Guidelines for Engineering Design for Process Safety P2 docx

Chapter 7. Heat Transfer Fluid Systems Chapter 8. Thermal Insulation Chapter 9. Process Monitoring and Control Chapter 10. Documentation The second half of the book deals with catastrophe avoidance through understanding and controlling chemical processing hazards. The order of the chapters in this section is first) understanding hazards, second) passive catastrophe prevention systems, and third) active protection systems. Chapter 11. Sources of Ignition Chapter 12. Electrical Hazards Chapter 13. Deflagration and Detonation Flame Arresters Chapter 14. Pressure Relief Systems Chapter 15. Effluent Disposal Systems Chapter 16. Fire Protection Chapter 17. Explosion Protection During the development of these Guidelines, it became clear to the authors that many interrelationships exist. It maybe difficult to address a safety issue in one system without affecting several other systems. The difficulty of fixing one problem without creating a problem in another system is frequently encountered. This overlap is also encountered from the perspective of hazard reduction: a single concept can often be applied to several systems. Because of these complexities, it is most effective to build safety into the initial design rather than adding it on. Specific references and applicable industry standards arc listed at the end of each chapter. Additional sources of information arc listed under Suggested Reading. It is not the intent of this book to make specific design recommenda- tions but to provide a good source of references where the interested rcader can obtain more detailed information. Nomenclature and units arc given after each equation (or set of equations); tables and figures adapted from other sources will use the units as originally published. A List of Acronyms and a Glossary arc provided. The readings listed at the end of Chapter 1 arc good general sources of information on chemical process safety. They arc recommended for use in combination with the CCPS Guidelines books. 1.5 REFERENCES 1.5.1 Regulations, Codes of Practice, and Industry Standards The editions that were in effect when these Guidelines were written are indicated below. Because standards and codes are subject to revision, users are encouraged to apply only the most recent edition. API (American Petroleum Institute) RP 750.1990. Management of Process Hazards. 1st ed. American Petroleum Institute, Washington, D. C. 29 CFR 1910.119. Process Safety Management of Highly Hazardous Chemicals. Oc- cupational Safety and Health Administration (OSHA). 1.5.2 Specific References CCPS (Center for Chemical Process Safety). 1992. Plant Guidelines for Technical Manage- ment of Chemical Process Safety, American Institute of Chemical Engineers, New York. ISBN 0-8169-0499-5. 1.5.3 Suggested Reading Carson, R. A. and C. J. Mumford. 1988. The Safe Handling of Chemicals in Industry. 2 Volumes, Longman Scientific & Technical (John Wiley & Sons, Inc.), New York. Journal of Loss Prevention in the Process Industries. Butterworth-Heinemann. London. King, R. 1990. Safety in the Process Industries. Butterworth-Heinemann, London and Stoneham, MA. King, R., and J. Magid. 1979. Industrial Hazard and Safety Handbook. Newnes-Butter- worths, London. Lees, F. P. 1980. Loss Prevention in the Process Industries. 2 Volumes. Butterworths, London. Loss Prevention Symposium Series. Papers presented at the Annual AIChE Loss Preven- tion Symposia. American Institute of Chemical Engineers (AIChE), New York. Process Safety Progress (formerly Plant/Operations Progress). T. A. Ventrone, ed., Quarterly publication of American Institute of Chemical Engineers (AIChE), New York. Responsible Care, Process Safety Code of Management Practices. 1990. Chemical Manufac- turers Association (CMA), Washington, D. C. 2 INHERENTLY SAFER PLANTS 2.1 INTRODUCTION In a 1988 report " Survey of Chemical Engineering Research: Frontiers and Opportunities/' the National Research Council identified inherently safer plant designs as a critical element for the continuing improvement of the good safety record of the chemical and petrochemical industries. The report par- ticularly recognizes the importance of process selection on safety, stating that "few basic decisions affect the hazard potential of a plant more than the initial choice of technology" ("Design" 1988). An inherently safer plant relies on chemistry and physics—the quantity, properties and conditions of use of the process materials—to prevent injuries, environmental damage and property damage rather than on control systems, interlocks, alarms and procedures to stop incipient incidents. In the long term, inherently safer plants are often the most cost effective. Smaller equipment operating at less severe temperature and pressure conditions will be cheaper and have lower operating costs. A process that does not require complex safety interlocks and elaborate procedures will be simpler, easier to operate, and more reliable. The need for an ongoing commitment of resources to maintain the safety systems will be eliminated. The safety of nuclear power plants relies heavily on complex instrumentation and safety systems, and the cost associated with those systems is high. Forsberg (1990) has estimated that 30-60% of the operating costs of a