This Technical Measures Document refers to secondary containment.
Temporary or mobile systems which are required to be put in place in
response to an emergency e.g. booms, absorbent materials, sandbags are
considered under the Technical Measures Document on:
Emergency response / Spill control
Also water sprays/curtains and foam blankets are considered under the
Technical Measures Document on
Active / Passive fire protection
Related Technical Measures Documents are:
The relevant Level 2 Criteria are:
Secondary containment is used on plant as a second line of defence for
preventing, controlling or mitigating major hazards events. It can take a
number of forms, the most common are:
- Drip trays
- Off-gas treatment systems
- Expansion vessels
- Double skinned tanks/vessels
- Concentric pipes
- Building structures/ventilation
Bunds are generally used around storage tanks or drum storage areas where
flammable or toxic liquids are held. Alternative measures may be earth dikes
(usually for very large tanks), sumps and interceptors. Bunds are also
sometimes used within plant buildings for reactors and other process
vessels. For materials that are normally gases at ambient conditions, bunds
are used where flash fractions are sufficiently low to merit them. Therefore
they are often used for refrigerated gases but not for the same gases stored
It is normal to limit the number of tanks in a single bund to 60,000 m3
total capacity. However, incompatible materials should have separate bunds.
Tanks often have individual bunds.
Bunds should be sized to hold 110% of the maximum capacity of the largest
tank or drum. This will allow some latitude for the addition of foam during
response to the emergency. There are no set rules on the ratio between wall
height and floor area and codes vary greatly with respect to recommendations
of bund wall height. Low wall heights (1-1.5 m) are often used to facilitate
firefighting but are poor defence against spigot flow (where a leak in the
wall of a tank passes over the bund wall) or the tidal wave effect of a
catastrophic tank failure. In some cases bunds up to height of the tank are
used, but these are quite unusual. For high walled bunds, consideration will
need to be given to the possibility of tanks floating as the bund fills,
causing catastrophic failure.
Bunds are generally fabricated from brick/mortar or concrete but where
liquids are being stored above their boiling point additional insulation,
e.g. vermiculite mortar, may be added as cladding to reduce the evaporation
rate. Such materials provide adequate chemical resistance to most liquids.
Maintenance of bunds is an important aspect, often overlooked,
particularly in remote locations. A system of inspection should be in place
to ensure the integrity of the bund. Also due consideration should be given
to drainage to allow the removal of rainwater. This is normally achieved by
incorporating a drain at a low point of a sloping floor with a manual valve,
normally kept closed. Operating schedules should include daily opening of
the valve to remove accumulated water, this will also assist in identifying
minor leaks. However, with this system there is the problem that the valve
may be left open or fail, thus reducing the effectiveness of the bund if a
tank failure occurs. Also in winter conditions, ice may form blocking the
drain. Failure to remove rainwater will reduce the capacity of the bund and
may result in overtopping and if the substance to be contained is
incompatible with water e.g. oleum, may result in an increased airborne
release. Consideration of these scenarios should be included in the Safety
Drip trays are often used beneath equipment liable to small leaks, such
as pumps, in process buildings and are effectively mini-bunds. They are
intended to prevent the spread of toxic or flammable substances to other
plant areas or to sumps and drains where secondary effects resulting in a
major accident could occur by domino effect. Drip trays vary greatly in size
and design. They are normally tailored to the individual item of equipment
but may serve a number of items. Materials of construction are often metals
such as stainless steel or strong rigid plastics that can be readily moved.
Drainage is not normally provided and liquid collected is normally removed
using absorbent material, after neutralisation or dilution (if required).
One variation on this theme is the use of sumps on drum stillages. These
are intended to hold the total contents of a drum in the event of a
catastrophic failure. They are normally limited to 1 or 2 drums and may be
used in drum transport by forklift truck.
HAZOP/HAZAN studies should determine where drip trays are required.
Off-gas treatment systems
Off-gas treatment systems which may act as secondary containment include:
- Catchpots/Knock-out drums
- Electrostatic precipitators
These systems may be used to reduce concentrations of hazardous gases and
vapours prior to discharge of the stream to atmosphere. Apart from
scrubbers, often such systems are part of the normal process but they may be
used in a secondary containment role. The latter two are used when discharge
streams may contain liquids or solids e.g. from reactor emergency venting,
which need to be removed, prior to further treatment. Catchpots may be
chilled or contain an absorbent liquid to remove contaminants. The worst
credible case discharge rate and volume should considered when designing
such systems. HAZOP/HAZAN should be used to establish the worst case
scenario. The Technical Measures Document Relief
Systems / Vent Systems provides more detail.
