Level 1, Unit 5, Section 2 – 2.5 Oxy-Fuel Operation

2.5.1 — Operation and Adjustment of Gas Pressure Regulators

Regulators reduce cylinder pressure to a controlled, stable working pressure and maintain consistent delivery as the cylinder empties and its pressure drops. Correct regulator operation ensures a stable flame that responds predictably to torch valve adjustments — which is essential for quality welds and cuts and for avoiding dangerous pressure surges.

Regulator Components

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High-Pressure Gauge

Indicates cylinder contents pressure. For oxygen cylinders this may read 15,000–20,000 kPa (2,200–3,000 psi) when full. For acetylene cylinders the reading is much lower, typically 1,500–1,800 kPa (220–260 psi). A falling high-pressure reading indicates the cylinder is being consumed.

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Low-Pressure (Working) Gauge

Indicates the delivery pressure set by the adjusting screw — the pressure supplied to the hose and torch. Typical working pressures for HVAC/R brazing: oxygen 70–200 kPa (10–30 psi); acetylene 35–100 kPa (5–15 psi); propane 35–70 kPa (5–10 psi), varying by tip size.

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Adjusting Screw (T-Handle)

Clockwise rotation increases delivery pressure by compressing the regulator spring and opening the seat. Counter-clockwise rotation reduces spring tension and lowers delivery pressure. Back out fully counter-clockwise before opening any cylinder valve to prevent a pressure surge that can damage the regulator seat or diaphragm.

Regulator Adjustment Procedure

Confirm both adjusting screws are backed out (fully counterclockwise, no spring tension) before opening either cylinder valve.
Open the oxygen cylinder valve slowly until the high-pressure gauge rises, then open fully. Watch for the low-pressure gauge to remain near zero, confirming the regulator seat is closed.
Open the fuel cylinder valve slowly — one to one-and-a-half turns maximum.
Set oxygen working pressure by turning the oxygen adjusting screw clockwise until the low-pressure gauge reaches the manufacturer-specified pressure for the selected tip.
Set fuel working pressure by turning the fuel adjusting screw clockwise to the specified pressure. For acetylene: never exceed 103 kPa (15 psi) working pressure.
Perform a leak check on all connections at working pressure before lighting (see Lesson 2.6).

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Acetylene Working Pressure Must Never Exceed 103 kPa (15 psi)

Acetylene becomes chemically unstable above approximately 103 kPa (15 psi). At higher pressures it can decompose explosively without an ignition source, especially in a hot hose. This limit applies to working (delivery) pressure — not cylinder storage pressure. Always verify the low-pressure gauge reads at or below the manufacturer limit and code requirement before lighting.

2.5.2 — Flame Types: Oxidizing, Neutral, and Carbonizing

The ratio of oxygen to fuel gas determines flame type, which controls both the temperature and the chemical environment at the joint. Selecting the correct flame type is essential: the wrong type can oxidize a joint (making it porous and weak), introduce carbon into the metal (altering its properties), or fail to reach the required temperature in the available time.

The Three Flame Types

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Neutral Flame

Equal volumes of oxygen and fuel produce balanced combustion. The inner cone is bright, well-defined, and blunt-tipped; there is no visible secondary feather beyond the inner cone. A neutral flame is the standard for most HVAC/R brazing with silver-alloy filler rod on copper and steel. It neither oxidizes nor carburizes the base metal.

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Oxidizing Flame

Excess oxygen: the inner cone is shorter, pointed, and paler; the flame produces a hissing sound. An oxidizing flame burns hotter than neutral but creates oxides on metal surfaces, preventing proper wetting by filler rod and producing a porous, brittle joint. Used intentionally only for brazing brass and bronze. Do not use on copper refrigerant piping or steel.

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Carbonizing (Carburizing) Flame

Excess fuel gas: a visible white feather — the acetylene feather — extends beyond the inner cone. The outer envelope is larger and softer. A carbonizing flame deposits carbon on metal surfaces; absorbed into steel it makes the joint harder and more brittle. Used intentionally for hard-facing and case hardening. Avoid on copper and most HVAC/R brazing applications.

