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Prudent Practices in the Laboratory: Handling and Disposal of Chemicals
reaction or unforeseen side reactions. All reactions under pressure should be shielded and should be carried out as remotely as possible, for example, with valve extensions and behind a heavy shield or with closed-circuit TV monitoring if needed.
Autoclaves and other pressure-reaction vessels should not be filled more than half full to ensure that space remains for expansion of the liquid when it is heated. Leak corrections or adjustments to the apparatus should not be made while it is pressurized; rather, the system should be depressurized before mechanical adjustments are made.
Immediately after an experiment in which low-pressure equipment connected to a source of high pressure is pressurized, the low-pressure equipment should either be disconnected entirely or left independently vented to the atmosphere. Either action will prevent the accidental buildup of excessive pressure in the low-pressure equipment due to leakage from the high-pressure side.
Vessels or equipment made partly or entirely of silver, copper, or alloys containing more than 50% copper should not be used in contact with acetylene or ammonia. Those made of metals susceptible to amalgamation (e.g., copper, brass, zinc, tin, silver, lead, and gold) should not come into contact with mercury. This includes equipment that has soldered and brazed joints.
Prominent warning signs should be placed in any area where a pressure reaction is in progress so that people entering the area will be aware of the potential risk.
All pressure or vacuum systems and all vessels that may be subjected to pressure or vacuum should be protected by properly installed and tested pressure-relief devices. Experiments involving highly reactive materials that might explode may also require the use of special pressure-relief devices and may need to be operated at a fraction of the permissible working pressure of the system.
Examples of pressure-relief devices include the rupture-disk type used with closed-system vessels and the spring-loaded safety valves used with vessels for transferring liquefied gases. The following precautions are advisable in the use of pressure-relief devices:
The maximum setting of a pressure-relief device is the rated working pressure established for the vessel or for the weakest member of the pressure system at the operating temperature. The operating pressure should be less than the allowable working pressure of the system. In the case of a system protected by a spring-loaded relief device, the maximum operating pressure should be from 5 to 25% lower than the rated working pressure, depending on the type of safety valve and the importance of leak-free operation. In the case of a system protected by a rupture-disk device, the maximum operating pressure should be about two-thirds of the rated working pressure; the exact figure is governed by the fatigue life of the disk used, the temperature, and load pulsations.
Pressure-relief devices that may discharge toxic, corrosive, flammable, or otherwise hazardous or noxious materials should be vented in a safe and environmentally acceptable manner such as scrubbing and/ or diluting with nonflammable streams.
Shutoff valves must not be installed between pressure-relief devices and the equipment they are to protect.
Only qualified persons should perform maintenance work on pressure-relief devices.
Pressure-relief devices should be inspected and replaced periodically.
The proper choice and use of a pressure gauge involve several factors, including the flammability, compressibility, corrosivity, toxicity, temperature, and pressure range of the fluid with which it is to be used. Generally, a gauge with a range that is double the working pressure of the system should be selected.
A pressure gauge is normally a weak point in any pressure system because its measuring element must operate in the elastic zone of the metal involved. The resulting limited factor of safety makes careful gauge selection and use mandatory and often dictates the use of accessory protective equipment. The primary element of the most commonly used gauges is a Bourdon tube, which is usually made of brass or bronze and has soft-soldered connections. More expensive gauges can be obtained that have Bourdon tubes made of steel, stainless steel, or other special metals and welded or silver-soldered connections. Accuracies vary from ±2% for less-expensive pressure gauges to ±0.1% for higher-quality gauges. A diaphragm gauge should be used with corrosive gases or liquids or with viscous fluids that would destroy a steel or bronze Bourdon tube.
Consideration should be given to alternative methods of pressure measurement that may provide greater safety than the direct use of pressure gauges. Such methods include the use of seals or other isolating devices in pressure tap lines, indirect observation devices, and remote measurement by strain-gauge transducers with digital readouts.