Part 5: Fire and Explosion Prevention and Hot Work

General

Prevent the ignition of combustible materials that may be near the welding process. If welding a metal wall or partition (or if welding near one), you should check what is behind it.

Know how to use gas equipment safely.

• Oxygen under pressure can cause the spontaneous combustion of oil or grease. Keep all regulators and air hoses free of oil and grease, and avoid getting grease/oil on hands, gloves and overalls.
• Use the right gases for the situation. Never substitute oxygen for compressed air.
• In confined spaces, the risk of fuel gases and air combining to ignite or explode is increased. Don't allow welding gas supply lines to lie in a confined space where they may leak.

Keep appropriate fire extinguishers available and visible at all times. Make sure staff are trained in their use.

Consult the relevant sections of TN7 (chapters 5 and 16), NZS 4781:1973 Code of practice for safety in welding and cutting and AS 2865 Hot work.

Work on Drums and Tanks

Severe explosions and fires, many resulting in fatalities, have been caused by welding, cutting, brazing, soldering on pipes, tanks, drums, and similar vessels which previously contained flammable materials.

Containers which have held petrol, white spirit or other flammable substances are highly dangerous to work on and a pin-point of heat can be enough to set off an explosion or fire. Equally dangerous are pipes or containers which have held substances like linseed oil, soap, diesel oil, acids that react with metals to produce hydrogen, or combustible solids which may have left a residue of dust. It is essential to remove all residues. The preferred method is to steam clean and then either fill with an inert gas such as carbon dioxide or nitrogen, or fill with water, leaving a very small vented space at the point where the repair is to be made. (Allow for the expansion of liquid in small-bore pipes.) Washing containers with cold or hot water, or blowing with air are both ineffective.

Cleaning with trichloroethylene must be carried out with care, since it is toxic and decomposes on heating and may form phosgene gas.

Welders should study the booklet Hot Work on Tanks and Drums (obtainable at any Department of Labour office) before carrying out such repairs. Also, see TN7 chapter 21. A sample hot work permit follows:

On the left is an example of a hot work permit which can be printed on a card and attached to a welding trolley.
On the right is an example of the hot work permit advocated in NZS 4781.

Part 6: Personal Protective Equipment (PPE)

Standard personal protective equipment (PPE) for the variety of welding processes includes:

• welding helmet with a suitable arc flash filter, not less than shade 10. (Note: A welding helmet does not protect against fumes.)
• eye protection - and use dark clothing to avoid ultraviolet (UV) rays
• flameproof overalls with long sleeves and neck fastenings
• gloves (AS/NZS 2161 - Occupational protective gloves)
• apron
• respiratory protection
• hearing protection

Welding without this protection may allow bare skin to be exposed to the welding arc.

Eye protection may be worn under a welding helmet for additional protection. Eye protection must be worn when using a tool such as a chipping hammer or powered grinder to prepare or deslag metal.

Additional items of protective clothing may include:

• a cap
• neck covering (for protection from arc flash in confined spaces)
• spats or leggings
• steel-capped boots (AS/NZS 2210 Guide to occupational protective footwear).

Additional respiratory protection may be required (e.g. when welding galvanised metal or when cadmium is present in welding rods). This may range from simple filter masks that fit under the welding helmet to powered air purifying respirators. Specialist help may be needed to select, fit and maintain these items. Please refer to the Department of Labour publication A Guide to Respiratory Protection; AS/NZS 1715:1994 Selection, use and maintenance of respiratory protective devices; and TN7, chapter 19.

Personal protective equipment must be paid for by the employer - see section 10(2) of the Health and Safety in Employment Act 1992. Some protective equipment is designed to protect people in the vicinity from arc flash - such as portable welding screens. These should be dark in colour as shiny surfaces can reflect a significant amount of the arc flash.

Part 7: Fume and Gas Control

Exposure to welding gases and fumes can be fatal. Unsafe atmospheres and toxic welding gases and fumes may create a risk of discomfort, suffocation, fire and poisoning.

Nuisance dusts include iron and aluminium oxides generated in large quantities, which should be controlled at source.

Examples of toxins are ozone, carbon monoxide, nitrogen oxides and fumes from highly toxic metals including cadmium, zinc, beryllium, lead, chromium, nickel, manganese and copper. Fumes from some of these metals can cause cancer.

Inert shielding gases may pose risks of oxygen depletion and therefore suffocation, especially in confined spaces.

Some welding gases present risks of fire and explosion.

Welding in confined spaces may increase the risks of each of these types of hazard (see Part 13: Welding in confined spaces).

Preparation of metal surfaces may cause toxic fumes or vapours to be released. Examples include:

• when paints or plastic coatings are heated
• if degreasing agents, oil or grease are not removed from metal surfaces
• welding car parts painted with isocyanate paints.

The interaction of the welding arc and certain degreasing agents can generate the highly toxic gas phosgene.

Fumes generated by fluxing agents in welding rods, welding pastes and silver brazing fluxes can cause allergic reactions.

You cannot rely on the sense of smell to detect any of these hazards. Some cannot be smelt at all and the sense of smell can become insensitive to those odours it can detect.

Special efforts to ensure adequate ventilation (especially in confined spaces) and to control fumes are necessary.

Basic Methods of Gas and Fume Control

Create Less Fumes

• Use a welding technology that creates less fumes (and is also more cost effective). For example, substitute manual welding with gas metal arc (MIG) welding, or substitute MIG with submerged arc welding.
• Use electrodes that create less fumes.
• Reduce the current to the minimum possible.
• An approximate decreasing order of fume generation is:
  • Manual metal arc welding (MMAW) - Most fume
  • Gas metal arc welding (MIG) - Less fume
  • Gas tungsten arc welding (TIG) - Least fume

Substitute Dangerous Work Methods

• Substitute a dangerous substance with a less dangerous one.
• Arrange the work so that hot gases and fumes do not rise into the worker's face.
• Automate where appropriate.

Use Dilution Ventilation

This method disperses fumes and gases from near the worker to the wider workplace. Placing extraction fans in walls and ceilings, keeping doors open or having a large volume of room space for each worker are common approaches.

This method is suitable only for low toxicity fumes and is not suitable where toxic substances or metals are generated or welded.

See TN7 Section 17.4 and Table 17.1.

Use Local Exhaust Ventilation

This is the preferred option.

Local exhaust ventilation systems capture fumes and gases near the point where they are generated. This method is essential for many welding processes.

See TN7 Chapter 17 and the WTIA Fume Minimisation Guidelines and part 8 of this guide for more detail.

Limit the Exposure Period

This administrative control may not prevent toxic effects from short-term exposure and is unsuitable for toxic contaminants.

It may be essential when working in confined spaces. See part 13: Welding in confined spaces.

Use Personal Protective Equipment (PPE)

Certain PPE is standard in welding processes, but should be regarded as a method of last resort for fume control, where other methods prove unsatisfactory. See details above and in part 8: Local exhaust ventilation for welding processes.

---

Contents | << Previous | Next >>