typical nuclear power plant are associated with safety. In recent years there has been considerable interest in inherently safer plants in the chemical process industries. A number of papers and two excellent books by Kletz (1983, 1984, 1989, 1990, 1991a,b,c) provide an over- view of the general concepts of inherently safer plants, and describe many specific examples. Recent papers by Englund (1990,1991a,b) and several other authors (Althaus and Mahalingam 1992; Dale 1987; Doerr and Hessian 1991; Hendershot 1988,1991a; Prugh 1992) also review inherently safer plants and processes and provide many specific examples of inherently safer designs. Although a process or plant can be modified to increase inherent safety at any time in its life cycle, the potential for major improvements is greatest at the earliest stages of process development. At these early stages, the process engineer has maximum degrees of freedom in the plant and process specifica- tion. The engineer is free to consider basic process alternatives such as fun- damental technology and chemistry and the location of the plant. Imperial Chemical Industries (ICI) describes six stages of hazard studies, including three during the process design phase and three during construction, startup and routine plant operation. The identification of inherently safer process alternatives is most effectively accomplished between the first and second process design hazard studies (Preston and Turney 1991). At this stage the conceptual plant design meets the general rule for an optimization process— that a true optimum can be found only if all of the parameters are allowed to vary simultaneously (Gygax 1988). 2.1.1 Process Risk Management Strategies Risk has been defined as a measure of economic loss or human injury in terms of both the incident likelihood and the magnitude of the loss or injury (CCPS 1989). Thus, any effort to reduce the risk arising from the operation of a chemical processing facility can be directed toward reducing the likelihood of incidents (incident frequency), reducing the magnitude of the loss or injury should an incident occur (incident consequences), or some combination of both. In general, the strategy for reducing risk, whether directed toward reducing frequency or consequence of potential accidents, falls into one of the following categories: • Inherent, or Intrinsic—Eliminating the hazard by using materials and process conditions that are nonhazardous (e.g., substituting water for a flammable solvent). • Passive—Eliminating or minimizing the hazard by process and equipment design features that do not eliminate the hazard, but do reduce either the frequency or consequence of realization of the hazard without the need for any device to function actively (e.g., the use of higher pressure rated equipment). • Active—Using controls, safety interlocks, and emergency shutdown systems to detect potentially hazardous process deviations and take correc- tive action. These are commonly referred to as engineering controls. • Procedural—Using operating procedures, administrative checks, emergency response and other management approaches to prevent incidents, or to minimize the effects of an incident. These are commonly referred to as administrative controls. Risk control strategies in the first two categories, inherent and passive, are more reliable and robust because they depend on the physical and chemical properties of the system rather than the successful operation of instruments, devices and procedures. Inherent and passive strategies are not the same and are often confused. A truly inherently safer process will completely eliminate the hazard (Kletz 199Ia). The discussion and examples in this chapter include both inherent and passive strategies to manage risk. Table 2-1 gives some Table 2-1 Examples of Process Risk Management Strategies Risk Management Strategy Category 1. Inherent 2. Passive 3. Active 4. Procedural Example An atmospheric pressure reaction using nonvolatile solvents which is in- capable of generating any pressure in the event of a runaway reaction. A reaction capable of generating 150 psig pressure in case of a runaway, done in a 250 psig reactor. A reaction capable of generating 150 psig pressure in case of a runaway, done in a 15 psig reactor with a 5 psig high pressure interlock to stop reactant feeds and a properly sized 15 psig rupture disk discharging to an effluent treatment system. The same reactor described in Example 3 above, but without the 5 psig high pressure interlock. Instead, the operator is instructed to monitor the reactor pressure and stop the reactant feeds if the pressure exceeds 5 psig. Comments There is no potential for overpressure of the reactor because of the chemistry and physical properties of the materials. The reactor can contain the runaway reaction. However, 150 psig pressure is generated and the reactor could fail due to a defect, corrosion, physical damage or other cause. The interlock could fail to stop the reaction in time, and the rupture disk could be plugged or im- properly installed, resulting in reactor failure in case of a runaway reaction. The effluent treatment system could fail to prevent a hazardous release. There is a potential for human error, the operator failing to monitor the reactor pressure, or failing to stop the