Plant design hazards

For many years the usual procedure in plant design was to identify the hazards, by one of the systematic techniques described later or by waiting until an accident occurred, and then add on protec tive equipment to control future accidents or protect people from their consequences. This protective equipment is often complex and expensive and requires regular testing and maintenance. It often interferes with the smooth operation of the plant and is sometimes bypassed. Gradually the industry came to resize that, whenever possible, one should design user-friendly plants which can withstand human error and equipment failure without serious effects on safety (and output and emciency). When we handle flammable, explosive, toxic, or corrosive materials we can tolerate only very low failure rates, of people and equipment—rates which it may be impossible or impracticable to achieve consistently for long periods of time. 

Operation includes nonual start-up, normal and emergency shutdown, and most activities performed by die production team. Whilst inlierently safe plant design limits inventories of hazardous substances, inherently safe operation ensures die number of individuals at risk are minimized. Access to die plant for non-essendal operational people such as maintenance engineers, post staff, administrators, quality control samplers, warehouse staff delivering raw material or plant items or collecting finished product, members of security, visitors etc., must be controlled. 

The design and operation of a proeess plant form an integral part of safety and systematie proeedures and should be employed to identify hazards and operability and, where neeessary, should be quantified. During die design of a new plant, die hazard identifieation proeedure is repeated at intervals. This is first performed on the pilot plant before the full-seale version as the design progresses. Potential hazards whose signifieanee ean be assessed with the help of experiments are often revealed by this study. 

My book Plant Design for Safety—A User-Friendly Approach [1] and References 12-15 describe many examples of ways in which plants can be made inherently safer. Note that we use the term inherently safer, not inherently safe, as we cannot avoid every hazard. 

Palmer, K. N. 1960. The Quenching of Flames hy Perforated Sheeting and Block Flame Arresters. Proc. Symposium on Chemical Process Hazards with Special Reference to Plant Design, pp. 51-57. Institution of Chemical Engineers, Rughy, England. 

Provision for protection and safety equipment should be incorporated in the original plant design. The size of the plant, nature of the hazards, and the e.xposure will determine the amount, kind, and location of this equipment. 

A hazard and operability study (HAZOP) is a systematic approach to recognizing and identifying possible hazards that may cause failure of a piece of equipment in new or e. isting facilities. This qualitative enterprise is conducted by a team of technical c.Kpcrts in plant design and operation, plus other e.vperts, as required. 

The first step in minimizing accidents in a chemical phuit is to evaluate the facility for potential fires, explosions, and vulnerability to other liazards, particularly those of a chemical miture. This calls for a detailed study of plant site and layout, materials, processes, operations, equipment, and training, plus an effective loss prevention program. The technical nature of industry requires detailed data and a broad range of experience. Tliis complex task, today becoming the most important in plant design, is facilitated by the safety codes, standiu ds, and practice information available. The technical approach to evaluating die consequences of hazards is discussed later in tliis cliapter and in Part V (Chapters 20 and 21). 

M.J.G. Wilson, ProcSympChem Process Hazards, Spec Refs Plant Design, Engl, 25—9 (1960) CA 54,10035 (1960) 15) D.J. Rasbash ... 

To assess the potential hazard of a new plant, the index can be calculated after the Piping and Instrumentation and equipment layout diagrams have been prepared. In earlier versions of the guide the index was then used to determine what preventative and protection measures were needed, see Dow (1973). In the current version the preventative and protection measures, that have been incorporated in the plant design to reduce the hazard-are taken into account when assessing the potential loss in the form of loss control credit factors. 

The data on probabilities given in this example are for illustration only, and do not represent actual data for these components. Some quantitive data on the reliability of instruments and control systems is given by Lees (1976). Examples of the application of quantitive hazard analysis techniques in chemical plant design are given by Wells (1996) and Prugh (1980). Much of the work on the development of hazard analysis techniques, and the reliability of equipment, has been done in connection with the development of the nuclear energy programmes in the USA (USAEC, 1975) and the UK. 

A good acronym in chemical plant design is KISS—Keep It Simple, Stupid This also applies to hazards. Complicated designs are almost always more hazardous than simple ones. 

In most cases, data that are obtained through theoretical approaches (literature, data bases, software programs) may not be sufficient for final plant design. Experimental work is usually required on various scales depending on the extent of reactivity. Therefore, the application of well designed experimental test methods is of prime importance to define hazardous conditions. Numerous test methods are available using a variety of sample sizes and conditions. 

PHA focuses on the hazardous materials and major plant elements in the process plant to provide a cost-effective hazard identification [2, 3]. It is intended for use in the early design stage and it can be very useful in site selection. It also provides early guidance to plant designers in considerations for reducing or eliminating potential hazards. 

The concept of inherently safer plant has been with us now for many years. But in spite of its clear potential benefits related to safety, health and the environment (SHE), as well as the costs, there has been few applications in chemical plant design. But as Kletz (1996) has written there are hurdles to be overcome. Inherently safer design requires a basic change in approach. Instead of assuming e.g. that we can keep large quantities of hazardous materials under control we have to try and remove them. Changes in belief and the corresponding actions do not come easily. 

The essence of the inherently safer approach to plant design is the avoidance of hazards rather than their control by added-on protective equipment (Kletz 1998). It particularly emphasizes eliminating large inventories of hazardous materials where feasible. 

Bretherick, L. in Chemical Process Hazards with Special Reference to Plant Design-V, 1-15, Kneale, M. (Ed.), Symp. Ser. No. 39a, London, IChE, 1975... 

The complexity of the plant design, the degree of sophistication, and the quality requirements of the fine chemicals to be produced the necessity to process hazardous chemicals the sensitivity of product specifications to changes of reaction parameters and the availability of a skilled workforce all determine the degree of automation that is advisable. 

---