13.3 Gas Welding, Electric Arc Welding & Hot Work Safety

13.3 Gas Welding, Electric Arc Welding & Hot Work Safety

Hot work operations—including oxy-acetylene gas cutting, brazing, and Shielded Metal Arc Welding (SMAW)—are conducted regularly aboard merchant ships for structural repairs, sea-fastening securing, and pipework maintenance. However, an open flame or electric arc operating inside a steel ship containing thousands of tonnes of combustible fuel and cargo represents an extreme fire and explosion hazard. General Purpose Ratings must understand gas chemistry, welding electrical circuits, flashback dynamics, personal safety gear, and hot work safety permits.

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1. Oxy-Acetylene Gas Welding & Cutting Equipment

Oxy-acetylene equipment combines pure oxygen and acetylene gas to generate an intense, concentrated flame reaching temperatures of $3,150^\circ\text{C}$, capable of melting structural steel and cutting thick hull plating.

                      OXY-ACETYLENE SYSTEM ARCHITECTURE
       +--------------------+                    +--------------------+
       |  OXYGEN CYLINDER   |                    | ACETYLENE CYLINDER |
       |  - Black Body      |                    | - Maroon Body      |
       |  - Right-Hand Thd  |                    | - Left-Hand Thd    |
       |  - High P: 200 bar |                    | - Low P: 15 bar    |
       +---------+----------+                    +---------+----------+
                 |                                         |
       [Pressure Regulator]                      [Pressure Regulator]
                 |                                         |
       [Flashback Arrestor]                      [Flashback Arrestor]
                 |                                         |
       Blue Hose (Oxygen)                        Red Hose (Acetylene)
                 |                                         |
       [Non-Return Valve]                        [Non-Return Valve]
                 \                                         /
                  +-------------------+-------------------+
                                      |
                              [ Cutting Torch ]
                                      |
                             ( Spark Lighter Only )

Gas Cylinders: Chemistry, Color Codes & Thread Conventions

To prevent catastrophic cross-connection of equipment between combustible fuel gas and pure oxidizer, international standards enforce strict physical differentiation:

• Acetylene Explosion Hazard & Acetone Solution: Acetylene ($C_2H_2$) can decompose violently into carbon and hydrogen without oxygen. Heat or a flashback can initiate this reaction; increased pressure makes decomposition more hazardous. It must not be treated like an ordinary compressed gas. A dissolved-acetylene cylinder contains a porous mass saturated with solvent, commonly acetone. The solvent holds a large volume of acetylene, while the porous mass helps prevent a decomposition reaction from spreading through the cylinder. A typical charged cylinder is around $15\text{ bar}$; the approved regulator reduces this to the much lower torch working pressure. Keep the cylinder upright during use to avoid solvent carry-over, and protect it from heat and flashback. A heated or fire-exposed cylinder remains dangerous after the external fire is out: raise the alarm, withdraw and follow the ship's emergency procedure.
• Storage and Handling Rules:
  • Cylinders must always be secured in dedicated, naturally ventilated lockers on the open weather deck outside engine and accommodation spaces.
  • Acetylene cylinders must ALWAYS be stored, transported, and operated in an upright (vertical) position. If an acetylene cylinder is operated horizontally or tilted at more than $45^\circ$, liquid acetone will be drawn out through the valve and hoses, dissolving the rubber hoses, ruining regulators, and generating a violently unstable, sputtering flame.
  • Cylinders must be chained securely to rigid bulkheads, fitted with screw-on valve protection caps during transit, and kept away from direct sunlight, steam pipes, and electrical cables.
  • Keep Oil and Grease Away from Oxygen: Compressed pure oxygen reacting with petroleum grease or lubricating oil causes instantaneous auto-ignition and explosive detonation. Never touch oxygen valves, regulators, or cylinder necks with oily hands or greasy rags.

