Role of Safety in chemical process industries

What is a Chemical Processing Industry ?

An industry in which the raw materials undergo chemical conversion during their processing into finished products, as well as (or instead of) the physical conversions common to industry in general; includes the traditional chemical,petroleum, and petrochemical industries. 

The Chemical Processing Industry encompasses a broad range of products, including petrochemical and inorganic chemicals, plastics, detergents, paints and pigments, and more.Major segments of our Chemical Processing include:

Inorganic and Organic Chemical ProducersProducers of olefins, alcohols, ethylene and ethylene-based chemicals, polymerization, cyclical compounds, and solvents. Major companies engaged in the production of acids, compounds, and specialty chemicals

Industrial Gas ProducersManufacturers of hydrogen, nitrogen, oxygen, and other industrial gases

Agricultural Chemical IndustryBulk liquid and solid (granular, powder) agricultural product producers, including fertilizers, herbicides, pesticides, fungicides, and intermediates, such as urea, ammonia, nitric acid, and ammonium nitrate

Detergents and other Household Product ProducersManufacturers of soaps and detergents, cleaners, bleaches, disinfectants, and surface agents

Plastics, Rubbers, Fibers, and Resins ManufacturersThe manufacturing of synthetic resins, plastics materials, nonvulcanizable elastomers, synthetic rubber by polymerization or copolymerization, cellulosic man-made fibers and the compounding of purchased plastics

Painting and Coating ProducersManufacturers of pigments, coatings, solvents, lacquers, enamels, stains, and varnishes for finishing

Producers of Other Chemicals

Producers of explosives to inks, dyes, glues, lubricants, fire retardants, and chemical preparations

Alkalies and ChlorineProducers of caustic soda, soda ash (not produced at mines), chlorine (compressed or liquefied), carbonates (potassium and sodium)

The Nature of the Accident Process

Chemical plant accidents follow typical patterns. It is important to study these patterns in order to anticipate the types of accidents that will occur. As shown in Table 1-6, fires are the most common, followed by explosion and toxic release. With respect to fatalities, the order reverses ,with toxic release having the greatest potential for fatalities.Economic loss is consistently high for accidents involving explosions. The most damaging type of explosion is an unconfined vapor cloud explosion, where a large cloud of volatile and flammable vapor is released and dispersed throughout the plant site followed by ignition and explosion of the cloud. An analysis of the largest chemical plant accidents(based on worldwide accidents and 1998 dollars) is provided in Figure 1-6. As illustrated, vapor cloud explosions account for the largest percentage of these large losses.The “other” category of Figure 1-6 includeslosses resulting from floods and windstorms.Toxic release typically results in little damage to capital equipment. Personnel injuries,employee losses, legal compensation, and cleanup liabilities can be significant.

The Nature of the Ac-cident Process

Fires OthersExplosionsVapour cloud explosions

Figure Types of loss for large hydrocarbon chemical plant accidents

Figure 1-7 presents the causes of losses for the largest chemical accidents. By far the largest cause of loss in a chemical plant is due to mechanical failure.Failures of this type areusually due to a problem with maintenance. Pumps, valves, and control equipment will fail ifnot properly maintained. The second largest cause is operator error. For example, valves arenot opened or closed in the proper sequence or reactants are not charged to a reactor in the correct order. Process upsets caused by, for example, power or cooling water failures account for 11% of the losses. Human error is frequently used to describe a cause of losses. Almost all accidents, except those caused by natural hazards, can be attributed to human error. For instance, mechanical failures could all be due to human error as a result of improper maintenance or inspection.

Mechanical operator error

Unknown Process upsets

natural hazards

Design Sabotage and arson

0

5

10

15

20

25

30

35

40

45

Accidents %

Figure 1-7 Causes of losses in the largest hydrocarbon-chemical plant accidents.

