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7. Design and installation

7.1 General principles

7.1.1. Plant and equipment should be designed and installed to contain or minimise the risks from chemicals used at work, e.g.:

(a) by reducing the release of harmful or flammable chemicals including the vapour and dust from such chemicals;

(b) by preventing the spread of fire and explosion in the workplace.

7.1.2. Containment of a hazard is best achieved by fully enclosing processes involving the use of chemicals. Full enclosure of processes can be more easily achieved where plant and equipment are automated or operated remotely. This should be a primary consideration during the design of plant and equipment and the process to be used. Bulk storage, with fixed pipework transfer suitably designed and fit for the purpose, should be used in preference to small container storage, where appropriate.

7.1.3. To reduce leaks, where particularly hazardous chemicals are used, enclosed plant and equipment should be fitted with extraction systems, which should be designed to ensure a slight negative pressure within the plant and equipment, where the process allows. Extraction systems should vent to a safe place, or contaminated air should be filtered or treated to ensure that exposure limits or other established criteria for the control of the working environment are not exceeded.

7.1.4. Working areas, plant and equipment should be so designed and installed as to avoid unnecessary exposure of workers to hazardous chemicals; this should include the provision of local exhaust ventilation, ensuring that cleaning can be kept to a minimum, and facilitating maintenance and cleaning procedures.

7.1.5. To further reduce risks from hazardous chemicals, plant, equipment and storage should be separated from other processes, from incompatible chemicals or other chemicals which may cause a hazard in the event of fire, from premises off site and other areas outside the control of the employer and, in the case of flammable, from fixed sources of ignition.

7.1.6. To prevent the spread of fire and explosion the following safety engineering techniques should be considered:

(a) design and construction to contain the effects of an explosion;

(b) limiting the effects of a fire or explosion by means of suitably sized and designed pressure-relief vents, explosion relief panels, etc., which vent to a safe place;

(c) methods that prevent or reduce the spread of fire, such as the use of non-combustible or fire-resistant materials of a specified standard;

(d) the use of chokes, baffles or similar means to contain the effects of a fire or explosion within areas of the plant;

(e) automatic means of extinguishing or suppressing a fire or explosion, such as the use of automatically operated inert gas systems to suppress an explosion, or automatically operated extinguishing systems, e.g. water sprays.

7.1.7. To prevent the spread of a hazardous chemical in the event of its release, a secondary means of containment should be provided in accordance with established criteria, such as bund walls for hazardous liquids, diversion walls and evaporation areas for heavier-than-air flammable gases at or near their boiling points at ambient temperature (e.g. butane), and containment areas for the evaporation of cryogenic liquids. A "bund wall" is a properly designed and constructed containment wall to contain the contents of a storage vessel enclosed by the wall. A "diversion wall" is a low wall adjacent to a storage vessel used to divert released flammable gas and liquid away from danger areas and to an area for safe evaporation.

7.1.8. Assessment of risks from harmful chemicals including monitoring where appropriate, should be made as soon as plant and equipment are installed in order to determine whether the criteria established by the competent authority have been met.

7.2 Local exhaust ventilation

7.2.1. Where total enclosure of a process involving hazardous chemicals is not reasonably practicable, local exhaust ventilation equipment should be provided and maintained to ensure that criteria such as exposure limits specified by the competent authority are not exceeded and that hazards such as flammable concentrations are eliminated or kept to a minimum.

7.2.2. The local exhaust ventilation should be so designed. constructed and installed as to ensure either the safe and effective removal of contaminated air from the workplace to a safe place, or the filtering or treatment of the contaminated air to avoid further hazard, taking into account exposure limits or other criteria for the control of the working environment established. approved or recognised by the competent authority. It should also be so designed as to prevent the spread of fire and explosion, following the principles outlined in paragraph 7.1.6 (prevention of the spread of fire).

7.2.3. The performance of the local exhaust ventilation should be checked upon installation against the design specification.

7.2.4. For efficient operation to prevent exposure of the worker, the exhaust ventilation should be located as close as possible to points of emission of hazardous chemicals. The length of ducting and the number of bends should be kept to a minimum to enable efficient operation.

7.3 General ventilation

7.3.1. Work areas should be supplied with clean air to balance the volume of extracted air as it is exhausted through the various extraction systems. This ensures efficient extractiOn and helps to reduce concentrations of chemicals.

7.3.2. The flow rates of general ventilation should be sufficient to change the air of the work area according to safety and health requirements, taking into account its size, the working conditions and numbers of workers.

7.3.3. Recirculation of extracted air into workrooms should be avoided, except under conditions acceptable to the competent authority. Where recirculation is allowed:

(a) effective methods should be used to decontaminate the air, which should be regularly checked and maintained;

(b) some air should be vented during recirculation and replaced by fresh air to avoid an accumulation of possible contamination;

(c) the rate of replacement by fresh air should be designed to ensure that hazardous limits or criteria for the control of the working environment, established, approved or recognised by the competent authority, are not exceeded in plant and workrooms;

(d) account should be taken in the design of the need to prevent any inadvertent release of hazardous chemicals from causing a hazard and spreading it to other working areas.

7.4 Elimination or control of sources of ignition

7.4.1. Where flammable chemicals are used, the primary consideration in design and installation should be to eliminate flammable atmospheres. Nevertheless, an assessment should be made of where flammable atmospheres may occur during the use of chemicals at all stages, and sources of ignition eliminated or minimised.

7.4.2. Areas should be classified according to the degree of probability of a flammable concentration occurring in the area. Unless classified as safe, electrical apparatus should not be used in these areas, where practicable. Where this is not practicable, electrical apparatus should be designed and constructed according to the classification of the hazard. The design and construction should be in accordance with standards recognised or approved by the competent authority.

7.4.3. Examples of ways in which sources of ignition may be eliminated include:

(a) the setting up and maintenance of "no smoking" areas;

(b) the prohibition of pumps and other electrical apparatus within the bunded areas of storage tanks (the pump should be located in its own containment area in case of leakage);

(c) the prohibition of electrical motors within ducts that contain flammable chemicals, e.g. replacing them with remote-driven fans;

(d) the prohibition of battery-charging operations for fork-lift trucks within storage areas or storage buildings.

7.4.4. The potential for creating static charges, e.g. with non-polar chemicals such as hydrocarbon solvents or certain dusts and solids such as sulphur, can be reduced by:

(a) avoiding free fall of the chemicals during filling of vessels from pipelines or from one container to another;

(b) a reduction in pumping rates at discharges;

(c) using anti-static electricity additives.

7.4.5. Special attention should be given to providing engineering measures to prevent a fire or explosion due to accumulation and discharge of static electricity. These measures should be periodically reviewed.

7.4.6. The type of heating provided in a workroom or storeroom should be suited to the likely conditions in that room. The following points should be observed where flammable chemicals are used:

(a) portable heaters such as oil and gas heaters, radiant electric fires and oil-filled electric radiators should be avoided;

(b) where oil and gas fired systems are used, they should be of the indirect type, i.e. the products of combustion should be safely flued to the outside atmosphere. The intake of air into such systems should come from safe locations where no spillage of flammable chemicals is likely to occur and enter the heating system.

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Updated by AS. Approved by EC. Last update: 30.11.2004.