Guidelines for Engineering Design for Process Safety

By CCPS (Center for Chemical Process Safety)

This updated version of one of the most popular and widely used CCPS books provides plant design engineers, facility operators, and safety professionals with key information on selected topics of interest. The book focuses on process safety issues in the design of chemical, petrochemical, and hydrocarbon processing facilities. It discusses how to select designs that can prevent or mitigate the release of flammable or toxic materials, which could lead to a fire, explosion, or environmental damage.

Key areas to be enhanced in the new edition include inherently safer design, specifically concepts for design of inherently safer unit operations and Safety Instrumented Systems and Layer of Protection Analysis. This book also provides an extensive bibliography to related publications and topic-specific information, as well as key information on failure modes and potential design solutions.

• Language: English
• Publisher: Wiley
• Release date: Nov 7, 2012
• ISBN: 9781118265468

Book preview

PREFACE

The Center for Chemical Process Safety (CCPS) was established in 1985 by the American Institute of Chemical Engineers (AIChE) for the express purpose of assisting the Chemical and Hydrocarbon Process Industries in avoiding or mitigating catastrophic chemical accidents. To achieve this goal, CCPS has focused its work on four areas:

Establishing and publishing the latest scientific and engineering practices (not standards) for prevention and mitigation of incidents involving toxic and / or reactive materials.

Encouraging the use of such information by dissemination through publications, seminars, symposia and continuing education programs for engineers.

Advancing the state-of-the-art in engineering practices and technical management through research in prevention and mitigation of catastrophic events.

Developing and encouraging the use of undergraduate education curricula which will improve the safety knowledge and consciousness of engineers.

This book, Guidelines for Engineering Design for Process Safety, Second Edition, is the result of multiple projects. The first project was the first edition of Guidelines for Engineering Design for Process Safety, which began in 1989 with volunteers from CCPS member companies working with engineers from the Stone & Webster Engineering Corporation. The intent was to produce a book that presented the process safety design issues needed to address all stages of the evolving design of a facility. The first edition discussed the impact that various engineering design choices have on the risk of a catastrophic accident, starting with the initial selection of the process and continuing through its final design.

The second project began in 1994 with volunteers from CCPS member companies working with Arthur D. Little Inc. to produce a book entitled Guidelines for Design Solutions for Process Equipment Failures. This book described the ways that major processing equipment could fail, causing a catastrophic accident. This second book identified available design solutions that might avoid or mitigate the failure in a series of options ranging from inherently safer / passive solutions to active and procedural solutions. By capturing industry experience in how major processing equipment can fail, this book provided a very useful tool for the selection of process safety systems. The inherently safer solutions that were suggested may, in some cases, have come as a surprise to the process and design engineer because they may have been the most cost-effective solution.

In 2009, both the Technical Steering Committee and the Planning Committee of CCPS recognized the need to consolidate these two works into one combined, expanded and updated volume. The result of this effort is the book you now hold in your hand.

Guidelines for Engineering Design for Process Safety, 2nd Edition, has been updated to provide design guidance and comprehensive references for process equipment in a number of different categories, including vessels, reactors, heat and mass transfer equipment, fluid transfer and separation equipment, fired equipment, dryers, and piping. Chapter 6 contains updated equipment failure tables from the Design Solutions book.

This book focuses on engineering design to reduce risk due to process hazards. It does not focus on operations, maintenance, transportation, or personnel safety issues, although improved process safety can benefit each area. Detailed engineering designs are outside the scope of this book, but the authors have provided an extensive guide to references and other literature to assist the designer who wishes to go beyond safety design philosophy to the specifics of a particular safety system design.

CHAPTER 1

INTRODUCTION

The Center for Chemical Process Safety (CCPS) has published a number of guidelines that focus on the evaluation and mitigation of risks associated with catastrophic events in process facilities. Originally published in 1993, the purpose of Guidelines for Engineering Design for Process Safety was to shift the emphasis on process safety to the earliest stage of the design where process safety issues could be addressed at the lowest cost and with the greatest effect. Almost 20 years later, this 2nd edition of Guidelines for Engineering Design for Process Safety continues to stress the importance of emphasizing process safety during Front-End Engineering and Design (FEED) to achieve the greatest risk reduction at the lowest cost – and also emphasizes the benefits of diligence to process safety design issues through the life of the facility. This updated book also incorporates material from Guidelines for Design Solutions for Process Equipment Failures, which was originally published by CCPS in 1998 (Ref. 1–1).