Most accidents follow a three-step sequence:

• initiation (the event that starts the accident),• propagation (the event or events that maintain or expand the accident), and• termination (the event or events that stop the accident or diminish it in size)

Inherent Safety

An inherently safe plant11,12 relies on chemistry and physics to prevent accidents rather than on control systems, interlocks,

redundancy, and special operating procedures to prevent accidents. Inherently safer plants are tolerant of errors and are often the most cost effective. A process that does not require complex safety interlocks and elaborate procedures is simpler, easier to operate, and more reliable. Smaller equipment, operated at less severe temperatures andpressures, has lower capital and operating costs. In general, the safety of a process relies on multiple layers of protection. The first layer of protection is the process design features. Subsequent layers include control systems, interlocks,safety shutdown systems, protective systems, alarms, and emergency response plans. Inherent safety is a part of all layers of protection; however, it is especially directed toward process design features. The best approach to prevent accidents is to add process design features to prevent hazardous situations. An inherently safer plant is more tolerant of operator errors and abnormal conditions. 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 the greatest at the earliest stages of process development. At these early stages process engineers and chemists have the maximum degree of freedom in the plant and process specifications, and they are free to consider basic process alternatives, such as changes to the fundamental chemistry and technology. The major approach to inherently safer process designs is divided into the followingcategories:• intensification• substitution• attenuation• limitation of effects• simplification/error tolerance

Inherent Safety Techniques

Type Typical techniques

Minimize Change from large batch reactor to a smaller continuous reactor Reduce storage inventory of raw materialsImprove control to reduce inventory of hazardous

intermediate chemicalsReduce process hold-up

Substitute Use mechanical pump seals vs. packing Use welded pipe vs. flangedUse solvents that are less toxicUse mechanical gauges vs. mercuryUse chemicals with higher flash points, boiling points, and other less hazardouspropertiesUse water as a heat transfer fluid instead of hot oil

Moderate Use vacuum to reduce boiling pointReduce process temperatures and pressures Refrigerate storage vessels Dissolve hazardous material in safe solventOperate at conditions where reactor runaway is not possiblePlace control rooms away from operationsSeparate pump rooms from other roomsAcoustically insulate noisy lines and equipmentBarricade control rooms and tanks

Simplify Keep piping systems neat and visually easy to follow Design control panels that are easy to comprehendDesign plants for easy and safe maintenance Pick equipment that requires less maintenancePick equipment with low failure ratesAdd fire- and explosion-resistant barricadesSeparate systems and controls into blocks that are easy to comprehend andunderstandLabel pipes for easy “walking the line”Label vessels and controls to enhance understanding

Vapor released from spills can be minimized by designing dikes so that flammable and toxic materials will not accumulate around leaking tanks. Smaller tanks also reduce the hazards of a release. While minimization possibilities are being investigated, substitutions should also be considered as an alternative or companion concept; that is, safer materials should be used in place of hazardous ones. This can be accomplished by using alternative chemistry that allows the use of less hazardous materials or less severe processing conditions. When possible, toxic or flammable solvents should be replaced with less hazardous solvents (for example,water-based paints and adhesives and aqueous or dry flowable formulations for agricultural chemicals).

Another alternative to substitution is moderation, that is, using a hazardous material under less hazardous conditions. Less hazardous conditions or less hazardous forms of a material include (1) diluting to a lower vapor pressure to reduce the release concentration, (2) refrigerating to lower the vapor pressure, (3) handling larger particle size solids to minimize dust, and (4) processing under less severe temperature or pressure conditions. Containment buildings are sometimes used to moderate the impact of a spill of an especially toxic material. When containment is used, special precautions are included to ensure worker protection, such as remote controls, continuous monitoring, and restricted access. Simpler plants are friendlier than complex plants because they provide fewer opportunities for error and because they contain less equipment that can cause problems. Often, the reason for complexity in a plant is the need to add equipment and automation to control the hazards. Simplification reduces the opportunities for errors and misoperation. For example, (1) piping systems can be designed to minimize leaks or failures, (2) transfer systems can be designed to minimize the potential for leaks, (3) process steps and units can be separated to prevent the domino effect, (4) fail-safe valves can be added, (5) equipment and controls can be placed in a logical order, and (6) the status of the process can be made visible and clear at all times. The design of an inherently safe and simple piping system includes minimizing the use of sight glasses, flexible connectors, and bellows, usingwelded pipes for flammable and toxic chemicalsand avoiding the use of threaded pipe, using spiral wound gaskets and flexible graphitetype gaskets that are less prone to catastrophic failures, and using proper support of lines to minimize stress and subsequent failures.

