Information on types of tanks for oil storage
There is some confusion about the terms ‘double skinned’,
‘integrally bunded’, ‘twin walled’ and ‘bunded’ plastic and steel
tanks in relation to above-ground oil storage, and which are
environmentally acceptable.
Unfortunately manufacturers and suppliers of these tanks have
used a variety of these terms with different interpretations.
Please refer to the list below for SEPA’s explanation.
Double skinned tanks
This is a primary tank with another ‘skin’ placed around it and
a very small gap between the two. None of the pipe work or
ancillary equipment is contained. The risk of oil being lost from
ancillary equipment and pipe work is high, so the Water Environment
(Oil Storage) (Scotland) Regulations 2006 requires that tanks have
all ancillary equipment such as sight gauges, taps and valves
retained within a secondary containment system.
Double skinned tanks are not compliant with the regulations
unless additional secondary containment is provided for the tank
and its ancillary equipment, such as an in-situ constructed
bund.
Double skinned tanks are recommended for underground storage of
oil/petrol etc. because the interstitial space between the tank
skins can be monitored for leaks. Underground installations should
also have twin walled, non-corrosive pipe work specified.
Proprietary tank systems
These come in a large range of designs and are produced by many
different manufacturers who may make certain claims about the
environmental performance of their products. To prevent the
confusion described above, ‘Proprietary Tank System’ or just ‘Tank
System’ is the preferred generic term for tanks often referred to
as integrally bunded or twin walled. Some tanks systems may have
adequate secondary containment to comply with regulation
requirements, but some may be regarded as ‘high specification
primary containers and would therefore be non-compliant without
additional containment.
Some proprietary integrally bunded tank systems take the volume
of the primary tank into account when calculating secondary
containment capacity, and provide additional secondary containment
capacity of less than 110% of actual primary tank capacity. This is
generally acceptable where, if there was a leak in the primary
container, oil could find its own level in both containers. In this
instance the primary container contributes to the total containment
capacity, which as long as it totals 110%, will be acceptable. A
similar position is achieved in a conventional bund, where the tank
is situated low down.
The regulations state that the secondary container ‘must have a
capacity of not less than 110% of the containers storage
capacity...’, but this can be interpreted to mean that the
secondary container must be ‘capable of containing 110% volume of
the primary container and, where applicable, the volume provided by
the primary container can be taken into account’.
An important issue in determining if a tank system is acceptable
is whether the ancillary equipment is adequately dealt with, such
as being within the secondary container, which is where some tank
systems have fallen down in the past.
To comply with the regulations, tank systems must have
containment to minimise damage from third party interference,
prevent pollution incidents from overfilling, leaking primary
container or ancillary equipment. They should be sited to minimise
the chance of damage by impact or collision or protected by
suitable physical barriers.
If well designed, manufactured, sited, installed, used and
maintained correctly, a tank system can be just as effective as a
conventional in-situ bunded tank.
SEPA’s ‘PPG2: Above Ground Oil Storage Tanks’ gives some basic
principles about the points to look out for when considering a tank
system, and products can be assessed against the checklist. Please
note that this guidance is currently being reviewed and will be
updated to reflect the requirements of the Scottish
Regulations.
Oil Storage Containers within Buildings
Storage of oil which falls within the regulations and is stored
within a building must meet the requirements of the
regulations.
Whilst SEPA would prefer use of ‘traditional’ tank secondary
containment system, the 110% secondary containment requirements of
the regulations may be met within the building itself (for example,
110% secondary containment may be provided within the building by
forming a ‘lip’ on doorways and calculating the floor area
capacity). All other requirements of the regulations must still be
met (e.g. additional requirements of the secondary containment
system relating to impermeability, no drains etc).