GENERAL DESIGN CONSIDERATIONS

1. GENERAL DESIGNCONSIDERATIONS Lecture 6

2. GENERAL DESIGN CONSIDERATIONS • The development of a complete plant design involves consideration of many different topics. • Quite understandably, the overall economic picture generally dictates whether or not the proposed facility will receive management approval. • However, the application of engineering principles in the design of such a facility in a safe and environmentally acceptable fashion, along with some general design considerations, will ultimately determine whether these earlier economic goals can be met.

3. Health and safety hazards • The potential health hazard to an individual by a material used in any chemical process is a function of the inherent toxicity of the material and the frequency and duration of exposure. • It is common practice to distinguish between the short-term and long-term effects of a material. • A highly toxic material that’ causes immediate injury is classified as a safety hazard while a material whose effect is only apparent after long exposure at low concentrations is considered as an industrial health and hygiene hazard.

4. The permissible limits and the precautions to be taken to ensure that such limits will not be exceeded are quite different for these two classes of toxic materials. Information on the effects of many chemicals and physical agents is accessible through computer databases such as MEDLAR and TOXLINE. • A number of health effects noted in these sources besides that of cancer are dermatitis, neuropathy, irritation, reproductive damage, and acute poisoning. • The inherent toxicity of a material is measured by tests on animals. The short-term effect is expressed as LDsO, the lethal dose at which 50 percent of the test animals do not survive. • Estimates of the LD,, value for humans are extrapolated from the animal tests.

5. On the other hand, the permissible limits of concentration for the long-term exposure of humans to toxic materials is set by the threshold limit value (TLV). The latter is defined as the upper permissible concentration limit of the material believed to be safe for humans even with an exposure of 8 hr per day, 5 days per week over a period of many years. • The handbook prepared by Sax? provides a comprehensive source of data as well as guidance on the interpretation and use of the data. Recommended TLV values are published in bulletins by the Occupational Safety and Health Agency (OSHA), the American Conference of Governmental Industrial Hygienists (ACGIH), the American Industrial Hygiene Association (ARIA), the National Institute for Occupational Safety and Health (NIOSH), and the United Ringdom Health and Safety Executive (HSE).

6. Sources of Exposure • The main objective of health-hazard control is to limit the chemical dosage of a chemical by minimizing or preventing exposure. • It is not practical to measure or control the chemical dosage directly; rather, exposure is measured and limits are set for the control of such exposure. • The most common and most significant source of workplace exposure to chemicals and also the most difficult to control is inhalation. • Workers become exposed when the contaminant is picked up by the air they breathe.

7. Thus, an understanding of the sources of contaminants to which workers are exposed is important for the recognition, evaluation, and control of occupational health hazards. • For example,mechanical abrasions of solid materials by cutting, grinding, or drilling can produce small particles which can form an airborne dust cloud or solid aerosol. Liquid aerosols, on the other hand, may be produced by any process that supplies sufficient energy to overcome the surface tension of the liquid. • This process occurs intentionally in spray coating and unintentionally when oil mist is generated from lubricants or coolants used on high-speed machinery. Liquid aerosols can also be produced by condensation.

8. Contaminant vapors are normally formed by allowing the liquid to evaporate into the air. A significant source of mercury poisoning is from worker exposure in laboratories where mercury has been spilled, trapped in cracks, and then evaporates at room temperature to exceed the TLV of 0.05 mg/m3. • Gases are usually stored or processed in closed systems. • Contamination of air with such gas occurs from fugitive emissions (leaks) or from venting. Essentially all closed systems leak to some degree. [The Environmental Protection Agency (EPA) through various studies has determined that emissions from just the synthetic organic chemical manufacturing industry in the United States are greater than 80,000 Mg/yr before emission controls are applied.] Obviously, the tightness of a system is directly related to the engineering and leak monitoring effort expended. This, in turn, depends on the consequences resulting • from these emissions.

9. High-value and very toxic materials are usually very tightly controlled. Contaminants that are neither valuable nor toxic but that create an undesirable atmosphere in neighboring communities are also controlled to maintain good public relations. • Flammable materials likewise are carefully controlled because a leak may lead to a fire and a possible major loss in life and facility. • Table 1 lists potential sources of air contamination in the chemical process industry, noting whether these are intermittent or continuous sources, whether workers are directly involved in the emission operation, the relative importance of the emission source, and the most probable control of the emission.

10. In typical well-maintained plants, pumps and valves are probably the major source of fugitive emissions. • Monitoring and maintenance efforts are therefore generally focused on these sources. Taken as a whole, fugitive emissions, even without major seal failure, are the origin of the continuous background exposure of workers. • This source of exposure may not, by itself, result in overexposure; but its presence reduces the margin within which other emissions may vary while still remaining under the acceptable TLV.

12. The continuous movement of materials through a process unit generally does not involve any situations for emission release and consequent exposure. • However, some material-handling steps are difficult to accomplish with total containment. • For example, whenever quantities of materials are allowed to accumulate in storage and then are removed for further processing, the possibility of release needs to be considered; for example, liquids entering fixed tankage generally displace air that must be vented to” avoid overpressuring” the tank. • Control of such liquid-transfer operations can be achieved by using variable volume tanks, particularly those with floating roofs, or by scrubbing, flaring, or recovering the vented gas stream.

13. Solids handling can provide considerable exposure to contaminants wheneverthe operation is performed in an open atmosphere. Where possible, such operations should be retrofitted with a closed system. • Even then, potential release problems exist, particularly during maintenance and repair of the system. • It should be recognized that the maintenance of any closed system can pose a hazardous exposure problem since most maintenance is performed while the plant is in operation and requires that workers be in close proximity to the operating equipment for long periods of time. • Under such conditions, it is necessary to consider not only local contaminant releases but also physical hazards that may be present, such as noise and thermal radiation.

14. In a closed system, equipment that must be repaired should first be cleaned to reduce exposure before the system is opened. • Where highly toxic process materials are present, it may be necessary to flush equipment with a low-toxicity stream, strip with steam, and then purge with nitrogen. • In such situations, the equipment design should include special fittings necessary for the flushing and purging procedures. • Turnarounds, or major periodic overhauls of chemical plant units, are a special case of plant maintenance. Since the units are shut down, some exposure risks are avoided.

15. However, since the unit is not in production, there is a time pressure to complete the turnaround and resume production. • In such an environment, there is the potential for disorganization and misunderstanding on the part of workers with the unanticipated release of contaminants. • To conduct a safe turnaround requires careful planning. • Contingencies need to be anticipated to the greatest extent possible and plans made to deal with them.

16. Fire and Explosion Hazards • Besides toxic emissions, fire and explosion are the two most dangerous events likely to occur in a chemical plant. Considerable resources are expended to prevent both of these hazards or control them when they do occur because of an accident. • These two hazards account for the major loss of life and property in the chemical and petroleum industry. For a fire to occur, there must be a fuel, an oxidizer, and an ignition source. In addition, the combustion reaction must be self-sustaining.