Design of drainage systems both within and outside process buildings
should take account of the need to segregate spillages of hazardous
materials. Drains systems to be considered may include:
- Stormwater drains
- Process effluent systems
- Firewater drainage systems
In many cases these functions are combined and often firewater and
process effluents are drained into main sewerage systems. Where there is a
possibility that hazardous substances could be discharged into a drainage
system, interceptors or sumps should be provided of sufficient capacity to
ensure that an offsite major accident does not occur. HAZOP studies or an
alternative hazard identification methodology should be used to identify
For process effluents arising from leaks or plant washdown, good practice
is to provide a local sump which is sampled before emptying. Such sumps
normally incorporate level indicators/alarms for monitoring. Discharge can
be to drums via submersible or mobile pumps for onward disposal or via
manual or manually operated automatic valves into main drainage systems if
the contents are non-hazardous. As for bund drainage consideration will need
to be given in the Safety Report on the possibility of valves being left
A particular concern is the discharge of non-water miscible flammable
liquids, which form a top layer. These could ignite considerable distances
from the plant after discharge. More sophisticated interceptors can be
provided to facilitate removal of floating flammable liquids. These tend to
be designed to meet individual needs and may incorporate conductivity-based
level sensors to distinguish between layers.
Firewater run-off is likely to involve very large quantities of
contaminated water (Lees quotes 900-2700 m3/hr). Risk Assessments
should be undertaken to consider the requirement for segregation of these
streams into lagoons or other catchment systems.
Expansion systems are used to prevent pressure build up, leading to loss
of containment, in the event of overfilling or temperature increases. They
are used mainly on liquefied gas storage systems, reactors and long runs of
Codes of practice for chlorine systems include the use of an expansion
vessel to allow for overfilling of the main storage tank. Depending upon the
arrangement, pressure, level or weight detection/alarms on the expansion
vessel may included to alert operators if liquid reaches this point.
Capacity of the expansion vessel is recommended as 10% of the capacity of a
Expansion vessels are sometimes provided for atmospheric storage tanks,
particularly where substances are particularly toxic or noxious. A liquid
scrubbing medium may be included in the expansion vessel to provide for
removal of fumes from air displaced on filling. The vent stream is sparged
into vessel below the liquid surface. The expansion vessel itself then vents
to either atmosphere or a scrubber. An alternative, where a number of tanks
are used for the same substance, is to arrange overflows from one tank to
Expansion tanks for reactors are described in the Technical Measure
Document Quench Systems.
Long pipelines containing liquids that have a high coefficient of
expansion should be provided with relief systems or expansion chambers to
prevent loss of containment due to overpressure. Relief systems should be
discharged into expansion vessels or off gas treatment plant if discharge
rates are within the design limits for such systems. Expansion chambers
should have a capacity of 20% of the pipeline volume. Chlorine is a
particular case to consider. Codes of practice recommend pressure relief
valves or bursting discs for liquid chlorine pipelines venting to the
expansion vessel or use of expansion chambers.
Double skinned vessels
Where there is particular concern about leakages occurring from tanks, an
alternative to bunding is to provide a second skin to collect material lost.
Monitoring of the annulus using specific analysers or level detection can
alert operators to the problem. Such systems are sometimes used for
underground or tanks in remote areas, where undetected leaks to the
environment may occur. Similarly tanks within process buildings may also be
Jacketed vessels including reactors and other process vessels are
primarily used to provide cooling or heating (using water, steam,
refrigerants, heating fluids etc.) to maintain temperatures of contained
substances. In some cases monitoring of the heat transfer medium is used to
detect loss of containment.
Pipes are sometimes provided with an outer shell or secondary pipe to
protect against loss of containment. As for double skinned tanks, these tend
to be used where the substance contained is particularly hazardous and no
alternative means i.e. bunding is available to contain any release. Such
methods are used in particular to protect pipes of less robust materials of
construction such as glass or plastic which are being used for very
corrosive substances e.g. bromine, strong acids. The outer pipe may be of
much stronger material, e.g. steel, which is sufficient to provide further
containment for a short duration without failure. Again monitoring of the
annulus is used to detect the initial failure and alert operators. Such
systems are often used where there are long runs of pipe on overhead
pipebridges. Pipes can be sloped to allow drainage to a collection pot
provided with level detection/alarms.
