Fundamentals of Process Safety Engineering

Samarendra Kumar Biswas, Umesh Mathur, Swapan Kumar Hazra

• Publisher: CRC Press
• Year: 2021
• Print ISBN: 9780367620769
• eBook ISBN: 9781000424584
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemical & Biochemical Engineering

This textbook covers the essential aspects of process safety engineering in a practical and comprehensive manner.

It provides readers with an understanding of process safety hazards in the refining and petrochemical industries and how to manage them in a reliable and professional manner. It covers the most important concepts: static electricity, intensity of thermal radiation, thermodynamics of fluid phase equilibria, boiling liquid expanding vapor explosion (BLEVE), emission source models, hazard identification methods, risk control and methods for achieving manufacturing excellence while also focusing on safety. Extensive case studies are included.

Aimed at senior undergraduate and graduate chemical engineering students and practicing engineers, this book covers process safety principles and engineering practice authoritatively, with comprehensive examples:

• Fundamentals, methods, and procedures for the industrial practice of process safety engineering.
• The thermodynamic fundamentals and computational methods for release rates from ruptures in pipelines, vessels, and relief valves.
• Fundamentals of static electricity hazards and their mitigation.
• Quantitative assessment of fires and explosions.
• Principles of dispersion calculations for toxic or flammable gases and vapors.
• Methods of qualitative and quantitative risk assessment and control.

1 Hazards in the Process Industries

DOI: 10.1201/9781003107873-1

The term “process industries” is used in this book to mean those industries that transform raw materials into intermediates or end products by physical or chemical means. Some examples are gas and oil exploration and production, petroleum refineries, petrochemicals, plastics, fiber, heavy and fine chemicals, fertilizer units, ferrous and nonferrous metal production units, agricultural production units, etc. The process industries engage in manufacturing, storage, handling, and transportation (bulk or packaged) of chemical compounds and mixtures. Although not strictly considered as process industries, many service or utility industries such as power plants or water treatment plants that use hazardous chemicals also need to implement process safety engineering and management procedures.

The term “hazard” refers to a material or a condition that has the potential to cause harm to human life, health, property, the environment, or some combination of these. Some common examples of hazards in our homes are LPG/natural gas (cooking/heating fuel), hot water, electric power, slippery floors, and weak roofs. In any process industry, hazards are classified as “process hazard” and “occupational hazard”.

Process hazards are caused by the release (or potential release) of materials or energy caused by processing activities that might lead to fire, explosion, or toxic exposure. Often, catastrophic consequences may result, such as personnel injuries or deaths, with severe environmental and financial impacts. Management failures in process safety result in process hazards, generally have a low probability of occurrence, and have high consequential impacts.

Occupational hazards generally refer to workplace incidents affecting a single individual or a small group of workers. These are caused by unsafe working practices, failure to use adequate personal protective equipment (PPE), or other human failures. Examples include falls while working at heights (no safety belt), head injury (no helmet), asphyxiation upon vessel entry (no breathing apparatus), electrocution (failure to observe lockout, tag-out procedures), hearing losses at high noise levels (no hearing protection), and so forth. Occupational hazard-related incidents generally have a high probability of occurrence and relatively low consequential impacts.

Process hazard analysis is a thorough, orderly, and systematic discipline for identifying and evaluating the dangers inherent in the processing of highly hazardous materials.

Process safety engineering (PSE) refers to a set of interrelated methods and procedures for managing hazards associated with the process industries. Its goal is to minimize, if not eliminate, the frequency and severity of incidents resulting from releases of chemicals and other energy sources (US Occupational Safety and Health Administration – OSHA, 1993).

Risk assessment is an essential discipline in process safety engineering and management and is addressed in some detail in Chapter 13. An early introduction to the term is necessary owing to its extreme importance. Risk is a function of the probability of a hazard being realized multiplied by the potential consequences. Risk is expressed in terms of an expected frequency or probability. The higher the frequency, the more likely is the event. The frequency of occurrence is the inverse of the probability of occurrence. Risk may also be defined as the ratio of potential consequence to frequency.

The process safety hazards commonly encountered in process plants are listed in Table 1.1.