This book focuses on process safety issues in the design of chemical, petrochemical, and hydrocarbon processing facilities. Enough information is provided on each topic to ensure that the reader understands:

The concept and issues

The design approach for process safety

Areas of concern

Where to go for detailed information

The scope of this book includes avoidance and mitigation of catastrophic events that could impact people and facilities in the plant or surrounding area. The scope is limited to selecting appropriate designs to prevent or mitigate the release of flammable or toxic materials that could lead to a fire, explosion, and impact to personnel and the community. Process safety issues affecting operations and maintenance are limited to cases where design choices impact system reliability. These Guidelines are intended to be applicable to the design of a new facility, as well as modification of an existing facility.

The scope excludes:

Transportation safety

Routine environmental control Personnel safety and industrial hygiene practices

Emergency response

Detailed design

Operations and maintenance

Security issues unrelated to process safety

These Guidelines highlight safety issues in design choices. For example, Section 7.1.1, Electrical Area Classification, covers the safe application of electrical apparatus in the process environment required for plant safety but does not address detailed design of the electrical supply or distribution system required to operate the plant.

It is clear that choices made early in design can reduce both the potential for large releases of hazardous materials and the severity of such releases, if they should occur.

1.1 ENGINEERING DESIGN FOR PROCESS SAFETY THROUGH THE LIFE CYCLE OF THE FACILITY

Engineering design for process safety must be an integral part of the life cycle of a facility. Process safety has been defined in previous publications as:

A discipline that focuses on the prevention and mitigation of fires, explosions, and accidental chemical releases at process facilities. Excludes classic worker health and safety issues involving working surfaces, ladders, protective equipment, etc. (Ref. 1–2).

Hazard evaluations are one method used to identify, evaluate, and control hazards involved in chemical processes. Hazards can be defined as characteristics of systems, processes, or plants that must be controlled to prevent occurrence of specific undesired events. Hazard evaluation is a technique that is applied repeatedly throughout the design, construction, and operation phases of a facility (Figure 1.1). Engineering design for process safety should be considered within the framework of a comprehensive process safety management program as described in Plant Guidelines for Technical Management of Chemical Process Safety (Ref. 1–3).

Figure 1.1 Identifying Hazards Through the Facility Life Cycle

Hazard evaluation is synonymous with process hazard analysis and process safety review. From conceptual design to decommissioning, no single method of hazard evaluation applies to all of the stages of a project. Different methods are required for different stages of a project, such as research and development, conceptual design, startup and operation. Table 1.1 presents some of the stages of facility life cycle and typical corresponding process hazard evaluation objectives. An objective shown for one stage may be applicable to another.

Table 1.1 Typical Hazard Evaluation Objectives at Different Stages of a Facility Life Cycle

As illustrated in Table 1.1, different types of hazards can be identified during the stages of a facility’s life cycle. Findings from the Baker Panel report (Ref. 1–4) associated with the 2005 Texas City Refinery Explosion illustrate the importance of engineering design for process safety:

Not all refining hazards are caused by the same factors or involve the same degree of potential damage. Personal or occupational safety hazards give rise to incidents—such as slips, falls, and vehicle accidents—that primarily affect one individual worker for each occurrence. Process safety hazards can give rise to major accidents involving the release of potentially dangerous materials, the release of energy (such as fires and explosions), or both. Process safety incidents can have catastrophic effects and can result in multiple injuries and fatalities, as well as substantial economic, property, and environmental damage. Process safety refinery incidents can affect workers inside the refinery and members of the public who reside nearby. Process safety in a refinery involves the prevention of leaks, spills, equipment malfunctions, over-pressures, excessive temperatures, corrosion, metal fatigue, and other similar conditions. Process safety programs focus on the design and engineering of facilities, hazard assessments, management of change, inspection, testing, and maintenance of equipment, effective alarms, effective process control, procedures, training of personnel, and human factors. The Texas City tragedy in March 2005 was a process safety accident. (Ref. 1–4).

1.2 REGULATORY REVIEW / IMPACT ON PROCESS SAFETY

The ideas presented here are not intended to replace regulations, codes, or technical and trade society standards and recommended practices. Specifically, implementation of these guidelines requires the application of sound engineering judgment because the concepts may not be applicable in all cases.

