Inherent safety hazards

The liberated azoimide attacks copper or its alloys (brass) and copper azides form on the metal surface. The concurrent use of LA and copper (or brass) in fuses or blasting caps therefore represents an inherent safety hazard. Although the LA-copper (copper alloys) reaction has been known since 1913, it has been a cause of many incidents decades later [49]. These incidents have been sometimes erroneously reported as spontaneous explosions. The reality, however, is that, in all known cases, they have been linked to some type of movement and therefore should not be considered spontaneous [70]. 

A more recent concept which could have significant impact on future designs is that of inherent safety (12). This basic principle states that what is not there cannot be blown up or leak into the environment. Thus, the idea is to avoid the hazard in the first place. 

Minimize inventory to the extent feasible. Expected benefits from minimum inventory may be offset by hazards resulting from more frequent and smaller shipments. The relative hazards should be reviewed (Englund, Design and Operate Plants for Inherent Safety—Part 1, Chem. Eng. Prog., vol. 87, no. 2, March 1991, pp. 85-91). 

Inherent safety Inventory reduction Fewer chemicals inventoried or fewer in process vessels. Chemical substitution Substitute a less hazardous chemical for one more hazardous. Fr ocess attenuation Use lower temperatures and pressures. 

Understanding the chemistry of the process also provides the greatest opportunity in applying the principles of inherent safety at the chemical synthesis stage. Process chemistry greatly determines the potential impact of the processing facility on people and the environment. It also determines such important safety variables as inventory, ancillary unit operations, by-product disposal, etc. Creative design and selection of process chemistry can result in the use of inherently safer chemicals, a reduction in the inventories of hazardous chemicals and/or a minimization of waste treatment requirements. 

Hazards can be reduced or eliminated by changing the materials, chemistry, and process variables such that the reduced hazard is characteristic of the new conditions. The process with reduced hazards is described as inherently safer. This terminology recognizes there is no chemical process that is without risk, but all chemical processes can be made safer by applying inherently safer concepts. This book occasionally uses the term inherent safety this does not mean absolute safety. 

In many cases, the inherent safety advantages of one process are clear when compared with alternatives. One or more hazards may be significantly reduced, while others are unaffected or only marginally increased. For example, aqueous latex paints are clearly inherently safer than solvent based paints, although there are applications where the increased performance of solvent based paints justifies their use, with the appropriate layers of protection. 

An inherently safer process offers greater safety potential, often at a lower cost. However, selection of an inherently safer technology does not guarantee that the actual implementation of that technology will result in a safer operation than an alternate process which is inherently safer. The traditional strategy of providing layers of protection for an inherently more hazardous process can be quite effective, although the expenditure of resources to install and maintain the layers of protection... 

Deciding among a number of process options having inherent safety advantages and disadvantages with respect to different hazards can be quite difficult. The first step is to understand thoroughly all hazards associated with the process options. Process hazard analysis and evaluation techniques are appropriate tools (CCPS, 1992). These include ... 

The combination of several unit operations into a single piece of equipment can eliminate equipment and simplify a process. There may be inherent safety conflicts resulting from this strategy (see Section 2.4). Combining a number of process operations into a single device increases the complexity of that device, but it also reduces the number of vessels or other pieces of equipment required for the process. Careful evaluation of the options with respect to all hazards is necessary to select the inherently safer overall option. 

Similarly, hazardous raw material storage should also be minimized, with greater attention being given to just in time supply. Inventory reduction lowers inventory costs, while increasing inherent safety. In determining appropriate raw material inventories, the entire raw material supply chain must be considered. Will the supplying plant have to increase inventories to provide just in time service, and will... 

Basic process chemistry using less hazardous materials and chemical reactions offers the greatest potential for improving inherent safety in the chemical industry. Alternate chemistry may use less hazardous raw material or intermediates, reduced inventories of hazardous materials, or less severe processing conditions. Identification of catalysts to enhance reaction selectivity or to allow desired reactions to be carried out at a lower temperature or pressure is often a key to development of inherently safer chemical synthesis routes. Some specific examples of innovations in process chemistry which result in inherently safer processes include ... 

The chemistry of side reactions and by-products may also offer opportunities for increasing the inherent safety of a process. For example, a process involving a caustic hydrolysis step uses ethylene dichloride (EDC 1,2-dichloroethane) as a solvent. Under the reaction conditions a side reaction between sodium hydroxide and EDC produces small but hazardous quantities of vinyl chloride ... 

Innovative chemical synthesis procedures have been proposed as offering potential for economical and environmentally friendly routes to a variety of chemicals. These novel chemical reactions also offer potential for increasing the inherent safety of processes by eliminating hazardous materials, eliminating chemical intermediates, or allowing... 

Synthetic rubber latex was made by a process with a large and hazardous inventory of butadiene and styrene. In a modified process, the reactor has an initial charge of water and emulsifier. Also, the monomers are added to the reactor as one premixed stream and the emulsified aqueous sodium persulfate is added as the other stream. The improved scheme, discussed by Englund (1991a) contains less hazardous material and at a lower, more controllable temperature. It illustrates that large and established processes may be made safer by applying inherent safety. 

Bayer (Pilz, 1995) uses a procedure based on hazard analysis, focusing on the application of inherent safety principles to reduce or eliminate hazards. 

