Hazard units

When considering release scenarios, the most hazardous unit in a plant should be chosen, based on inventoiy and process conditions. The idea is to imagine the release of material in the fastest way that is reasonably possible. The worst realistic scenario should be considered. This can be based on the outcome of a review, from a HAZOP study or a hazard analysis. The time a scenario will take is almost always considered to be continuous, because after a few minutes a stable dispersion distance exists. Making the time longer will not necessarily change the hazard distance. 

The first step in the procedure is to conceptually divide the process into separate process units. A process unit is a single pump, a reactor, or a storage tank. A large process results in hundreds of individual units. It is not practical to apply the fire and explosion index to all these units. The usual approach is to select only the units that experience shows to have the highest likelihood of a hazard. A process safety checklist or hazards survey is frequently used to select the most hazardous units for further analysis. 

Extra Hazard Units with light material, vapor cloud explosion potential, liquid inventory over 10,000 gal... 

The chemical reactor is the most hazardous unit in any chemical plant because most accidents occur by uncontrolled reaction, either within the reactor or after reactants have escaped the reactor and perhaps reacted with oxygen in air. Obviously no reactor or piping can withstand the temperatures and pressures of total combustion unless designed specifically for these conditions. We will consider the energy balance and temperature variations in continuous reactors in more detail in Chapters 5 and 6, while flames and explosions will be considered in Chapter 10. 

Control Room. The control room location can be critical to the efficient operation of a facility. One prime concern is to locate it the maximum distance from the most hazardous units. These units are usually the units where LPG or other flammables, eg, hydrocarbons that are heavier than air, can be released and accumulate at grade level. Deadly explosions can occur if a pump seal on a light-ends system fails and the heavier-than-air hydrocarbons collect and are ignited by a flammable source. Also, the sulfur recovery unit area should be kept at a healthy distance away as an upset can cause deadly fumes to accumulate. 

At the species level, the TU approach — a point-estimate approach — has been used to express the toxicity of one compound as a fraction of another with the same mode of action. Transfer of the TU principle to species sensitivity distributions (SSDs), by scaling compounds in a similar way, results in hazard units (HUs). The scaling is done on point estimates taken from an SSD, such as the HC5 or the HC50, or on any other point estimate that is considered relevant for the assessment (such as legal quality criteria). 

Alternatively, when using a whole-curve approach for compounds with allegedly the same mode of action, a single SSDs can be derived using (relative) concentration addition quantified by hazard units. This SSD represents the separate compounds and any mixture of these compounds. It is assumed that the mode of action in this case applies to all species from which the SSD is derived. The msPAF calculations for concentration addition (msPAFCA) are performed according to the protocols given in Section 5.6.2 (below). These protocols require toxicity data and SSDs for all components of the concentration-additive mixture for a variety of species. 

Toxicity data are scaled into dimensionless hazard units, preferably based on bio-available concentrations. A hazard unit is defined as the concentration where the effect criterion (e.g., NOEC) is exceeded for 50% of all species tested, that is, the median of the toxicity data of the whole data set, IHJJ = NOECj / NOEC, for i = 1 to n compounds and for / = I to m species, with HU/ = the scaled NOECs in dimensionless hazard units (mg-L 1 / rng-E ), and NOEC, = the median NOEC for substance i. The SSDs for each compound are obtained by fitting a log-logistic or log-normal model to the log toxicity data in hazard units. For the log-normal... 

Transfer of the TU principle to the community level results in hazard units or cumulative criterion units (CCUs). The most common types of hazard assessment use this concept. 

There is always some type of risk involved in a manufacturing facility. Risk is the possibility of loss or injury. Safety considerations are involved early in the design phase of a new plant to eliminate risk. Hazards are identified and plans designed to reduce or remove them. Emergency planning takes into account floods, hurricanes, inclement weather, evacuation routes, and the location of hazardous units. For example, in designing a new plant, the most hazardous process units would be located away from the most traveled and populated area of the plant. 

Berkowitz, J. B., Funkhouser, J. T., and Stevens, J. I., Unit Operations for Treatment of Hazardous Industrial Wastes, Noyes Data Corporation, Park Ridge, N.J., 1978. 

R. E. Hiachee, G. D. Sayles, and R. S. Keen, eds.. Biological Unit Processes for Hazardous Waste Treatment, BatteUe Press, Columbus, Ohio, 1995. 

Adipic acid is shipped in quantities ranging from 22.7 kg (50-lb bags) to 90.9 t (200,000-lb hopper cars). Upon long standing, the soHd material tends to cake, dependent on such factors as initial particle size and moisture content. Shipping data in the United States are "Adipic Acid," DOT-ID ALT 9077, DOT Hazard Class ORM-E. It is regulated only in packages of 2.3 t (5,000 lb) or more (hopper cars and pressure-differential cars and tmcks) (157). 

For chemical faciUties in the United States, hazard analysis is not an option if inventories of hazardous chemicals are maintained in amounts greater than the threshold quantities specified by the Occupational Safety and Health Administration (OSHA) regulation 1910.119. Many faciUties are finding that hazard analysis has many benefits. The process or procedure often works better, the quaUty of the product is improved, the process experiences less down time, and the employees feel more comfortable in the work environment after a hazard analysis has been completed. 

Shipment of hydrazine solutions is regulated in the United States by the Department of Transportation (DOT) which classifies all aqueous solutions between 64.4 and 37% N2H4 as "Corrosive" materials with a subsidiary risk of "Poison". Hydrazine has been identified by both the Environmental Protection Agency and the DOT as a hazardous material and has been assigned a reportable quantity (RQ) of 0.450 kg (1 lb) if spilled. Dmms for the shipment of these solutions must bear both the DOT specification "Corrosive" and "Poison" labels in association with the markings "RQ Hydrazine Aqueous Solution UN 2030." Aqueous solutions of 37% concentration or less are a hazard Class 6.1, UN 3293, Packing Group III and require "Keep Away From Food" placards and labels. 

C. C. Haun and E. R. Kiokead, Chronic Inhalation Toxicity ofHydra ne, University of California, Irviue, Toxic Hazards Research Unit, Dayton, Ohio, Jan. 1975. 

California and Minnesota have placed restrictions on the disposal of fluorescent light tubes, which contain from 40—50 mg of mercury per tube, depending on size. After batteries, fluorescent lamps are the second largest contributor of mercury in soHd waste streams in the United States (3,14). A California law classifies the disposal of 25 or more fluorescent lamp tubes as hazardous waste. In Minnesota, all waste lamps generated from commercial sources are considered hazardous waste. Private homes are, however, exempt from the law (14). Other states have proposed similar regulations. Several companies have developed technologies for recovering mercury from spent lamps (14). 

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