Toxic gases and vapours

This technique detects substances qualitatively and quantitatively. The chromatogram retention time is compound-specific, and peak-height indicates the concentration of pollutant in the air. Detection systems include flame ionization, thermal conductivity and electron capture. Traditionally gas chromatography is a laboratory analysis but portable versions are now available for field work. Table 9.4 lists conditions for one such portable device. 

Portable or fixed multipoint colorimetric detectors are available which rely on paper tape impregnated with reagent. A cassette of the treated paper is driven electrically at constant speed over a sampling orifice and the stain intensity measured by an internal reflectometer to provide direct readout of concentration. Such instruments are available for a range of chemicals including those in Table 9.7. 

The basis of this technique is absorption of ir radiation by molecules over a wide spectrum of wavelengths to give a characteristic fingerprint spectrum providing both qualitative and quantitative data on the substance. This versatile technique owes its success in occupational hygiene to the development of a portable spectrometer. Table 9.8 lists some compounds detectable by one type of portable ir analyser. 

Several instruments are available that are designed to monitor a specific compound rather than a wide range of substances. The detection system varies according to the pollutant. A selection is given in Table 9.9. 

Centuiy ot nic vapour analysers are factory calibrated to measure total organic vapours according to a standard (methane). Since different organic vapours interact with the flame ionization detector (FID) to varying extents, it is vital that the instrument user be aware of the magnitude of the variation in order to obtain the most accurate data. Each user must determine relative responses for the individual instrument. 

Compound Relative response ( Column temperature1. Column packing retention time (mintsecs) B-8 C-8 T-8 

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Table 4.1 Densities of some toxic gases and vapours relative to air at 20°C ...

BS EN 45544 Electrical apparatus for the direct detection and measurement of toxic gases and vapours ... 

Toxic gases and vapours 218 Llammable gases 237 Toxic particulates 237 Sampling strategies 260... 

Hazardous substances present in the process are identified on the basis of their flammability, explosiveness and toxicity. The flammability of gases and vapours of flammable liquids is a great concern in the process industries. The result of an ignition can be a fire or an explosion or both. Accidental fires and explosions of flammable mixtures with air often follow the escape of combustible materials or inlet of air into process equipment. 

Industrial poisoning. The production of silicone products uses substances harmful for human health. These are inorganic substances (ammonia, chlorine, sodium and potassium hydroxides, sulfuric and hydrochloric acids, hydrogen chloride) and organic compounds of various types, such as hydrocarbons (methane, benzene and its homologues), chlorine derivatives (methyl- and ethylchloride, chlorobenzene), alcohols (methyl, ethyl, n-butyl, hydrosite), acetone, pyridine, etc. The information about their toxicity, explosion hazard, effect on human body, as well as maximum allowable concentrations of gases and vapours in the air at workplace can be found in special references.(Ryabov 1970). A comprehensive description of silicone substances is given in Table 29. 

Values for inhalation toxicity are based on 4 hours tests in laboratory animals. When experimental values are taken from tests using a 1 hour exposure, they can be converted to a 4 hour equivalent by dividing the 1 hour value by a factor of 2 for gases and vapours and 4 for dusts and mists. 

ISO TR 9122-3, 1933, Toxicity testing of fire effluents. Part 3, Methods for the analysis of gases and vapours in fire effluents. 

The decomposition of self-reactive substances can be initiated by heat, contact with catalytic impurities (e.g. acids, heavy-metal compounds, bases), friction or impact. The rate of decomposition increases with temperature and varies with the substance. Decomposition, particularly if no ignition occurs, may result in the evolution of toxic gases or vapours. For certain self-reactive substances, the temperature shall be controlled. Some self-reactive substances may... 

The decomposition of self-reactive substances can be initiated by heat, contact with catalytic impurities (e.g. acids, heavy-metal compounds, bases), friction or impact. The rate of decomposition increases with temperature and varies with the substance. Decomposition, particularly if no ignition occurs, may result in the evolution of toxic gases or vapours. For certain self-reactive substances, the temperature must be controlled. Some self-reactive substances may decompose explosively, particularly if confined this characteristic may be modified by the addition of diluents or by the use of appropriate packagings. Some self-reactive substances bum vigorously. Self-reactive substances include some of the following types of compounds aliphatic azo compounds (-C-N=N-C-) organic azides (-C-N3) diazonium salts (-CN2 Z ) N-nitroso compounds (-N-N=0) and aromatic sulphohydrazides (-SO2-N-NH2). ICAO 2-4.1.3.2, lATA 3.4.1.2.4... 

Hazard Zone means one of four levels of hazard (Hazard Zones A through D) assigned to gases, as specified in 173.116(a) of this subchapter, and one of two levels of hazards (Hazard Zones A and B) assigned to liquids that are poisonous by inhalation, as specified in 173.133(a) of this subchapter. A hazard zone is based on the LC50 value for acute inhalation toxicity of gases and vapours, as specified in 173.133(a). US 171.8... 

Use/Application These are required for continuous (or as per need) evacuation of big reactors, distillation columns, oil storage tanks to remove toxic, inflammable or corrosive gases and vapours for subsequent venting out through appropriate scrubbers or condensation if the material is to be recovered, especially for maintenance work inside the vessels. 

CONFINED SPACE A spacc which is substantially, although not always entirely, enclosed and where there is a reasonably foreseeable risk of serious injury from hazardous substances or conditions within the space or nearby. The risks may include flammable substances oxygen deficiency or enrichment toxic gases, fume or vapour ingress or presence of liquids free-flowing solids presence of excessive heat. For the purpose of the Confined Spaces Regulations 1997 a confined space means any place, including any chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or other similar space in which, by virtue of its enclosed nature, there arises a reasonably foreseeable specified risk. 

A fire involving foam rubber mattresses in a storeroom led to an unexpected and serious explosion. Subsequent investigation showed that an explosion risk may exist when the flammable smoke and vapour from smouldering of large amounts of foam rubber are confined in an enclosed space. Suitable strict precautions are recommended [1], Methods of quantitatively evaluating potential fire hazards from release rates of heat, smoke and toxic gases from heated foam rubber and plastics materials were developed, and their application to real situations discussed [2],... 

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