Plasma cutting and tube cutting tools

Back at Riverbend, the furnace is cool, the scaffold is up to the leak and Peg Lindqvist has marked the repair on the wall. The leak is on the left sidewall, about 60 ft above the furnace floor, in tube 47 counting from the front wall. A crack at the toe of a membrane weld, next to a buckstay attachment, has let steam out, and the jet of steam has cut a groove in the side of tube 48 next to it. Peg's ultrasonic thickness readings show that both tubes must be replaced over a length of about 4 ft. The repair is a two-tube panel section: the old section of tubes 47 and 48, with the membrane between them, is cut out of the wall, and a new section, made in Ironbridge's shop, is welded in.

Cutting that section out takes two kinds of cutting. The membrane bars that join tube 46 to 47, and 48 to 49, have to be split along their length, and plasma cutting is the tool for that. The tubes themselves have to be cut across, square and clean, at the top and bottom of the section, and that is a job for mechanical cutters. This lesson covers both.

Plasma cutting on boiler work

WLD-110 explained how a plasma torch works: an arc between an electrode in the torch and the work is squeezed through a small nozzle with a flow of gas, which heats the gas to a plasma that melts the metal and blows it out of the kerf. Plasma cuts any metal that conducts electricity, including stainless steel and the alloys that oxyfuel cannot cut, and it cuts thin and medium plate much faster than oxyfuel, with a narrower kerf and a smaller heat-affected zone. On boiler and vessel work, boilermakers use plasma for:

• Splitting membrane bars between tubes, where the cut must run straight along a bar about 1/2 in wide without touching the tubes on either side.
• Cutting casing, lagging sheet, ductwork and seal boxes on the outside of the boiler.
• Cutting stainless and alloy parts: liners, clad plate, stainless tube shields and sootblower components.
• Gouging, with gouging consumables, where an air carbon arc torch would be too heavy or too dirty.

Plasma has limits. It needs compressed air or bottled gas and a power supply at the job, and its consumables, the electrode and nozzle, wear and must be changed before the cut quality falls off. On thick carbon steel, above about 1 in for a hand torch, oxyfuel is usually more practical. And a plasma arc does not stop at the back of the plate: whatever is behind the cut is in its path.

Splitting a membrane

A membrane cut runs down the center of the bar, leaving half the bar on the tube that stays and half on the tube that comes out. The cutter uses a drag shield or a standoff guide that rides on the bar, keeping the nozzle at the right height and the cut on the center. Because the arc goes straight through the bar, the insulation and casing behind it must be protected; on Riverbend, the casing side has been opened and the insulation removed back from the repair for the welders' access, and a steel shield plate is slipped behind the bar as the cut proceeds. The cut must stop short of the ends of the section, about 1 in beyond where the tube cuts will be, so the membrane on the tubes that stay is not split farther than needed.

The most important rule in membrane cutting is to stay off the tubes. A plasma arc that wanders onto a tube that is staying in the wall gouges it, and that gouge must then be ground out, thickness checked and possibly built up or the tube replaced. Kiera practices membrane splits on a scrap panel in the yard before she is allowed to make one in the furnace, and on the day, Dale makes the cuts while she watches the arc and moves the shield.

Choosing amperage and consumables

Plasma systems are rated by current, and each consumable set, the electrode, nozzle and shield, is made for a current. The maker's chart gives the thickness each current cuts at production quality and the speed for each thickness. The membrane bar is 1/4 in thick, so the smallest consumables comfortably cut it; higher currents would only widen the kerf and put more heat into the tubes on either side. The cutter uses compressed air from a dryer, because water in the air shortens consumable life and roughens the cut.

Kiera works out the plasma time for the two membrane splits and the time each tool would take for the four tube cuts, to help Dale decide which tools to bring up the scaffold.

