the vessel must be of the proper size and configuration needed for the process. Reactions should never be carried out in, nor heat applied to, an apparatus that is a closed system unless it has been designed and tested to withstand pressure.
Pressure-containing systems designed for use at elevated temperatures should have a positive temperature controller. Manual control using a simple variable autotransformer, such as a Variac, is not good practice. The use of both a back-up temperature controller capable of recording temperatures and shutting down an unattended system is strongly recommended.
(See section 6.D.2 above.)
In some localities, adherence to national codes such as the ASME (American Society of Mechanical Engineers) Boiler and Pressure Vessel Code (ASME, 1992) is mandatory. Selection of containers, tubing, fittings, and other process equipment, along with the operational techniques and procedures, must adhere to the constraints necessary for high-pressure service. The proper selection and assembly of components in a pressure system are critical safety factors. Compatibility of materials, tools used for assembly, and the reliability of connections are all key considerations.
Each pressure vessel in a laboratory should have a stamped number or fixed label plate that uniquely identifies it. Information such as the maximum allowable working pressure, allowable temperature at this pressure, material of construction, and burst diagram should be readily available. Information on the vessel's history should include temperature extremes it has experienced, any modifications and repairs made to the original vessel, and all inspections or test actions it has undergone. Similarly, the relieving pressure and setting data should be stamped on a metal tag attached to installed pressure-relief devices, and the setting mechanisms should be sealed. Relief devices used on pressure regulators do not require these seals or numbers.
All pressure equipment should be tested or inspected periodically. The interval between tests or inspections is determined by the severity of the usage the equipment has received. Corrosive or otherwise hazardous service requires more frequent tests and inspections. Inspection data should be stamped on or attached to the equipment. Pressure vessels may be subjected to nondestructive inspections such as visual inspection, penetrant inspection, acoustic emissions recording, and radiography. However, hydrostatic proof tests are necessary for final acceptance. These tests should be as infrequent as possible. They should be performed before the vessel is placed in initial service, every 10 years thereafter, after a significant repair or modification, and if the vessel experiences overpressure or overtemperature.
Testing the entire apparatus with soap solution and air or nitrogen pressure to the maximum allowable working pressure of the weakest section of the assembled apparatus can usually detect leaks at threaded joints, packings, and valves.
Final assemblies should be pressure-tested and leak-tested to ensure their integrity. Laboratory workers are strongly advised to consult an expert on high-pressure work as they design, build, and operate a high-pressure process. Finally, extreme care should be exercised when disassembling pressure equipment for repair, modification, or decommissioning. Protective equipment should be worn just in case a line or vessel that is opened contains material under pressure.
For any reaction run on a large scale (more than 10 g total weight of reactants) or at a maximum pressure in excess of 690 kPa (100 psi), only procedures involving a suitable high-pressure autoclave or shaker vessel should be used. Whenever possible, metal reactors with glass liners should be used instead of sealed glass tubes. Fisher-Porter-type tubes with a pressure gauge and release device are preferred for pressure reactions in glass equipment. However, it is sometimes convenient to run very small-scale reactions at low pressures in a small sealed glass tube or in a thick-walled pressure bottle of the type used for catalytic hydrogenation. For any such reaction, the worker should be fully prepared for the significant possibility that the sealed vessel will burst. Removal of any gas should be prepared for by appropriate ventilation. Every precaution should be taken to prevent injury from flying glass or from corrosive or toxic reactants by using suitable shielding. Centrifuge bottles should be sealed with rubber stoppers clamped in place, wrapped with friction tape and shielded with a metal screen or wrapped with friction tape and surrounded by multiple layers of loose cloth toweling, and clamped behind a good safety shield. Some bottles are typically equipped with a head containing inlet and exhaust gas valves, a pressure gauge, and a pressure-relief valve. When corrosive materials are being used, a Teflon pressure-relief valve should be used. The preferred source of heat for such vessels is steam, because an explosion in the vicinity of an electrical heater could start a fire and an explosion in a liquid heating bath would scatter hot liquid around