Dow (Sheffler, 1996 Gowland, 1996a,b) describes the use of the Dow Fire and Explosion Index (Dow, 1994b) and the Dow Chemical Exposure Index (Dow, 1994a) as measures of inherent safety, along with the use of inherently safer design principles to reduce hazards. 

The first major objective for the inherent safety review is the development of a good understanding of the hazards involved in the process. Early understanding of these hazards provides time for the development team to implement recommendations of the inherent safety effort. Hazards associated with flammability, pressure, and temperature are relatively easy to identify. Reactive chemistry hazards are not. They are frequently difficult to identify and understand in the lab and pilot plant. Special calorimetry equipment and expertise are often necessary to fully characterize the hazards of runaway reactions and decompositions. Similarly, industrial hygiene and toxicology expertise is desirable to help define and understand health hazards associated with the chemicals employed. 

Reducing and eliminating hazards and their associated risks is the second major objective. Applying inherent safety principles early in the product/process development effort provides the greatest opportunity to achieve the objectives of the inherent safety review process for the project at hand. If these principles are applied late in the effort the results may have to be applied to the project after next as the schedule may not permit implementation of the results. 

During process hazards reviews (such as HAZOP), inherent safety concepts are also considered. Mistake proofing the design should receive attention and each safety critical device (last line of defense) and safety critical procedure should be examined to see if there is a way to eliminate the need for the device or procedure. 

Experience has shown that reactive chemistry hazards are sometimes undetected during bench scale and pilot plant development of new products and processes. Reactive chemistry hazards must be identified so they can be addressed in the inherent safety review process. Chemists should be encouraged and trained to explore reactive chemistry of "off-normal operations. Simple reactive chemicals screening tools, such as the interactions matrix described in Section 4.2, can be used by R D chemists. 

Companies may wish to develop workshops to train potential team members in the inherent safety review process. The workshop can provide background information on inherent safety concepts, the extensive systems required to manage hazardous materials, and information on the inherent safety review process. Videos, problems, examples, and team exercises can be included to enliven the education process. 

Implementing an inherent safety review process is one mechanism companies can use to institutionalize inherent safety. The review process should integrate well with company systems for process safety management, new product development, and project execution. Safety, health, and environmental considerations in the new product or process development effort can be strengthened via the introduction of the inherent safety review. Companies may also build inherently safer design concepts into their existing process safety management system and process hazard reviews. 

The following checklist contains a number of questions which can aid in identifying inherently safer process options. The list is adapted from CCPS (1993a). Other checklists, particularly the extensive checklist in Appendix B of the Guidelines for Hazard Evaluation Procedures, 2nd Edition with Worked Examjzles (CCPS, 1992) contain many questions which are related to inherent safety. 

Does the aetivity inherently expose workers to hazardous substanees, or to health and safety hazards from a hazardous waste operation ... 

The easiest means for assessing occupational exposure hazards associated with materials used in a process is through the use of Permissible or Occupational Exposure Limits (OEL or PEL) which go by a variety of names for example, TLV (U.S. - American Conference of Government Industrial Hygienists), MAK (Germany), or individual company established values. Occupational exposure limits are usually set based on a combination of the inherent toxicological hazard of a chemical and a series of safety factors such as intraspecies variability in test results, nature and severity of the effect, adequacy and quality of... 

The approach to developing metrics for process safety is analogous to those that might be used to assess Occupational Exposure risk. One can cite as well several indices that have been developed as metrics for estimating and ranking the safety of a given process or chemical reaction, such as the DOW fire and explosion index,the Stoessel index ° for hazard assessment and classification of chemical reactions, the Inherent Safety Index, the Prototype Index for Inherent Safety, amongst others. ... 

The worst hazard scenarios (excessive temperature and pressure rise accompanied by emission of toxic substances) must be worked out based upon calorimetric measurements (e.g. means to reduce hazards by using the inherent safety concept or Differential Scanning Calorimetry, DSC) and protection measures must be considered. If handling hazardous materials is considered too risky, procedures for generation of the hazardous reactants in situ in the reactor might be developed. Micro-reactor technology could also be an option. Completeness of the data on flammability, explosivity, (auto)ignition, static electricity, safe levels of exposure, environmental protection, transportation, etc. must be checked. Incompatibility of materials to be treated in a plant must be determined. 

Inherent safety concept. This route requires the deepest understanding of the chemical process, and, accordingly, of the potential hazards. It is not easy, even at a high degree of understanding, to eliminate mistakes in assessment. However, this approach is strongly supported by many specialists in problems of safety in the chemical industry. According to Kletz(1983) ... 

Most of the materials used in the manufacture of chemicals are poisonous, to some extent. The potential hazard will depend on the inherent toxicity of the material and the frequency and duration of any exposure. It is usual to distinguish between the short-term effects (acute) and the long-term effects (chronic). A highly toxic material that causes immediate injury, such as phosgene or chlorine, would be classified as a safety hazard. Whereas a material whose effect was only apparent after long exposure at low concentrations, for instance, carcinogenic materials, such as vinyl chloride, would be classified as industrial... 

Here, safety and health considerations will be restricted to features of inherent safety that can be included as the design is developing, rather than the detailed hazard and operability studies that take place in the later stages of design when the design is nearing completion. 

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