# Splitting the membrane and cutting the tubes for the panel cut-out (US units)

# plasma system: (amps, rated production thickness, typical hand-cut speed) - illustrative
table = [(45, 0.5, 20), (65, 0.75, 18), (85, 1.0, 15), (105, 1.25, 12)]
bar = 0.25                               # membrane bar thickness
amps, rated, speed = next(row for row in table if row[1] >= bar)
print(f"membrane {bar} in -> {amps} A consumables (rated to {rated} in), about {speed} in/min by hand")

splits = [52, 52]                          # vertical membrane cuts, each this long
length = sum(splits)
minutes = length / speed * 1.6            # restricted access, drag shield, repositioning
print(f"membrane to split: {len(splits)} cuts, {length:,} in in all, about {minutes:.0f} min of plasma work")

# tube cuts: tool, minutes per cut on this tube, setup minutes per cut
tools = {"clamshell cutter": (6, 10), "portable band saw": (4, 6), "reciprocating saw": (9, 2)}
cuts = 4                                  # two tubes, cut at the top and the bottom
for tool, (each, setup) in tools.items():
    print(f"{tool:18} {cuts} cuts -> {cuts * (each + setup):3d} min (cut ends square, no slag inside)")

membrane 0.25 in -> 45 A consumables (rated to 0.5 in), about 20 in/min by hand
membrane to split: 2 cuts, 104 in in all, about 8 min of plasma work
clamshell cutter   4 cuts ->  64 min (cut ends square, no slag inside)
portable band saw  4 cuts ->  40 min (cut ends square, no slag inside)
reciprocating saw  4 cuts ->  44 min (cut ends square, no slag inside)

The membrane splitting is only minutes of plasma time. The tube cuts take longer, and most of the time is set-up: getting the tool onto the tube, square to it, in a space where the next tube is a few millimeters away. Dale chooses the portable band saw for the bottom cuts, where the scaffold gives room, and the clamshell cutter for the top cuts, under the buckstay, where a saw will not fit.

Mechanical tube cutting

Tubes are cut mechanically for three reasons. A torch puts slag, oxide and spatter inside the tube, and debris left in a waterwall tube can lodge at a bend or orifice and starve the tube of water, causing an overheating failure. A torch cut leaves a rough, hardened and oxidized end that has to be ground back to sound metal anyway. And a mechanical cut can be made square to the tube axis, which the end preparation tool needs. The common tools are:

Two habits go with every tube cut. First, before the cut is made, the tube ends that will stay in the boiler are plugged or capped, or a foam plug is pushed into the bore below the cut, so chips and filings cannot fall into the tube. The plug has a string or a tag, and every plug put in is written on the traveler so that every plug is taken out before the joint is welded. Plugs forgotten inside tubes have caused failures. Second, the cut piece is supported before the last cut, because a 4 ft length of two tubes with membrane is heavy and the cut closes on the blade as the piece starts to drop. On Riverbend, Dale hangs the section on a chain fall from the scaffold structure before the final cut, as in BLR-120.

The cut-out sequence

With plasma for the membrane and mechanical cutters for the tubes, the cut-out at Riverbend follows a set order that Peg approves in the repair plan.

• Mark the cut lines on both tubes, square to the tube axis, at the elevations on the repair drawing. The new section will be made to fit between them.
• Split the membrane on both sides of the section with plasma, stopping 1 in beyond the tube cut lines.
• Make rough cuts with the reciprocating saw 2 in inside the final lines to free the section, after rigging it.
• Lower the section and send it to Peg for examination; the failed tube is evidence for the cause of the leak.
• Plug the tube ends that stay, then make the final cuts square at the marked lines with the band saw and clamshell cutter.
• Check that the cut ends are square and at the right elevation before end preparation begins in lesson 7.

The rough cut first, final cut second sequence is common in tube work. It lets the heavy section come out without a tool trapped in a closing cut, and the final cut can be made carefully, square and exactly on the line, in a tube end that is free.

Looking after the cutters

Tube cutting tools are only as good as their blades and bits. A dull band saw blade wanders and leaves a cut that is not square, and a chipped clamshell bit tears the tube end instead of cutting it. Dale's crew carries spare blades and bits up the scaffold with the tools, checks each one before a cut, and changes it at the first sign of wandering or chatter. Cutting fluid is used sparingly on tube cuts, because whatever goes on the cut ends up in the weld zone and must be cleaned off before welding; many crews cut dry with blades made for it. Chips are swept from the scaffold boards and the furnace floor below after every cut, because a steel chip in a boot sole is a slipping hazard on a scaffold, and chips left on the furnace floor end up in the ash hopper. Plasma consumables are checked the same way: a worn nozzle makes a wide, beveled kerf that can drift toward the tube, and the cutter changes the nozzle and electrode as a set.