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New abrasive must be added regularly to replace what is lost. The result is a mixture of sizes called a work mix or operating mix of large particles that do the heavy work of cracking and breaking up the contaminants and small particles that do the cleanup work of scouring and removing them. Maintaining a properly balanced work mix

important for efficient blast cleaning. This can be done by hand or with a continuous automated system of replenishing spent abrasives.

Mineral abrasives — From the beginning, sand has been used for abrasive blasting. Silica sand, in particular, is readily available, which makes it economical. Its range of angular and spherical shapes gives it good cleaning action. However, it breaks down quickly and produces a large amount of silica dust, which causes a dangerous lung disease known as silicosis when the dust is small enough to be respirable (10 microns or less). Nonsilica or heavy mineral sands used in blasting include magnetite, staurolite, and olivine rutile, which are tough, dense, and smaller than silica sand but contain no dangerous free silica. These abrasives generally are not recycled because of how quickly they break down in blasting. Garnet, a tough abrasive, can be recycled several times. Other mineral abrasives, such as zircon and novaculite, are generally used for specialty blasting because of their cost and the finish they leave. Zircon, for example, gives a smooth, matte finish, and novaculite leaves a satin luster.

Byproduct abrasives — Slag, the refuse from smelting or other processes, makes a good abrasive because of its availability, relatively low cost, and low silica content. Its sharp, angular shape gives it good cutting power, and it comes in a full range of sizes. Blasting slags include coal slag, copper slag, and nickel slag. Byproduct abrasives are not recycled because they break down too rapidly.

Special purpose abrasives — There are specialty abrasives made with walnut shells o] peach pits that will remove light deposits without damaging the surface. Corncob and sodium bicarbonate abrasives can also be used for this purpose or for removing surface( grease. Other special abrasives include sponge, silicon carbide, aluminum oxide, cu’ glass, and plastic or glass and ceramic beads. Plastic and glass abrasives will clean small delicate surfaces. Plastic media blast cleaning is also used to remove old paint from aircraft, military vehicles, and radar dishes because it does no damage to the substrate Aluminum oxide and silicon carbide are tough, hard, aggressive abrasives that can be recycled. Aluminum oxide works on stainless steel, and silicon carbide does specialty etching. A polyurethane sponge, sometimes incorporating various abrasives, can be used to remove oil and other contaminants while also cleaning and roughening a surface for painting. Most of these abrasives can be recycled a limited number of times.

Shape, size, density, and hardness are the other factors to consider in selecting the proper abrasive material. As noted, abrasives come in spherical and angular shapes, as well as irregular shapes that have features of both. Rounded or spherical abrasive particles clear by hitting the surface; sharp or angular abrasives clean by cutting or gouging it. Large abrasive particles remove thick, heavy contaminants, while small ones work on the fine residual debris. A mix of sizes is often best. Generally, the denser the abrasive, the more effective it is, and the harder it is, the deeper it will clean the surface. All abrasives should be kept clean and dry before blasting.

Vancouver Industrial Painting:  Centrifugal blasting and Blasting Media

Centrifugal blasting is cleaner than air abrasive blasting. It can also be quicker and easier, making it more cost effective. Since the abrasive action is automated, it can clean with more consistency and uniformity than handheld blasting systems in which air pressure and volume, speed, nozzle distance, and other variables determine the quality of the operation. On the other hand, the equipment is cumbersome and expensive, and does not perform as well when cleaning irregular surfaces. Portable centrifugal blasting is generally limited to large, accessible, and relatively smooth, flat areas. This system is the only acceptable preparation method for concrete floors prior to application of high-performance floor coatings.

BLASTING MEDIA

Abrasives come in a range of types, shapes, sizes, and degrees of hardness. Given the multitude of options, selecting the right abrasive for the job becomes a matter of determining which material will give a surface the desired finish, which means the proper degree of cleanliness and roughness as well as the type of texture needed for coating. Other factors to consider in choosing an abrasive are the type of surface to be blasted and its present condition, the consumption rate of the abrasive, its cost, and any environmental regulations that may affect the blasting operation.

