Hot work

Hot work conducted outside of a workshop environment must have a permit issued.

The University of Otago Hot Work Permit can be found here

Hot work includes:

• gas cutting
• torch welding
• arc welding
• brazing
• soldering (other than limited soldering of electronic components)

The main hazards from these operations are fire, burns (flame, surface contact or radiation), toxic fumes and eye damage.

Hot work on enclosed vessels such as drums and tanks may give rise to ignition of their internal atmosphere. Even substances not normally considered flammable at room temperature (eg grease) may explode under such circumstances.

Managing hazards

Before you begin

• Alternatives to hot work should always be considered, such as manual cutting, or cold soldering (adhesives and fillers).
• Only suitably trained and equipped personnel may use gas cutting and welding operations.
• Before starting hot work, the operator must be satisfied that the equipment is in good working order, and that the surrounding environment and other people are protected from hazards which may arise from the proposed work.
• Flashback arresting devices must be fitted to gas bottles used in welding operations.
• All equipment must be checked thoroughly before use. In particular, gas hoses must be checked for leaks using a a detergent/water mix, and the insulation on arc welding sets checked.
• Oil or grease must not be used to lubricate the threads on oxygen cylinders, as high pressure oxygen may react explosively with it.

Hazardous substances/gas

• Acetylene bottles must never be tilted more than 45 degrees from their upright position. This is because the liquid within them may foul the valve assembly, and cause a dangerous situation.
• When transporting gas cylinders other than on an appropriate trolley, fit valve caps and ensure the cylinders are securely mounted.
• Do not drag, bump, drop, roll, heat, or otherwise damage compressed gas cylinders.
• Empty gas cylinders must be treated with the same caution as full ones, as they may still contain an explosive residual atmosphere.

Preventing fire

• A fire extinguisher must be available for all hot work. Sparks from gas cutting operations may be an ignition hazard within a 10m radius of the work. Conduction of heat may also cause ignition of adjacent combustibles. All combustible materials within this radius should thus be removed, shielded or otherwise protected.
• Screen torch operations to reduce the risk of fire, and to avoid eye irritation to those nearby. When are welding, protection is required for all those within 15m of the work.

Personal protective equipment

• Overalls, gloves, apron, eye protection, filter respirators and long boots are required to be worn for welding operations. Overalls should fit tightly at the neck and wrist. Nylon overalls or jackets and clothing with exposed pockets may not be worn. Goggles must be worn for gas welding operations, and a face shield for arc welding. Head protection is optional. Gloves or gauntlets are necessary when arc welding to protect against shock, burns and radiation exposure.
• When soldering using lead solder, a lead fume is generated. For prolonged or routine soldering operations, it is necessary to install local exhaust ventilation to remove this fume or to wear respiratory protection equipment specific for lead fumes.
• Never point a torch toward clothing, whether lit or otherwise, as gas trapped in the fabric of clothing can cause severe burns if ignited.

General welding/cutting safety

• Dispose of hot welding rod stubs, slag and off-cuts in a safe manner, such as into sand.
• Welding and cutting operations may generate toxic metal fumes, ozone, nitrogen dioxide and carbon monoxide. Such work must therefore be done in a well ventilated area, preferably using a local exhaust ventilation unit. Respirators should always be worn by the operator.
• Electric arc welding units must have an effective earthing arrangement using an earthing clamp or bolted terminal, which should be checked regularly. Arc welding must not be attempted if hands are wet or floors are damp. When arc welding ensure that the unit is served by an isolating transformer, (whether integral or separate).
• Brazing in particular produces corrosive and toxic fumes, whilst the vaporisation of grease, paint and other coatings on the metal will add further components to the fume generated.

Hazardous/flammable materials

Never weld or cut a barrel or container that contains hazardous or flammable material, residues as a flammable or toxic atmosphere may be generated during the heating process. Flammable vapours may arise from:

• volatile liquids
• non-volatile oil, tar, grease soap
• acids which react with metal to form hydrogen
• combustible solid powders (eg wood dust, coal dust)

Cleaning drums & tanks

Drums and tanks should not be welded or cut without special safety precautions. If they contain even a small amount of flammable liquid, sludge or solid they may explode. To avoid this danger such containers need to be thoroughly cleaned and tested before any hot cutting or welding is carried out.

Recommended cleaning methods are:

• Soluble residues - repeatedly fill and drain with water or an appropriate solvent.
• Non-soluble residues - inject low pressure steam into the upturned container for at least 30 minutes, until condensate is clean. Earth the tank and steam pipe if highly flammable residues. - completely immerse tank in a boiling water bath so that the tank fills. (30 minutes minimum).
• If it is not possible to effectively remove hazardous residues, water or an inert gas or continuous steaming must be used to fill the vessel during the hot work.