Primary and Secondary Flame Cones

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Understanding Flame Structure

An oxy-fuel flame has two distinct zones. Understanding them lets you position heat correctly and identify flame type by visual inspection:

Primary (inner) cone: the bright, luminous zone immediately at the tip orifice where initial combustion occurs; this is the hottest zone of the flame (up to 3,500°C for oxy-acetylene) and is where heat is concentrated during cutting and brazing
Secondary (outer) envelope: the larger, softer surrounding flame where combustion products react with atmospheric oxygen; this zone is cooler and provides partial shielding of the joint against atmospheric contamination
Position the tip of the inner cone close to but not touching the work surface for maximum heat transfer; touching the inner cone to the metal surface causes backfire
Inner cone length indicates flame type: longer visible feather = excess fuel (carbonizing); shorter, pointed inner cone = excess oxygen (oxidizing); blunt, well-defined, no feather = neutral

2.5.3 — Steel Cutting Procedures and Cylinder Transportation

Oxy-fuel cutting uses a preheat flame to bring steel to its ignition temperature, then introduces a high-pressure oxygen jet that rapidly oxidizes (burns) the steel along the kerf. Correct technique produces a clean, square cut face with minimum slag; incorrect technique produces slag-clogged, rough cuts that require rework. Cylinder transportation procedures prevent impact and valve damage during movement.

Torch Setup and Safe Working Procedures for Cutting

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Pre-Cut Setup Checklist

Verify the material to be cut through drawings or markings; confirm no utilities, electrical conduit, or refrigerant piping runs behind or beneath the cut line
Wear required PPE: cutting goggles (shade 5 minimum), leather gloves, flame-resistant clothing, safety boots, and a face shield where spatter is expected
Select cutting tip size based on material thickness per the manufacturer's tip chart; verify correct pressures for the selected tip and fuel gas
Set oxygen and fuel working pressures per the tip chart; cutting oxygen pressure is higher than brazing pressure — follow the chart exactly
Perform a full leak check at cutting pressures before lighting
Light the preheat flame using a friction lighter; adjust to a neutral flame on the preheat orifices
Test the cutting oxygen lever before approaching the work: depress and observe the cutting jet — it must be a straight, narrow, high-velocity stream; a cone-shaped or off-centre jet indicates a dirty or damaged tip

Cutting Technique

Preheat the starting point: position the preheat inner cones approximately 3–5 mm above the steel surface at the starting edge; hold the torch at 90° to the plate. Allow the steel to reach bright orange (ignition temperature) before introducing the cutting jet.
Introduce the cutting oxygen: once the starting point is at ignition temperature, depress the cutting oxygen lever smoothly and begin moving the torch along the marked cut line at a steady, controlled pace.
Maintain consistent travel speed: too slow produces excess heat, wide kerf, and slag buildup; too fast extinguishes the cut and requires a restart. A clean kerf with slag falling cleanly below the plate indicates correct speed.
Maintain tip-to-work distance: keep the inner preheat cone tips 3–5 mm above the surface throughout the cut. Lifting too high drops heat input and extinguishes the cut; too close causes tip overheating and backfire.
Complete the cut: release the cutting oxygen lever before lifting the torch from the cut zone. Allow the cut edge to cool before handling — freshly cut steel retains heat well beyond its visible colour.

Start-Up and Shut-Down for Cutting

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Start-Up Sequence

Open oxygen cylinder valve (fully); open fuel cylinder valve (one to one-and-a-half turns). Set regulator pressures per the tip chart. Perform leak check. Light preheat flame with a friction lighter; adjust to neutral. Test cutting oxygen jet before approaching the work. Confirm fire-watch is in position and extinguisher is accessible.

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Shut-Down Sequence

Close the fuel torch valve first to extinguish the flame; then close the oxygen torch valve. Close both cylinder valves. Open torch valves to bleed both hoses until all four gauges read zero. Close torch valves. Back out both regulator adjusting screws fully counterclockwise. Allow tip to cool before storing.

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Cylinder Transportation Procedures

Always use a proper cylinder cart or hand truck for moving cylinders, even short distances on a job site; do not roll, drag, or drop
Replace all valve caps before any move; the valve is the most vulnerable part of the cylinder — a damaged valve can cause a catastrophic uncontrolled release
Separate oxygen and fuel cylinders when transporting: in a vehicle they must be secured upright in a ventilated cargo area, chained or strapped so they cannot contact each other or fall
Never transport in a closed passenger compartment — even a minor leak in a confined vehicle can rapidly produce an explosive atmosphere
Do not drop cylinders or strike them together; even a small dent near a weld seam can compromise structural integrity
Fully depressurize (both gauge sets at zero) and disconnect all hoses and regulators before transporting cylinders off-site

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Documentation for Cutting Work

Record in your job documentation: the material specification and thickness, the tip size used, the working pressures set, the hot-work permit number where applicable, and the fire-watch sign-off at completion. Confirm in the record that the cutting area was inspected for buried utilities and combustibles before work began.