LOSS PREVENTION

Evaluation and Specification of Secondary Containment

Secondary containment is used as a second defense line to control or mitigate the consequences of events related to loss of containment. It can take several forms, and the most common are containment basins, channels, pits, catch-pots, double wall tanks and concentric tubes.

Containment basins are generally used around storage tanks or drum storage area, where toxic or flammable liquids are maintained. Slopes or channels may be adopted as alternative measures. Sometimes basins are employed at industrial plants to contain reactors and other process vessels. Incompatible compounds should be stored in separate basins. Containment basins shall be built with concrete or with reinforced block, and suitable coating to prevent corrosive attack in the event of a spill.

Whenever there is any bigger concern with tanks and vessels leaks, an alternative to containment basin is build a second wall around the equipment. Monitoring the annular region by means of specific analyzers or level switches can alert the operation team about a leak.

Double wall is adopted for buried tanks or tanks on remote locations, where a leak not detected can dramatically jeopardize the environment. This kind of protection can also be applied to process vessels located inside industrial plants.

Jacketed reactors or vessels have as main function provide cooling or heating (using water, steam, refrigerants and thermal fluid) to maintain the temperatures within the primary containment. In some cases, monitoring of the heat transfer medium is used in order to detect any loss of containment.

Piping provided with an external wall or an additional external piping offer an additional barrier against loss of containment. As in the case of double wall tanks, this type of arrangement is recommended where the substance to be transported is very hazardous and there is no other means to contain the spillage. This kind of protection is particularly used to protect piping with a less robust material, such as glass or plastic, used for corrosive compounds (e.g. bromine, strong acids). The outside piping shall be of a stronger material (e.g. carbon-steel), which is enough to provide additional containment without failure for a short time. Monitoring the annular area can be used to detect an initial failure and alert the operating team.

Design of drainage systems, inside and outside industrial plants, should take into account the need to segregate the spills of hazardous substances. The design should consider spills, rainwater, effluents and firefighting. In many cases, these functions are combined, such as firewater and process effluents routed to a containment pond. Where there is the possibility of hazardous substances reach the drainage system, one should provide interceptors, culverts and pits with enough capacity to avoid a major accident outside the plant limits. Some methodology for hazard identification can be applied to assess the system.

For process effluents from small leaks and floor washing, the good practice is to provide a local pit, which is sampled before emptying.