1-7 Inherent Safety
An inherently safe plant11, 12 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.
In general, the safety of a process relies on multiple layers of protection. The first layer of protection is the process design features. Subsequent layers include control systems, interlocks, safety shutdown systems, protective systems, alarms, and emergency response plans. Inherent safety is a part of all layers of protection; however, it is especially directed toward process design features. The best approach to prevent accidents is to add process design features to prevent hazardous situations. An inherently safer plant is more tolerant of operator errors and abnormal conditions.
Although a process or plant can be modified to increase inherent safety at any time in its life cycle, the potential for major improvements is the greatest at the earliest stages of process development. At these early stages process engineers and chemists have the maximum degree of freedom in the plant and process specifications, and they are free to consider basic process alternatives, such as changes to the fundamental chemistry and technology.
The following four words are recommended to describe inherent safety:
- Minimize (intensification)
- Substitute (substitution)
- Moderate (attenuation and limitation of effects)
- Simplify (simplification and error tolerance).
The types of inherent safety techniques that are used in the chemical industry are illustrated in Table 1-9 and are described more fully in what follows.
Table 1-9. Inherent Safety Techniques
Change from large batch reactor to a smaller continuous reactor
Reduce storage inventory of raw materials
Improve control to reduce inventory of hazardous intermediate chemicals
Reduce process hold-up
Use mechanical pump seals vs. packing
Use welded pipe vs. flanged
Use solvents that are less toxic
Use mechanical gauges vs. mercury
Use chemicals with higher flash points, boiling points, and other less hazardous properties
Use water as a heat transfer fluid instead of hot oil
Moderate (attenuation and limitation of effects)
Use vacuum to reduce boiling point
Reduce process temperatures and pressures
Refrigerate storage vessels
Dissolve hazardous material in safe solvent
Operate at conditions where reactor runaway is not possible
Place control rooms away from operations
Separate pump rooms from other rooms
Acoustically insulate noisy lines and equipment
Barricade control rooms and tanks
Simplify (simplification and error tolerance)
Keep piping systems neat and visually easy to follow
Design control panels that are easy to comprehend
Design plants for easy and safe maintenance
Pick equipment that requires less maintenance
Pick equipment with low failure rates
Add fire- and explosion-resistant barricades
Separate systems and controls into blocks that are easy to comprehend and understand
Label pipes for easy "walking the line"
Label vessels and controls to enhance understanding
Minimizing entails reducing the hazards by using smaller quantities of hazardous substances in the reactors, distillation columns, storage vessels, and pipelines. When possible, hazardous materials should be produced and consumed in situ. This minimizes the storage and transportation of hazardous raw materials and intermediates.
Vapor released from spills can be minimized by designing dikes so that flammable and toxic materials will not accumulate around leaking tanks. Smaller tanks also reduce the hazards of a release.
While minimization possibilities are being investigated, substitutions should also be considered as an alternative or companion concept; that is, safer materials should be used in place of hazardous ones. This can be accomplished by using alternative chemistry that allows the use of less hazardous materials or less severe processing conditions. When possible, toxic or flammable solvents should be replaced with less hazardous solvents (for example, water-based paints and adhesives and aqueous or dry flowable formulations for agricultural chemicals).
Another alternative to substitution is moderation, that is, using a hazardous material under less hazardous conditions. Less hazardous conditions or less hazardous forms of a material include (1) diluting to a lower vapor pressure to reduce the release concentration, (2) refrigerating to lower the vapor pressure, (3) handling larger particle size solids to minimize dust, and (4) processing under less severe temperature or pressure conditions.
Containment buildings are sometimes used to moderate the impact of a spill of an especially toxic material. When containment is used, special precautions are included to ensure worker protection, such as remote controls, continuous monitoring, and restricted access.
Simpler plants are friendlier than complex plants because they provide fewer opportunities for error and because they contain less equipment that can cause problems. Often, the reason for complexity in a plant is the need to add equipment and automation to control the hazards. Simplification reduces the opportunities for errors and misoperation. For example, (1) piping systems can be designed to minimize leaks or failures, (2) transfer systems can be designed to minimize the potential for leaks, (3) process steps and units can be separated to prevent the domino effect, (4) fail-safe valves can be added, (5) equipment and controls can be placed in a logical order, and (6) the status of the process can be made visible and clear at all times.
The design of an inherently safe and simple piping system includes minimizing the use of sight glasses, flexible connectors, and bellows, using welded pipes for flammable and toxic chemicals and avoiding the use of threaded pipe, using spiral wound gaskets and flexible graphitetype gaskets that are less prone to catastrophic failures, and using proper support of lines to minimize stress and subsequent failures.