The principles of centrifugal blastcleaning.
Developed nearly 70 years ago, centrifugal blastcleaning consists of the following stages. Abrasive is fed into a centrifugal wheel through the impeller, which meters the abrasive out onto the blades. The control cage guides the abrasive onto the castings. After impacting the castings, the abrasive falls into the cabinet hopper along with the sand, scale and dust. The abrasive handling system lifts the contaminated abrasive to the separator. The separator cleans the abrasive and returns it to the storage hopper, where it is again fed into the centrifugal wheel.
Because of the high energy dispensed in blastcleaning, the equipment requires periodic maintenance and adjustment. By observing your operations and making adjustments outlined in this article, you can better control your cleaning costs.
As shown in Fig. 1, blast equipment is composed of five basic parts. These are: the bladed airless abrasive wheel that propels the abrasive at the casting; the cabinet that confines the abrasive; the material handling system that positions the casting; the abrasive cleaning and recycling system; and a dust collector and vent pipe system.
Because the material handling system and cabinet vary considerably from machine to machine, they must be examined on an individual basis. Generally, as long as the material system properly positions the work to the blast, only normal maintenance of its mechanical parts is required. The cabinet should be kept abrasive-tight. If abrasive resistant liners protect the ceiling and walls, maintenance of the cabinet will be a minor problem.
The airless abrasive blast wheel is the propulsion means for throwing the abrasive at the work. The size and type of abrasive, and the quantity, direction and Velocity it is thrown determines the effectiveness of the blast. The velocity of the thrown abrasive by the wheel is directly proportional to the wheel speed. The impact force of the abrasive is proportional to the velocity squared. For most blastcleaning operations, a velocity of about 245 fps, belt driven at 2250 rpm, and 300 fps, direct drive at 3600 rpm, is used.
Assuming the operating mixture of abrasive thrown by the wheel is the same, efficiency of casting cleaning depends on only three elements: the quantity, velocity and direction of the thrown abrasive.
The quantity of abrasive thrown is determined by the ammeter, which reflects the loading on the wheel motor. As the amount of abrasive fed through the wheel increases, the ammeter reading increases. If an ammeter wheel motor reads full load amperage when blasting, the maximum rated capacity of abrasive being thrown is being utilized. If the ammeter reads less than this rated amount, the operation is below maximum efficiency and a correction should be made.
There are two fundamental causes of a low amperage reading. The first is a "starved" wheel [ILLUSTRATION FOR FIGURE 2 OMITTED], in which the wheel does not pull full amperage because it lacks abrasive. A solution is to check the abrasive supply in the storage hopper to determine if it is adequate. Also, check the abrasive conveying system to ensure that it is delivering the abrasive as it should. Further, check the scalping screens, feed spout and feed funnel for obstructions. A small piece of wire or paper impedes normal abrasive flow to the wheel and seriously reduces its working efficiency. Excessive or sharp bends in the abrasive feed pipe to the wheel unit(s) can reduce its ability to deliver the necessary abrasive to the wheel. Finally, the elevator may not be delivering an adequate abrasive supply. It should be checked for slippage and worn or missing buckets.
The second general cause of low amperage is a "flooded" or "choked" wheel [ILLUSTRATION FOR FIGURE 2 OMITTED]. This choked condition may arise when abrasive is fed at too rapid a rate and the feed spout is choked with abrasive. To correct this condition on manually operated abrasive controls, an adjustable stop or set screw should be positioned on the dipper valve to limit the amount of abrasive released to the wheel. On the soundproof-type valves it is necessary to adjust the stop provided for this purpose. In all cases, the wheel should be cleared of abrasive before repositioning the stop.
A worn impeller or foreign matter in the impeller can reduce its capacity to feed the wheel with abrasive and cause the same condition. Inspect the impeller periodically to ensure it is clean and in good condition.
Finally, if the belts driving the wheel are not under correct tension, they will slip and create a flooded wheel condition. Unless the wheel and impeller revolve at their established rates, they will be unable to handle the abrasive fed to them and pass it on to the blades.