Identifying and addressing relevant process safety standards, codes, regulations, and laws over the life of a process is one of the five elements in the Risk Based Process Safety pillar of committing to process safety (Ref. 1–5). Companies should establish a process for maintaining adherence to applicable standards, codes, regulations, and laws. Guidelines for Risk Based Process Safety (Ref. 1–5) recommends establishing a standards system to achieve this objective, including:

Establishing a system to identify, develop, acquire, evaluate, disseminate, and provide access to applicable standards, codes, regulations, and laws that affect process safety

Promoting consistent interpretation, implementation, and efficiency in the initial identification of and ongoing monitoring of changes in standards

Safe operation and maintenance of facilities that manufacture, store, or otherwise use hazardous chemicals require robust process safety management systems. The primary objective of establishing a standards system is to ensure that a facility remains in conformance with applicable standards, codes, regulations, and laws, including voluntary ones adopted by the company over the life of the facility. Long-term conformance to such standards of care helps ensure that the facility is operated in a safe and legal fashion. Key principles and essential features of maintaining a dependable standards system include:

Ensuring consistent implementation of the standards system

Identifying when standards compliance is needed

Involving competent personnel

Ensuring that standards compliance practices remain effective

The Baker Panel also emphasizes the importance of implementation of external good engineering practices and a corporate safety management system that supports and improves process safety importance (Ref. 1–4).

For detailed information on establishing a system to comply with standards, readers are referred to Chapter 4, Compliance with Standards, of Guidelines for Risk Based Process Safety (Ref. 1–5).

Table 1.2 provides some examples of the types of process safety standards, codes, and regulations that many facilities comply with.

Table 1.2 Examples and Sources of Process Safety Related Standards, Codes, Regulations, and Laws

It is important to note that regional or local laws and regulations often mandate more stringent requirements than similar federal regulations. For example, the State of California’s Accidental Release Prevention Program requires compliance by facilities with over a threshold quantity of 100 lb of chlorine, while the U.S. EPA Risk Management Program’s threshold quantity for compliance is 2,500 lb of chlorine. Different global, federal, and regional requirements pose challenges to facilities that operate in different geographic locations.

1.3 WHO WILL BENEFIT FROM THESE GUIDELINES?

Process safety is an important part of risk management and loss prevention. Although these Guidelines do not provide all the answers, they do highlight the process safety issues that must be addressed in all stages of design. These Guidelines will benefit many different people within an organization:

Corporate Leadership - Senior executives define the basis for the development of design philosophies. Their commitment and recognition of the value of integrating process safety at all levels of the design process is essential.

Project Managers - Project Managers are responsible for executing projects, usually from design through startup and commissioning. A Project Manager is responsible for determining the basic protection design concepts to apply in the execution of a project. The Project Manager is responsible for implementing the decisions and abiding by the process safety systems associated with the design.

Engineers - Engineers are responsible for specifying and designing process units and protection systems that meet their company’s requirements. This still leaves room for making decisions when designing process units and protection systems.

HSE Professionals - Health, Safety, and Environmental (HSE) Professionals provide technical guidance to engineers and typically are in an assurance role for process safety systems.

1.4 ORGANIZATION OF THIS BOOK

Figure 1.2 provides an overview of the contents of these Guidelines and also provides examples of how each chapter can assist in integrating process safety throughout the life of the process. Each chapter has been updated to include state-of-the-art information, industry experience, and references to other CCPS publications.

Figure 1.2 Overview of Guideline Contents

Specific references and applicable industry standards are listed at the end of each chapter. It is not the intent of this book to make specific design recommendations, but to provide a good source of references where the interested reader can obtain more detailed information.

