When the combustible gas content of a pipe, tank, or other enclosure is directly replaced by air, a mixture of gas and air within the flammable limits forms and remains during part of the operation. A similar mixture within flammable limits occurs when air is directly replaced by gas.

Carefully controlled purging of air from pipelines by direct displacement with natural gas has been safely practiced for many years with the recognition that some flammable mixture is present. Purging of natural gas from pipelines by direct displacement with air also has been similarly practiced.

There are many situations in which formation of flammable mixtures during purging should be prevented. As an example, flammable mixtures in large pipelines, tanks and other large volume containers cannot be safely controlled or tolerated even though there may be no apparent source of ignition present. Whenever flammable mixtures cannot be tolerated, their formation should be prevented by means of an inert purge, which involves using an inert substance to keep separate the two media being interchanged.

The basic requirement for a successful and safe purging operation is knowledge of the principles concerning the formation, analysis and control of gas mixtures. Additional requirements include a thorough preliminary study of the application of these principles for each situation; a well prepared procedure detailing the sequence of events, a predetermined rate of introduction of the purge medium and verification of end-points. Finally, the steps of the procedure must be followed and carried out by capable, wellinformed people.

Chapters 1, 2, 3 and 4 cover general principles of purging. Subsequent chapters discuss application of the principles to particular situations and provide some examples of typical procedures. The appendices include information on purging facilities that were common in the natural gas industry but for the most part, are no longer in use. This information is included for historical reference.