Regulations for secondary containment.
Size of secondary containment for a single oil container
For one tank, mobile bowser or intermediate bulk container (IBC), the secondary containment must be able to hold 110% of the volume that the container can hold. For one storage drum you can store it on a drip tray.
The drip tray must be able to hold 25% of the drum's total volume – not 25% of the amount of oil the drum has in it.
Size of secondary containment if more than one container in a storage area
More than one tank in the same secondary containment system
If the tanks aren’t hydraulically linked but are in the same secondary containment system, the containment capacity must be a minimum of 25% of the total capacity, or 110% of the largest tank; whichever is greatest.
If the tanks are hydraulically linked but situated in the same containment system, they should be treated as one tank. The containment capacity should be a minimum of 110 per cent of the total capacity of the tanks.
Tanks in different secondary containment systems
If the tanks are hydraulically linked, but in separate containment systems, containment of at least 110% of the total volume stored is required at each location.
There may be benefits in hydraulically linking the secondary containment systems.
Storage of multiple oil drums
If you are storing more than one drum on a drip tray the drip tray must be able to hold 25% of the total volume for the number of drums that can be stored on it.
Regulations for double-skinned tanks
A double-skinned tank, or a twin-walled tank, is unlikely to provide adequate secondary containment on its own.
Double-skinned tanks have an inner tank surrounded by an outer skin for extra strength. Don’t confuse double-skinned tanks with integrally- bunded tanks; they don’t give the same protection against oil loss from overfilling, or damage to tank and pipework damage, and require extra secondary containment when installed above ground.
Effective secondary containment will include ancillary equipment as well as the storage tank. This means that sight gauges and valves should also be inside the secondary containment facility. Vent pipes should discharge vertically in the secondary containment facility.
The Construction Industry Research and Information Association (CIRIA) report 163, on the construction of bunds for oil storage tanks, explains how to build good quality bunds.
You need to make sure your bund is built properly. It must be able to withstand total failure of a full tank.
Impermeability levels for constructed bunds
The regulations state that a container's base and walls are both 'impermeable to water and oil'. The regulations don't specify any particular construction materials for a secondary container.
A conventional bund constructed from bricks and mortar is unlikely to be impermeable without rendering or coating to achieve a permeability coefficient of not less than 10-9 metres per second.
Calculating secondary containment capacity
Take the volume of the primary tank into account when calculating secondary containment capacity.
Most proprietary tank systems are designed so that the oil finds its own level in both containers if the primary container leaks. In this instance, the primary container contributes to the total containment capacity. You should take into account that these systems may only provide a 10% containment capacity in the event of overfilling.
Oil will also find its own level in a conventional bund when the tank is situated low down.
For example, a maximum volume delivery made to a tank more than 10% full will result in a loss of oil from the secondary containment.
The use of an overfill prevention device is good practice and you should consider these or other additional pollution prevention safeguards, such as additional containment capacity in sensitive locations.
Other secondary containment options
An oil separator isn’t a form of secondary containment. We consider this to be tertiary containment. It will not be accepted as containment under the Oil Storage Reguilations England.