1.5 OTHER CCPS RESOURCES

Other CCPS Guidelines provide additional resources for topics discussed in these Guidelines. Some of these include:

Continuous Monitoring for Hazardous Material Releases

Deflagration and Detonation Flame Arresters

Guideline for Mechanical Integrity Systems

Guidelines for Analyzing and Managing the Security Vulnerabilities of Fixed Chemical Sites

Guidelines for Chemical Process Quantitative Risk Analysis, Second Edition

Guidelines for Chemical Reactivity Evaluation and Application to Process Design

Guidelines for Developing Quantitative Safety Risk Criteria

Guidelines for Facility Siting and Layout

Guidelines for Fire Protection in the Chemical, Petrochemical and Hydrocarbon Processing Industries

Guidelines for Hazard Evaluation Procedures, Third Edition

Guidelines for Integrating Process Safety Management, Environment, Safety, Health and Quality

Guidelines for Pressure Relief and Effluent Handling Systems

Guidelines for Preventing Human Error in Process Safety

Guidelines for Process Safety Documentation

Guidelines for Process Safety in Batch Reaction Systems

Guidelines for Risk Based Process Safety

Guidelines for Safe and Reliable Instrumented Protective Systems

Guidelines for Safe Handling of Powders and Bulk Solids

Guidelines for Safe Storage and Handling of Reactive Materials

Inherently Safer Chemical Processes a Life Cycle Approach, Second Edition

Plant Guidelines for Technical Management of Chemical Process Safety

Safe Operation of Process Vents and Emission Control Systems

Additional information on these publications can be found at www.aiche.org/ccps/.

1.6 REFERENCES

1-1. CCPS. Guidelines for Design Solutions for Process Equipment Failures. Center for Chemical Process Safety of the American Institute of Chemical Engineers. New York, NY. 1998.

1-2. CCPS. Guidelines for Investigating Chemical Process Incidents, Second Edition. Center for Chemical Process Safety of the American Institute of Chemical Engineers. New York, NY. 2003.

1-3. CCPS. Plant Guidelines for Technical Management of Chemical Process Safety. Center for Chemical Process Safety of the American Institute of Chemical Engineers. New York, NY. 1992.

1-4. Baker, et al. The Report of the BP U.S. Refineries Independent Safety Review Panel. January 2007.

1-5. CCPS. Guidelines for Risk Based Process Safety. Center for Chemical Process Safety of the American Institute of Chemical Engineers. New York, NY. 2007.

1-6. American Chemistry Council, 1300 Wilson Blvd., Arlington, VA 22209. www.americanchemistry.com

1-7. European Chemical Industry Council (Cefic), Avenue E. van Nieuwenhuyse, 4 box 1, B-1160 Brussels. www.cefic.org

1-8. American Petroleum Institute, 1220 L Street, NW, Washington, D.C. 20005. www.api.org

1-9. American National Standards Institute, 25 West 43rd Street, New York, NY, 10036. www.ansi.org

1-10. American Society of Mechanical Engineers, Three Park Avenue, New York, NY, 10016. www.asme.org

1-11. The Instrumentation, Systems, and Automation Society, 67 Alexander Drive, Research Triangle Park, NC 27709. www.isa.org

1-12. National Fire Protection Association, 1 Batterymarch Park, Quincy, MA, 023169. www.nfpa.org

1-13. Process Safety Management of Highly Hazardous Chemicals (29 CFR 1910.119), U.S. Occupational Safety and Health Administration, May 1992. www.osha.gov

1-14. Flammable and Combustible Liquids, Occupational Safety and Health Standards (29 CFR 1910.106), U.S. Occupational Safety and Health Administration. www.osha.gov

1-15. PSM Covered Chemical Facilities National Emphasis Program, OSHA Notice, 09-06 (CPL 02), U.S. Occupational Safety and Health Administration, July 2009. www.osha.gov

1-16. Petroleum Refinery Process Safety Management National Emphasis Program, OSHA Notice, CPL 03-00-010, U.S. Occupational Safety and Health Administration, August 2009. www.osha.gov

1-17. Accidental Release Prevention Requirements: Risk Management Programs Under Clean Air Act Section 112(r)(7), 40 CFR Part 68, U.S. Environmental Protection Agency, June 20, 1996 Fed. Reg. Vol. 61[31667–31730]. www.epa.gov

1-18. California Accidental Release Prevention (CalARP) Program, CCR Title 19, Division 2, Office of Emergency Services, Chapter 4.5, June 28, 2004. www.oes.ca.gov

1-19. Contra Costa County Industrial Safety Ordinance. www.co.contra-costa.ca.us

1-20. Extremely Hazardous Substances Risk Management Act, Regulation 1201, Accidental Release Prevention Regulation, Delaware Department of Natural Resources and Environmental Control, March 11, 2006. www.dnrec.delaware.gov