There are four types of abrasives: metallic, mineral, byproduct, and special purpose.

Metallic abrasives — The most common metallic abrasive is cast steel. Others include malleable iron and chilled cast iron. All come in the form of shot and grit and in different degrees of hardness. Abrasive made from wire cut into short pieces is also available. Shot is rounded, which makes it appropriate for cleaning heavy, brittle deposits and producing a rounded profile or anchor pattern. Both grit, which is made by crushing hardened shot pellets, and cut wire both have an irregular, angular shape that effectively removes rust, mill scale, and paint, and leaves a rough profile that feels like abrasive paper. Shot, grit, and cut wire are screened according to size.

Metallic abrasives can be recycled many times before breaking down into a size that is too small to be reused. The repeated blasting impact causes the abrasive to fracture or flake. Generally, the harder the abrasive, the faster it breaks down. The particles become smaller and smaller until they are eventually filtered out in the recycling process.

Rick Anderson

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BLAST SUIT AND GLOVES

A durable, high-quality, one- or two-piece suit constructed of heavyweight canvas weave material with elastic or strap-closed wrist and ankle cuffs is necessary to provide protection: from rebounding abrasive and dust laden air. Some suits have leather patches on the fronts” of the arms and legs for additional protection and to help extend the life of the suit. These designs provide good protection without the weight and heat associated with all-leather or all-rubber suits.  Gauntlet-type leather gloves are used to protect hands from abrasive and rough material handling.

PORTABLE CENTRIFUGAL BLAST EQUIPMENT

Centrifugal blasting is a very different approach to surface preparation. It is related to the other blasting methods discussed in this module to the extent that it produces a clean surface by pounding it with abrasive material. However, the method of achieving this goal is much different.

Instead of pressurized air or water, centrifugal blast cleaning uses a motorized wheel to propel high-velocity abrasive against a surface. The abrasive material is fed to the center of the wheel, and from there onto blades that radiate from its hub. As the wheel turns, centrifugal force accelerates the abrasive to the ends of the blades, which hurl it against the surface. The rate and degree of cleaning depend on the number of wheels in eacj machine, as well as the size, type, and quantity of abrasive. The higher the horsepower turning the wheel, the more efficient the operation becomes. The abrasive media commonly used is recyclable steel shot. This comes in various sizes and can be tailored to a specific project.

The self-contained abrasive recycling system of centrifugal blasting machinery also distinguishes it from other blasting methods. The abrasive is captured along with the dust and contaminant debris. They are then separated. Reusable abrasive is recycled through the system, while dust and debris are drawn into a dust collector.

Centrifugal blasting will clean, profile, and remove burrs, flashing, or defects. It is used on different forms of steel and on other surfaces, such as stone and concrete.

In addition to large, stationary blasting units found in factories and fabricating ships, portable machines are used for onsite centrifugal blasting during construction or maintenance work, such as cleaning the decks of ships or parking garages, and other flat steel or concrete surfaces.

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Article on Communications and Hearing Protection By:

Vancouver Industrial Painters

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COMMUNICATIONS AND HEARING PROTECTION

In the past, the most common method used to get the attention of a blast operator was to turn off the blast feed to the operator’s nozzle or, if the operator was working on steel, to signal him/her by rapping on the steel with something hard (often an easily damaged, expensive blast nozzle). In any event, the operator had to remove his/her helmet, remove any hearing protection, breath dust, and attempt to shout over the noise created by any nearby operations in order to communicate.