Occupational safety and health protection and chemical industryenterprises

Chemical industry enterprises are aware of the fact that they are monitored and evaluated by the state authorities, business partners, the public and also by their staff members according to how they care for occupational safety and health protection. Their long-term approach to this area is characterized by high responsibility, with emphasis laid on prevention and primarily on minimizing risks to the health and life of employees. Already in 1985 a worldwide voluntary programme Responsible Care (hereinafter RC) was launched in Canada, whose aim is to reduce risks related with chemical production processes and to openly communicate with the state administration bodies and the public about approaches to chemical industry safety improvements. Very intensive attention to the occupational safety and health protection issues is paid at the level of European Chemical Industry Council (CEFIC). With regard to high importance of these issues, in the past a number of European chemical industry enterprises and institutions published more or less extensive reports on approaches to occupational safety and health protection and on the

progress of performance in this area. The chosen indicators were differing Human Resources Management & Ergonomics 2/2008 from enterprise to enterprise and from state to state, in dependence on national and local priorities and definitions. For this reason, in 1998 CEFIC prepared a new document „Responsible Care, Health, Safety and Environmental Reporting Guidelines“, which forms a common framework for the reporting of results and monitoring and allows to summarize the data at European level, as all member federations in publishing the data follow common basic parameters and their definitions. Results of the European chemical industry are published in regular reports that are released by CEFIC. The data displayed cover more than 895 000 employees of the European chemical industry.

Are there any differences in approach to occupational health and safety management in small and large chemical businesses?

At the beginning of 2008, research in two selected Czech chemical industry enterprises (with place of business on the territory of the Pardubice Region) was conducted, having focused on identifying the approach of these enterprises to occupational safety and health protection issues. Of concern were, in particular, occupational health and safety management and the matter of informing the interested parties of corporate approach to these issues. Within health and safety management, attention was primarily paid to the following aspects: - The importance of occupational health and safety management. - Reasons for implementation or non-implementation of HSMS. - Assistance in implementation of HSMS. - Problems in implementation and system benefits. - Occupational risk assessment, the provision of personal protective aids. - Regular health and safety inspections, trainings and health and safety documentation.

Within the reporting, particularly the following areas were examined: - Making the information available to the public through regular reports, the reasons for preparing such reports, the way of reporting. - Interest in information.

The research was conducted in a small and in a large chemical business. The small business is specialized in dyeing and refining of plastics; the main

corporate operations include the manufacture of chemical substances and chemical preparations, wholesale and retail of chemicals. The business has been in existence since 1950 and has approx. 40 employees. The large business is a leading European manufacturer of sophisticated chemical products with more than eighty years´ tradition and employs over 2 200 staff members. It manufactures more than 1000 products and exports to 54 countries worldwide. Business activities are targeted at three market segments: advanced organic intermediates, cellulose derivatives, and pigments and dyes. The research was conducted in the form of in-depth interview. In the small business, interview proceeded with applications engineer who has been working in the firm for a number of years and the inquired areas fall within his competence. In the large business, discussion (with regard to much broader differentiation of competences) proceeded with two staff members who are engaged in these issues, namely head of occupational safety department and staff manager of the company. In the area of occupational health and safety management, the following findings were identified . a) For a small business, improvements in occupational safety and health protection are very important. The business has its HSMS implemented since 1999; this system is not certified. Implementation of the system was suggested by company management, mainly driven by efforts to achieve higher level of occupational safety and avoid the occurrence of occupational injuries. The need to build up the system also resulted from the requirements of business partners. In the process of the system implementation, assistance from external subjects (safety engineer and adviser) was made use of. Occupational risk assessment together with assessment of risks for the provision of personal protective aids as well as operational risk assessment was carried out. Technological procedures, given by Labour Code, have been elaborated. Based on this, measures aiming to minimize and/or eliminate the risks have been adopted. The business ensures safe condition of the technical equipment and facilities through maintenance and regular checks and revisions, it has also first aid system for the sake of accidents and breakdowns (a so-called traumatology and evacuation plan which forms part of occupational health and safety documentation) elaborated. The business performs regular trainings of staff members and managers in the occupational health and safety issues. It also performs entry trainings in the occupational health and safety issues that are provided by safety engineer and recorded in every employee’s file. Preventive checks on occupational health and safety are performed by contract physician. The business also performs checks on its staff members whether they are not intoxicated. From the listed activities it is evident that the main aim of the system is to achieve compliance with the legislation.