1-21. Chemical Accident Prevention Program (CAPP), Nevada Division of Environmental Protection, NRS 459.380, February 15, 2005. http://ndep.nv.gov/bapc/capp/capp.html

1-22. Toxic Catastrophe Prevention Act (TCPA), New Jersey Department of Environmental Protection Bureau of Chemical Release Information and Prevention, N.J.A.C. 7:31 Consolidated Rule Document, April 17, 2006. www.nj.gov/dep

1-23. Australian National Standard for the Control of Major Hazard Facilities, NOHSC: 1014, 2002. www.docep.wa.gov.au/

1-24. Environmental Emergency Regulations (SOR / 2003-307), Environment Canada. www.ec.gc.ca/CEPARegistry/regulations

1-25. Control of Major-Accident Hazards Involving Dangerous Substances, European Directive Seveso II (96 / 82 / EC). http://ec.europa.eu/environment/seveso/legislation.htm

1-26. Korean Occupational Safety and Health Agency, Industrial Safety and Health Act, Article 20, Preparation of Safety and Health Management Regulations, Korean Ministry of Environment, Framework Plan on Hazards Chemicals Management, 2001–2005. http://english.kosha.or.kr/main

1-27. Malaysia, Department of Occupational Safety and Health (DOSH) Ministry of Human Resources Malaysia, Section 16 of Act 514. http://www.dosh.gov.mv/doshV2/

1-28. Control of Major Accident Hazards Regulations (COMAH), United Kingdom Health & Safety Executive, 1999 and 2005. www.hse.gov.uk/comah/

CHAPTER 2

FOUNDATIONAL CONCEPTS

Understanding basic, foundational concepts is essential in establishing a system that identifies hazards and manages risk. To be effective, this system must continuously loop-back and question What can go wrong? at all stages in a facility’s life cycle. Identifying the hazards associated with the facility and providing engineering measures to prevent or mitigate the consequences are the basic principles of engineering design for process safety. Most effective when it is performed during conceptual and detailed design, this process also provides substantial value through construction, startup, operation, and decommissioning.

This chapter, Foundational Concepts, provides an overview of understanding hazards and risk-based design. Table 2.1 identifies the topics found in this chapter, where the reader can find more information on the topic in this book, and finally where detailed information may be found in other sources.

Table 2.1 Foundational Concepts and Detailed Resources

2.1 UNDERSTANDING THE HAZARD

2.1.1 Dangerous Properties of Process Materials

Safe handling of materials in both process and storage begins with understanding their physical and chemical properties. This concept applies to all chemical substances used by or formed in a process, including reactants, intermediates, products, catalysts, solvents, adsorbents, etc. Some important material characteristics are listed in Table 2.2 and discussed in the following pages.

Table 2.2 Typical Material Characteristics

2.1.1.1 General Properties

Information describing the general properties of most chemical substances is usually found on the Material Safety Data Sheets (MSDSs) which are provided by manufacturers. Information is also available in handbooks, such as the CRC Handbook of Chemistry and Physics (Ref. 2-12) or Perry’s Chemical Engineers’ Handbook (Ref. 2-13). The Design Institute for Physical Property Data (DIPPR®) has developed critically evaluated thermophysical property data for pure components and mixtures (Ref. 2-14) that is periodically updated.

Boiling point and freezing point data establish whether a substance is a solid, liquid, or gas at atmospheric pressure. Comparison of boiling points or volatilities relative to process conditions provides insight into a number of potentially significant issues, such as flammability or ease of separation by distillation. Vapor pressure data are more difficult to obtain but are more useful in predicting volatility-related behavior. Freezing point data reveal that some relatively common substances may require special handling for cold weather.

Molecular weight provides a quick comparison of gas densities, which indicate whether a vapor released to the atmosphere will rise and disperse or travel along the ground. Critical pressure and temperature are needed for developing thermodynamic expressions using the laws of corresponding states. Since vapors cannot be compressed into liquids at temperatures above their critical regions, substances that can exist only as vapor are indicated by critical temperatures below ambient or processing temperature.

Fluid density and viscosity determine the difficulty of transporting substances inside piping. This information is also useful in other transportation-related issues, such as overloading tank trailers with high density liquids and design of relief systems. In the event of spills, density and solubility relative to water are important issues. Electrical conductivity often indicates the degree to which static charges might build in flowing systems. Enthalpy or specific heat data predict temperature rises for heated substances, critical information when vessels containing volatile flammable liquids are subjected to fire. Heat-of-mixing data indicates pronounced thermal effects that might occur when mixing substances, such as two different concentrations of sulfuric acid.

