October 14, 2008
Abrasive waterjets are powerful cutting tools. As is the case with all industrial machinery, waterjet machines have certain inherent hazards that go beyond the obvious cuts. Knowledge of these hazards and proper protective measures are necessary for safe operation. This article presents an overview of the hazards and ways to ensure safety.
Are you being green when operating your abrasive jet? Can you feel righteous—like someone getting 40+ MPG in his car—or should you feel a bit guilty about enjoying your high productivity? What are the environmental impacts of abrasive jet cutting?
Abrasive waterjet cutting inherently is an environmentally friendly (green) process. Unlike traditional mechanical cutting systems, waterjet cutting requires no cooling or lubricating oils, so there are no chemically contaminated chips to dispose of. Unlike thermal processes such as plasma or laser, waterjet generates no noxious fumes during the cutting process.
The cutting "tools," water and garnet abrasive, are environmentally benign and available worldwide with minimal preprocessing.
The abrasive waste generally is inert garnet with a small fraction of particles from the material being cut. This means that unless the material being cut is a highly hazardous material, such as lead, the waste abrasive can be disposed of safely at a dry landfill, with no processing and no environmental risk.
The abrasive waterjet's narrow cut width allows for close nesting of parts to maximize material utilization. Moreover, the minimal scrap that remains is free of chemical contamination and can be recycled, which results in raw material, energy, and cost savings for recyclable materials such as aluminum, steel, and titanium.
Abrasive jet cutting consumes abrasive, electric power, and water in order of decreasing expense. The three resulting waste streams are heat, water, and solids in order of ease of disposal. Let's calculate the effect of cutting 10 feet of 1-in.-thick steel plate.
Suppose you are cutting with 50,000 PSI, a 0.014-in. water orifice, a garnet flow of 0.75 lbs./min. With a relatively smooth surface finish, the cut will take 57.1 minutes at 2.71 inches per minute (IPM) and use 42.2 lbs. of garnet. The waterjet will consume about 25 kWH of electricity and about 52 gal. of water. The jet will grind about 1 lb. of the steel in the kerf of the cut into fine dust. So, what is the impact of all this?
The most expensive component is the garnet, and a major cost component in the garnet is transportation. An average transit distance for garnet by truck may be 1,000 miles. A truck loaded with 40,000 lbs. of garnet may get 8 MPG, so transporting the 42.2 lbs. for 1,000 miles would burn about 0.13 gal. of diesel fuel. Generating 25 kWH in a modern power plant would burn the equivalent of another 1.2 gal. So, we could say that the carbon footprint on the consumption side from the 10-ft. cut is 1 1/3 gal. of diesel, about the same as for driving a moderately efficient car about 30 miles.
Of course, this statement is very dependent on the exact source of the electric power, but is roughly correct for coal or oil-fired plants. The 52 gal. of water consumed is about the same as would be used in 10 minutes of lawn watering and has little impact.
The heat produced is the most benign of the waste streams and in fact is not a waste if the jet cutting is conducted within a building that is being heated. The 25 kWH ends up as heat and simply reduces the heating fuel consumption by about 0.5 gal. On the other hand, if the jet cutting is performed in an air-conditioned shop, the extra heat load is a real waste because running the air conditioners requires extra power.
Water often is simply run down the drain without any treatment as it leaves the machine. In this state, it is cloudy with very fine particles in suspension. In some places, this is OK and in others, it isn't. A first stage of treatment is to run the waste through a weir tank to clarify it. Often this process alone is sufficient.
In other cases, a municipality may have a rule that no machine can run waste water into a drain. Or, it may be that the fabricator is cutting some poisonous or hazardous material that contaminates the water. In this final case, a complete recycling system is used, and the water is filtered, deionized, cooled, and finally run back through the cutting nozzle.
The waterwaste stream thus has little environmental impact, no matter how it is handled.
The solid waste from the cut described previously consists of 42 lbs. of wet garnet with about 1 lb. as powdered steel. Garnet in sufficiently large quantities can be recycled economically. One European company collects spent garnet and recycles it into jet cutting abrasive, sandpaper, and other garnet products. Heavy users also can buy garnet recycling machines that reclaim up to 50 percent of the used garnet. Most U.S. fabricators simply send the solid waste to a landfill where it is accepted as nonhazardous waste.
The same things that affect costs also affect the environmental impact, but the effect is small for the cutting job. Table 1 compares material and energy consumption for a 0.010-in.jet size and garnet flow of 0.35 lb per minute with the larger jet described above.
|Resource||Jet Diameter (in.)|
The power level of the jet is reduced in half, but the energy consumed is slightly larger because the cut now takes more than twice as long. The water and garnet consumption are also slightly larger, but the over all impact is not very sensitive to big changes in operating parameters. A far greater effect is found when examining alternate processes.
Perhaps the most energy-efficient way to make a 10-ft. cut in a 1-in. plate is with a band saw. A simple test was performed in which it took 27.7 seconds to cut through a 1-in. bar, 1 ½ in. thick with a band saw having only a 1-HP motor. At this rate, a 10-ft. cut took about 37 minutes, but because of the low power used only about 0.5 kWH. There is little other impact from this cut, but of course it is difficult to make intricately shaped precision parts with a band saw.
At the other extreme is wire EDM cutting, which is good for making precise, intricate parts. Table 2 lists the parameters for a typical EDM setup, using hard brass wire (65 percent copper, 35 percent zinc), 0.012 in. dia., which is suitable for cutting 1-in.-thick steel quickly:
|Linear speed||0.25 IPM|
|Wire consumption||1.04 lb./hr.|
With these parameters, a 10 -ft. cut would take 480 minutes and consume about 80 kWH.
An environmental concern using this process is disposal of the water filters, which contain the residue from the copper-zinc alloy wire and the steel, and disposal of the deionizationresin. Most companies use a deionization resin-regenerating service instead of disposing of it. The regeneration service must have an EPA hazardous waste generator license.
Table 3 examines the three cutting methods for a 10-ft. cut.
|Environmental Impact of|
Various Cutting Processes
|Parameter||Band Saw||Abrasive Jet||Wire|
|Energy||0.5 kWH||25 kWH||80 kWH|
|Effects||None||Waste Garnet||Wire and|
The main environmental impact of abrasive jet cutting is energy usage. Cutting with an abrasive jet requires about 50 times the energy required for a saw cut of the same length and one-third that of a wire EDM, but the environmental impact is small and about the same as driving a car for the same amount of time.