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ENGINEERING CONTROLS

문서에서 TABLE OF CONTENTS (페이지 30-33)

Engineering controls are physical in nature, including mechanical devices or processes that eliminate or minimise the generation of chemicals, suppress or contain chemicals, or limit the area of contamination in the event of spills and leaks. They often involve partial enclosure, use of exhaust ventilation or automation of processes. Examples of engineering controls include:

„ using intrinsically safe electrical equipment in hazardous areas

„ using robots to minimise operator exposure, for example, spraying in coating operations

„ partially enclosed and ventilated spray booths or fume cupboards

„ fully enclosed ventilation booth (see diagram 1)

4. CONTROLLING RISKS

Local exhaust ventilation is designed to capture airborne contaminants close to the source of generation. This prevents them contaminating the working environment. The ventilation should be arranged to prevent contaminants from entering the breathing zone of the operator. The exhaust extraction shown in diagram 4 is a well-designed system, while that shown in diagram 5 is poorly designed as it carries contaminants directly through a person’s breathing zone.

DIAGRAM 1: Abrasive blasting cabinet DIAGRAM 2: Side hood ventilation for an open surface tank

DIAGRAM 3: Enclosure around a grinding wheel

DIAGRAM 4: Good design carries away contaminants from the operator’s through the breathing zone

DIAGRAM 5: Poor design carries contaminants through the breathing zone.

4. CONTROLLING RISKS

Design considerations for ventilation systems

Ventilation is a means of maintaining a safe atmosphere by the introduction or recirculation of air; by natural, forced or mechanical means. Maintaining a safe atmosphere in the storage and handling area of hazardous chemicals is an important control measure. Recirculation should be avoided unless precautions are taken to detect and avoid harmful contamination, and prevent accumulation of airborne contaminants. Recirculation should only be used where temperature control is required.

A ventilation system should operate exclusively for the particular building, room or space.

Where this is not practicable, the system may be linked to another area provided that this does not increase the risk to exposure of hazardous chemicals, for example by recirculating hazardous or flammable vapours or spreading them into other areas where that chemical is not being used.

Ventilation systems should be suitable for the types of hazardous chemicals on the premises.

For example, if a hazardous chemical has vapours which are denser than air, these will accumulate in low lying areas. In this case, extraction of vapours should be from the lowest point and fresh air introduced from above.

Exhaust systems and ducting should be resistant to the vapours, mists or dusts being extracted. The risk of fire propagation can be reduced by installing self-closing fire dampers, for example in laboratory fume cupboards. Extraction ducting should not be linked to multiple items of plant if there is any risk of fire spreading through the ducting. Provision against flashback, for example by installing flame arresters, may be necessary.

Exhaust gases and air should be discharged where it will not cause other hazards. For example, fume cupboard extraction systems should not exhaust close to air intakes and should be in compliance with any local building or environment protection requirements.

Exhaust systems can also be fitted with means to reduce airborne contaminants which may be harmful to the environment or people prior to discharge to the atmosphere. This might include particulate filters, absorbents and adsorbents (e.g. carbon), catalysts, scrubbers or burners.

Regular checks of these systems should be included in planned maintenance schedules to ensure that vents remain unobstructed.

Mechanical ventilation

Inlet and outlet vents located on opposite sides of the storage area at low levels provide airflow across the floor. Where both inlet and exhaust are mechanically assisted, capacities and rates should be adjusted to ensure that the pressure inside the store or room never exceeds that outside and airflow into any adjoining work areas and offices is prevented.

Local exhaust ventilation

Local exhaust ventilation is used to remove airborne contaminants before they reach the breathing zone of workers in the area. It is used for effective control of more highly toxic contaminants created in large quantities and is applied close to the source of generation.

It is more effective than increasing general ventilation to try to dilute toxic contaminants.

4. CONTROLLING RISKS

Natural ventilation

Natural ventilation can be used to control small amounts of relatively low toxicity

contaminants including dusts, fumes, gases and vapours which have low and steady rates of generation. It requires a large building space for dilution and relatively large air flow through open doors, windows or ceiling exits. For solvent storage areas, where heavier than air vapours may accumulate in lower regions (e.g. near floor level) with a subsequent build-up of hazardous concentrations, vents should be provided at a level immediately above any spill containment, on the opposite sides of a room or space, to provide for airflow across the storage or handling area. High level ventilation may be necessary for temperature control (e.g. roof vents to allow the escape of warm air).

Note: Vents in a screen wall may negate any fire protection or vapour barriers.

To ensure the effectiveness of ventilation systems, these should be designed in accordance with appropriate technical standards, and installed and maintained by qualified or

experienced persons, such as engineers or occupational hygienists.

Further information on the design of ventilation systems can be found in:

„ AS 1940: The storage and handling of flammable and combustible liquids

„ AS/NZS 60079.10.1: Explosive atmospheres – Classification of areas – Explosive gas atmospheres [IEC 60079-10-1, Ed 1.0 MOD]

„ HSG258 Controlling airborne contaminants at work: A guide to local exhaust ventilation (LEV); 2nd Edition 2011; Health Safety Executive (Great Britain)

„ ACGIH Industrial Ventilation: A Manual of Recommended Practice, 25th Edition; American Conference of Governmental Industrial Hygienists.

문서에서 TABLE OF CONTENTS (페이지 30-33)