2.1.1.2 Reactivity

The reactivity of a chemical substance not only influences process reactions, it also influences the hazard potential in accidental releases or inadvertent mixtures. Exothermic reactions can pose hazards because the heat evolved raises the temperature of the reactants leading to increased reaction rate or vaporization of materials. When high temperature is reached in an open system, the materials may ignite or explode. In a closed system, high temperature can lead to vessel rupture from overpressurization caused by gas evolution or vapor pressure.

Some materials react violently upon contact with water, generating considerable heat. For example, some strong acids may evolve large amounts of hazardous fumes when contacted with water or moisture in the air. It is important to recognize this aspect when preparing fire fighting contingencies.

Pyrophoric substances react violently with air, resulting in spontaneous ignition. Such substances are typically handled by methods that prevent contact with air, often by submerging the substance in a compatible solvent, water or oil.

Other chemicals react violently with oxidizing or reducing agents. Oxidants may generate heat, oxygen, and flammable or toxic gases. Reducing agents react with a variety of chemicals and may generate hydrogen, as well as heat, and flammable or toxic gases. Storage and usage of strong oxidizing and reducing agents require special precautions that are unique to the particular substance in question. Generally, each supplier provides complete packages of safety-related information to its customers.

Some chemicals polymerize or decompose at elevated temperature or if contaminated by polymerization initiators or catalysts. Common substances, such as water, rust, or other contaminants, can initiate polymerization reactions. When polymerization is initiated, exothermic reaction may occur leading to high temperature and pressure, possibly resulting in explosion or release of flammable or toxic substances. Such decomposition and polymerization reactions may be prevented by incorporating safety systems, inhibitors, and safe operating procedures.

Because chemical reactivity is extremely complex, hazardous materials should be examined on a specific case-by-case basis. Chemical reactivity data are available in:

Handbook of Reactive Chemical Hazards (Ref. 2-15)

EPA’s Chemical Compatibility Chart (Ref. 2-16)

Guidelines on Chemical Reactivity Evaluation and Applications to Process Design (Ref. 2-5)

Sax’s Dangerous Properties of Industrial Materials (Ref. 2-17)

Chemical Reactivity Worksheet (Ref. 2-18)

Fire Protection Handbook (Ref. 2-19)

CCPS Reactivity Evaluation Screening Tool (Ref. 2-20)

2.1.1.3 Flammability

Another important material characteristic requiring attention in early stages of process design is flammability. The most common measures of flammability potential for materials are:

Autoignition temperature

Conductivity

Fire point

Flammable limits

Flash point

KSt

Minimum / limiting oxygen concentration

These are discussed further in Chapter 3, Basic Physical Properties / Thermal Stability Data.

2.1.1.4 Toxicity

Toxic releases generally have a greater impact on humans than fire or explosion; therefore, recognizing the toxicity of materials is important in process design. Humans can be exposed to toxics by inhalation, ingestion, and dermal contact. Toxic exposure is influenced by the airborne concentration and the duration of exposure. Toxic exposures are described as:

Acute - Acute exposures represent brief contacts with potentially lethal concentrations, typically experienced during sudden large discharges of toxic materials.

Chronic - Chronic exposures occur due to prolonged exposure, usually over a period of time.

Various sources of recognized exposure limits for airborne contaminants are presented in Table 2.3. These sources can be used to determine exposure limits under a variety of circumstances. The Subcommittee on Consequence Assessment and Protective Actions (SCAPA) of the Department of Energy also maintains a hierarchal listing of chemicals’ Protective Action Criteria (PAC) in the order priority of AEGL, ERPG, then TEEL, whichever has been defined (Ref. 2-21).

Table 2.3 Selected Primary Data Sources for Toxic Exposure Limits

2.1.1.5 Effect of Impurities

Impurities in process streams may jeopardize desired reactions and possibly pose threats to plant safety. These impurities may be traces of compounds typically present in raw materials (e.g., pyrophoric iron sulfides in petroleum or catalyst poisoning agents). Sometimes impurities are the same substance in a different physical form, such as solids in a liquid stream or liquid slugs in a gas stream.

