Lances and underwater cutting

Some cutting jobs are beyond every method so far: a steel mill’s ladle skull a metre thick, a frozen-in pin in a crusher’s housing, a tapping hole in a furnace, reinforced concrete foundations that must come out of a running plant, thick steel under water on a wreck or a wharf. For these, cutters use the oldest principle in thermal cutting in its most extreme form: burning iron in pure oxygen. An oxygen lance is a steel pipe through which oxygen flows; once its end is lit, the pipe itself burns, and its fierce, molten tip melts and burns its way through steel, concrete and rock. A thermic lance packs the pipe with rods that burn too, making it hotter still. Underwater, oxygen-based torches and electrodes do the same work with the water kept away from the flame.

This chapter explains how lances work, calculates the oxygen and heat of burning iron, describes the early underwater torch Viall recorded in 1921 and its modern successors, and sets out the hazards of lance work, which are among the most severe in any metal cutting.

Burning iron in oxygen

When iron burns in pure oxygen, it combines with the oxygen to form iron oxides and releases a great deal of heat. In oxygen cutting, the burning iron in the kerf supplies most of the heat of the cut, and the oxide, which melts at a lower temperature than the steel, is blown out by the jet. The chemistry is simple enough to calculate, as an engineer quoted in Viall’s book did for an oxy-hydrogen cutting job, working out the weight of oxygen theoretically needed to convert the iron from the kerf into oxide, and comparing it with the much larger amount actually used, most of which went into blowing out the slag and heating.

Figure 1Oxygen needed to burn iron to oxide

oxygen per kg of iron ≈ (4 × 16) ÷ (3 × 56) for the oxide Fe₃O₄

Oxide

three atoms of iron to four of oxygen

Mass ratio

168 parts of iron to 64 of oxygen

1. oxygen per kg of iron ≈ 64 ÷ 168 ≈ 0.38 kg
2. by volume ≈ 0.38 ÷ 1.43 ≈ 0.27 m³ of oxygen per kg of iron burned
3. the rest of the oxygen supplied blows slag out and burns away unused

The theoretical figure is surprisingly small, about a quarter of a cubic metre of oxygen for each kilogram of iron. Real oxygen cutting and lancing use several times that, because the jet must also blow the molten oxide and metal out of the cut, and much of the oxygen passes through without reacting. The comparison of theoretical and actual oxygen is a useful measure of a process’s efficiency, and Oxy-Fuel by the Numbers takes it further for oxygen cutting.

The oxygen lance

An oxygen lance is a length of steel pipe, typically a few metres long, connected through a handle and a valve to a supply of oxygen. Its end is heated to ignition with a torch or a special igniter, and the oxygen is turned on; the pipe’s end begins to burn, and as long as oxygen flows, it keeps burning, consuming the pipe from the tip back towards the handle. The burning tip reaches temperatures far above the melting point of steel, and its molten iron and oxide, driven by the oxygen jet, melt and burn through whatever the tip is pressed against. When the lance burns short, the operator stops, fits a new length or couples one on, and continues.

Figure 2Heat released by a burning oxygen lance

heat rate = mass of pipe burned per second × heat of burning iron

Pipe burned

0.5 m/min × 0.8 kg/m ≈ 0.4 kg/min

Heat of burning iron

about 6.7 MJ/kg

1. heat released ≈ 0.4 × 6.7 ≈ 2.7 MJ a minute
2. about 44 kW from the burning pipe alone, concentrated in its tip
3. the oxygen theoretically needed for the pipe is only about 0.1 m³ a minute; the flow supplied is many times more

Forty kilowatts concentrated in the tip of a pipe is why a lance cuts what nothing else will. It does not depend on the material being cut burning, as oxygen cutting does; it melts concrete, rock, slag and refractory with its own heat, and burns through steel and cast iron alike. Its drawbacks are just as plain as its power: it is slow, it consumes lance pipe and large volumes of oxygen, it produces dense fume and a shower of molten metal and slag, and its tip is far hotter than anything else in the shop.