A simple test can be used to determine the cause of low wheel amperage:
1. With the wheel running, turn on abrasive and turn off abrasive flow.
2. If amp reading jumps to full load capacity before falling to "no load," the problem is a "choked" or "flooded" wheel.
3. If amperage falls to "no load," the wheel is "starved."
Unless the abrasive strikes the work, it cannot clean - therefore blasting efficiency also depends on what proportion of the abrasive is thrown onto the work. This is determined by the control cage setting.
The control cage is a sleeve that fits around the impeller. The impeller is cast with blades resembling the wheel itself - except that it is much smaller in size. The impeller is attached to the shaft and rotates with it and the wheel. It receives the abrasive from the feed spout and gives it a circular motion. The abrasive is prevented from going onto the blades except through the control cage opening. Abrasive is pumped by the impeller onto the blades through the control cage opening and in turn propels it toward the work.
There are many ways to determine where the blast pattern is hitting. This concentrated area is called a hot spot because a stationary piece of work or a metal target plate mounted in line with the blast becomes hot when subjected to a blast of 30 sec or longer.
The best way to determine the location of the hot spot is to put a steel plate target in line and on top of the work and blast it for 30 sec [ILLUSTRATION FOR FIGURE 3 OMITTED]. You can then actually feel the target to determine where the blast is hitting. The control cage can then be moved one way or the other to adjust the hot spot to the proper location [ILLUSTRATION FOR FIGURE 4 OMITTED] and a second check can be performed. Control cages are available with different sizes and shapes of openings to suit the machine or application.
Once the control cage is set, it continues to deliver the blast to the proper area until wear alters the size of the opening. Wear occurring on any one of the wheel elements - the impeller, control cage or blades - can cause the movement of the hot spot. The wear on the control cage alters the hot spot because it allows more room for the abrasive to be thrown. Wear on the impeller fibs and control cage disturbs the hot spot. Abrasive leaving the worn ribs of the impeller causes the abrasive to hit the back edge of the respective blades and to land high on the face of the following blades rather than landing on the throwing face of the respective blades. This causes the hot spot to be badly diffused, with a resultant loss in velocity and impact force.
Badly pitted or worn blades offer more resistance to abrasive flow so that the abrasive spends more time on the blades than it would on a smooth set of blades. Because of this, badly worn blades tend to lengthen and shift the hot spot, with a loss in abrasive velocity and impact force.
Increasing the percentage of fines in the operating mixture also causes the hot spot to change, because fines tend to hang up on the blades for a longer time.
The airless abrasive blast wheel is the heart of blast equipment and how well it performs determines how well the equipment operates.
A good job of cleaning can't be achieved unless the abrasive is the right size and is hitting the work in the fight area, at the right flow and velocity. By keeping these points in mind, you can cope with most cleaning problems.
Operating Mix Makeup
The final element determining blastcleaning efficiency is the makeup and condition of the abrasive operating mix, which is controlled by the abrasive separator.
The separator is the most important element of any piece of blast equipment from the standpoint of determining operating costs from worn parts. The separator:
* controls the sizing of the abrasive operating mix, which in turn has a strong influence on cleaning efficiency.
* removes sand and other contaminants from the abrasive stream so that only good, clean abrasive is fed to the wheels. Clean abrasive is a determining factor on maintenance costs.
* controls abrasive consumption rates. Abrasive consumption is determined by a size of the abrasive pellets removed from the machine. The separator makes this selection.
You can see that the setting of the separator is of major importance in controlling all phases of operating costs. Most separators use the air wash principles because of the ultra fine adjustments possible with this type of device to remove undersized abrasive.
Heavier components fall right through the air stream without being disturbed by it. Obviously, the greater suction there is, the larger the size of the pellet that is pulled out by the airstream.
