respirators at all times. Enclosures must be ventilated to bring in fresh air and extract dust in sufficient volume to maintain a low concentration of dust. (See: OSHA 29 CFR 1910.1000.) Do not use silica sand in enclosed areas because it generates harmful dust. The Occupational Safety and Health Administration (OSHA) enforces stringent regulations on supplied-air respirators and pressure-demand respirators for abrasive blasting. All manufacturers must submit their respirators to NIOSH, OSHA’s agency for testing, to obtain approval. Manufacturers must display a copy of the approval certificate in their instruction manual. Using, a nonapproved respirator (or using a respirator with nonapproved replacement parts) violates OSHA regulations, and subjects the respirator’s owner to stiff penalties. Employers must supply all required safety clothing and decontamination equipment and enforce their use. Also, employers must train their employees-even when that means reading the instructions to the illiterate and translating instructions into foreign languages. TRAINING
THE
ABRASIVE
BLAST
OPERATOR
Since 1971, OSHA regulations have made it clear that employers are solely responsible for providing safe working environments for their employees. Employers must provide training, supply all personal Metal
Finishing
protection equipment, and enforce effective safety programs. Employers take on a tremendous amount of work to train and equip employees, but in reality, everyone wins. The properly trained employees become craftsmen, taking pride in their work, and become valuable assets to their employer. Employers benefit from the increased productivity, which increases profit. SSPC: The Society for Protective Coatings markets a set of four training videotapes on surface preparation and coatings application. The first is devoted to surface preparation equipment, which includes hand and power tools, and high-pressure water blasting, in addition to wet and dry abrasive blast equipment. For effective in-house training programs, display the actual components and allow the attendees to handle them as they are being discussed. Make cutaways of couplings and blast hose to show how
)crModel510
Model 1600
Aubin’s Ty-Nee Tumbler cleans, tumbles, deburrs, polishes, strips or coats small parts, taking less fhan two square feet of bench space. Portable electric-motor-driven or air-powered units have standard hex shaped polypropylene barrels. The Ty-Nee Tumbler is available in two models: the 510, with load capacity of 5 Ibs., and the 1600, with load capacity of 20 Ibs. and optional spherical shaped barrel.
Visit our web site: www.aeaubin.com
A.E. AUBIN
COMPANY
345 North Main St., P.O. Box 456 Marlborough, Connecticut 06447 800-423-0697, in CT 860-295-9597 Fax 860-295-8962 Circle July
2000
001 on reader
information
card
proper installation prevents turbulence and leakage. Supervised assembly and disassembly of components gives hands-on training on what to do in the field. Hands-on demonstrations of equipment teach proficiency. CONCLUSION
The expression “No chain is stronger than its weakest link” applies to abrasive blast equipment. Within the major components of a blast system, the quality and performance of each element determines the effectiveness of the entire system. Even with an efficient compressor, high-quality blast machine, and the right abrasive, it takes a trained and skillful operator behind the nozzle to make the system perform to its potential. This article was excerpted from “Blast Off 2-Your Guide to Safe and Efficient Abrasive Blasting,“published by Clemco Industries Corp. NW
Compounds
Supplies
* Bar Compounds (all sues) for met&. plastics &wood * GreaselesCoqxwnd * Greosestrk(Abrawe Lubricant) * LQuid Compound - Rouges
* Abrasive Cleanen * Belly Pads - Belt Lubricants * M Rakes & Fillers . Hot Glue * Cold Cement ’ Steel Flanges
‘/ k%
Abrasives * Abrasive - Abrasive * Abrasive * Abrasive
Belts Cleaners Discs Flap Wheels
* Contact Wheels * Full Disc Polishing Wheels * Nylon Abrasive Brushes * Pieced Polshlng wheels caste”.
Circle
We’ll Make You Shine! 800 642-7454 202 SEWARD AVENUE UTICA. NEW YORK 13503 PHONE: 315/797-0470 FAX 315/797-w55 co**r Dl”iwn
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card 37