Last reviewed 5 September 2014

In light of the HSE’s current revision of its guidance on flammable liquids, Dr Roger K Bentley (CFIOSH) looks at the ideal conditions for storing flammable solvents.

Background guidance

The Health and Safety Executive (HSE) has recently re-issued not only the Approved Code of Practice for Dangerous Substances and Explosive Atmospheres Regulations (DSEAR), but is also in the process of revising its guidance on flammable liquids. Preferably, flammable chemicals should be kept in external stores, with the amounts in laboratories reduced to what is necessary for the efficient performance of the work. Guidance is for a maximum 50 litres of highly flammable liquid in closed vessels and kept in a 30-minute fire-resisting cupboard, cabinet or bin, with a sump to collect spillage. It should be labelled with a triangular flammability warning sign.

Highly flammable liquids, as defined in the Chemicals (Hazard Information and Packaging for Supply) Regulations (CHIP), are effectively those with a flash pointbelow ambient temperature. The new classification by the European Regulation on Classification, Labelling and Packaging (CLP) terms these flammable liquids of categories 1 and 2.

Flammable liquids under CHIP, or flammable liquid category 3 under CLP, with flashpoints up to approximately 60°C, pose a lesser fire and explosion risk (unless the liquid is heated above its flashpoint or released as a mist or spray, none of which are likely during storage). The guidance allows 250 litres of these to be stored in a laboratory or other workroom.

Where the inventory consists of both flammable liquids and highly flammable liquids, then the quantity should be restricted to 50 litres. For waste solvents, these must be taken into account in the quantities quoted in the guidance.

Application of DSEAR in laboratories

The above quantities are not legally-enforceable limits, but recommended maxima representing industrial safe practice. In large or open-plan laboratories, where the design of the workroom and the pattern of work make it difficult to keep within these limits, and where it is proposed to store quantities in excess of the recommended maxima, a robust justification should be recorded in the risk assessment required by the DSEAR regulations.

For small laboratories that can keep within the recommendations, note that DSEAR does not have a minimum threshold for a risk assessment, so that should always be undertaken and recorded, although it may conclude that additional measures beyond the normal fire precautions are not required. There is some overlap with fire legislation: both require a risk assessment, and for smaller laboratories it may well be expeditious to combine these. DSEAR requires both control (prevent incident) and mitigation (limit extent of effects) measures, as well as general safety measures and emergency procedures.

Design

If external storage is not an option (for security, aesthetic or contractual reasons), an internal storeroom may be provided. It should be designed as a separate fire compartment, minimum 30 minutes’ fire resistance (60 minutes if residential accommodation is in the same building), and should have a self-closing fire-door. This applies irrespective of whether the storeroom is accessed from the laboratory or from a corridor. Storerooms need to be marked to indicate the hazards associated with their contents.

Management and housekeeping

Usually storage will be in the containers in which the chemical is supplied (often “Winchesters” for solvents), otherwise in suitable proprietary containers, kept closed. Some secondary containment might be appropriate in the event of leakage or spillage, in the form of a tray or sump under the containers, or a sill across the doorway perhaps with a ramp for wheeled trolleys.

A spill kit containing inert absorbent should be readily available to deal promptly with spills and leaks: use a plastic scoop to deposit it in a metal container for removal. Personnel should be briefed to follow a written procedure, and to avoid their clothing soaking up the flammable liquid. Any contaminated clothing should be changed promptly, even if it appears to have dried, as flammable vapour can remain in clothing for a long time and it can be easily ignited, for example, by standing close to a heater or by smoking materials.

Combustible materials such as easily ignitable packaging should not be kept in a flammable liquid storeroom. Place wipes and other items contaminated with flammable liquid in a metal bin, suitably labelled and fitted with a self-closing metal lid, and regularly emptied.

Segregation

A laboratory storeroom will sometimes need to contain chemicals of other classifications, which may need to be segregated from flammable solvents. A helpful chart can be found in HSG 71 Chemical Warehousing: the Storage of Packaged Dangerous Substances; HSE, second edition, 1998. Most importantly, oxidising chemicals should not be stored with (highly) flammable solvents. Attention should also be given to chemicals classified as corrosive: if these come into contact with containers of other materials, they can be corroded, allowing the contents to escape. Beware of unstable chemicals such as di-isopropyl ether which, when exposed to air, is prone to form explosive peroxide. Some proprietary chemicals contain stabilisers, which usually have a limited period of effectiveness: make a periodic inspection of the storeroom contents, and dispose of anything liable to become unstable.

Explosive atmospheres and Hazardous Area Classification

Ventilation should be adequate to avoid the formation of an explosive atmosphere, although often a far better standard will be required to comply with other safety and environmental legislation not covered in this article. Where failure of forced ventilation would lead to a hazardous situation, then an airflow detector can be fitted in the ducting to give an alarm. The mechanical ventilation should be designed to cope with the maximum leak anticipated.

Where highly flammable liquids are stored, then the extent of an explosive atmosphere that could result from leaks and spillages should be determined, and ensure all ignition sources are excluded from this area. This is referred to as a hazardous area classification (HAC).

Within a storeroom, the extent of the hazardous area will usually be defined by the walls of the room, although it might be necessary to extend it to immediately outside the door and around any vents, also in ventilation ducts which will have implications for the fan. Unless otherwise made obvious, yellow triangular warning signs marked “EX” are located at the entry points of places that have been classified as hazardous areas.

Such hazardous areas are classified into zones, depending on the likelihood of an explosive atmosphere occurring, and its persistence. For storerooms where no other activities take place, zone 2 is appropriate, reflecting that an explosive atmosphere will only be present in abnormal circumstances.

Ignition sources

Within such a hazardous area, lighting and any other electrical or mechanical equipment should be certified as complying with the Equipment and Protective Systems intended for Use in Potentially Explosive Atmospheres Regulations 1996 (EPS). The corresponding standard of protected equipment for a zone 2 is designated category 3 (although categories 1 and 2 can also be used as they are manufactured to a higher standard). Also, it must be for use in the presence of gas or vapour (marked G) as there is also category 3 equipment for use in dusty environments (marked D). Equipment is also given a temperature rating between T1 and T6, and this should be selected taking into account the lowest autoignition temperature of the substances to be stored.

If heating is required in the storeroom, this is best provided by indirect means such as radiators. The avoidance of ignition sources applies not only to fixed equipment, but should include consideration of personal items such as mobile phones.

Static electricity

Another possible ignition source is an electrostatic charge, which may accumulate on the person due to friction, and if not leaking to earth, can result in a spark. Consideration should be given to providing laboratory workers with “anti-static” shoes and clothing which will dissipate the charge. The flooring of the storeroom also needs to be conductive.