b) The large business has an occupational health and safety management system implemented and strives for the certification according to the guideline OHSAS 18001:1999 (Occupation Health and Safety Management Systems). The main reasons for implementation of the system include efforts to achieve higher level of occupational safety, efforts to achieve a lower number of occupational accidents and injuries and reduce costs resulting from compensations for occupational accidents and injuries. Implementation of the system was suggested directly by corporate management, having been performed by company itself, unassisted. Within this system, emphasis is primarily laid on making company staff members acquainted with the risks resulting from the nature of production, namely through initial training in the form of educational film and assisting materials prepared by occupational safety department. Permanent attention is paid by enterprise to the trainings and periodical examinations of skills and qualifications of employees who for performance of their work must have special professional qualifications, prescribed by generally valid legislation (autotruck drivers, gas facilities operating staff, welders, scaffolders, etc.). The company ensures regular checks on occupational health and safety as well as prescribed revisions to be performed by engineering inspector, and also provides employees with personal protective aids. The company has first aid system elaborated for the sake of accidents and breakdowns, and strives to continuously update the system. The company cooperates with 5 contract physicians who perform regular preventive examinations and it is also in permanent contact with the regional hospital and with the regional hygiene station. The company has put into operation alert system which through SMS informs of danger the population living in the surroundings. Corporate principles regarding the occurrence of impaired/intoxicated individuals are followed very strictly, for that purpose, spot checks are performed at the building entrance. The business keeps occupational health and safety documentation (book of accidents, records of occupational accidents and records of personal protective aids). To reach compliance with the legal regulations, the business implemented HSMS and strives for its certification. The business also uses another voluntary tool – it professes the RC principles and is a holder of the RC logo. As regards the area of reporting, the following can be stated: a) Small business makes available to the public the basic information on its activities in the field of environmental protection and on work law relations – this is included in annual report; such obligation ensues from the Accounting Act. Annual report is compiled every year as of 31.12. It is available to the general public. It is made available on the intranet and also in printed form. b) Factual and full disclosure of information on corporate activities in the area of environmental protection and protection of working environment has become a matter of course for a large business. The main aim in this area is the maximal openness towards the employees and all other partners, such as the public,

public media, public administration, non-governmental organizations and business partners. The main corporate targets in the area of environmental protection and occupational safety and health protection as well as the way of achieving them are presented by the business in regularly released Reports on Corporate Environmental Impacts. The reports are distributed in printed form and are also available to the public on corporate website. The business is a holder of RC logo and based on its participation in this programme it also meets all obligations in the area of disclosure of information. In addition, the basic information on corporate environmental approach and on work law relations is included in annual report. Annual report is placed on corporate website and is also available in the form of CD version. The business regularly organizes meetings with mayors of the municipalities in the region, where the participants are made acquainted with various activities not only in the area of environmental protection but also in the area of occupational safety and health protection. Up-to-date information is also made available to the public by means of corporate periodical. From the listed activities in the area of communication with the interested parties it is evident that the business goes above the framework of legal requirements. This means that it is aware of the importance of communication for improving credibility of the company and its attractiveness as employer. To address the interested parties, the business also chooses different communication tools. Such communication activity is also, beyond all doubt, a response to the requirements of the interested parties.