Pressure Regulators, Hoses & Flashback Arrestors

• Pressure Regulators: Reduce cylinder pressure to the correct torch working pressure. Set each gas pressure using the maker's nozzle-size and flow instructions; welding and cutting tips require different settings. Use only an approved acetylene regulator and observe its working-pressure limit. COSWP warns that acetylene above about one atmosphere gauge pressure becomes increasingly hazardous.
• Flashback Protection: A flashback is flame travelling back into the torch or gas supply system. It can damage hoses and equipment and threaten the cylinders.
  • Locations: COSWP advises non-return valves at the torch connections and flashback arrestors normally near the low-pressure regulator outlets. Additional torch-mounted arrestors may be appropriate. Use the approved arrangement and gas-flow ratings specified for the equipment.
  • Different Safety Functions:
    1. Non-Return Valve (NV): Prevents reverse gas flow and reduces the risk of oxygen and fuel mixing in the hoses. It does not itself quench a flame.
    2. Flame Arrestor (FA): A porous sintered-metal element removes heat from an approaching flame and quenches it.
    3. Thermal Cut-Off (TV), where fitted: Shuts off gas when its temperature limit is reached. It is separate from the reverse-flow valve. A heat-operated cut-off must be dealt with according to the maker's instructions before reuse.

                     DISTINCT FLASHBACK SAFETY FUNCTIONS
            NV: reverse-flow prevention   FA: flame quenching
            TV: heat-operated gas shut-off, where fitted

            Regulator -- Arrestor assembly -- Hose -- Check valve -- Torch
                         (approved functions and mounting as specified)

Torch Lighting, Flame Chemistry & Gas Cutting

• Safe Lighting Procedure:
  1. Inspect connections, hoses and safety devices; check for leaks with the approved leak-detection solution. Set the correct working pressures for the fitted nozzle.
  2. Purge each gas line separately in a well-ventilated area, clear of ignition sources. The time depends on hose length and flow and may range from seconds to a minute or longer. Close each torch valve after purging.
  3. Open the torch fuel valve as instructed and ignite with a friction spark lighter. Keep matches and cigarette lighters away.
  4. Adjust the fuel flow, then introduce oxygen and set the required flame according to the torch instructions.
• The Three Flame Types:

                             THE THREE FLAME TYPES
         NEUTRAL FLAME (Equal O2 and Acetylene, 3150 deg C)
         +-----------------[ Rounded Inner Cone ]====( Outer Envelope )=====>
         Welding mild steel, stainless steel, cast iron, copper.

         OXIDIZING FLAME (Excess Oxygen, 3300 deg C, Hissing/Pointed)
         +-----------------[ Sharp Pointed Cone ]===( Short Envelope )=====>
         Welding brass and bronze (forms protective zinc oxide coat).

         CARBURIZING / REDUCING FLAME (Excess Acetylene, 3000 deg C, Acetylene Feather)
         +-------[ Inner Cone ]===[ Acetylene Feather ]===( Outer Envelope )=====>
         Hardfacing, silver soldering, high-carbon steels.

• Oxy-Fuel Gas Cutting Principle: Gas cutting is not a melting process; it is a rapid chemical oxidation (burning) process. The torch preheats the steel to its ignition (kindling) temperature (approximately $900^\circ\text{C}$, bright cherry red) using the preheating flame. Once at temperature, the high-pressure cutting oxygen lever is depressed. A stream of pure, high-velocity oxygen strikes the cherry-red steel, causing the iron to burn vigorously: $$3\text{Fe} + 2\text{O}_2 \longrightarrow \text{Fe}_3\text{O}_4 + \text{Heat}$$ The reaction generates immense exothermic heat, melting the iron oxide, which is blown away by the oxygen jet to form a clean kerf (cut slot). Note: Gas cutting works only on metals whose oxide melting point is lower than the parent metal's melting point (mild steel); it cannot cut stainless steel, aluminum, or cast iron without specialized flux powders.
• Normal Shutdown: Following COSWP Annex24.1, close the torch fuel valve first, then the oxygen valve. Close both cylinder valves, vent each hose safely through its torch valve until the gauges read zero, release the regulator adjusting screws and close both torch valves. Follow the separate emergency procedure for backfire or flashback.

