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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

Type

Typical techniques

Minimize (intensification)

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

Substitute (substitution)

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.

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