To overcome this problem, most high-production blasting contractors today use two-way portable radio sets that are designed especially for use in blasting operations. Most of these radio sets are ruggedized, frequency modulated (FM), ultra-high frequency (UHF) radios with one or more operator-selected channels and rechargeable batteries. These radio sets allow a supervisor to communicate with one or more blast operators or allow blast operators to communicate with each other under noise conditions as high as 115 decibels and at distances as great as 1 mile (1.6 km).

The supervisor’s set usually includes a handheld radio with a sound-deadening headset equipped with a boom-mounted microphone. The blast operator’s radio set usually consists of a radio and holster, a pressure-actuated talk switch, and a sound-deadening earphone set with a small boom-mounted microphone designed to fit inside a supplied-air helmet. The operator responds to communications by pressing one arm against the talk actuator switch located under the blast suit or cape. This permits hands-free communications.

Radio communication is more than a convenience. It speeds the training of new operators, increases productivity, allows coordination of operations in the work area, and allows announcements of breaks, shift changes, or impending bad weather. It is also a valuable safety feature when blasters work outside of direct visual range or when dust obscures vision. Blasters can alert supervisors to any production problems or request help in the event of an accident. The time-saving and safety benefits make communications equipment a vital component of a blast system.

Due to the high noise levels that blast operators are exposed to in high-production blasting, OSHA requires that NIOSH-approved hearing protection be used. If radio communications equipment with sound-deadening earphones is not adequate or is not used, then operators must use adequate, NIOSH-approved hearing protection.

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BREATHING AIR FILTERS

OSHA requires that breathing air filters comply with the requirements for Grade-D breathing air. Filters that do not meet these requirements must not be used. The filter must use high-capacity, super-efficient, easily replaced filter cartridges specifically designed for breathing air systems. The filter must have a pressure regulator and gauge, not only for adjusting pressure to the control valve of the helmet, but also to indicate when the cartridge needs replacement. The gauge will show declining pressures as the cartridge becomes saturated with liquid and solid matter. The filter must also be sized to supply the volume of air required for all the respirators connected to it.

Remember, these filters cannot handle large quantities of oil or moisture-laden air from the compressor for any length of time and they will not remove carbon monoxide. Compressor maintenance, high-temperature detection shutdown or alarm, carbon monoxide monitoring or conversion, and maximum cooling and filtering of breathing air at the compressor is an absolute must.

CARBON MONOXIDE DETECTION ALARMS AND CONVERTERS

To help prevent operator exposure to carbon monoxide, two methods are available, each usint, a different technology. One monitors the air and triggers an alarm if carbon monoxide reaches an unacceptable level. The other method monitors and sounds alarms as well but, up to certain levels, it converts carbon monoxide to carbon dioxide that can be tolerated by the human body.

Alarms — Carbon monoxide detectors continuously test samples of air in an air line for the presence of carbon monoxide. If the detected gas exceeds the permissible level of 10 ppm for Grade-D air, the detector activates loud alarms. Their design should be such that they do not restrict the flow of air to blast operators. They should also be equipped with outputs for remote alarms, monitors, and strip chart recorders (required by OSHA for some applications). These systems are normally powered by 120 volts AC or 12 volts DC and are the least expensive of the two methods. However, a relatively expensive sensing element in the detector must be replaced every two to five years.

Converters — These units use chemicals to change carbon monoxide (CO) to carbon dioxide (CO2). The human body can tolerate much more CO2 than CO. The permissible level of CO2 is 1,000 ppm for Grade-D breathing air versus 10 ppm for CO. These electrically- operated units use elaborate air drying and moisture purging equipment to keep the reactive conversion chemicals dry. It is absolutely necessary that the incoming air from the compressor be as free as possible of oil and moisture in order to prevent unnecessary malfunctions of the unit.

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Breathing Air Supply Article

Compressed air cylinder: An air cylinder fitted with a pressure regulator can supply breathing air in remote areas, or when a contractor prefers the extra insurance of contamination-free air. Make sure that the cylinders are obtained from a qualified supplier that certifies that the compressed air meets or exceeds Grade-D specifications.

