Inherently Safer Chemical Processes

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Zamir Khan

UW-11-Ch.E-BSc-54

Submitted by

Course Instructor

Engr. Kareem Bakhsh

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Contents

Introduction

A Way of Thinking

The Role of Inherently Safer Design Concepts in Process Risk Management

When Do We Consider Inherently Safer Options

Inherent Safety Trade offs

Resolving Inherent Safety Issues

Inherently Safer Design Strategies

CHAPTER 2

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Introduction

What is inherently safer "chemical” process?

• "Inherent“ has been defined as "existing in something as a permanent and in separable element, quality, or attribute“. According to (American College Dictionary,1967).

• A chemical manufacturing process is inherently safer if it reduces or eliminates the hazards associated with Materials and operations used in the process, and this reduction or elimination is permanent and in separable.

• Hazard is defined as a physical or chemical characteristic that has the potential for causing harm to peoples , the environment , or property (adapted from CCPS,1992).

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Some specific examples of hazards include:

• Chlorine is toxic by inhalation.

• Sulfuric acid is extremely corrosive to the skin.

• Ethylene is flammable.

• Steam confined in a drum at 600 psig contains a significant• Amount of potential energy(Pv energy).

• Acrylic acid can polymerize ,releasing large amounts of heat

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Growl and Louvar (1990 ) describe a three-step process which most accidents Occur:

• Initiation: the event that starts the accident

• Propagation: the events that maintain or expand the accident

• Termination: the events that stop the accident or diminish it in size

• Inherently safer strategies can impact the accident process at any of the three stages .The most effective strategies will prevent initiation of the accident. Inherently safer design can also reduce the potential for propagating an accident, or provide an early termination of the accident sequence before the reare major impacts on people; property, or the environment.

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The Role of Inherently Safer Design Concepts in Process Risk Management

• Risk is defined as a measure of economic loss ,human injury, or environmental damage in terms of both the incident likelihood and the magnitude of the loss ,injury, or damage.

• In general, the strategy for reducing risk, weather directed toward reducing frequency or consequence of potential accidents, can be classified in to four categories.

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These categories ; in decreasing order of reliability, are:

• Inherent— Eliminating the hazard by using materials and process conditions which are non hazardous; e.g. substituting water for a flammable solvent.

• Passive—Minimizing the hazard by process and equipment design features which reduce either the frequency or consequence of the hazard with out the active function in of any device; e.g the use of equipment rated for higher pressure.

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These categories ; in decreasing order of reliability, are:

• Active: Using controls, safety interlocks, and emergency shutdown system to detect and correct process deviations; e.g a pump that is shutoff by a high level switch in the down stream ank when the tank is 90% full. These systems are commonly referred to as engineering controls.

• Procedural: Using operating procedures, administrative checks, emergency response, and other management approach to prevent incidents, or to minimize the effects of an incident. e.g hot-work procedures and permits. These approaches are commonly refer red to as administrative controls.

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III. When Do We Consider Inherently Safer Options?

• It is never too late to consider inherently safer alternatives.

• Major enhancements to the inherent safety of plants which have been operating for many years have been reported .

• Table 2.2 is a summary of some of the key questions that should be asked at various stages in the development of a plant design, as suggested by the INSIDE Project, a major European government/

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When Do We Consider Inherently Safer Options?

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Inherently Safer Design Strategies

• Approaches to the design of inherently safer processes and plants have been grouped into four major strategies by IChemE and IPSG (1995) and Kletz (1984, 199Ib).

• Minimize: Use smaller quantities of hazardous substances (also called Intensification)

• Substitute: Replace a material with a less hazardous substance

• Moderate, Use less hazardous conditions, a less hazardous form of a material, or facilities which minimize the impact of a release of hazardous material or energy (also called

• Attenuation and Limitation of Effects).

• Simplify Design facilities which eliminate unnecessary complexity and make operating errors less likely, and which are forgiving of errors which are made (also called ErrorTolerance).

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Inherently Safer Design Strategies

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• Assign a weighting factor (1 to 10) to the various parameters based on your judgment of the relative importance of this Safety, Health and Environmental, or other issue.  For each option, assign a performance factor from 1 to 10 based on the relative perform-ance of that option with respect to the particular parameter. This can be based on judgment, or scaled based on some kind of quantitative analysis.  Multiply the weighting factor by the performance factor for each parameter and process option combination.  Sum the products for each process option.  The highest total is most desirable. NOTES: See Kepner and Tregoe (1981) or CCPS (1995a) for additional discussion, particularly on how potential negative consequences may impact the scoring matrix.  The weighting factors in this table are for purposes of illustrating the methodology only, and do not represent recommendations on the relative importance of the factors listed.

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References

• Inherently safer processes :a life cycle approach by Robert E. Bellinger.[etal.]: edited by DanielA.Crowl

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