Inherently safe plants

Operation includes normal start-up, normal and emergency shutdown, and most activities performed by the production team. Whilst inherently safe plant design limits inventories of hazardous substances, inherently safe operation ensures the number of individuals at risk are minimized. Access to the plant for non-essential operational people such as maintenance engineers, post staff, administrators, quality control samplers, warehouse staff delivering raw material or plant items or collecting finished product, members of security, visitors etc., must be controlled. 

Hendershot, D. C. 1991. Design of inherently safer process facilities. Texas Chemical Council Safety Seminar, Session D, Inherently Safe Plant Design, pp. 2-22. 

VI. The design of inherently safe plants is feasible, running such a plant in a safe way is not. 

There is no doubt that the ultimate development of process intensification leads to the novel field of microreaction technology (Figure 1) (7-9). Because of the small characteristic dimensions of microreaction devices, mass and heat transfer processes can be strongly enhanced, and, consequently, initial and boundary conditions as well as residence times can be precisely adjusted for optimizing yield and selectivity. Microreaction devices are evidently superior, due to their short response time, which simplifies the control of operation. In connection with the extremely small material holdup, nearly inherently safe plant concepts can be realized. Moreover, microreaction technology offers access to advanced approaches in plant design, like the concept of numbering-up instead of scale-up and, in particular, the possibility to utilize novel process routes not accessible with macroscopic devices. 

An inherently safe plant is safe by its nature and by the way it is constituted. 

Under this act the designer is charged with building an inherently safe plant. He is charged with building it in accordance with the best safety standards available. No plant should be designed that requires employees to wear earmuffs or ear plugs or requires that temporary barriers should be erected for safety purposes when safety can be achieved by some other means. 

UMIST (1982) Inherently safe plant . Proceedings of Safety in the Chemical Industry 1982, University of Manchester Institute of Science and Technology. 

The MARS (Multipurpose Advanced Reactor, inherently Safe) plant is a nuclear plant designed for electric power generation and/or for industrial heat generation. 

Inherently safer plant uses basic design measures to achieve hazard elimination, prevention and reduction. The classic definition of an inherently safe plant or activity is one that cannot under any circumstances cause harm to people or the environment. This may be because ... 

The term, inherent means belonging by nature, or the essential character of something. An inherently safe plant is safe by its nature and by the way it is constituted. 

Snyder, P. G. (1996). Inherently Safe(r) Plant Design. Process Plant Safety Symposium, Volume 1, April 1-2,1996, Houston, TX, ed. H. Cullingford, 203-215. Houston, TX South Texas Section of the American Institute of Chemical Engineers. 

The view is therefore growing that we should try to design plants so that they are safe even if there is a fault in the software. This can be done by adding on independent safety systems, such as relief valves and hardwired trips and interlocks, or by designing inherently safer plants that remove the hazards instead of controlling them (see Chapter 21). 

My book Plant Design for Safety—A User-Friendly Approach [1] and References 12-15 describe many examples of ways in which plants can be made inherently safer. Note that we use the term inherently safer, not inherently safe, as we cannot avoid every hazard. 

T. Kletz, Process Plants A Handbook for Inherently Safe Design , Taylor Francis, London, 1998. 

Fig. 5.4-66 outlines the probability and consequences of a thermal runaway in case of a plant incident. For the solvent process, failure results in a temperature rise from 27 °C to 119 °C. This is far from the onset temperature of secondary processes, which only start at 150 °C or higher. Consequently, the solvent process can be considered safe. A failure of the water process can cause a temperature rise from 50 to 95 C, i.e. higher than the onset temperature (90 °C) of the secondary decomposition of the di-nitro compound. The decomposition would start before the reaction mixture started boiling. Hence, the water process cannot be considered inherently safe. 

Almost all the papers and committees have overlooked the fact that there may be a better and cheaper solution to the problem. If we could design our plants so that they use safer raw materials and intermediates, or not so much of the hazardous ones, or use the hazardous ones at lower temperatures and pressures, then we would avoid rather than solve a lot of our problems. Such plants can be described as intrinsically or inherently safe, while the conventional plant, in which the hazards are kept under control, is extrinsically safe."... 

Preventive and protective measures. Technical safety measures must always be developed no matter how inherently safe we think the process is. The scope of preventive and protection measures is large for the whole plant. Below, some preventive measures to be taken to prevent thermal runaways when a dangerous situation is recognized are listed ... 

The design of inherently safe process plant is discussed by Kletz in a booklet published by the Institution of Chemical Engineers, Kletz (1984) and Keltz and Cheaper (1998). He makes the telling point that what you do not have cannot leak out so cannot catch fire, explode or poison anyone. Which is a plea to keep the inventory of dangerous material to the absolute minimum required for the operation of the process. 

An inherently safe plant1112 relies on chemistry and physics to prevent accidents rather than on control systems, interlocks, redundancy, and special operating procedures to prevent accidents. Inherently safer plants are tolerant of errors and are often the most cost effective. A process that does not require complex safety interlocks and elaborate procedures is simpler, easier to operate, and more reliable. Smaller equipment, operated at less severe temperatures and pressures, has lower capital and operating costs. 

The global rates of heat generation and gas evolution must be known quite accurately for inherently safe design.. These rates depend on reaction kinetics, which are functions of variables such as temperature, reactant concentrations, reaction order, addition rates, catalyst concentrations, and mass transfer. The kinetics are often determined at different scales, e.g., during product development in laboratory tests in combination with chemical analysis or during pilot plant trials. These tests provide relevant information regarding requirements... 

Kletz, T. A. Make Plants Inherently Safe Hydrocarbon Processing Magazine. Houston, TX Gulf Publishing Company, September 1985. 

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