Effects of impurities should be critically analyzed before beginning process design. Engineering solutions that prevent impurities from entering the process include filters and strainers, adsorbent beds (one-time and regenerative), and guard beds.

2.1.2 Process Conditions

Process conditions, such as pressure and temperature, have their own characteristic problems and hazards. High pressures and temperatures create stresses that must be accommodated by design. Extreme temperatures or pressures individually are usually not the problem, but rather their combination. A combination of extreme conditions results in increased plant cost due to the need for material with high mechanical strength and corrosion resistance.

High pressure increases the amount of potential energy available in a process facility. For these facilities, in addition to the energy of compressed gases and of fluids kept under pressure in the liquid state, there may also be a concern of chemical reactivity under pressure or an adverse reaction from rapid depressurization. Leakage is much more pronounced in high pressure operations. Because of the large pressure difference, the amount of fluid that can discharge through a given area is greater. A high pressure difference has a considerable impact on the consequences of a release, as the hazard zone extends to a larger area.

High temperature also poses material failure problems, most frequently due to metal creep. The use of high temperature conditions usually increases plant cost, not only due to materials of construction but also due to the requirement for special supports to handle the stresses generated. Process design should take these stresses into account. The design should minimize such stresses, especially during startup and shutdown.

High temperatures are often obtained with the use of fired heaters, which have additional hazards like tube rupture and explosions. It is a good idea to consider using steam heaters, where possible, instead of fired heaters to prevent such hazards.

Low pressure operation usually does not pose much of a hazard in comparison with other operating conditions. However, in the case of vacuum applications where flammable materials are present, the potential for ingress of air does create a hazardous situation. This can result in the formation of a flammable mixture inside equipment leading to fire and / or explosion. It is essential that this aspect is reviewed and adequate measures provided in the process design to prevent air ingress. For equipment not designed for full vacuum, damage frequently occurs because of failure to vent while draining or steaming out, allowing heated equipment to cool while blocked-in, or failure of a vacuum relief device due to pluggage.

Low temperature engineering design considerations include:

Build-up of ice on equipment and drain systems

Low temperature caused by J-T effect (e.g., natural gas pressure reducing stations)

Low temperature embrittlement or loss of elasticity due to inadvertent flow of low temperature fluids into systems constructed of materials not fit for low temperature services

Low temperature in flare header application (e.g., LPG)

Possibility of failure of refrigerant or coolant systems which are normally provided to maintain low temperature

Thermal stresses (contraction and expansion)

Chapter 5, General Design, and Chapter 6, Equipment Design, contain details on design solutions.

2.1.3 Inventory

A common factor in major disasters in the chemical industry is a large release of a hazardous material. One of the best ways to make a plant safer is to minimize the quantity of hazardous materials. The principal approach is to minimize inventory, so that even if there is a leak or explosion, the consequences are minimized (Ref. 2-8).

Low inventories result in safer and more cost-effective process facilities. Lower inventories can be achieved by using smaller or fewer vessels. If fewer vessels are used, fewer protective devices, such as alarms, valves, trips, and smaller flare systems, may be required, further reducing facility costs.

Other methods to limit inventory include:

Reducing reactor volumes by improving mixing conditions or better understanding reaction kinetics

Reducing inventory by integrating plant operation, especially for storage tanks and day tanks that usually contain large inventories

Using continuous reactors instead of batch reactors

Reducing holdup in distillation columns by using low holdup equipment internals, e.g., packing has less holdup than conventional trays

Reducing onsite storage by using just-in-time delivery

Laying out equipment and pipe to reduce pipe rack toxic material holdup

Improving the performance of the reactor (reducing by-product production) so that subsequent operations, e.g., distillation, become easier, further reducing holdup

Making highly toxic material generation (e.g., phosgene) a subprocess just prior to using the material in the main process, shifting inventory to less toxic materials

Producing on-demand from less hazardous materials

Substituting a less hazardous material or limiting the inventory of hazardous materials is usually the first choice in risk reduction. For example, consider using steam as heat transfer medium instead of a flammable material. If reduction of the inventory or substitution of hazardous materials is not feasible, attempts should be made to use less hazardous conditions, such as low pressure and temperature storage; use of material in its gas phase instead of its liquid phase; or use of a safer solvent.