If the curtain is disturbed and is made heavier at one end and thinner at the other, the air follows the line of least resistance and passes through the thin curtain area [ILLUSTRATION FOR FIGURE 5 OMITTED]. This causes the velocity of the air in that end to be substantially greater than what it would be if spread equally across the whole area. Therefore, heavy pellets are pulled at in the sparse end and substantially light contaminants are left at the dense end.
While the actual separators on blast equipment are much different from the example of the vacuum and the curtain, they only differ with respect to how the uniform curtain and suction is obtained. For instance, the standard gravity separator has a series of flat plates that are designed to spread the abrasive so that the curtain flowing past the air wash is always uniform [ILLUSTRATION FOR FIGURE 6 OMITTED]. The first element that does this is the tramp metal screen that catches tramp metal to keep it from lodging in the lower portion of the separator and disrupting the flow of abrasive.
The adjustable baffle spreads the abrasive across the full length of the separator. The swinging baffle and its counterweight(s) controls the thickness of the abrasive curtain. The lower adjustable baffle along with the slide gate, in the vent line, is then adjusted to remove the contaminants from the abrasive curtain.
The air wash itself flows from the open side of the separator up through the wash area into a plenum chamber, from which it heads into the dust collector. The plenum chamber is primarily used to settle out the large contaminants that were pulled out of the air wash so that the dust load to the dust collector does not become unnecessarily excessive.
What usually goes wrong with the separator? First, the draft on the separator can be too great, causing the removal of too large a size pellet. If the draft is too little, fines are retained. This problem can be adjusted and corrected by the blast gate located between the dust collector and the separator.
The next most common problem within a separator is uneven abrasive flow, which is caused by tramp metal lodging under the swinging baffle; holes in the tramp metal scalping screen; a warped or missing swinging baffle; or a worn or missing stationary baffle. Each of these causes abrasive to channel, which, in turn, causes too large a pellet to be pulled out on the sparse end while leaving contaminants in the mixture on the heavy-flowing end of the curtain.
Because the effective operation of the separator depends on the airflow following a certain channel, it is important that air leaks (which may be caused by holes in the metal case or at the weldments) are prohibited. If air enters the separator at other than the proper openings, the volume of air at the separation point is reduced and the cleaning efficiency of the separator suffers.
The best way to check to determine if a separator is functioning properly is to screen a sample of the operating mixture and separator discharge. A screen analysis is recommended over a visual analysis because it is extremely difficult to get a quantitative measure of the condition of the abrasive mixture or discharge by just looking at it. The screen analysis is an exact breakdown of the various mixtures.
Consider the effect of the dust collector on the setting of the separator. The dust collector is the vacuum cleaner in our example. Like a vacuum cleaner, it works efficiently as long as the bags are not clogged with dirt or as long as there are no air leaks between the suction point in the separator and the vacuum cleaner itself.
Dust collector bags must be cleaned or shaken down regularly (if used) and the ductwork to the dust collector must be free from holes so that the suction at the separator is not reduced. You must also be concerned with the ventilation of the machine proper, as a change in the amount of air being fed through the machine alters the amount of air going through the separator. Thus, the separator affects cleaning efficiency, maintenance and abrasive consumption.
Experience shows that the most efficient abrasive cleaning mixture consists of large, medium and small pellets. The larger pellets are used to impact the surface of the work to loosen tough sand, scale and other contaminants. The medium and smaller particles, because of the increased number of pellets to the pound, provide the coverage to completely scour the surface and remove these loosened components so that the work is clean. Just as a hunter uses a rifle slug to kill a deer and fine shot to shoot a quail, the size of the abrasive is adjusted according to the type of contaminants to be removed.
A rule of thumb is to select the smallest size abrasive to remove scale, bum-in and casting defects to obtain acceptable castings. This is also based on being able to remove the scale and sand from the operating mix. Since there is always a range of contaminants, from tightly bonded, to loose, easy to remove materials, there should always be a range of sizes of abrasive to do the job.