reactant feeds in time to prevent a runaway reaction. Note: These examples refer only to the categorization of the risk management strategy with respect to the hazard of high pressure due to a runaway reaction. The processes described may involve trade-offs with other risks arising from other hazards. For example, the nonvolatile solvent in the first example may be extremely toxic, and the solvent in the remaining examples may be water. Decisions on process design must be based on a thorough evaluation of all of the hazards involved. examples of the four risk management strategy categories. The categories are not rigidly defined, and some strategies may include aspects of more than one category. Marshall (1990,1992) discusses managerial approaches to accident prevention, control of occupational disease and environmental protection in terms of strategic and tactical approaches. Strategic approaches have a wide signif- icance and represent "once and for all" decisions. The inherent and passive categories of risk management would usually be classified as strategic approaches. In general, strategic approaches are best implemented at an early stage in the process or plant design. Tactical approaches, the active and procedural risk management categories, include safety interlocks, operating procedures, protective equipment and emergency response procedures. These approaches tend to be implemented much later in the plant design process, or even after the plant is operating, and often involve much repetition, increasing the costs and potential for failure. In general it is probably not appropriate to talk about an inherently safe plant, but rather to use the term inherently safer. An absolute definition of safe is difficult, and risk cannot be reduced to zero. However it is possible to say that one process alternative is inherently safer than another alternative. For example, under the wrong circumstances water can be an extremely hazardous chemical—thousands of people drown every year. However, for the potential exposure scenarios in a chemical plant, water is clearly an inherently safer solvent than other materials. Process alternatives may also involve trade-offs, where the increased inherent safety from the viewpoint of one hazard results in a less safe process from the viewpoint of a different hazard. The note to Table 2-1 describes a possible scenario where the increased inherent safety of a process option based on the risk of runaway reaction pressure may result in a less safe process with respect to the toxicity of the materials used. Another example, described by McQuaid (1991) considers the safety tradeoffs of one and two story houses. A one story house is inherently safer with regard to the risk of falling down steps. However, in an incident in Belgium in the 1970s, people woke up one morning in their second floor bedrooms and found that their domestic animals on the ground had been killed by a dense gas cloud from a chlorine leak at a nearby chemical plant. Considering the risk of being exposed to a dense toxic gas cloud, it is inherently safer to sleep in a second floor bedroom. Another example of tradeoffs, frequently in the news in recent years, is the use of chlorofluorocarbon refrigerants in place of other materials such as ammonia and propane. Chlorofluorocarbons are clearly inherently safer from the viewpoint of acute toxicity (compared to ammonia) and flammability (compared to ammonia or propane). However, the suspected long term environmental impact of chlorofluorocarbon discharges to the atmosphere is resulting in their phase out in many applications. This illustrates the importance of understanding all of the hazards associated with material, process or plant design options. Then all hazards can be evaluated so that the best decision on which alternative results in the greatest overall benefit can be made. 2.1.2 Safety Layers Process safety relies on multiple safety layers, or defense in depth, to provide protection from a hazardous incident (Drake and Thurston 1992; CCPS 1993; Johnson 1990). These layers of protection start with the basic process design and include control systems, alarms and interlocks, safety shutdown systems, protective systems and response plans as illustrated in Figure 2-1. Inherent and passive approaches to safety can be a part of several layers of protection. For example, proper dike design can minimize the evap- oration of a spilled material. However, a truly inherent safety approach will be directed at the innermost layer of protection—the process design. The best first line of defense is to design a process in which hazardous incidents cannot happen. If such a process can be designed, or if potential incidents are small enough that they cannot hurt anybody, damage the environment or damage property if they do occur, then there will be no need for many of the additional layers of protection. 