Avoid Chemical Reactivity Incidents in Warehouses

Chemical Engineering Progress, Feb 2008 by Chastain, J Wayne, Doerr, William W, Berger, Scott, Lodal, Peter N

Use this approach for the initial evaluation of potential reactivity hazards in storage applications.

Chemical reactivity issues, with their potential to generate heat, overpressure and/or harmful chemical reaction products, present a significant threat in many aspects of chemical commerce (1). In April 2004, the Reactivity Management Roundtable (RMR) was formed as a special project of AIChE's Center for Chemical Process Safety (CCPS), and was charged with working "cooperatively to assimilate, implement, maintain and update effective practices for managing chemical reactivity" (2).

While reactivity concerns are more apparent in situations where chemicals are intentionally mixed and processed, these same concerns also occur in other areas of chemical handling and use - where they are not always recognized. Since reactivity issues can be found throughout the chemical manufacturing and distribution process, the RMR addressed key parts of the chemical supply chain: reaction processing; mixing; and storage or warehousing.

This article focuses on reactivity issues in venues where chemicals are stored. In many cases, reactivity concerns may be adequately addressed by physical separation and/or isolation with the intent to reduce the potential for ignition or escalation of a hazardous situation due to proximity of incompatible materials. The methods set forth here assume that the storage facility is properly designed for the task (e.g., to National Fire Protection Association (NFPA) warehousing standards), and that no intentional processing of the materials is being done (e.g., there is no deliberate breaching of the original storage container). Although this guidance is targeted at companies whose primary business is chemical storage, the information could also be useful to businesses where the storage occurs as part of a manufacturing operation, which could include physical processing of materials (i.e., distillation, blending, drying, etc.) as well as intentional reactive-chemistry operations.

The RMR approach presented here is adapted from a method recommended in the U.K. Health and Safety Executive's (HSE) publication "Chemical Warehousing The Storage of Packaged Dangerous Substances" (3). It uses DOT/UN shipping classifications (both primary and secondary) as a means of determining the nature of a chemical's hazard(s) and to identify where potentially harmful interactions or self-reactivity hazards may exist. It controls reactivity hazards by separation and/or isolation of the chemical from other chemicals, as shown in Table 1.

This approach has two major advantages. First, it makes use of information that is universally available for chemicals in commerce, i.e., the DOT/UN shipping classification. The Material Safety Data Sheet (MSDS) or International Safety Card (ISC) for these materials will contain the shipping classification. second, it provides an easy-to-understand visual display of how materials should be separated.

However, it does have several disadvantages. The method does not evaluate chemical interactions explicitly, as morequantitative methods do. Known deficiencies in DOT/UN classifications are not corrected within the methodology. One example of such a deficiency is chlorine, which is well-known as an oxidizer but does not have a secondary classification as an oxidizer (5.1). Other materials whose shipping classification may not fully represent the potential reactivity hazard include ethylene oxide, styrene, tetrafluoroethylene and vinylidene chloride.

The quantity of material, either collectively or in individual shipping units, is not a factor in the analysis. In addition, incidents that can domino to involve much more than the original quantity were not considered.

Some codes, as well as local, state and federal regulatory requirements, may require additional separation. For example, NFPA 55 requires 20 ft of separation for quantities of flammable gas greater than 7,500 lb, and one German regulation requires 20 ft of separation in its jurisdiction. These differences may arise because the reactive chemical's hazard is assessed differently, or because one of the materials may also be a flammable hazard.

The RMR matrix

The heart of this method is the evaluation matrix shown in Figure 1, which was adapted from the HSE Warehousing Guide (3) and augmented with information from other sources, primarily NFPA guidance documents (4-8). Unlike the HSE guide, the matrix includes all primary categories of shipping classifications.

The evaluation matrix uses binary combinations of chemicals based on their shipping classifications. Rows and columns represent the two chemicals, and the cell in the matrix represents the separation category from Table 1 that applies. For each binary pair of chemicals, four combinations of shipping classifications are checked, primary-primary, primary-secondary, secoixiaiy-primary, and secondary-secondary. The highest separation category indicated for these interactions is used as the storage configuration. Putting the matrix into a spreadsheet or database format allows for automated comparison of all binary pairs of a given chemical list