Safety Through Design in the Chemical Process Industry:Inherently Safer Process Design

A chemical manufacturing process is described as inherently safer if it reduces or eliminates hazards associated with materials and operations used in the process, and this reduction or elimination is a permanent and inseparable part of the process technology. A hazard is defined as a physical or chemical characteristic that has the potential for causing harm to people, the environment, or property (adapted from CCPS, 1992). The key to this definition is that the hazard is intrinsic to the material, or to its conditions of storage or use. For example, chlorine is toxic by inhalation, gasoline is flammable, and steam at 600 psig contains significant potential energy. These hazards are basic properties of the materials and the conditions of usage, and cannot be changed. An inherently safer process reduces or eliminates the hazard by reducing the quantity of hazardous material or energy, or by completely eliminating the

hazardous agent. A traditional approach to managing the risk associated with a chemical process is by providing layers of protection between the hazardous agent and the people, environment, or property which is potentially impacted. This approach is illustrated in Figure 1 (CCPS, 1993b; Bollinger, et. al., 1996). The protective layers may include one or more of the following:· The process design· Basic controls, alarms, and operator control· Critical alarms, operator control, and manual intervention· Automatic actions — emergency shutdown systems and safety interlock systems· Physical protection equipment such as pressure relief devices· Physical mitigation systems such as spill containment dikes· Emergency response systems — for example, fire fighting· Community emergency response — for example, notification and evacuationThe layers of protection are intended to reduce risk by reducing either the likelihood of potential incidents resulting in an impact on people, the environment, or property, or by reducing the magnitude of the impact should an incident occur. The risk can be reduced to very low levels by providing a sufficient number of layers of protection, and by making each layer highly reliable (Figure 2). However, the basic process hazards remain, and there is always the potential — perhaps very small, but never zero — that all layers will fail simultaneously and the hazardous incident will occur. Furthermore, the layers of protection require significant expenditure of resources, both to design and build them initially, and to maintain their reliability throughout the life of the plant. Failure to adequately maintain the layers of protection may result in a significant increase in the process risk .The inherently safer design approach is to eliminate or reduce the hazard by changing the process itself, rather than by adding on additional safety devices and layers of protection. Ideally, hazards would be reduced to a level where no protective systems are required because the hazard is too small to be of concern Even if this is not possible, an inherently safer process will allow the numberof layers of protection to be reduced. The overall design is therefore more robust from a safety and environmental viewpoint, and is likely to be less expensive to build and operate because of the elimination of complex safety systems.

Relationship to Safety in Design

Safety in Design can be based on either of the approaches described. Add-on safety features and layers of protection can be identified and incorporated during design. In fact, they should be incorporated into the process design — we should anticipate potential accidents during design and provide the appropriate protective systems, procedures, and devices, rather than discovering the need for these layers of protection later on as a result of accidents and near misses. However, the greatest potential for realizing an inherently safer process design is early in development. At this time, the designer still has considerable freedom in technology selection. For a chemical process, perhaps the greatest opportunities lie in the selection of the chemical synthesis route to be used,including the raw materials, solvents, chemical intermediates, reaction steps, and other physical and chemical operations to be used.

However, it is important to remember that it is never too late to consider inherently safer design options. In an existing plant, there will be different kinds of opportunities for modifications to improve inherent safety, but these opportunities can result in significant improvements. It may not be feasible to change the basic process chemistry and technology, but it may be possible to reduce inventory, simplify the plant, or otherwise make the plant more “user friendly.” Significant improvements in the inherent safety of plants which have operated for many years have been reported.

