Purging, as implied in this manual, is the process of a flammable gas being either removed from or introduced into a system. When a flammable gas is purged into or out of service by air, the interface between the gas and the air may create a flammable mixture inside the system as well as outside when it is discharged into air. A flammable gas, when purged into or out of service by an inert gas, may create a flammable mixture when it is discharged outside into the air. Proper purging operation will eliminate or minimize the formation and discharge of a flammable gas mixture, thus helping to avoid conditions for hazardous uncontrolled ignition.
Controlling formation of a flammable mixture during purging is necessary even though no apparent source of ignition is present. This can be accomplished by means of controlled injection of purge gas to prevent formation of a flammable mixture of gas and air. Figure 1-1 represents the concentration profile at the interface between the purge gas and the purged gas. The mixed gas region length (usually starting as 98% purge gas concentration at the beginning to 98% purged gas concentration at the end of the mixed gas region) grows as the gas interface moves along the pipe with increasing percentage of purge gas. Carefully controlled purging of air from pipelines by direct displacement with natural gas has been safely conducted for many years with the recognition that a controllable flammable mixture is present. Purging of natural gas from pipelines by direct displacement with air under controllable conditions is also considered an acceptable industry practice. Direct displacement purging techniques may be applicable to system other than pipelines where applicable.
An essential requirement for developing a successful and safe purging procedure is knowledge of fluid dynamics associated with pipe flow. These requirements include an understanding of pressure drop and flow through pipelines, flow velocity, fluid stratification and gas dispersion processes. Additional requirements include an understanding of basic gas chemistry characteristics including flammable limits of gas mixtures and ignition characteristics. A thorough understanding of the application of these principles for each site-specific situation is required in addition to a well prepared procedure and hazard assessment detailing the sequence of events, a predetermined rate of introduction of a purge medium and verification of end-points at properly vented locations. Finally, the steps of the procedure must be followed and carried out by properly trained and qualified individuals.
Chapters 1, 2, 3 and 4 cover purging theory and detailed discussions on purging control, instrumentation used to monitor purge status and the use of various inert purge media. Subsequent chapters apply the theories covered in chapters 1- 4 and provide examples of some practical applications. The appendices contain current practices as well as historical information on purging that were common in the natural gas industry but for the most part are no longer in use. Appendix A includes a glossary of terms used in this manual. Appendix B includes GRI Report â€œPipeline Purging Principles and Practices Research,â€ a technical guide that provides supplemental engineering information including a detailed sample calculation to determine purge time and velocity based on first principles of fluid dynamics and gas chemistry. Appendix C includes sample purging safety checklist, sample site-specific purge procedure for electricity generation operations, typical set up of purging operations by direct displacement method and typical set up of purging operations by slug purge method. Appendix D includes historical purge procedure examples for facilities such as gas holders and manufactured gas plant piping along with a series of charts describing flammable limits of gases associated with these facilities. Appendix E includes additional technical references.