INTRODUCTION TO INHERENT SAFETY
An inherently safer design is one that avoids hazards instead of controlling them, particularly by Removing or reducing the amount of hazardous material in the plant or the Number of hazardous operations. The best way of dealing with a hazard is to remove it completely.\
As Lees (1996 ) has said the aim should be to design the process and plant so that they are inherently safer. Inherent safety was first widely expressed in the late I970's by Trevor KJetz. The basic principles are common sense and include avoiding the use of hazardous materials, minimising the inventories of hazardous materials, and aiming for simpler processes with more benign and moderate process alternatives.
THE PRINCIPLES OF INHERENT SAFETY
Kletz (1984. 1991) has given Basic Principles of Inherent Safety as follows:
1. Intensification
"What you don't have, can't leak." Small inventories of hazardous materials reduce the consequences of leaks. Inventories can often be reduced in almost all unit operations as well as storage. This also brings reductions in cost, while less material needs smaller vessels, structures and foundations.
2. Substitution
If intensification is not possible, an alternative is substitution. It may be possible to replace flammable refrigerants and heat transfer with non-flammable ones, hazardous products with safer ones, and processes that use hazardous raw materials or intermediates with processes that do not. Using a safer material in place of a hazardous one decreases the need for protective equipment and thus decreases plant cost and complexity.
3. Attenuation
If intensification and substitution are not possible or practicable, an alternative is attenuation. This means carrying out a hazardous reaction under less hazardous conditions, or storing or transporting a hazardous material in a less hazardous form. Attenuation is sometimes the reverse of intensification, because less extreme reaction conditions may lead to a longer residence time.
4. Limitation of Effects
If it is not possible to make plants safer by intensification, substitution, or attenuation, the effects of a failure should be limited. For instance, equipment is designed so that it can leak only at a low rate that is easy to stop or control. For example, gaskets should be chosen to minimize leak rates. Also limitation of effects should be done by equipment design or change in reaction conditions rather than by adding on protective equipment.
5. Simplification
Simpler plants are inherently safer than complex plants, because they provide fewer opportunities for error and contain less equipment that can go wrong. Simpler plants are usually also cheaper and more user friendly.
6. Change Early
Change Early means identification of hazards as early as possible in the process design. The payback for early hazard identification can make or break the capital budget of a new' process. This can be achieved by dedicated safety evaluation methodologies which are designed for preliminary process design purposes.
Avoiding Knock-on Effects
Safer plants are designed so that those incidents, which do occur, do not produce knock-on or domino effects. For example safer plants are provided with fire breaks between sections to restrict the spread of fire, or if flammable materials are handled, the plant is built out-of-door so that leaks can be dispersed by natural ventilation.
7. Making Status Clear
Equipment should be chosen so. that it can be easily seen, wheather it has been installed correctly or wheather it is in the open or shut position. This refers to ergonomics of the plant. Also clear explanation of the chemistry involved in the process helps operating personnel to identify possible hazards.
8. Making Incorrect Assembly Impossible
Safe plants are designed so that incorrect assembly is difficult or impossible. Assembled components must meet their design requirements. A loss of containment may result from using eg. a wrong type of gaskets.
9. Tolerance
Equipment should tolerate maloperation. poor installation or maintenance without failure. E.g. expansion loops in pipework are more tolerant to poor installation than bellows. The construction materials should be resistant to corrosion and physical conditions. For most applications metal is safer than glass or plastic.
10. Ease of Control
A process should be controlled by the use of physical principles rather than added-on control equipment (i.e. the dynamics of the process should be favourable ). If a process is difficult to control, one should look for ways of changing the process or the principles of control before an investment in complex control system is made.
11. Administrative Controls Procedures
Human error is the most frequent cause of the loss of containment. Training and certification of personnel on critical procedures are permanent considerations. Also some other inherent safety principles, like ease of control, making status clear, tolerance and making incorrect assemply impossible, come into play here. An inherent safety design should contain the mentioned principles. They should also cover the whole design process. In the early stages of process design these principles help to choose the safest materials, process conditions and even process technology. The difficulty at the moment is the lack of rutines tounplement these inherent safety principles into reality.