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2. Shielded Metal Arc Welding (SMAW / Stick Welding)

Shielded Metal Arc Welding (SMAW or Manual Metal Arc Welding MMAW) utilizes an electric arc struck between a consumable flux-covered electrode and the base metal workpiece to generate temperatures exceeding $6,000^\circ\text{C}$, melting the electrode wire and joint edges into a unified weld pool.

                    SMAW ELECTRIC ARC WELDING CIRCUIT
            +------------------------------------------------+
            | Welding Power Source (CC Drooping Inverter/AC) |
            +----------+--------------------------+----------+
                       | Electrode Lead           | Welding Return Lead
                       v                          v
             [ Fully Insulated ]           [ Heavy Return Clamp ]
             [ Holder & Stinger]           (Directly on Workpiece!)
                       |                          |
                       v                          v
                 ( Electrode ) ---------> [ Workpiece Joint ]
                 [Arc 6000°C ]

The Welding Circuit, Return Lead & Protective Earth

• Welding Power Source: Modern ships use DC inverter or transformer-rectifier power sources with a drooping (constant current) volt-ampere characteristic. If arc length varies slightly due to hand tremors, the welding current remains virtually constant, ensuring uniform weld penetration.

Consumable Electrodes & Flux Coating Functions

The electrode consists of a solid steel core wire surrounded by a concentric extruded flux coating. The flux performs four essential metallurgical roles:

1. Shielding Gas Generation: The heated flux produces gases that shield the arc and molten weld pool from surrounding air, reducing contamination by oxygen and nitrogen. These shielding gases can include carbon dioxide; they need not be chemically inert.
2. Slag Blanket Formation: Molten flux forms slag over the weld pool and cooling bead. It protects the hot metal from air, helps shape and support the bead, and influences cooling. The correct electrode, heat input and any required preheat remain important in controlling hardening and cracking.
3. Deoxidation & Alloying: Flux contains manganese and silicon to deoxidize the weld pool and scavenge impurities into the slag.
4. Arc Stabilization: Potassium and sodium salts in the flux ionize easily, stabilizing the electric arc and preventing arc flutter.

• AWS Classification Example (E6013 and E7018):
  • E6013: E = Electrode; 60 = Minimum tensile strength ($60,000\text{ psi}$ or $415\text{ N/mm}^2$); 1 = All welding positions (flat, horizontal, vertical, overhead); 3 = Rutile (titanium dioxide) coating for general repair with AC or DC.
  • E7018: 70 = Minimum tensile strength ($70,000\text{ psi}$); 1 = All positions; 8 = Low-hydrogen iron powder coating. Used for structural hull plating and high-stress machinery frames. Keep low-hydrogen electrodes dry. After opening sealed containers, use the maker-specified holding oven, commonly $120\text{ to }150^\circ\text{C}$ ($250\text{ to }300^\circ\text{F}$). Redrying damp electrodes is a separate procedure with its own temperature and time. Moisture can cause porosity and hydrogen-induced cracking.

Health Hazards & Personal Protective Equipment (PPE)