Approaches to Inherently Safer Design in the Chemical Industry

Chemical process risk management approaches can be classified into four categories:Inherent — Eliminating the hazard by using materials and process conditions which are non-hazardous.Passive — Minimizing the hazard by process and equipment design features which reduce either the frequency or consequence of the hazard without the active functioning of any device.Active — Using controls, safety interlocks, and emergency shutdown systems to detect and correct process deviations. These systems 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.

Marshall (1990) categorizes risk management strategies as strategic and tactical. Strategic approaches include measures which have “wide significance, and which represent a ‘once and for all’ policy decision.” Inherent and passive approaches tend to fall into the “strategic” category. Tactical approaches include measures “which are added on at a late state or those which entail frequent repetition.” Active controls and procedural approaches tend to be “tactical.” Safety in Design can entail both strategic and tactical risk management approaches, but the greatest benefits are realized from the early consideration of strategic risk management measures. There are four basic strategies for implementing inherently safer chemical processes:Minimize — Use smaller quantities of hazardous substancesContinuous reactors (stirred tanks, loop reactors, tubular reactors) in place of batchreactorsReduced inventory of raw materials and in-process intermediatesHigh efficiency heat exchangersExample: A 50-liter loop polymerization reactor has a capacity equal to that of a 5000-liter batch reactor (Wilkinson and Geddes, 1993).Substitute — Replace a material with a less hazardous substanceWater based paints and coatingsAlternative chemistry using less hazardous materialsLess flammable or toxic solvents

Example: Acrylic esters were formerly manufactured using the Reppe process, usingacetylene, carbon monoxide, and a nickel carbonyl catalyst. The newer propylene oxidationprocess uses significantly less hazardous materials (Hochheiser, 1986).Moderate — Use less hazardous conditions, a less hazardous form of a material, orfacilities which minimize the impact of a release of hazardous material or energyDilutionRefrigeration of volatile hazardous materialsGranular agricultural product formulations in place of powdersExample: The distance to an atmospheric concentration of 500 ppm of mono-methylamine in the event of the failure of a 2-inch pipe is reduced from 1.9 miles to 0.6 miles by reducing the temperature of the monomethylamine from 10ºC to -6ºC .Simplify — Design facilities which eliminate unnecessary complexity and make operating errors less likely, and which are forgiving of errors which are madeDevelop fundamentally simpler technology with fewer reactions and processing operations

Eliminate unnecessary equipment (question need for each device or feature)Remove unused or abandoned equipmentHuman factors considerations in designExample: Design vessels to withstand the maximum pressure which can be generated, rather than providing complex emergency relief systems, including devices such as scrubbers, catch tanks, or flares to contain the relief system effluent.

CASE STUDIES

Four Significant Disasters

The study of case histories provides valuable information to chemical engineers involved with safety. This information is used to improve procedures to prevent similar accidents in the future. The four most cited accidents (Flixborough, England; Bhopal, India; Seveso, Italy; and Pasadena, Texas) are presented here. All these accidents had a significant impact on

public perceptions and the chemical engineering profession that added new emphasis and standards inthe practice of safety. Chapter 13 presents case histories in considerably more detail. The Flixborough accident is perhaps the most documented chemical plant disaster. The British government insisted on an extensive investigation.

