ments. All cylinders containing flammable gases should be stored in a well-ventilated place. Reserve stocks of such cylinders should never be stored in the vicinity of cylinders containing oxygen, fluorine, chlorine, or other oxidizing gases. Reaction vessels should be equipped with pressure-relief devices.

Empty Cylinders

A cylinder should never be emptied to a pressure lower than 172 kPa (25 psi) because the residual contents may become contaminated with air if the valve is left open. Empty cylinders should never be refilled by the user. Rather, the regulator should be removed, and the valve cap should be replaced. The cylinder should be clearly marked as empty (MT) and returned to a storage area for pickup by the supplier. Empty and full cylinders should not be stored in the same place.

Cylinder discharge lines should be equipped with approved check valves to prevent inadvertent contamination of cylinders that are connected to a closed system where the possibility of flow reversal exists. Backflow is particularly troublesome in the case of gases used as reactants in a closed system. A cylinder in such a system should be shut off and removed from the system while the pressure remaining in the cylinder is still greater than the pressure in the closed system. If there is a possibility that a cylinder has become contaminated, it should be so labeled and returned to the supplier.

6.D.2 Other Equipment Used with Compressed Gases

6.D.2.1 Records, Inspection, and Testing

High-pressure operations should be carried out only with equipment specifically built for this use and only by those trained especially to use this equipment. 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. To ensure that the equipment has been properly designed, each pressure vessel should have stamped on it, or on an attached plate, its maximum allowable working pressure, the allowable temperature at this pressure, and the material of construction. Similarly, the relief 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 frequency of tests and/or inspections varies, depending on the type of equipment, how often it is used, and the nature of its usage. Corrosive or otherwise hazardous service requires more frequent tests and inspections. Inspection data should be stamped on or attached to the equipment.

Testing the entire assembled 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.

Before any pressure equipment is altered, repaired, stored, or shipped, it should be vented, and all toxic, flammable, or other hazardous material removed completely so it can be handled safely. Especially hazardous materials may require special cleaning techniques, which should be solicited from the distributor.

(See section 6.E.1 for further information.)

6.D.2.2 Assembly and Operation

During the assembly of pressure equipment and piping, only appropriate components should be used, and care should be taken to avoid strains and concealed fractures resulting from the use of improper tools or excessive force. Tubing in place in a pressure apparatus should not be used to support any significant weight.

Threads that do not fit exactly should not be forced (refer to section 6.D.1.2.1). Thread connections must match; tapered pipe threads cannot be joined with parallel machine threads. Teflon tape or a suitable thread lubricant should be used when assembling the apparatus (see section 6.D.2.2.6). However, oil or lubricant must never be used on any equipment that will be used with oxygen. Parts having damaged or partly stripped threads should be rejected (also see section 6.D.2.2.5).

In assembling copper tubing installations, sharp bends should be avoided and considerable flexibility should be allowed. Copper tubing hardens and cracks on repeated bending. Many metals can become brittle in hydrogen (H2) or corrosive gas service. Nickel alloys can generate Ni(CO)4 in some carbon monoxide atmospheres. All tubing should be inspected frequently and replaced when necessary.

Stuffing boxes and gland joints are a likely source of trouble in pressure installations. Particular attention should be given to the proper installation and maintenance of these parts, including the proper choice of lubricant and packing material.

Experiments carried out in closed systems and involving highly reactive materials, such as those subject to rapid polymerization (e.g., dienes or unsaturated aldehydes, ketones, or alcohols) should be preceded by small-scale tests using the exact reaction materials to determine the possibility of an unexpectedly rapid

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