TABLE 1.1
List of Common Process Plant Hazards

Chemical hazards	Flammable chemicals Explosive chemicals Reactive chemicals Toxic chemicals
Physical hazards	Physical explosion Electrostatic charges Rollover of liquids Boilover of liquids
Environmental hazards	Air pollution Water pollution Hazardous wastes
Other hazards	Static electricity-induced ignition Faulty electrical system hazards

1.1 Chemical Hazards

1.1.1 Flammable Chemicals

A flammable chemical is a gas or a vaporized liquid which, when ignited in the presence of oxygen, continues to react with oxygen, giving rise to a flame that emits heat and light. The formal definition of a flammable substance is one having a flash point below 93°C.

A flame is the visible portion of the volume within which the oxidation of fuels occurs in gaseous form. Thus, a flame is a phenomenon that occurs in the gaseous phase only. A flammable liquid must first vaporize; the vapors so generated can undergo combustion in a flame. Chemical decomposition or pyrolysis on solid surfaces yields flammable, volatile products that can sustain combustion.

In the combustion of solids, if pyrolysis does not yield volatile products at a rate higher than some minimum value required for sustaining the flame, the combustion proceeds at the surface of the solid, producing smoke but no flame. Such non-flaming combustion is known as smoldering.

Flammable gases or vapors burn in air only over a limited range of composition. Below the lower flammability limit (LFL), the mixture is too lean to ignite and propagate the flame. Similarly, above the upper flammability limit (UFL), the mixture is too rich to ignite and propagate the flame. The concentrations between these two limits constitute the flammability range. Further, by lowering the oxygen concentration (LOC) in air by adding an inert gas, the UFL can be brought down, as explained in Chapter 3.

For flammable liquids, the flash point (discussed in detail in Section 3.1.2) is a crucial property determining how easily ignition would occur. At the flash point temperature, the liquid’s vapor pressure is sufficient to yield a vapor concentration in the air that corresponds to the LFL. Therefore, regulatory bodies use the flash point as an essential parameter for classifying the hazard category of flammable chemicals, using terms such as:

1. Flammable gases
2. Extremely flammable liquids
3. Very highly flammable liquids
4. Highly flammable liquids
5. Flammable liquids.

1.1.2 Explosive Chemicals

Upon initiation by shock, impact, friction, fire, chemical reaction, or other ignition sources, an explosive material releases energy at a very high rate, causing a sudden increase in atmospheric pressure in the surroundings and, typically, a flash or a loud noise. An explosive may be a pure substance (such as nitroglycerine, trinitrotoluene (TNT), or pentaerythritol tetranitrate (PETN)) or a “preparation” (such as dynamite that contains nitroglycerine, nitrocellulose, and ammonium nitrate as essential ingredients). Such formulations are known as “condensed explosives” and are used for military purposes or as commercial blasting explosives for mining and demolition work. They are generally safer to handle than their main ingredients.

Condensed explosives fall into two categories: “high explosives” such as dynamite or TNT are known as “detonating explosives” whose decomposition reactions proceed very rapidly. On the other hand, in black powder (a mixture of potassium nitrate, sulfur, and charcoal), the decomposition reaction occurs relatively slowly, simulating rapid burning or combustion. Such explosives are known as “deflagrating explosives”. The terms “detonation” and “deflagration” are explained in greater detail in Chapter 9.

The manufacture, storage, and handling of condensed explosives are governed by strict safety norms stipulated in the appropriate laws and regulations in various countries. People working in such industries are generally well trained and have a high safety awareness level compared to those in other process industries. Despite this, accidents do occur in explosives plants owing to the inherent properties and instability of explosives.

However, unintended and accidental explosions also occur in the process industries and isolated storage facilities of flammable liquids and liquefied gases. Some accidents have resulted in large numbers of casualties. Explosions in this second category are described as follows:

• Confined explosions in reaction vessels caused by uncontrolled chemical reactions
• Confined explosions in systems involving reactive materials, such as ammonium nitrate, chlor...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Foreword
• Preface
• Acknowledgments
• List of Figures
• List of Tables
• Acronyms and Abbreviations
• Authors
• Chapter 1 Hazards in the Process Industries
• Chapter 2 Overview of Some Major Accidents in the World
• Chapter 3 Fundamentals of Fire Processes
• Chapter 4 Static Electricity
• Chapter 5 Pool Fire
• Chapter 6 Jet Fire
• Chapter 7 Vapor Cloud Fire
• Chapter 8 Fireball
• Chapter 9 Explosion
• Chapter 10 Toxic Releases
• Chapter 11 Dispersion of Gases and Vapors
• Chapter 12 Hazard Identification
• Chapter 13 Risk Assessment and Control
• Chapter 14 Human Factors in Process Safety
• Chapter 15 Process Safety and Manufacturing Excellence
• Index