Some secondary effects of reducing inventories may need to be considered, such as:

A reduction in residence time could result in poor separation of materials

Increased potential for cavitation of pumps

Less time for operator response to a low level alarm

2.2 RISK-BASED DESIGN

Process or equipment design often involves deciding between alternative designs with differing process efficiency, safety, environmental controls, cost, and schedule implications. To accomplish this, the formation of a multidisciplinary design team is required at the beginning of a project in order to obtain total integration of process safety with process design and environmental protection considerations. Sometimes safety considerations clearly dominate and decisions are made in the form of special design approaches (e.g., design of facilities manufacturing or using nitromethane, ethylene oxide, hydrogen fluoride, phosgene, etc.). In some instances, codes and standards exist that either mandate or suggest design approaches to known high risks.

In a majority of situations, however, no single factor dominates. In the process of arriving at a design basis decision, the risks of each option are typically dealt with judgmentally or qualitatively (Ref. 2-22). In some instances, one component of risk is quantified (i.e., either consequence or frequency) to justify the design selection. For large projects, full risk quantification is sometimes used to assess the combined impacts of multiple hazards.

Risk-based design begins at the earliest stages of a project. After the general configuration of the process has been established and the design is defined in terms of heat and material balances and basic process controls, the process design can be evaluated for quality, safety, health and environmental impact. The design team begins brainstorming how the process can deviate from normal conditions (i.e., failure scenarios) by asking questions, such as:

What can go wrong? What failure scenarios can we realistically expect with this process?

What impact can those failure scenarios have?

How frequently might they occur?

What is the risk?

Is this risk acceptable?

What design features can be put in place to minimize the risk?

If posed at the conceptual stage of a process design, these questions offer great opportunity for the application of inherently safer design solutions. While inherently safer solutions should emerge as recurring themes throughout the design process, the earlier the application of inherently safer solutions, the more cost-effective and easier to implement these solutions will be.

It is important to recognize that, irrespective of the specific approaches and the level of effort, engineers and technical managers are already directly or indirectly factoring risk into the selection of design options. The process used to assess risk should be systematic and comprehensive.

A systematic technique can provide a consistent risk management framework for process safety system design basis decisions. Inconsistencies in approach can develop not only between different processes and facilities, but also in the case of large, complex design projects, and different design engineers may follow different risk management philosophies.

Consistency with respect to risk acceptance decisions is necessary to assure all stakeholders (e.g., owners, employees, customers, and the general public) that risks are being properly managed. In some countries, governments are also explicit stakeholders in the effort to reduce the risk of chemical industry accidents, providing such regulations as:

Australia, Australian National Standard for the Control of Major Hazard Facilities, NOHSC: 1014, 2002. (Ref. 2-23)

Korea, Korean Occupational Safety and Health Agency, Industrial Safety and Health Act, Article 20, Preparation of Safety and Health Management Regulations. (Ref. 2-24)

Malaysia, Department of Occupational Safety and Health (DOSH), Ministry of Human Resources Malaysia, Section 16 of Act 514. (Ref. 2-25)

United Kingdom, Control of Major Accident Hazards Regulations (COMAH), United Kingdom Health & Safety Executive. (Ref. 2-26)

United States, Environmental Protection Agency, Accidental Release Prevention Requirements: Risk Management Programs Under Clean Air Act. (Ref. 2-27)

United States, Occupational Safety and Health Administration, Process Safety Management of Highly Hazardous Chemicals. (Ref. 2-28)

Consequently, having a consistent, documented technique for the selection and design of process safety systems is not only prudent management; in many countries it is a regulatory requirement.

However, systematic does not necessarily imply quantitative. In many simple design situations, qualitative approaches will satisfy the requirements of the technique for selecting process safety system design bases. More complex design cases may occasionally require rigorous quantitative risk analysis approaches. But even in these complex cases, quantitative approaches should only be employed to the degree required to make an informed decision. This concept of the selective use of quantitative risk analysis has been incorporated into the technique presented later in the chapter and in Chapter 4, Analysis Techniques.

2.2.1 The Concept of Risk

The design basis selection technique for process safety systems described later in this chapter is a risk-based technique. Risk is defined as a measure of loss in terms of both the incident likelihood and the magnitude of the loss (Ref. 2-7).

This concept of risk combines an undesirable outcome, i.e., a consequence such as safety impact or financial loss, with the likelihood of