WARNING! Never use oxygen tanks to supply breathing air.

Prolonged exposure to high concentrations of oxygen will cause lung damage.

• Breathing air pumps — Electric or air-motor powered, oil-free ambient air pumps are specifically designed to supply breathing air at about 15 psi (103 kPa). They are essentially blowers that push air through a line. Air pumps do not produce carbon monoxide and are usually equipped with built-in inlet and outlet filters. Because they do not compress air, very little or no heating occurs and they do not require high- temperature or carbon monoxide detection alarms. They are suitable for use by one, or

possibly two, blast operators using low-pressure, supplied-air helmets.

• Oil-less air compressors — Available as stationary or portable packages, these are the

best choice for providing high-pressure, supplied-air helmets. Depending on their size, they can supply multiple operators with the proper air volume at a sustained pressure of 95 psi (655 kPa) for high-pressure, conditioned-air helmets. Generally, these compressor packages produce little or no carbon monoxide or contaminants and are equipped with pre-filters and after-filters for the air output. However, they produce hotter air than a lubricated compressor. Unless the manufacturer has rated the compressor package as NIOSH-approved to supply Grade-D or better breathing air, appropriate after-coolters, filters, and high-temperature/carbon monoxide detection alarms must be used on the output.

WARNING! Do not use piston-type, oil-lubricated compressors for

breathing air. They can produce unacceptable levels of carbon monoxide.

• Lubricated air compressors — Most contractors use one compressor to supply both blast and breathing air. When lubricated compressors are used, extra precautions must be observed when using the output for breathing air. OSHA regulations require that any lubricated air compressor used for breathing air be equipped with a high-temperature detection shutdown or alarm and/or a carbon monoxide detection alarm, or both. It is recommended that both a high-temperature detection shutdown or alarm and a carbon monoxide detection alarm or converter be used at all times. In addition, OSHA requires breathing air filters to remove particulate matter, oil mist, and moisture. It is highly recommended that a compressor of this type be equipped with proper after-coolers, receivers, coalescing filters, filter traps, and air dryers. These should be installed ahead of the air outlets used for breathing air in order to reduce the air temperature and the amount of moisture and oil reaching any carbon monoxide converters or the breathing air filters.

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SUPPLIED AIR AND PRESSURE-DEMAND RESPIRATORS

The continuous flow, supplied-air respirator protects the operator from breathing dust-laden air. These respirators are fed Grade-D breathing air which is first filtered, monitored for carbon monoxide, pressure regulated, and then fed into a helmet. A positive pressure is created inside the helmet by the pressurized air. This prevents dust and media from entering. Supplied-air respirators are used only in work environments where the surrounding air is not immediately hazardous. This means that the operator would have time to exit the area if the helmet air supply were to be interrupted.

Two supplied-air, Type C respirator designs are available. The full-helmet design offers maximum protection and comfort. A large faceplate window, low-noise air distribution, hard-hat protection, and replaceable waist-length cape with a removable cloth inner collar provide maximum productivity and full protection for the operator and any equipment worn under the cape. This is the preferred respirator for prolonged, high-production blasting. For small blast jobs, blasting in confined areas, and for non-blasting personnel at a blast site, lightweight hoods offer adequate protection for short periods.

Grade-D breathing air is an OSH A requirement for supplied-air respirators. A partial definition of Grade-D air includes:

• Oxygen content — Atmospheric 19.5 to 23.5%

- Carbon monoxide — Maximum 10 parts per million (ppm)

• Carbon dioxide — Maximum 1,000 ppm

- Hydrocarbons (condensed) — Maximum 5 milligrams/cubic meter (mg/m3) of gas

For work environments where concentrations of toxic dust exceed the level of protection offered by a supplied-air respirator, OSHA requires that an operator use a pressure-demand respirator which is worn over the operator’s face inside a special, supplied-air respirator helmet (Type CE respirator).

