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Prudent Practices in the Laboratory: Handling and Disposal of Chemicals
The use of glassware for work at high pressure should be avoided whenever possible. Glass is a brittle material subject to unexpected failures due to factors such as mechanical impact and assembly and tightening stresses. Poor annealing after glassblowing can leave severe strains. Glass equipment, such as rotameters and liquid-level gauges, incorporated in metallic pressure systems should be installed with shutoff valves at both ends to control the discharge of liquid or gaseous materials in the event of breakage. Mass flowmeters are available that can replace rotameters in desired applications.
Except as noted below, the use of plastic equipment for pressure or vacuum work should be avoided unless no suitable substitute is available. These materials can fail under pressure or thermal stress.
Tygon and similar plastic tubing have quite limited applications in pressure work. These materials can be used for hydrocarbons and most aqueous solutions at room temperature and moderate pressure. Reinforced plastic tubing that can withstand higher pressures is also available. However, loose tubing under pressure can cause physical damage by its own whipping action. Details of permissible operating conditions must be obtained from the manufacturer. Because of their very large coefficients of thermal expansion, some polymers have a tendency to expand a great deal on heating and to contract on cooling. This behavior can create a hazard in equipment subjected to very low temperatures or to alternating low and high temperatures.
Piping, Tubing, and Fittings
The proper selection and assembly of components in a pressure system are critical safety factors. Considerations should include the materials used in manufacturing the components, compatibility with the materials to be under pressure, the tools used for assembly, and the reliability of the finished connections. No oil or lubricant of any kind should be used in a tubing system with oxygen because the combination produces an explosion hazard.
All-brass and stainless steel fittings should be used with copper or brass and steel or stainless steel tubings, respectively. It is important that fittings of this type be installed correctly. Different brands of tube fittings should not be mixed in the same apparatus assembly because construction parts are often not interchangeable.
Teflon Tape Applications
Teflon tape should be used on tapered pipe thread where the seal is formed in the thread area. Tapered pipe thread is commonly found in applications where fittings are not routinely taken apart (e.g., general building piping applications).
Teflon tape should not be used on straight thread where the seal is formed through gaskets or by other metal-to-metal contacts that are forced together when the fitting is tightened (e.g., CGA gas cylinder fittings or compression fittings). Metal-to-metal seals are machined to tolerances that seal without the need of Teflon tape or other gasketing materials. If used where not needed, as on CGA fittings, Teflon tape only spreads and weakens the threaded connections and can plug up lines that it enters accidentally.
WORKING WITH HIGH/LOW PRESSURES AND TEMPERATURES
Work with hazardous chemicals at high/low pressures and/or high/low temperatures requires planning and special precautions. For many experiments, extremes of both pressure and temperature, such as reactions at elevated temperatures and pressures and work with cryogenic liquids and high vacuum, must be managed simultaneously. Procedures at high/low pressures should be carried out with protection against explosion or implosion by appropriate equipment selection and the use of safety shields. Appropriate temperature control and interlocks should be provided so that heating or cooling baths cannot exceed the desired limits even if the equipment fails. Care must be taken to select and use glass apparatus that can safely withstand thermal expansion or contraction at the designated pressure and temperature extremes.
High-pressure operations should be performed only in special chambers equipped for this purpose. Laboratory workers should ensure that equipment for operations using pressure vessels is appropriately selected, properly labeled and installed, and protected by pressure-relief and necessary control devices. Vessels must be strong enough to withstand the stresses encountered at the intended operating pressures and temperatures. The vessel material must not corrode when it is in contact with the material(s) it contains. The material should not react with the process being studied, and