2.1.3 Design Approaches for Inherently Safer Plants Approaches to the design of inherently safer plants have been categorized into five major groups by Kletz (1984,199Id): • Intensification—Using small quantities of hazardous substances • Substitution—Replacing a material with a less hazardous substance •. Attenuation—Using less hazardous conditions or a less hazardous form of a material • Limitation of Effects—Designing facilities that minimize the impact of a release of hazardous material or energy • Simplification/Error Tolerance—Designing facilities that make operating errors less likely, and that are forgiving of errors that are made The remainder of this chapter will discuss strategies for inherently safer plant design in more detail and provide some specific examples, using these categories to organize the discussions. NOTE: Protection layers for a typical process are shown in the order of activation expected as a hazardous condition is approached. ESD - Emergency Shutdown SIS - Safety Interlock System Figure 2-1 Typical layers of protection in a modern chemical plant (CCPS 1993). COMMUNfTY EMERGENCY RESPONSE PUNT EMERGENCY RESPONSE PHYSICAL PROTECTION (DIKES) PHYSICAL PROTECTION (RELIEF DEVICES) AUTOMATIC ACTION SIS OR ESD ! CRlTICALALARMS, OPERATOR SUPERVISION, AND MANUAL INTERVENTION BASICCONTROLS 1 PROCESSALARMS 1 AND OPERATOR SUPERVISION PROCESS DESIGN 2.2. INTENSIFICATION 2.2.1 Reactors Reactors often represent a large portion of the inventory of hazardous material in a chemical process. A reactor maybe large because the chemical reaction is slow. However, in many cases the chemical reaction actually occurs very quickly, but it appears to be slow due to inadequate mixing and contacting of the reactants. Innovative reactor designs that improve mixing may result in much smaller reactors. Such designs are usually cheaper to build and operate, as well as being safer due to smaller inventory. In many cases, improved product quality and yield also result from better and more uniform contacting of reactants. A complete understanding of reaction mechanism and kinetics is essential to the optimal design of a reactor system. With a thorough understanding of the reaction, the designer can identify reactor configurations that maximize yield and minimize size, resulting in a more economical process, reducing generation of by-products and waste, and increasing inherent safety by reducing the reactor size and inventories of all materials. 2.2.1.1 Continuous Stirred Tank Reactors Continuous stirred tank reactors (CSTR) are often much smaller for a specific production rate when compared to a batch reactor. In addition to reduced inventory, a CSTR usually results in other benefits which can also enhance safety, reduce costs, and improve product quantity. For example: • Mixing in the smaller CSTR is generally better, which may improve product uniformity and reduce by-product formation • Greater heat transfer surface per unit of reactor volume can be provided improving temperature control and reducing the risk of thermal runaway • It may be more practical to build a small reactor for a high design pressure, allowing containment of a runaway reaction. In one reported example, the same quantity of a material can be manufactured either in a 3000 gallon (-11 m 3 ) batch reactor or a 100 gallon (-0.4 m 3 ) CSTR. The reaction is exothermic and a runaway reaction could result in reactor rupture. Table 2-2 compares the overpressure resulting from reactor rupture at distances of 50 (15 m) and 100 feet (30 m) from the reactor for the two cases, assuming both reactors have the same design and rupture pressure (Hendershot 199Ia). To put these numbers into perspective, 1 psig overpressure is sufficient to cause partial demolition of houses, and 2-3 psig overpressure shatters unreinforced concrete or cinder block walls (CCPS 1989). In considering the relative safety of batch and continuous processing it is important to fully understand any differences in chemistry and processing conditions, which may outweigh the benefits of reduced size of a continuous reactor. For example, Englund (199Ia) describes continuous latex processes which have enough unreacted monomer in the continuous reactor that they maybe less safe than a well designed batch process. Kletz (199Id) discusses a generic case where more severe processing conditions may result in a more severe hazard from a smaller reactor. 2.2.1.2 Tubular Reactors Tubular reactors often offer the greatest potential for inventory reduction. In addition, they are usually extremely simple in design, containing no moving parts and a minimum number of joints and connections that could leak. In many cases a relatively slow reaction can be completed in a long tubular reactor. There are many devices available for providing mixing in tubular reactors, including jet mixers, eductors, and static mixers. Caro's acid is an equilibrium mixture of sulfuric acid, water and peroxy- monosulfuric acid (HfeSOs) that can be used in the metal extraction and separation industries and other applications where an extremely powerful oxidizing agent is needed. It is manufactured by reacting concentrated sulfuric acid with hydrogen peroxide. Whiting (1992) describes a process for the manufacture of 300 kg/day of Caro's acid using a 30 liter agitated isothermal reactor with a 30 minute residence time. The reactor must operate at less than O 0 C to avoid product decomposition. An improved process uses an adiabatic tubular reactor with a volume of 20 milliliters and a residence time of less than 1 second to produce 1000 kg/day of Caro's acid, a reactor size reduction of 1500:1. The process requires an elevated temperature, but the short residence time, and immediate reaction of the product with the solution to be treated, minimize decomposition at the elevated temperature. A batch process for the manufacture of a nonhazardous product from several hazardous raw materials is shown in Figure 2-2. The batch stirred tank reactor has a volume of several thousand gallons. A new process, as shown in Figure 2-3, was developed using a tubular reactor containing static mixing Table 2-2 Effect of Size on Overpressure Due to Vessel Rupture 8 Distance (feet) 50 100 a Henderehot!991a Overpressure from Vessel Rupture (psig) 3000 Gallon Batch Reactor 3.4 1.1 1 0O Gallon Continuous Reactor 0.62 0.27 [...]