ABRASIVE BLASTING

The faceplate of a helmet must be protected with disposable outer lenses. At the start of a shift, an operator can affix several outer lenses to cover the faceplate and carry enough spare lenses to last the day. As each outer lens becomes frosted because of pitting caused by abrasive, it can be pulled off, exposing a fresh lens. The helmet should also be equipped with an adequate chin strap so that it stays securely in place as the operator moves.

All air-fed helmets are furnished with a flexible breathing hose and an air control valve. The air control valve is set to provide a minimum of 6 CFM to a maximum of 15 CFM (0.2 to 0.4 m3/min.) at operating pressures of 14 to 25 psi (96 to 172 kPa), as required by NIOSH. The operator can only adjust the airflow within these limits. If damaged, the air control valve or the flexible breathing hose from the control valve to the helmet must be replaced with only the type recommended by the manufacturer of the helmet.

AIR CONDITIONING CONTROL VALVES

Once considered a luxury, air conditioning control valves are now recognized as essential to achieving maximum productivity from an operator in high-production blast operations. Operators breathing hot air on a hot day or cold air on a cold day can be extremely uncomfortable and will be distracted from the task at hand. As a result, productivity, safety, and job quality may suffer. Most helmet manufacturers have optional NIOSH-approved air conditioning control valves available that replace the standard control valve supplied with the helmet. Some supply only cool air, while others can supply either warm or cool air, as desired. The valve doesn’t actually, cool or warm the air. Instead, the valve takes incoming pressurized air and separates hot air from cold air by means of a vortex tube. Then, either hot or cold valves divert the desired warm or cool air to the helmet while discharging the unwanted cool or warm air, respectfully.

While these valves supply the NIOSH required 6 to 15 CFM (0.2 to 0.4 m3/min.) to the helmet, they consume about 20 CFM (0.6 m3/min.) at 85 to 95 psi (586 to 655 kPa), which is considerably more pressure and volume than is required by a standard control valve. Therefore, they will require a substantial breathing air supply. Do not combine one manufacturer’s equipment with another manufacturer’s helmet because substitution will void the NIOSH approval.

BREATHI1NG AIR SUPPLY

Sources for breathing air range from air cylinders to large air compressors. Breathing air must not contain any toxic contaminants at levels which make the air unsafe. No matter what the source, breathing air supplied to a helmet must be Grade-D or higher breathing air as described in the Compressed Gas Association Commodity, Specifications G7.1

Rick Anderson

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Needle Gauge and Protective Gear

A needle gauge can be used to check the air pressure of the blast system. Inject the needle into the blast hose behind the nozzle and pointed away from the airflow. The air pressure should be about 90 to 100 psi for regular blasting operations. For brush-off blast cleaning, the pressure may not need to be so high. If the pressure is low, check the following for the cause:

• Is the compressor capacity large enough for the project?

• Is the compressor operating properly?

• Are the air lines too long?

• Is the nozzle the right size for the available air volume and pressure?

• Are any couplings leaking?

• Are there any obstructions in the system?

OPERAT0R PROTECTIVE GEAR

Blast cleaning propels abrasive media to speeds in excess of 400 mph (644 km/hr.). This process turns paint, rust, contaminants, and in some cases, part of the substrate and/or the media, into microscopic dust particles. To an unprotected, untrained operator, blasting can be a dangerous, even deadly occupation. The air/abrasive jet from a blast nozzle can cause serious injury if unprotected flesh comes in contact with it at close range. Rebounding abrasive can cause eye injury. Inhalation of the dust from blasting operations can cause permanent lung injury and may also be toxic, especially if silica, asbestos, and lead or other heavy metal particles are present in the dust. For these reasons, an operator working in ar average blasting environment must be equipped with National Institute for Occupational Safety and Health. This equipment is explained in the following posts.