The thermic lance

A thermic lance, sometimes called a burning bar, packs the steel pipe with rods or wires of steel, often with some aluminium or magnesium, through which the oxygen flows. The packing burns along with the pipe, raising the heat released per metre and the temperature of the tip, so that a thermic lance cuts faster and through harder materials than a plain pipe. Thermic lances are used for cutting and piercing thick reinforced concrete, removing frozen steel from furnaces and ladles, cutting seized parts in heavy machinery, and demolition work where other methods would be too slow or would need too much access.

Figure 3Lances and their uses

	How it works	Typical use	Notes
Plain oxygen lance	steel pipe burning in its own oxygen stream	tapping furnaces; opening ladles; cutting heavy steel scrap	simple; slower than thermic lances
Thermic lance or burning bar	pipe packed with steel and other rods, all burning	thick concrete, rock, refractory; seized heavy parts	very hot; fast; heavy fume and spatter
Exothermic cutting rod	thin consumable rod lit by a battery or arc, burning in oxygen	field cutting and piercing without fuel gas or large power	portable; used on site and underwater
Oxy-arc electrode	tubular steel electrode with oxygen through its bore, lit by an arc	underwater cutting	electrical and diving hazards

Lancing concrete and rock

Concrete and rock do not burn; the lance cuts them by melting. The lance’s molten iron and oxide flux the concrete’s minerals into a fluid slag that runs out of the hole, and the operator works the lance in and out to clear it. Reinforced concrete is lanced readily, because the steel bars burn too and add heat. Lancing pierces holes for anchors and demolition charges, cuts openings in thick walls and slabs, and removes concrete foundations from inside running plants where vibration from breakers or noise from saws would be unacceptable. It is slow by the volume removed, and the molten slag and silica-bearing fume need careful control, but it works where little else can. Diamond wire and wall saws compete with it on large straight cuts in concrete, with less heat and fume; the lance’s place is in irregular shapes, deep holes and thick reinforced sections.

Planning a lance job

Because lance work is so hazardous and so slow, it is planned in detail before it starts. The plan identifies what is to be cut and in what order, so that sections do not fall or shift unexpectedly; where the molten slag and spatter will go, and how it will be caught; the oxygen supply, its pressure and flow, and the number of lance lengths needed, from the maker’s burn rates; the fire watch, extinguishers and the area cleared of combustibles; ventilation and fume control; the protective clothing and equipment; and how the operator will be relieved, because lancing is physically exhausting in heat and heavy clothing. The service centre’s specialist contractor estimated the crusher pin job from the pin’s volume, the lance’s cutting rate in steel and the number of lengths it would burn, and arrived with half as many lances again as the estimate.

Exothermic rods in the field

Exothermic cutting rods, thin consumable lances lit by a spark from a battery or an arc and burning in a stream of oxygen from a cylinder, give repair crews a portable way to cut, pierce and gouge almost any material on site without fuel gas or a large power source. They cut seized bolts, pins and bushes, pierce holes in thick steel and cast iron, and remove frozen parts, and they work under water. They are noisy, smoky and wasteful of oxygen, and their molten spray needs the same precautions as a full lance, but they reach places other equipment cannot. The service centre’s site crews carry an exothermic kit for seized pins and bushes on mining equipment.

Slag and spatter

A lance produces a large volume of molten slag and iron, which runs from the cut, spatters around it and can travel a long way. It sets fire to anything combustible it meets, damages concrete floors and machinery, and burns through hoses and cables. Lance work is done over sand, steel plate or a slag pan where possible, with the area around it cleared or protected by fire blankets, and the slag is allowed to cool fully before it is handled and removed.

Lancing without fuel gas

All these tools burn steel in oxygen without any fuel gas, which is the same principle the Imperial handbook of 1916 used for a much gentler job: burning carbon deposits out of engine cylinders with a stream of oxygen alone, once the carbon had been lit. Once a reaction with pure oxygen has started and is releasing enough heat, it keeps itself going as long as the oxygen flows. That is why lancing equipment can be so simple, and also why it must be handled with such care: the reaction does not need anything but the oxygen to keep going.

Cutting under water

Cutting under water must overcome the water’s cooling, which quenches a flame or arc, and the pressure, which rises with depth. Viall’s book of 1921 described an underwater torch used at a United States navy yard, made by fitting a hood over the tip of an ordinary cutting torch. The hood was pressed against the metal, and air at about 100 pounds per square inch forced the water back from the flame; an electrical device lit the torch under water. Viall reported a cut by a diver 22 feet under water, plate 1 inch thick cut at about 6 inches a minute, and a claim that the torch would cut down to 200 feet.