Flixborough, England

The accident at Flixborough, England, occurred on a Saturday in June 1974. Although it was not reported to any great extent in the United States, it had a major impact on chemical engineering in the United Kingdom. As a result of the accident, safety achieved a much higher priority in that country.The Flixborough Works of Nypro Limited was designed to produce 70,000 tons per year of caprolactam, a basic raw material for the production of nylon. The process uses cyclohexane, which has properties similar to gasoline. Under the process conditions in use at Flixborough (155°C and 7.9 atm), the cyclohexane volatilizes immediately when depressurized to atmospheric conditions.The process where the accident occurred consisted of six reactors in series. In these reactors cyclohexane was oxidized to cyclohexanone and then to cyclohexanol using injected air in the presence of a catalyst. The liquid reaction mass was gravity-fed through the series of reactors. Each reactor normally contained about 20 tons of cyclohexane. Several months before the accident occurred, reactor 5 in the series was found to be leaking. Inspection showed a vertical crack in its stainless steel structure. The decision was made to remove the reactor for repairs. An additional decision was made to continue operating by connecting reactor 4 directly to reactor 6 in the series. The loss of the reactor would reduce the yield but would enable continued production because unreacted cyclohexane is separated and recycled at a later stage.The feed pipes connecting the reactors were 28 inches in diameter. Because only 20-inch pipe stock was available at the plant, the connections to reactor 4 and reactor 6 were made using flexible bellows-type piping, as shown in Figure 1-10. It is hypothesized that the bypass pipe section ruptured because of inadequate support and overflexing of the pipe section as a result of internal reactor pressures. Upon rupture of the bypass, an estimated 30 tons of cyclohexane volatilized and formed a large vapor cloud. The cloud was ignited by an unknown source an estimated 45 seconds after the release.The resulting explosion leveled the entire plant facility, including the administrative offices. Twenty-eight people died, and 36 others were injured. Eighteen of these fatalities occurred in the main control room when the ceiling collapsed. Loss of life would have been substantially greater had the accident occurred on a weekday when the administrative offices were filled with

employees. Damage extended to 1821 nearby houses and 167 shops and factories. Fifty-three civilianswere reported injured. The resulting fire in the plant burned for over 10 days. This accident could have been prevented by following proper safety procedures.

Bhopal, India

The Bhopal, India, accident, on December 3, 1984, has received considerably more attention than the Flixborough accident. This is due to the more than 2000 civilian casualties that resulted. The Bhopal plant is in the state of Madhya Pradesh in central India. The plant was partially owned by Union Carbide and partially owned locally. The nearest civilian inhabitants were 1.5 miles away when the plant was constructed. Because the plant was the dominant source of employment in the area, a shantytown eventually grew around the immediate area. The plant produced pesticides. An intermediate compound in this process is methyl isocyanate (MIC). MIC is an extremely dangerous compound. It is reactive, toxic, volatile, and flammable. The maximum exposure concentration of MIC for workers over an 8-hour period is 0.02 ppm (parts per million). Individuals exposed to concentrations of MIC vapors above 21 ppm experience severe irritation of the nose and throat. Death at large concentrations of vaporis due to respiratory distress. MIC demonstrates a number of dangerous physical properties. Its boiling point at atmospheric conditions is 39.1°C, and it has a vapor pressure of 348mmHgat 20°C.Thevapor is about twice as heavy as air, ensuring that the vapors will stay close to the ground once released. MICreacts exothermicallywithwater.Although the reaction rate is slow, with inadequate cooling the temperature will increase and the MIC will boil. MIC storage tanks are typically refrigerated to prevent this problem. The unit using the MIC was not operating because of a local labor dispute. Somehow a storage tank containing a large amount of MIC became contaminated with water or some other substance.Achemical reaction heated theMIC to a temperature past its boiling point. TheMIC vapors traveled through a pressure relief system and into a scrubber and flare system installed to consume theMICin the event of a release.Unfortunately, the scrubber and flare systemswere not operating, for a variety of reasons. An estimated 25 tons of toxic MIC vapor was released. The toxic cloud spread to the

adjacent town, killing over 2000 civilians and injuring an estimated 20,000 more. No plant workers were injured or killed. No plant equipment was damaged. The exact cause of the contamination of the MIC is not known. If the accident was caused by a problem with the process, a well-executed safety review could have identified the problem. The scrubber and flare system should have been fully operational to prevent the release. Inventories of dangerous chemicals, particularly intermediates, should also have been minimized. The reaction includes the dangerous intermediate MIC. An alternative reaction scheme is shown at the bottom of the figure and involves a less dangerous chloroformate intermediate. Another solution is to redesign the process to reduce the inventory of hazardous MIC. One such design produces and consumes the MIC in a highly localized area of the process, with an inventory of MIC of less than 20 pounds.