As for jacketed vessels, coolant/heating medium flow through jacketed
pipes may be used to detect leaks also.
Building ventilation systems can be arranged such that flow is maintained
from less contaminated to areas that may become contaminated following a
loss of containment, before discharge via off-gas systems, to provide some
degree of secondary containment. Such systems are used routinely in the
The Technical Measures Document Relief
Systems / Vent Systems considers ventilation systems.
Status of guidance
There are no design codes specifically covering secondary containment
measures, however HSE guidance notes on particular substances cover relevant
aspects. These are listed below, together with general references.
Codes of Practice relating to secondary containment
- HS(G)140 Safe use and handling of flammable
liquids, HSE, 1996.
Paragraph 26 discusses hydrostatic relief valves for pipelines, venting
back to the storage tank or a sump.
Paragraph 34 recommends that pumps are located outside bunds.
- HS(G)176 The storage of flammable liquids in
tanks, HSE, 1998.
Supersedes both HS(G)50 The storage of
flammable liquids in fixed tanks (up to 10000 cu. m in total capacity)
and HS(G)52 The storage of flammable liquids
in fixed tanks (exceeding 10000 cu. m in total capacity).
Paragraph 82 discusses hydrostatic relief valves for pipelines, venting
back to the storage tank or a sump.
Paragraphs 141 to 152 give guidance on the provision of bunds.
Individual bunds are recommended for large tanks, bund walls should be
0.5 m or greater and bunds should hold 75% of the volume of the largest
tank. Recommendations for spacing and construction are also given.
- HS(G)186 The bulk transfer of dangerous
liquids and gases between ship and shore, HSE, 1999.
Paragraph 117 recommends that hydrostatic relief valves on pipelines
should normally discharge back to the storage vessel, or discharge via
lines to safe places, such as slop tanks, sumps, flare stacks or other
vessels suitably designed for the recovery or disposal of the substance.
- HS(G)71 Chemical warehousing: the storage of
packaged dangerous substances, HSE, 1998.
Paragraph 108 recommends bunds are used to ensure segregation of
incompatible materials should leaks occur.
- EH70 The control of fire-water run-off from
CIMAH sites to prevent environmental damage, HSE, 1995.
Paragraphs 31 to 41 give guidance on possible methods of containment of
fire water, including bunds, lagoons, tanks, sacrificial areas,
flexi-tanks, catch pits, interceptors, drain seals (to prevent material
reaching sewers), booms and sand bags. It recommends that permanent
provision for the containment of large quantities of fire-water run-off,
typically several thousand cubic metres and above, may be achieved by
lagoons and tanks. These can be remote from, and may serve, several
storage areas, receiving the run-off via a gravity flow or pumped
drainage system. For bunds, it recommends bund capacity as 110% of the
largest tank, allowing 10% for foam addition. Recognition is given that
bunds cannot be sized to allow for fire-water. Bund walls should be
strong enough to resist hydrostatic pressure which arises when a loss of
- HSE, ‘Bulk storage and handling of high strength potable alcohol’,
National Interest Group Publication, 1990.
Paragraphs 24 and 25 recommend bunds of 110% capacity of the largest
tank, with a maximum wall height of 1.5 m.
- HS(G)28 Safety advice for bulk chlorine
installations, HSE, 1999.
Paragraph 83 recommends that bunds for storage tanks have a capacity for
the contents of the largest tank.
Paragraphs 119-132 provide guidance on relief systems for bulk chlorine
installations, recommending the use of a closed expansion vessel into
which pipelines and storage tanks are vented.
- HS(G)146, ‘Dispensing petrol : assessing and controlling the risk of
fire and explosion at sites where petrol is stored and dispensed as a
fuel’, HSE, 1996.
Recommends that lines and tanks should be double skin system monitored
by a leak detection system.
Further reading material
- Lees, F.P., 'Loss Prevention in the Process Industries: Hazard
Identification, Assessment and Control', Second Edition, 1996.
- Barnes, D., ‘Bund overtopping - The consequences following catastrophic
failure of large volume liquid storage vessel’, SRD R500, 1990.
- Mecklenburgh, J.C., ‘Process plant layout’, Godwin, London, 1985.
- Institute of Petroleum, ‘Refining safety code’, 2nd Edition, 1976
Case studies illustrating the importance of secondary containment