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Can you plasma cut Aluminum on a water table?
Plasma + Aluminum + Water = Hydrogen Explosion Hazard!
Dry plasma arc cutting of Aluminum plate may require expensive and complicated dust removal systems. It is also a very loud process, producing high intensity ultra-violet light. Therefore, many companies prefer to cut Aluminum on a water table. Most Aluminum alloys can easily be plasma cut on a water table, Aluminum-Lithium alloys being the exception - see the note below. But even for the majority of alloys that can be cut on a water table, the user has to be aware of a very real and dangerous explosion hazard, and take steps to avoid it.
Explosion Hazard - Never Cut Aluminum-Lithium Alloys In The Presence Of Water. Certain molten aluminum lithium (Al-Li) alloys can cause explosions when plasma cut in the presence of water. Do NOT plasma cut Al-Li alloys with or near water. These alloys should only be dry cut on a dry table. Do NOT dry cut over water. Do NOT use water injection plasma torches. Contact your aluminum supplier for additional safety information regarding hazards associated with these alloys.
When aluminum metal is submerged in water, it will generate hydrogen gas. Similar to the carbon dioxide bubbles that seem to form from nowhere on the inside surface of your glass of Coca Cola, the surface of a piece of aluminum will chemically react with water to produce tiny amounts of hydrogen gas. The tiny bubbles of hydrogen will eventually break loose from the metal surface and float upwards.
The amount of hydrogen produced depends on many factors, but the most significant in this case are the surface area that is exposed to water, and the length of time it is submerged. In the case of a small part cut from aluminum, it may fall into the water for a short period of time and then be retrieved. It has a relatively small surface area, and therefore generates little hydrogen. However, plasma arc cutting produces a high volume of molten aluminum droplets that fall into the water table. Those droplets, especially the fine particles, have a cumulative surface area that becomes very large over time, and they can sit on the bottom of a water table for days, weeks, or months, all the while generating hydrogen gas bubbles.
What to Avoid When Plasma Cutting Aluminum
The biggest danger occurs if a plate is left submerged on the table for an extended period of time. Perhaps an operator loads a plate on the cutting table, raises the water level, and starts cutting a few parts. Then his shift ends and he turns off the machine and goes home. When he comes in the next day, there could be a significant pocket of hydrogen gas trapped below the plate. If he pierces the plate with the plasma torch and ignites that bubble of hydrogen, a serious explosion can occur, with the potential to damage equipment and injure or kill personnel.
The Simple Solution
The simple solution to this hazard is an aerator. But it also requires training. An aerator is simply a series of pipes with small holes arranged on the bottom of the table through which a constant small flow of compressed air bubbles up to the surface of the table. This prevents pockets of highly concentrated hydrogen from accumulating under a plate by diluting and dissipating gas bubbles.
Training Is Essential
Operator training is also essential, so that the operator understands the hazard, how and where hydrogen may accumulate, and what steps he should take to prevent explosions. One simple step is to never leave a plate sitting on a water table for any extended period of time. For tables with water level raise and lower capability, the operator can simply drop and raise the water prior to cutting, in order to allow any bubbles of hydrogen to be broken up or dissipated.
Other solutions can include a “bubble muffler”, or a water filtration system. A bubble muffler is a device that clamps onto the plasma cutting torch, like an air curtain. It uses compressed air through an inner nozzle to protect the plasma arc, and also pumps a pretty high volume of table water through an outer nozzle to create a water shroud around the plasma arc. A bubble muffler is usually used on a water table that does not have water level controls, and so cannot submerge the plate. The bubble muffler reduces arc glare and noise, captures most of the plasma smoke, and also agitates the water to break up bubbles of Hydrogen. Since the plate is not submerged, Hydrogen pockets don’t get trapped under the plate.
Water filtration systems, such as the Ebbco GRS system, use a high volume pump and a series of nozzles in the table to keep fine particles suspended in the water and then remove them from the water with a centrifugal separator. This will reduce generation of Hydrogen in the table by removing much of the aluminum metal particles from the water. It also keeps the water very well agitated, which will prevent any accumulation of Hydrogen.
Regardless of which method you choose, plasma arc cutting of aluminum on a water table is a viable option, as long as a system is put in place to eliminate the hydrogen explosion hazard.