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How Thick Can You Cut With Plasma Under Water
October 7, 2013
Plasma Arc Cutting Underwater
Under water plasma cutting is nothing new - it was first done in the mid 1970's. But it still has many advantages that lead many fabricators, steel service centers, and manufacturers to prefer it over dry cutting. An "Air Curtain" or "Bubble Muffler" is used to create a pocket of air around the front end of the torch, shielding the arc from the water. But not every thickness of every material can be cut under water. So what are the limits, and what happens if you push them?
Mild steel can be cut under water up to 2 inches thick. This is achieved at 450 Amps with Oxygen plasma, and the plate is submerged below 2 – 3 inches of water. Less than 2 inches will not reduce the noise and arc glare below acceptable limits.
Fortunately, 2 inches is the piercing limit for plasma cutting on mild steel, so there's really no need to push the limit. If you did try to go thicker and edge start, say all the way up to 3 inch thick, you will see a significant reduction in cut speed just to make the cut, as well as a lot more dross on the bottom of the cut.
Stainless Steel and Aluminum up to 2 inches thick can be cut under water. This is achieved at 600 Amps with Nitrogen plasma, or up to 720 Amps with water-injection plasma. As with mild steel, the plate is submerged below 2 – 3 inches of water. Less than 2 inches will not reduce the noise and arc glare below acceptable limits.
Under water cutting should not be done with flammable gases, such as Argon-Hydrogen (H35) or Methane (CH4), because excess gas can become trapped under the plate, causing an explosion hazard.
There are a few things to be aware of when cutting under water:
When cutting under water, you may need to reduce the cut speed on some materials. However, on most materials 1" thick and under, the same speeds are used when cutting both above water and under water.
The air curtain has to be installed correctly and pressure set appropriately, or else the cut edge quality may be adversely affected.
Due to the possibility of plate movement, plate thicknesses under 1/4 inch are not generally recommended for underwater cutting..