... much safer The use of polystyrene sulfonic acid catalyst for CH3OH + CH2=C(CHO2 Pd^rem Sulfmic Ad 4 CH3OC(CHs)3 Sherrington (1991) provides several additional examples and suggestions for future development Chemistry of side reactions and by-products may also offer opportunities for increasing the inherent safety of a process For example, a process involving a caustic hydrolysis step uses ethylene... material volume Table 2-5 compares the surface compactness of a variety of heat exchanger types Process inventory can be minimized by using heat exchangers with the minimum volume of hazardous process fluid for the heat transfer area required 2,3 SUBSTITUTION 2.3.1 Chemistry Inherent safety of the manufacturing process for a material can be greatly increased by development of alternate chemistry using less... sulfuric acid) for sulfonation reactions If a chemical process requires the concentrated form of a material, it may be feasible to store it as a more dilute form and concentrate the material, by distillation or some other technique in the plant prior to introduction to the process This reduces the inventory of material with greater intrinsic hazard to the minimum amount required to operate the process ... to inherent safety, involving synthesis routes, reagents, catalysts and solvents 2.3.2 Solvents Replacement of volatile organic solvents with aqueous systems or less hazardous organic materials improves safety of many processing operations and final products Some examples include: • Water based paints and adhesives in place of solvent based products • Aqueous or dry flowable formulations for agricultural... eliminated chlorofluorocarbons, chloroform, methylene chloride, and other hazardous solvents, replacing them with nonhazardous materials (Kelley 1992) Apple Computer reports the elimination of all chlorofluorocarbons for cleaning electronic assemblies and has converted to water based processes (Chemical WeekNewswire 1992) • The United States Air Force is evaluating a process called Coldjet which removes... pharmaceutical industry, allowing the process designers to identify optimal reactor configurations using novel designs including tubular reactors with static mixing elements 2.2.1.3 Gas-Liquid Reactions Mass transfer is often the rate limiting step in gas-liquid reactions, and novel reactor designs that increase mass transfer can reduce reactor size and also improve process yields As an example (Koestler... intensification through improved reactor design 2.2.2 Storage and Material Transfer Raw material and in -process storage tanks often represent a major portion of the risk of a chemical plant Hazardous material transfer lines can also be a significant hazard Attention to the design of storage and transfer equipment can reduce hazardous material inventory 2.2.2.1 Storage Storage tanks for raw material and intermediates... formulations for agricultural chemicals instead of organic solvent formulations • British computer manufacturer ICL has eliminated chlorofluorocarbons from its manufacturing processes, replacing them with aqueous cleaning systems for flux removal ("Technology" 1991) In the United States, IBM Table 2-6 Some Examples of Solvent Substitutions3 Chloroform -+ Acetone —* Ethyl Acetate -* Ethanol Dichloromethane... the Bhopal accident, or that generate only small quantities of methyl isocyanate as an in -process intermediate (Kletz 199Id) DuPont has developed a proprietary process for manufacture of carbamate insecticides which generates and immediately consumes methyl isocyanate Total methyl isocyanate inventory in the process is no more than 10 kilograms (Kharbanda and Stallworthy 1988) Acrylonitrile can be... solvents Table 2-6 lists some of the substitutions identified by this industrial survey 2.3.3 Utility Systems Utility and plant services systems must also be examined for options to increase the inherent safety of a plant or process For example: • Use water or steam as a heat transfer medium rather than flammable or combustible oils (Kharbanda and Stallworthy 1988; Kletz 199Id) • Use high flash point . (OSHA). 1.5.2 Specific References CCPS (Center for Chemical Process Safety) . 1992. Plant Guidelines for Technical Manage- ment of Chemical Process Safety, American Institute of Chemical. of protection—the process design. The best first line of defense is to design a process in which hazardous incidents cannot happen. If such a process can be designed, or if . first and second process design hazard studies (Preston and Turney 1991). At this stage the conceptual plant design meets the general rule for an optimization process that a true

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