2.5 Plasma Arc Cutting (PAC): Torch Anatomy, Consumables, Gas Selection, Cut Quality & Safety

Plasma Arc Cutting (PAC): Torch Anatomy, Consumables, Cut Quality & Safety

Core Trade Concept: Oxyfuel cutting depends on the base metal burning in a stream of oxygen, which is why it stops working on stainless steel, aluminum, copper and nickel alloys — those metals form refractory oxides that will not sustain the reaction. Plasma Arc Cutting (PAC) does not care. It forces a gas through a constricting orifice and a high-current arc, ionizing the gas into a plasma jet at temperatures far above the melting point of any structural metal, then blows the melt out mechanically. For a boilermaker, PAC is the process that severs stainless tower internals, austenitic tube stubs, alloy trays and thin waterwall panel material where oxyfuel would fail or distort.

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1. How the Process Works

                 TRANSFERRED ARC PLASMA CUTTING

   Power supply (-) ---> ELECTRODE (hafnium insert in copper)
                              |
                         [ swirl ring ]  <-- imparts vortex to gas
                              |
                         [ NOZZLE ]  <-- constricting orifice: the
                              |            single most important
                              |            dimension in the torch
                       ==== PLASMA JET ====
                              |
   Power supply (+) ---> WORKPIECE   (arc transfers to the work)

• Constriction is the whole idea. Forcing the arc through a small nozzle orifice raises its current density and velocity enormously. A wider or wallowed-out orifice immediately degrades every quality parameter.
• Transferred vs. non-transferred arc. In a transferred arc the work is part of the circuit and the arc jumps electrode-to-work; this is what all metal cutting uses. In a non-transferred arc the circuit is electrode-to-nozzle, used for spraying and for cutting non-conductive material. If a PAC torch will only pilot and will not transfer, suspect the work lead clamp.
• Swirl. The swirl ring spins the gas so the hottest column stays centered. The vortex direction means one side of the kerf is always squarer than the other — on most torches, moving forward, the right-hand side is the good side. Lay out cuts so the scrap takes the bevelled side.

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2. Torch Anatomy and Consumable Wear

Replace the electrode and nozzle as a matched set. Running a new nozzle on a worn electrode simply destroys the new nozzle. The classic field mistake is to change only the part that looks worst.

Air quality is a consumable issue. Plasma air must be clean, dry and oil-free. Moisture and compressor oil are the leading cause of short consumable life, double arcing (an arc that jumps electrode-to-nozzle-to-work and burns the nozzle out), and dirty cut faces.

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3. Gas Selection

For boilermaker field work, compressed air covers most cutting; nitrogen or argon–hydrogen appear on shop tables cutting thick stainless internals.

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4. Cut Quality: Speed, Standoff and Dross

Three variables control the cut, and two of the three failures look similar until you learn the difference.

      TRAVEL SPEED TOO SLOW          CORRECT SPEED           TOO FAST
      +-------------------+       +-----------------+     +-----------------+
      | Wide kerf         |       | Narrow kerf     |     | Narrow kerf     |
      | LOW-SPEED DROSS:  |       | Little/no dross |     | HIGH-SPEED      |
      | heavy, bubbly     |       | Arc lag 5-10 deg|     | DROSS: thin,    |
      | slag, chips off   |       |                 |     | hard, welded on |
      | Excess heat/taper |       |                 |     | Cut may not sever|
      +-------------------+       +-----------------+     +-----------------+

• Low-speed dross forms a heavy, bubbly bead on the underside that usually chips off with a scraper. It signals travel too slow, amperage too high, or standoff too small.
• High-speed dross is a thin, hard, tightly adherent bead that must be ground. It signals travel too fast, amperage too low, or standoff too great — and is frequently accompanied by an incomplete cut and a trailing arc lag greater than about 10°.
• Standoff (nozzle-to-work distance) is set by the manufacturer's chart and held constant with a drag shield or a stand-off guide. Increasing standoff widens the kerf, increases bevel and shortens consumable life.
• Piercing. Never pierce with the nozzle sitting on the plate. Start at an angle so the initial blowback is directed away from the torch, then rotate upright. Rated pierce thickness is typically about half the rated severance thickness for a given machine — a unit that severs 1 in. will usually only pierce 1/2 in. reliably.

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5. PAC Compared With Oxyfuel and CAC-A

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6. Safety

• Electric shock is the differentiating hazard. PAC power supplies run high open-circuit DC voltage and the torch carries it. Never change consumables with the machine energised; never operate with a damaged torch lead; never bypass the torch safety interlock that prevents firing with the retaining cap loose.
• Eye protection scales with amperage. From AWS Z49.1 Table 1, plasma arc cutting minimum protective shades are: less than 20 A — shade 4; 20–40 A — 5; 40–60 A — 6; 60–80 A — 8; 80–300 A — 8 (suggested 9); 300–400 A — 9 (suggested 12); 400–800 A — 10 (suggested 14). Do not carry the shade 12–14 habit from gouging down to a 45 A portable unit, and do not carry a shade 5 habit up to a 400 A table.
• Fume. Cutting stainless and other chromium-bearing alloys generates hexavalent chromium. Local exhaust or a water table is required, and inside a vessel this becomes a respiratory protection question, not a ventilation preference.
• Noise. A plasma jet is a supersonic gas jet; hearing protection is mandatory.
• Fire. Sparks from arc cutting travel in excess of 35 ft. Hot work permit, fire watch, and coverage of everything below the cut.
• Compressed gas. Air lines at 90–120 psi whip when they part; use whip checks and safety pins on couplings exactly as for pneumatic tools.

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7. Realistic Trade Scenario: Removing 304 Stainless Tray Rings From a Fractionation Tower

A turnaround crew must remove twelve corroded Type 304 stainless tray support rings, seal-welded to the inside wall of a carbon steel column, without gouging the shell.

• Oxyfuel is ruled out — 304 stainless forms chromium oxide with a melting point far above the base metal, so the cut will not sustain itself.
• CAC-A is ruled out for this cut — it would remove metal aggressively and risks carbon and copper contamination on the shell wall that must later be re-welded.
• PAC on compressed air is selected: narrow kerf, minimal heat input, controllable depth. The crew sets standoff with a drag shield, plans each cut so the square side of the kerf falls on the shell side, and stops the cut short of the shell, finishing the last 1/16 in. with a grinder so the pressure boundary is never arced.
• Confined space controls dominate the plan: hexavalent chromium fume from the 304, forced local exhaust at the cut, continuous atmospheric monitoring, and an attendant at the manway. The electrical hazard of the torch inside a wet, conductive steel vessel drives the use of a GFCI-protected supply and a rigorous consumable-change lockout.