The number of abrasive pellets per pound varies greatly with the size of the abrasive. Reducing the abrasive (diameter) size by 50% increases the number per pound by a multiple of eight. Each pound of the smaller size gives many more impacts per pound than do the larger size abrasives. However, since the heavy abrasives hit with more force they are also required for speedy cleaning. There must be a balance between the abrasive sizes before we can have an efficient operating mix.
The machine automatically forms this desirable operating mix if small regular abrasive additions of the proper size abrasive are made as frequently as required and if the separator is permitted to remove the abrasive as it is worn down below usable size.
The separator also has a strong effect on maintenance costs. As explained, one of its prime functions is to remove sand, scale and foreign contaminants from the operating mix. Experiments and experience have shown that as little as 2% sand by volume in an abrasive operating mixture increases the wear rate on wheel parts by up to 50%.
The presence of contaminants in an operating mixture makes a tremendous difference in operating costs and in the amount of maintenance required on a piece of blastcleaning equipment.
The setting of the separator controls abrasive consumption. Regardless of the type of abrasive used, whether chilled iron, malleable iron or steel, abrasive consumption increases as the size of particle removed by the separator increases. If the size of pellet removed is increased by 20%, shot consumption also increases by 20%.
As a final point, the waste tube where the sand and fine contaminants drop from the settling chamber of the separator is sealed at the bottom by a small rubber flapper or dribble valve.
This is a small item-yet it is extremely important.
Here's how it works [ILLUSTRATION FOR FIGURE 6 OMITTED]: The dust collector fan pulls a current of air through the separator. If the flapper is not in place and the waste tube is not sealed, air flows up the waste tube and short - circuits the separation process. Thus, the air goes up the discharge tube and directly out the dust collector piping. The area where separation is supposed to occur is bypassed to a great extent and the effectiveness of the separator is sharply reduced. The sand, which should fall down this tube, is buoyed by the upward air current and returned to the plenum chamber. This sand eventually goes to the dust collector, causing excessive wear to the ductwork and the dust collector as well as increasing the dust load to the dust collector.
Basic, Yet Misunderstood
The blastcleaning principles and problems described in this article are common to all blast equipment. These basics, however, are generally misunderstood.
Unfortunately, blast equipment operates and performs satisfactorily - even when the wheels or separators are badly malfunctioning.
Soaring operating costs and cleaning time, however, will result. Therefore, it's important that both the wheels and the separator are checked regularly to ensure that they are functioning in the most efficient manner. A routine check of these elements must be conducted at regular intervals whenever the equipment is in operation.
Daily Walk - Around Checklist
Listed here are suggestions to reduce your foundry's blastcleaning operating costs through a planned maintenance program.
* Record the hour meter reading. Do this at the beginning of each shift for consistent readings. Use these readings to determine tune-up life kit (blades, impeller and control cage), abrasive consumption/additions and maintenance requirements.
* Check and record blast wheel amperage. Make note of any changes since prior reading. Verify monthly by checking with a clamp on amprobe at the starter that what the ammeter is reading is accurate.
* While the machine is operating check the amp meter for a steady reading. A bouncing meter indicates abrasive flow problems that can be attributed to not enough abrasive in the system or a feed line obstruction.
* Check the fines discharge pipe at the dribble valve. If good abrasive is visible, the separator operation must be checked.
* Check the coarse refuse discharge pipe. Good abrasive should not be visible.
* Listen to the machine for wheel vibration or elevator buckets hitting the casing. Strange noises must be reported to maintenance for appropriate action.
* Check for abrasive leaks and note so repairs can be made quickly.
* Keep the area around the machine clean and free of obstructions and abrasive. Abrasive should be swept up on a regular basis, screened and put back into the machine.
* Check the dust collector hoppers to ensure they are emptied at the beginning of the shift. Also, check the dust collector hopper discharge for good abrasive - it indicates a problem in the ventilation system.
* Check the manometer/magnehelic gauge for the dust collector. Note any changes since previous readings. An increase or decrease alerts you to a possible problem.
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|Author:||Tarabek, Eugene A.|
|Date:||Jan 1, 1998|
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