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Typical Components Of A Single Tank

The typical pressurized abrasive tank (sometimes referred to as a pot) for a single-tank blast machine must be ASME-coded [usually for 125 psi (862 kPa) operation] and should be equipped with a conical bottom, a concave top with a hopper that will accommodate about 60% of the rated abrasive capacity, and a rain cover. The conical bottom should have at leas a 35° slope for most abrasive media. If the machine is designed to handle lightweight media such as plastic, the conical bottom may have a 60° slope. If the machine will be used with bulk or reused abrasive, the concave top should be equipped with a large, screened opening to allow rapid filling of the tank while screening out oversized media clumps or particles. Some smaller machines are also equipped with a bag ripper to aid in rapid filling of the hopper. The opening at the top should be shielded by an umbrella and sealed by an automatic pop-up valve when air pressure is applied to the tank. The pop-up valve allows automatic filling of the tank from the hopper when air pressure is released from the tank. The shield acts to relieve abrasive pressure on the pop-up valve so that it can seal properly. The pop- up valve should also be equipped with large vent openings so that the tank will repressurize quickly to prevent blowback through the metering valve. The tank should also be equipped with a large side inspection/clean-out port to allow quick access to the tank interior for maintenance or cleaning.

Dual-tank pressurized blast machines are continuous-action machines that allow blasting to continue while the machine is being refilled. The machine actually consists of two separate pressurized sections within one tank, one above the other. While blasting is being conducted using the lower section, the upper section can be depressurized and refilled with abrasive. Once the upper section is again pressurized, there will be an automatic transfer of abrasiv, from that section to the lower section. A cycle timer or some other device is used to control pressurization of the upper section. One important requirement for dual-tank machines is that the air compressor receivers have enough capacity to repressurize the upper section without causing low-pressure surges to the blasting operations being conducted from the lower section.

HOSE

Hoses tie the key components of the blast equipment together—the compressor to the blast pot and the blast pot to the nozzle. The air supply hose, sometimes called the bull hose, feeds air from the compressor to the blast pot. Its inside diameter should be three or four times as large as the nozzle orifice. The blast hose is a thick, reinforced hose that carries the air and abrasive mix from the blast pot to the nozzle. It is made to be grounded as a guard against static electricity that may result from the abrasive flow and shock the equipment operator. It should also be three or four times as large as the nozzle orifice.

Sometimes, a whip hose, which is a short length of hose with a smaller diameter than the blast hose, is used near the nozzle for better handling and flexibility. However, a whip hose dramatically reduces the air volume, which substantially affects the rate of blast cleaning. Therefore, a whip hose is not usually used by professional blast cleaners.

A personal air hose feeds air to the blast hood, which is a safety helmet with shroud worn by the blaster for breathing, cooling, and maintaining positive air pressure to help keep out harmful blasting dust and debris.

Control hoses are usually dual lines (electric or pneumatic) that form a complete circuit between the control valve on the blast pot and the control handle (deadman switch) (Figure 7). The deadman switch is a safety lever at the nozzle that lets the operator stop and start the blasting operation. Hold the deadman switch down to blast, release it to stop. This device is legally required for safe blasting operations. It should never be tampered with, and should always be checked before each job to ensure that it works properly.

Couplings connect hoses together. External couplings require no tools to connect the hoses. They have universal joints for joining hoses of different sizes and rubber gaskets to form tight seals. They are recommended instead of internal couplings, which reduce the inside diameter of the hoses and restrict airflow. Wire the couplings together to prevent them from disconnecting accidentally, or use a whip check (a safety cable that connects hoses near the coupling to keep them from flying around if the joint breaks).

ABRASIVE BLASTING

Turbulence, Friction, And Wear Caused By Internal Couplings

The blast nozzle with an opening measured in fractions of an inch, has the fins effect on the abrasive being blasted at the surface. The size, type, length, and lining of the nozzle all make a difference in how it works. Needless to say, there are a variety of nozzle types.

