Vapours, hazardous equipment

Flashing of vapour eontaining entrained mist may oeeur on venting equipment or vessels eontaining volatile liquids. This may ereate a toxie or flammable hazard depending on the ehemieal with steam the risk is of sealding. Rupture of equipment ean produee a similar effeet. 

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. 

Quenching the vapour with cold air in the chamber may increase the rate of heat removal although excessive nucleation is likely and the product crystals will be very small. Condenser walls may be kept free of solid by using internal scrapers, brushes, and other devices, and all vapour lines in sublimation units should be of large diameter, be adequately insulated, and if necessary, be provided with supplementary heating to minimise blockage due to the buildup of sublimate. One of the main hazards of air-entrainment sublimation is the risk of explosion since many solids that are considered safe in their normal state can form explosive mixtures with air. All electrical equipment should therefore be flame-proof, and all parts of the plant should be efficiently earthed to avoid build-up of static electricity. 

Also according to Van Dolah ammonium nitrate-oil mixtures offer a certain dust explosion hazard and any electric equipment (switches controls, motors, lights) located in the plant should conform to the safety requirements or should be installed outside the plant. In order not to increase the dust explosion hazard no liquid hydrocarbon fuel with higher volatility than No. 2 Diesel fuel (minimum flash point of 145°F, ASTM closed-cup procedure) should be used as an admixture to ammonium nitrate. More volatile fuels, such as gasoline, kerosine or No. 1 Diesel fuel cannot be recommended according to Van Dolah, as they would seriously increase the hazard of a vapour explosion. 

Due to the quite different structures of industrial plants equipped with electrical installations, there are different probabilities for the existence of potentially explosive atmospheres formed by gas-air, vapour-air or dust-air mixtures. With respect to economical reasons, the types of explosion protection for electrical apparatus and systems will depend strongly on the explosion risk at the installation point. It is very unusual to construct and build all explosion protected electrical apparatus in such a manner that they can operate permanently in a surrounding hazardous atmosphere with combustible gases, vapours and dusts in air. The common way is to classify different areas in industrial plants according to the probability for the existence of a hazardous atmosphere and to establish adequate types of explosion protection. In other words, industrial plants with potentially explosive atmospheres are divided and classified into zones. ... 

To complete Chapters 6 and 7 describing zone 1 equipment, this chapter is focused on electrical apparatus for zone 0 and zone 2 application, i.e. hazardous areas endangered by combustible gases, vapours or mist. The relevant standards and their technical content have been covered in Sections 2.3 for zone 0 and 2.4 for zone 2. 

A hazardous area is one in which a fiammable mixture of gases or vapours may exist. Upon ignition the fiammable mixture will bum or explode, usually the latter. A similar situation can arise with combustible dust. The ignition can be caused by one of two methods. The usually attributed method is by a spark. The second is by a high surface temperature being attained, usually by operating equipment. 

Vapour baths may be used at temperatures up to about 600 K. They consist of a boiler flask connected to a vapour condenser in which the sample tube is suspended. The temperature is controlled by regulating the pressure at which the vapour condenses. If a pure liquid is used in the boiler this type of equipment gives good temperature control but is fairly complex to operate and can be hazardous if a sample tube breaks. 

Table 4.20 Types of ignition protection for electrical equipment exposed to hazards by gases and vapours (after [47])... 

These are not self-contained units, and extraction of volatiles is achieved by placing in a well-ventilated room or in a fume cupboard. Excellent ventilation and sensing equipment to detect vapour levels in the vicinity of the equipment are essential from the point of view of fire hazard and toxicity. 

Most flammable liquids only form an explosive mixture between certain concentration limits. Above and below this level of concentration the mix will not explode. The lowest temperature at which sufficient vapour is given off from a flammable substance to form an explosive gas-air mixture is called the flashpoint. A liquid which is safe at normal temperatures will require special consideration if heated to flashpoint. An area in which an explosive gas-air mixture is present is called a hazardous area, as defined by the British Standards, and any electrical apparatus or equipment within a hazardous area must be classified as flameproof. 

Compared with TDI, MDI is relatively safer in the working environment because it has a much lower vapour pressure. In practice, free MDI is unlikely to be a hazard at temperatures below 40 °C, especially if adequate extraction is provided at the point of application. If the application temperature is above 40 °C, or if the adhesive is spray applied, a vapour mist will always be formed and exhaust extraction must be used. To ensure a safe working environment, monitoring equipment should always be used to determine the level of isocyanate in the atmosphere, the maximum permitted level for MDI being 0.02 parts per million (0.2 mg m ). See Health and safety. 

---