Secondary containment

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General principles
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:

Bunds
Drip trays
Off-gas treatment systems
Interceptors/Sumps
Expansion vessels
Double skinned tanks/vessels
Concentric pipes
Building structures/ventilation
Bunds
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 under pressure.

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 Report.

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Drip trays
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:

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Scrubbers
Flares
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.

Interceptors/sumps
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:

Sewers
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 such hazards.

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 open.

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
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 pipelines.

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 storage tank.

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 another.

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 doubled skinned.

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.

Concentric pipes
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
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 nuclear industry.

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.

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