Civil Engineering Reference
In-Depth Information
Ventilation of storage areas
Ventilation has been discussed as part of the storage
requirements; however, all areas where fl ammable liquids
are handled should be adequately ventilated to ensure
that any released vapours are diluted to a safe level.
Whichever ventilation system is used it needs to
be capable of providing a minimum of six complete
air changes per hour. The system's capacity should
ensure that the amount of vapour in the atmosphere is
not only diluted to well below its fl ammable limit, but
also reduced to a level below any relevant workplace
exposure limit (HSE's guidance note EH 40 gives advice
on workplace exposure limits).
Dependent upon the concentrations and volumes
in use good, natural, well-positioned ventilation may be
adequate, but where this cannot be achieved, mechan-
ical ventilation and/or local exhaust ventilation (LEV) is
almost certain to be needed.
Positive pressure mechanical systems can be
used to force air into an area; diluting and venting any
potential build-up of fl ammable vapours. Any area
provided with a positive pressure mechanical system
will also need to be provided with suffi cient openings
to release the atmosphere created. LEV systems are
designed to capture airborne contaminants such as
fl ammable vapours, fi lter them and ventilate outside a
building to a place of safety in the open air.
Any vents provided for the release of pressure or
contaminants should be a minimum of 3 m above ground
level, at least 3 m from building openings, boundaries
and sources of ignition, and away from building eaves
and other obstructions where they could become
trapped. The ducting taking the fl ammable vapours
away should be arranged so that they cannot condense
and collect at low points within them.
Ventilation/fume cupboards
- in certain operations
(e.g. solvent-based spraying operations or decanting
of fl ammable liquids) booths or cabinets are required
to control the build-up of fl ammable vapours. In such
instances the airfl ow rate into all openings in the
enclosure should be suffi cient (about 1 m/s) to prevent
vapours entering the work area. These ventilation booths,
cabinets and other enclosed equipment are generally
designed to ensure that the vapours are kept below 25%
of their LEL.
As the outcome or failure of any ventilation system
or part of the system may result in a build-up in vapour
concentrations with the potential for both fi re and
explosion, the system must be provided with an alarm
system to provide warning.
There are also a number of storage systems that
are combined with booths and cabinets which use LEV
for vapour control. Regardless of the system used each
must be subject to rigorous maintenance testing and
examination regimes to ensure that fl ammable vapours
are controlled.
Any electrical system used as part of the ventila-
tion or extraction systems must meet rigid electrical
standards and as in the case of all such equipment
be intrinsically safe. As discussed earlier, classifying
hazardous or potentially explosive atmospheres is not
only a legal requirement under DSEAR but has been
widely used to determine the extent of hazardous zones
created by fl ammable concentrations of vapours.
Over recent years the process of classifying hazard-
ous areas in such a way has been extended from its
initial purpose of selecting fi xed electrical equipment
for use in the area, to helping to identify and eliminate
potential ignition sources, including portable electrical
equipment, vehicles, hot surfaces, etc. from fl ammable
atmospheres.
Wind
direction
Figure 8.22
The use of wall and roof vents together, give
good natural ventilation
Figure 8.23
Example of a fume cupboard