• Welding Helmet & Filter Shade: The intense electric arc radiates high-intensity ultraviolet (UV) and infrared (IR) radiation. Welders and assistants must use a certified welding helmet fitted with a Shade 10 to 12 filter lens (for currents between $75\text{ and }200\text{ A}$), backed by a clear protective cover lens.
• Arc Eye (Welder's Flash): Direct exposure to UV radiation causes acute corneal inflammation (photokeratitis), known as arc eye. Symptoms appear 6 to 12 hours after exposure: intense gritty sensation (feeling like sand inside the eyelids), severe throbbing eye pain, extreme sensitivity to light (photophobia), and uncontrollable tearing. First aid: Rest in a darkened room, apply cold wet compresses over closed eyes, instill soothing ophthalmic lubricating drops, and seek medical attention from the ship's medical officer.
• Protective Clothing: Welders must wear chrome-tanned leather welding gauntlets, a heavy leather apron, leather spats over steel-toe safety boots, and pure heavy cotton or flame-retardant overalls. Never wear synthetic fabrics (polyester or nylon); synthetic fibers melt upon contact with welding spatter, bonding to the skin and inflicting catastrophic third-degree burns.
• Fumes & Ventilation: Welding galvanized steel releases zinc oxide fumes that cause acute metal fume fever (chills, nausea, fever). Welding in enclosed machinery spaces requires continuous mechanical extraction fans with suction trunks positioned directly over the weld.

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3. Shipboard Hot Work Safety Procedures & Fire Watch

Hot work includes open flames, electric arcs, grinding and other operations capable of igniting flammable gas, vapour or combustible material. Heat and sparks can reach spaces beyond the welder's view.

                         HOT WORK CONTROL SEQUENCE
       +--------------------------------------------------------------+
       | SMS authorization and permit; risk assessment of the job     |
       | Remove/protect combustibles; inspect affected adjacent spaces|
       | Required gas tests, ventilation and firefighting equipment    |
       +------------------------------+-------------------------------+
                                      |
                  +-------------------+--------------------+
                  | DURING WORK       | AFTER WORK         |
                  | Dedicated watch   | Watch per permit   |
                  | Stop if unsafe    | COSWP: frequent    |
                  |                   | checks at least 2h |
                  +-------------------+--------------------+

The Shipboard Hot Work Permit

Follow the vessel's SMS and obtain authorization from its designated officer. COSWP recommends a permit for hot work outside the workshop. The permit identifies the task, location, personnel, validity period, precautions and responsibilities. Stop when it expires or conditions change; obtain reassessment before restarting.

• Pre-Work Checklist:
  1. Clear and Protect the Area: Remove dunnage, cardboard, oily rags, paint, chemicals and other combustibles to a safe distance determined by the risk assessment. Protect items that cannot be moved with suitable fire-resistant barriers. Control sparks through deck openings and gratings. If protection is inadequate, do not start.
  2. Atmospheric Testing: Test the work area and affected adjoining spaces where the permit requires it, especially near tanks or spaces that have contained flammables. A trained person uses calibrated equipment and the specified acceptance limits. Do not work in oxygen-enriched or deficient atmospheres, or where flammable gas or harmful contaminants make the job unsafe. Maintain ventilation and repeat or continuously monitor tests as required.
  3. Inspect Adjacent Spaces: Steel conducts heat to the opposite side of a bulkhead or deck. Inspect both sides, nearby compartments and concealed spaces for fuel, cargo, insulation or other combustibles. Clean, isolate, gas-free and protect affected spaces as required before authorizing the work. An apparently empty tank can still contain flammable vapour.
  4. Prepare Firefighting Equipment: Place suitable extinguishers and any required charged fire hose where the fire watch can use them immediately. The permit specifies equipment appropriate to the materials and hazards. Confirm access, communications and escape routes before starting.

The Dedicated Fire Watch

A trained fire watch concentrates on the fire risk rather than assisting with welding or fitting. The watch must see or cover all affected areas, including the opposite sides of boundaries where necessary, and know how to raise the alarm and stop the work.

• During Work: Watch for sparks, slag, smoke and heating of nearby material. Keep suitable extinguishing equipment ready and maintain the agreed communication with the responsible officer. Stop the job if the permit conditions fail or an unsafe situation develops.
• After Work: Sparks or slag can lodge in bilges, crevices and insulation and smoulder before a visible fire develops. Maintain the watch required by the permit. COSWP advises frequent checks for at least two hours after work stops, including affected adjacent areas; the risk assessment can require longer monitoring. Inspect the site and close the permit only when the prescribed checks confirm safe conditions.