Propelling Abrasive Media

Abrasive blast cleaning began commercially with air or steam directed through a conduit of pipe or hose with a final nozzle to direct the impacting abrasive stream. Both pressure blast and suction blast nozzle systems require high power to generate the compressed air or pressurized steam that is used to accelerate and propel the abrasive. This requirement is due to aerodynamic inefficiencies in accelerating the spherical and angular abrasive particles, especially the higher-density ferrous abrasives.

Wheels. Airless abrasive propelling wheels that use blades or vanes require about 10% of the horsepower required by air blast systems to throw equal volumes of abrasive at the same velocities. The power losses in an airless system are the friction between the abrasive and vanes, the impeller-control cage interference, and the wheel-drive system.

Airless abrasive blast wheels are generally of the blade type, as shown in Fig. 1. These wheels may have one or two side plates, one of which is attached to a hub, shaft bearings, and belt drive, or the side plate may be attached directly to the shaft of a suitable motor. The side plate holds four to twelve throwing blades, depending on the size of the wheel. Blade tip diameters range from 205 to 660 mm (8 to 26 in.) and blade widths range from 40 to 125 mm (1.5 to 5 in.). Rotational speeds range from 500 to 4000 rev/min or more. Usable abrasive velocities range from 15 m/s (50 ft/s) to 122 m/s (400 ft/s), with 75 m/s (245 ft/s) the most widely used velocity. Abrasive flow rates with steel shot range from 23 kg/min (50 lb/min) up to 1040 kg/min (2300 lb/min) with a 100 hp motor.

Fig. 1 Blade-type airless centrifugal abrasive blast wheel

Figure 1 also shows the operation of a blade-type wheel. A controlled flow of abrasive (through a valve not shown) is fed by gravity into an abrasive feed spout from which it flows into a rotating vaned impeller. The impeller rotates at the same speed as the bladed wheel, and the number of vanes is equal to the number of wheel blades. The impeller rotates in a stationary cylinder (referred to as a control cage or impeller case) that is equipped with an opening that may be rotated and locked in a preferred position. As the impeller forces the abrasive out of the control cage opening, each of the blades picks up a metered amount of abrasive at the inner end of the blade and accelerates the abrasive to produce a tent blast pattern, as shown.

Centrifugal blast wheel units are enclosed in housings to prevent the discharge of stray abrasive. The principal wearing parts of the blast wheel assembly are the impeller, control cage, wheel blades, and housing liners. These parts are most economically made of high-alloy cast iron, and each can be individually replaced. Unalloyed cast iron parts, although less expensive, have a very short life under normal operating conditions.

The life of these parts is influenced primarily by the type and condition of the abrasive medium and contaminants picked up in the cleaning process. Abrasive materials are discussed in depth later in this section. Clean steel shot provides the longest useful life of wheel and guard housing liners. Much greater wear results from the use of nonmetallic abrasives such as sand, aluminum oxide, and silicon carbide. Table 1 shows the effects of abrasive in various conditions on the life of the components of a centrifugal blast wheel unit. Relatively little wear on wheel parts and housing liners is caused by glass beads, nonferrous shot, or the agricultural abrasives frequently used in deburring and special finishing applications.

Table 1 Effect of abrasives on life of components of a centrifugal blast wheel unit

Blades

Impeller

Control cage/case

Alloy housing liners

100% steel shot (few fines)

600

600

600

3000

Steel shot, 1% sand

100-200

100

100

2000

Steel shot, 3% sand

15-50

50

50

1500

100% steel grit(b)

125-150

150

150

1000-1500

100% sand

4-6

4-8

4-8

Life based on running time of centrifugal blast wheel 495 mm (19 in.) diam and 65 mm (2 in) wide, 30 hp drive and flow rate of 375 kg/min (830 lb/min).

Centrifugal wheel-type blast machines may be relatively simple, having a single blast wheel, a simpler work conveyor, an abrasive recycling system, and a dust collection device.

Pressure blast nozzle systems generally rely on a 685 kPa (100 psig) air supply to propel the abrasive through a special nozzle. A typical intermittent pressure tank (Fig. 2) has dimensions of 610 by 610 mm (24 by 24 in.) and an abrasive discharge capacity of 0.12 m3 (4.2 ft3). This capacity is adequate to operate one 6 mm (-4 in.) diameter blast nozzle for 30 to 60 min. This type of tank is refilled through the filling valve by gravity when the air supply is shut off. Without air pressure in the tank, the filling valve is pushed down and open by the weight of the abrasive. When the air pressure is turned on again, the valve rises and stops the flow of abrasive into the tank. The abrasive in the now-pressurized tank moves into a mixing chamber. Mixing chambers usually are equipped with an adjustable control to regulate the flow rate of abrasive into the mixing chamber and on through the hose and nozzle assembly. The pressure tank and filling valve may be vertically doubled with a timer and proper valving to provide a continuous automatic pressure tank.

Fig. 2 Double-chamber abrasive blast pressure tank. Courtesy of Bob Thompson, Schmidt Manufacturing Inc.

Airblast nozzles are used in a variety of shapes, some as simple as a piece of pipe. Most systems are replaceable nozzles of metal alloys or nozzles with wear-resistant ceramic inserts. The latter nozzles may be of straight bore or venturi cross section. All types of abrasive may be handled with the pressure blast system in a variety of environments. In exceptional cases, air pressure blasting is performed in an open field with sand as the abrasive. Protective clothing and a helmet with air supply are the only health precautions taken. Quite often the sand is not recovered after use.

Suction blast cabinets are generally considered the simplest form of abrasive blast equipment. They may be used manually or have fixed or oscillating nozzles. Figure 3(a) illustrates a 1220 by 915 by 840 mm (48 by 36 by 33 in.) suction blast cabinet.

Fig. 3 Suction blast equipment (a) cabinet. (b) Nozzle assembly

Figure 3(b) illustrates a suction blast nozzle assembly. The nozzle in the suction cabinet is an induction nozzle that creates a blasting mixture by the siphon effect of the air discharged through the nozzle body. This effect pulls abrasive through the abrasive hose from the cabinet hopper, and the blast mixture is formed within the nozzle body. Because only compressed air flows through the air nozzle, the air consumption remains constant. The air nozzle is cast of a wear-resistant alloy. The nozzle can be used until considerably enlarged without affecting the efficiency of the blast. This cannot be done in a direct-pressure blast nozzle without seriously affecting air consumption. The amount of abrasive or the mixture of air and abrasive can be controlled in the suction cabinet by changing the relative position of the end of the abrasive hose to the abrasive flowing from the cabinet hopper.

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