Abrasive Blasting Nozzles

The size of the nozzle orifice is measured in 1/16-inch increments. For example, a No. 3 nozzle has a 3/16-inch orifice and a No. 4 nozzle has a 1/4-inch orifice. The size of the nozzle use( will depend on the available air pressure and volume as well as the object to be blasted The higher the pressure and volume, the larger the nozzle that can be used. The larger the nozzle, the more abrasive that is consumed. The more abrasive used, the more surface tha can be cleaned. Obviously, it pays to use the largest possible nozzle orifice that availably air volume and pressure will support. The larger the nozzle, the faster and more efficientl: the work goes. A 1/2-inch nozzle will clean four times as much surface in an hour as 1/4-inch nozzle. However, smaller nozzles are often preferred when cleaning small parts o surface areas because they provide better control.

There are two primary designs of nozzles: the straight bore and the Venturi style .

The conventional straight bore nozzle has about half the output velocity as the same size Venturi nozzle. The power in a straight bore nozzle is concentrated in the center of a large blast pattern, and diminishes toward the edges. The bore of the Venturi nozzle has a large mouth, tapered midsection, and flared opening. It can propel abrasive at a speed that is more than twice as fast as the straight bore nozzle. It also has a more even blast pattern, which means the work is done more efficiently and with less abrasive.

Venturi And Straight Bore Nozzles

Several other types of nozzles are designed for special purposes, such as blasting at an angle to clean hard-to-reach areas or in a 360° pattern to clean the inside surface of pipe or tubing.

The nozzle length, which ranges from about 3 to 9 inches, relates to cleaning strength, with longer nozzles being used for hard-to-clean, tightly-adhered contaminants.

The nozzle lining determines how long the nozzle will last. The flow of abrasive eventually wears down the lining. Tungsten carbide, a conventional lining material, will last several hundred hours. Boron carbide and silicon nitride have proven to be even more durable. Using a nozzle orifice gauge inspect the nozzle for wear regularly, and replace it when the orifice increases by 1/16 of an inch.

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AIR COMPRESSORS

The air compressor is critical to conventional air abrasive blasting operations because it must supply the right amount of air at the right pressure to propel the abrasive against the surface. Air pressure is measured in pounds per square inch (psi). Standard atmospheric pressure, which is the normal pressure of air from the weight of the Earth’s atmosphere, is 14.7 psi. A compressor creates high air pressure by forcing or compressing large quantities of air into a small space. The air pressure needed to blast steel effectively is 90 to 100 psi at the blast nozzle. In order to obtain this air pressure, a compressor must pressurize its receiver tank to approximately 125 psi.

Another important factor is how much pressurized air a compressor can deliver. This volume or output is measured in CFM (cubic feet per minute), and is determined by the size of the compressor. The more air the compressor can force into the air lines of the blast equipment, the higher the resulting air pressure. The air volume required depends on the size of the blast nozzle(s) and how many of them are used at the same time. If other equipment is being run off a compressor simultaneously, take this into consideration when adjusting the CFM.

There are both stationary compressors, which are found in fixed blasting sites, and portable compressors (Figure 2), which can be moved to jobs as needed. Compressors are either powered by an electric motor or an internal combustion engine that runs on gasoline or diesel fuel.

All compressors must have a label that indicates that certain high-pressure components (receivers, etc.) are built to the National Board of Pressure Inspectors Standards and are registered to meet the American Society of Mechanical Engineers (ASME) code. The compressor must also be equipped with ASME-approved pressure safety relief valve(s) that will vent air pressure if the maximum pressure rating of the equipment is exceeded. If the compressor package does not carry the ASME label and the safety relief valve(s), do not use it. The label certifies to the user that the compressor package can be used anywhere in the U.S. without fear that a State Pressure Vessel Inspector will shut down the job because of non-coded, high-pressure components or inadequate safety devices.

Rick Anderson

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