Seveso, Italy

Seveso is a small town of approximately 17,000 inhabitants, 15 miles from Milan, Italy. The plant was owned by the Icmesa Chemical Company. The product was hexachlorophene, a bactericide, with trichlorophenol produced as an intermediate. During normal operation, a small amount of TCDD (2,3,7,8-tetrachlorodibenzoparadioxin) is produced in the reactor as an undesirable side-product. TCDD is perhaps the most potent toxin known to humans. Animal studies have shown TCDD to be fatal in doses as small as 10_9 times the body weight. Because TCDD is also insoluble in water, decontamination is difficult. Nonlethal doses of TCDD result in chloracne, an acne-like disease that can persist for several years. On July 10, 1976, the trichlorophenol reactor went out of control, resulting in a higher than normal operating temperature and increased production of TCDD. An estimated 2 kg of TCDD was released through a relief system in a white cloud over Seveso. A subsequent heavyrain washed the TCDD into the soil. Approximately 10 square miles were contaminated. Because of poor communications with local authorities, civilian evacuation was not started until several days later. By then, over 250 cases of chloracne were reported. Over 600 people were evacuated, and an additional 2000 people were given blood tests. The most severely

contaminated area immediately adjacent to the plant was fenced, the condition it remainsin today. TCDD is so toxic and persistent that for a smaller but similar release of TCDD in Duphar, India, in 1963 the plant was finally disassembled brick by brick, encased in concrete and dumped into the ocean. Less than 200 g of TCDD was released, and the contamination was confined to the plant. Of the 50 men assigned to clean up the release, 4 eventually died from the exposure.The Seveso and Duphar accidents could have been avoided if proper containment systems had been used to contain the reactor releases. The proper application of fundamental engineering safety principles would have prevented the two accidents. First, by following proper procedures, the initiation steps would not have occurred. Second, by using proper hazard evaluation procedures, the hazards could have been identified and corrected before the accidents occurred.

Pasadena, Texas

A massive explosion in Pasadena, Texas, on October 23, 1989, resulted in 23 fatalities, 314 injuries, and capital losses of over $715 million. This explosion occurred in a high-density polyethylene plant after the accidental release of 85,000 pounds of a flammable mixture containing ethylene, isobutane, hexane, and hydrogen. The release formed a large gas cloud instantaneously because the system was under high pressure and temperature. The cloud was ignitedabout 2 minutes after the release by an unidentified ignition source. The damage resulting from the explosion made it impossible to reconstruct the actual accident scenario. However, evidence showed that the standard operating procedures were not appropriately followed. The release occurred in the polyethylene product takeoff system.Usually the polyethylene particles (product) settle in the settling leg and are removed through the product takeoff valve. Occasionally, the product plugs the settling leg, and the plug 13Occupational Safety and Health Administration is removed by maintenance personnel. The normal—and safe—procedure

includes closing the DEMCO valve, removing the air lines, and locking the valve in the closed position. Then the product takeoff valve is removed to give access to the plugged leg.The accident investigation evidence showed that this safe procedurewas not followed; specifically, the product takeoff valvewas removed, the DEMCO valvewas in the open position, and the lockout device was removed. This scenario was a serious violation of well-established and well-understood procedures and created the conditions that permitted the release and subsequentexplosion.TheOSHAinvestigation13 found that (1) no process hazard analysis had been performed in the polyethylene plant, and as a result, many serious safety deficiencies were ignored or overlooked; (2) the single-block (DEMCO) valve on the settling legwas not designed to fail to a safe closed position when the air failed; (3) rather than relying on a single-block valve a double- block-and-bleed valving arrangement or a blind flange after the single-block valve should havebeen used; (4) no provision was made for the development, implementation, and enforcement of effective permit systems (for example, line opening); and (5) no permanent combustible gas detection and alarm system was located in the region of the reactors. Other factors that contributed to the severity of this disaster were also cited: (1) proximity of high-occupancy structures (control rooms) to hazardous operation, (2) inadequate separation between buildings, and (3) crowded process equipment.