IMPROPER GLASSWARE IN A CRYOGENIC FLUID
A thin-walled Pyrex NMR sample tube containing absorbed hydrocarbons on platinum on an alumina support, which had been sealed under vacuum and annealed, was placed in a dry ice and chloroform mixture in a Dewar flask in a hood with horizontal sliding sashes. The tube exploded after approximately one minute in the bath, apparently due to thermal shock. Although the Dewar was not damaged, the researcher suffered severely irritated eyes and had to be transported to the trauma center. The researcher had been wearing glasses and a laboratory coat as personal protection, and the hood sash had been slid to the side. Face shields, goggles, gloves, and acrylic shielding were available in the laboratory but had not been used.
The 18% chromium/8% nickel stainless steels retain their impact resistance down to approximately -240 °C, the exact value depending heavily on special design considerations. The impact resistance of aluminum, copper, nickel, and many other nonferrous metals and alloys increases with decreasing temperatures. Special alloy steels should be used for liquids or gases containing hydrogen at temperatures greater than 200 °C or at pressures greater than 34.5 MPa (500 psi) because of the danger of weakening carbon steel equipment by hydrogen embrittlement.
Liquid cryogen transfer lines should be designed so that liquid cannot be trapped in any nonvented part of the system. Experiments in supercritical fluids include high pressure and should be carried out with appropriate protective systems.
Vacuum work can result in an implosion and the possible hazards of flying glass, spattering chemicals, and fire. All vacuum operations must be set up and operated with careful consideration of the potential risks.
Although a vacuum distillation apparatus may appear to provide some of its own protection in the form of heating mantles and column insulation, this is not sufficient because an implosion could scatter hot, flammable liquid. An explosion shield and a face mask should be used to protect the worker, and the procedure should be carried out in a hood.
Equipment at reduced pressure is especially prone to rapid pressure changes, which can create large pressure differences within the apparatus. Such conditions can push liquids into unwanted locations, sometimes with undesirable consequences.
Water, solvents, and corrosive gases should not be allowed to be drawn into a building vacuum system. When the potential for such a problem exists, a water aspirator with a solvent collection device and a trap with a check valve installed between the water aspirator and the apparatus, to prevent water from being drawn back into the apparatus, should be used as the vacuum source.
Mechanical vacuum pumps should be protected by cold traps, and their exhausts should be vented to an exhaust hood or to the outside of the building. If solvents or corrosive substances are inadvertently drawn into the pump, the oil should be changed before any further use. (Oil contaminated with solvents, mercury, corrosive substances, and so on, must be handled as hazardous waste.) It may be desirable to maintain a log of pump usage as a guide to length of use and potential contaminants in the pump oil. The belts and pulleys on vacuum pumps should be covered with guards.
(See section 6.C.2 for a discussion of vacuum pumps.)
Although glass vessels are frequently used in low-vacuum operations, evacuated glass vessels may collapse violently, either spontaneously from strain or from an accidental blow. Therefore, pressure and vacuum operations in glass vessels should be conducted behind adequate shielding. It is advisable to check for flaws such as star cracks, scratches, and etching marks each time a vacuum apparatus is used. Only round-bottomed or thick-walled (e.g., Pyrex) evacuated reaction vessels specifically designed for operations at reduced pressure should be used. Repaired glassware is subject to thermal shock and should be avoided. Thin-walled, Erlenmeyer, or round-bottomed flasks larger than 1 L should never be evacuated.
Dewar flasks are under high vacuum and can collapse as a result of thermal shock or a very slight mechanical shock. They should be shielded, either by a layer of fiber-reinforced friction tape or by enclosure in a wooden or metal container, to reduce the risk of flying glass in case of collapse. Metal Dewar flasks