Part 8: Local Exhaust Ventilation for Welding Processes

Chapter 17 of TN7 and the Fume Minimisation Guidelines cover this topic fully. These notes aim to give a brief overview of welding ventilation best practice.

The amount of fume generated depends on:

• the process being used
• the type of welding consumables (rods) used
• what is being welded
• any coatings on it
• the temperature
• the amperage and voltage of the welding current
• gas and arc temperatures
• heat input and
• the duration and frequency of the work.

After these aspects of the task have been chosen to minimise fume generation, some fumes may remain. The best way to prevent exposure is to provide appropriate ventilation.

Mechanical Dilution Ventilation

This will be adequate only in the circumstances described in TN7 Section 17.4 and Table 17.1.

Portable fans blowing fume away from the welder's breathing zone are one way of providing exhaust fume control. A capture velocity of 0.5 m/s is required. Velocities up to 2.0 m/s should not interfere with shielding gases. This may not be suitable in some circumstances.

Local Exhaust Ventilation

Local exhaust ventilation has the following components:

• A hood, a duct system, an air-cleaning device, an exhaust fan and a stack.
• A ventilation slot at the rear of the welding bench. Precise specifications for the air-flow rates for different types and sizes of capture hoods can be found in TN7.
• Specific purpose ventilation. Fixed hoods can be located at points where welding occurs for example, where components to be welded are held in jigs.
• Ventilation equipment mounted on the welding gun. Care needs to be taken over the design (for worker acceptance) and the disturbance of shielding gases, if they are used. Regular maintenance is essential if this equipment is to work properly.
• Relocatable exhaust hoods connected to fans and air-cleaners by flexible tubing. These may connect to a fixed fan or vacuum cleaner-type unit. They allow the welder to reposition the suction inlet as welding proceeds.

Capturing fumes when the point of welding moves may be more difficult, but is important when exposure is significant.

Local exhaust ventilation may be required whenever:

• welding takes place in confined spaces
• the metals listed in TN7 Table 17.2 are being welded.

Whenever workers may be exposed to beryllium or cadmium, local exhaust ventilation and personal respiratory equipment must be provided, such is their toxicity, no matter where welding is carried out.

Coatings on metals often give rise to hazardous exposures. The correct identification of metal coatings and their removal is the preferred control measure. Local exhaust ventilation (and possibly personal protective equipment) will be essential to control toxic dusts/fumes if these coatings cannot be removed.

Special measures to control toxic products (phosgene and phosphine) that can be generated when metals contaminated with degreasing agents and phosphate paints (respectively) are welded, are described in TN7 part 17.11.

Equipment may be needed to measure the levels of gases such as carbon monoxide, ozone and nitrogen oxides.

The fume control worksheet shown in part 21 can indicate the level of protection required in other circumstances.

Part 9: Electrical Safety

See TN7 chapters 4 and 14 for details about general electrical safety and the specific electrical safety measures required for each different welding process. Ensure equipment is constructed to the relevant standards (AS 1966 and AS/NZS 3195).

Ask the advice of the welding equipment supplier or an appropriately qualified specialist when purchasing and/or installing welding equipment. Some basic precautions follow.

Equipment Electrical Safety

• ensure the equipment has the correct current capacity
• provide an isolating switch
• where a flammable gas or solvent is present, an electrical spark might cause an explosion, so welding should not occur
• take care to preserve electrical polarity when using two welding machines on electrically connected metals
• use a welding machine with an automatic cut-out to ensure the duty cycle cannot be exceeded
• inspect equipment regularly (AS/NZS 3100:2002 Approval and test specification - General requirements for electrical equipment and
  AS/NZS  3195:2002 Approval and test specification - Portable machines for electric arc welding and allied processes). Do not use equipment with frayed or cracked leads, connectors or fittings or broken switches and cover plates. Do not allow them to be used while waiting for repairs (see AS/NZS 1995:2003 Welding cables). Leads need to be tested and tagged
• select an appropriate rod holder (AS 2826-1985: Manual metal-arc welding electrode holders)
• check the electrical safety of the rod holder or welding hand-piece regularly - maintain or replace as required
• use the shortest possible leads and ensure they are capable of carrying the required current safely
• use a residual current device (RCD) when using hand-held power tools
• use appropriately-rated powerboards rather than double adaptors or piggyback plugs.

Safe Use of Equipment

• be aware of welding where water may be present (showers, kitchens, boats, spas, swimming pools etc.)
• never twist or knot a lead, bend it sharply, tack it to a wall or drape it over your body
• dry your hands before welding. If you get sweaty, dry off, take a break and use a wooden duckboard to insulate yourself
• disconnect electrical equipment immediately after use
• pull on the plug, not the lead, to unplug equipment
• use welding gloves - keep them and any protective clothing dry. Don't work in the rain. Don't hold electrodes under the armpits.

If you use a petrol motor generator to power a welding set, be aware that carbon monoxide can kill quickly. Don't use in confined spaces, including ones that appear to be ventilated, such as a parking garage.

TN 22, Welding Electrical Safety, is also available from the Welding Technical Institute of Australia.

Part 10: Safe Use of Welding Gases

Maintenance of Gas Bottle Integrity

• cylinders should be correctly labelled
• cylinders should be stored in a ventilated area
• cylinders should be properly secured against falls
• fuel gas cylinders and oxygen cylinders should be stored separately
• gas cylinders should be kept away from electrical apparatus and sources of heat
• acetylene cylinders should be stored upright
• gas cylinders should be regularly examined for signs of defects, rusting or leakage
• empty cylinders should be appropriately marked and stored with their valves closed.

Integrity of Equipment Connected to Gas Cylinders

• flashback arrestors must be fitted to fuel and oxygen bottles
• the integrity of the equipment fitted to gas cylinders should be inspected routinely
• cylinders, cylinder valves, couplings, regulators, hoses and apparatus should be kept free of oily or greasy substances
• red should be used to identify acetylene (and other fuel gas) hoses, blue for oxygen hoses, and black for inert gas and air hoses
• copper piping should not be used with acetylene.

Correct Usage of Gas Equipment

Operators should know:

• the correct assembly procedures for attaching equipment to gas cylinders
• the correct procedures and materials (detergent, not soap) for leak testing
• the correct procedures for lighting gas torches and shutting them off
• the signs of a flashback, what to do in response, how to check if damage has occurred to equipment and the actions necessary if it has occurred
• never to crack a fuel gas cylinder valve near a source of ignition
• before a regulator is removed, to close the valve and release gas from it
• NOT to use the oxygen cylinder to dust off clothing
• NOT to use the oxygen to 'sweeten' the atmosphere.

The use of LPG should be avoided in holes and trenches, where it can pool at a low level.

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