Guidelines for Engineering Design for Process Safety 2nd Edition

Book Preface

Engineers like to think of their discipline as a rigorous application of scientific and mathematical principles to the problem of creating a useful object. To a certain extent, this is an appropriate description of the tools of engineering – those techniques that we use to translate a concept in the mind of the designer into a physical object. But, where does that mental image of the object to be built come from? At its heart, engineering is intuitive, and an art form. The engineer / designer’s accumulated experience, and that of others, is applied to a defined problem. By intuitive and creative problem solving processes, the engineer develops and refines a conceptual design, and uses the mathematical and scientific tools of engineering to translate a mental concept into reality. The selection of the design basis for a process safety system is a problem like any other engineering problem. There is no equation or formula, no scientific principle, which will define the “best” design. Yes, there are scientific and mathematical tools which will help convert a design concept into something which can actually be constructed. But there is no general answer to the question “What is the best design?” Each system must be considered on its own, with a thorough evaluation of all of the details of its environment and required functions, to determine what the optimal design will be.

The number of potential solutions to any engineering problem is large, as anybody who has ever visited an automobile show quickly realizes. Sometimes, for a specific problem, there will be some solutions which clearly meet the overall objectives of nearly all stakeholders better than others. In these situations it is easy to select an optimum design. However, in other cases, different stakeholders have significantly different objectives, or will differ significantly in the relative importance of the different objectives of the design. This is one of the reasons why there are so many different kinds of cars at the automobile show, giving each potential purchaser a chance to find a design that best meets his or her objectives. But this is not possible in the design of a process plant – there is one plant which impacts many stakeholders with their different objectives and priorities. How can we best find the optimal solution? While this is not entirely a technical question, but also includes social and political aspects, I believe that the critical first step is to consider a large number of potential solutions. This increases the likelihood that the solution most acceptable to as many stakeholders as possible will be among those identified. Where do we get those potential solutions? One important source is accumulated experience our own, and that of others who have faced similar problems in the past. This book collects much of that accumulated experience from a large number of experts in the chemical process industry. Use of the tables which make up the heart of this book will allow the reader to take advantage of many years of practical experience. By considering a large number of potential solutions to the problem of specifying the design basis for safety systems, the design engineer is more likely to be able to identify the solution, or combination of solutions, which best meets most people’s needs.

This book, a combination, update, and expansion of two earlier CCPS Guideline publications, emphasizes a risk-based approach to the evaluation of safety system design. Potential safety systems suggested are categorized as inherently safer / passive, active, and procedural, in decreasing order of robustness and reliability. Inherently safer approaches are often preferred, but there can be no general answer to the question of which approach or specific solution is best for a particular situation. Instead, the design engineer must take a very broad and holistic approach to the complete design, accounting for the many different, and often competing, objectives which the design must accomplish. Safety, health effects, environmental impact, loss prevention, economic and business factors, product quality, technical feasibility, and many other factors must be considered. This book challenges the engineer to adopt a risk-based approach to evaluating many competing goals when deciding among a number of potential design alternatives.

This book can be extremely useful in conducting process hazard analysis studies. The failure mode tables in Chapter 6 can be the basis for hazard identification checklists and also offer a variety of potential solutions for identified concerns. However, the book will be even more beneficial if used by the individual engineer at the earliest stages of the design process, before any formal hazard reviews. The message of this book can be summarized very briefly:

• Consider a large number of design options Identify opportunities for inherent and passive safety features early
• Fully understand all of the hazards and resulting risks associated with design alternatives
• Use a risk-based approach to process safety systems specification

I hope that this book will find a home on the desk (not gathering dust on the bookshelf!) of every chemical process designer, particularly those involved in the earliest phases of conceptual design where the basic chemistry and unit operations are defined. It should be consulted frequently in the course of the designer’s day-to-day work in specifying and designing process facilities. If you are a process safety professional, make sure that all of the process design engineers in your organization read and use this book. It will make your job a lot easier!

Dennis C. Hendershot
CCPS Staff Consultant