Modern underwater cutting mostly uses oxy-arc electrodes or exothermic rods: a tubular electrode through which oxygen flows, struck as an arc against the work and then burning in its own oxygen, cuts steel at depth with equipment a diver can carry. Underwater cutting is done under occupational diving rules, such as those set out in AS/NZS 2299 for occupational diving operations, by commercial divers specifically trained and qualified for it. Its hazards include electric shock in the water, hydrogen and oxygen collecting in pockets under structures and exploding when the arc reaches them, and the ordinary hazards of diving.

Hazards of lance work

Figure 4Hazards and controls for lance work

• Avoid: Extreme heat and molten spatteraluminised or heavy fire-resistant clothing, face shield, gloves; spatter direction controlled
• Avoid: Oxygen enrichmentno oil or grease near oxygen; leaks checked; work area ventilated; clothing never "freshened" with oxygen
• Avoid: Firearea cleared of combustibles; fire watch during and after; hot work permit
• Avoid: Fumedense iron oxide and material fume; extraction or respiratory protection; concrete adds silica dust
• Avoid: Flashback and burnbacklance never burned to the handle; correct oxygen pressure; flashback arrestors on the supply
• Avoid: Structural collapsecutting sequence planned so cut members do not fall unexpectedly
• Do: ? Noisevery loud; hearing protection

Oxygen enrichment deserves particular emphasis. Lances use large flows of pure oxygen, and oxygen that escapes, through leaks or after passing through the cut, can enrich the air in a confined or poorly ventilated area. In oxygen-enriched air, clothing, hair and oily materials ignite easily and burn fiercely, and people have died when their clothes caught fire. Lance work is therefore done only with good, continuous ventilation, with the oxygen supply checked for leaks, and with clothing kept free of oil and grease.

The running case: the crusher pin

This is an illustration. A crusher’s eccentric shaft pin had seized in its housing so firmly that presses and heating could not move it, and the housing was too valuable to cut apart. The service centre engaged a specialist lancing contractor, who planned the job with the centre’s repair supervisor: the housing was set up in the open yard, away from buildings, with a fire watch and screens; a thermic lance was used to burn out the centre of the pin from one end, leaving a thin shell that was then collapsed and removed; the housing’s bore was protected by keeping the lance on the pin’s axis and stopping short of the bore. The pin came out in about two hours of lancing, and the bore needed only light machining. Lance work was not attempted by the centre’s own staff, who had neither the training nor the equipment for it, and the contractor’s plan, permits and records were kept in the job file.

A failure: the oxygen-soaked overalls

This is an illustration. At another site, an operator lancing in a pit used the oxygen hose to blow dust off their overalls between lances. A spark from the next lance landed on the oxygen-soaked cloth, which burst into flame and burned the operator badly before the fire could be put out.

The investigation found that the operator had not been trained on oxygen enrichment and that the pit had little ventilation. The site’s procedures now forbid using oxygen for anything other than the lance, require ventilation and oxygen monitoring in pits, and include oxygen enrichment in every lance operator’s training.

Exercise: oxygen for a lance

A thermic lance burns back at about 0.6 m a minute, with pipe and packing together weighing about 1.6 kg per metre. Using the ratio above, how much oxygen is theoretically needed to burn the iron of the lance each minute? If the lance maker’s data give an oxygen flow of about 1.5 m³ a minute, what share of the oxygen is used in burning the lance, and where does the rest go?

The lance burns about 0.6 × 1.6 ≈ 0.96 kg of iron a minute, needing about 0.96 × 0.38 ≈ 0.36 kg of oxygen, about 0.36 ÷ 1.43 ≈ 0.26 m³ a minute, treating the packing as iron.

At 1.5 m³ a minute supplied, that is about 0.26 ÷ 1.5 ≈ 17% of the oxygen. The rest blows the molten iron, oxide and melted material out of the cut, burns some of the material being cut if it is steel, and escapes, which is why lance work produces so much fume and why oxygen enrichment of the surrounding air is a real hazard.