AGA PPP-2001 Purging Principles and Practice (Third Edition XK0101)《清洗原则和规范.第三版 XK0101》.pdf
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1、 PURGING PRINCIPLES AND PRACTICE THIRD EDITION June 2001 American Gas Association 400 N. Capitol Street N.W. Washington, DC 20001 Catalog No. XKO101 Third Edition Copyright O 2001 Registered by American Gas Association Printed in the United States of America PURGING PRINCIPLES AND PRACTICE PREFACE T
2、O THIRD EDITION This manual helps provide principles and practices for pipeline purging and describes equipment encountered in the natural gas industry. The information provided is based on sound engineering principles and good operating practices. The intent is to provide the operator with guide ma
3、terial to help safely and successfully plan and implement a purging operation. The operator should use this information with caution and recognize that the information may not be adequate for all conditions encountered. The material included provides guidelines for maintaining safe atmospheres insid
4、e pipes, holders, and other facilities that are to be purged into service or taken out of service. Good operating practice as well as federal and state laws require that precautions be taken to minimize or control mixtures of combustible gas in the air during purging, welding and cutting operations.
5、 New information presented includes information for purging pipelines developed by the Gas Research Institute (GRI), now known as the Gas Technology Institute (GTI). In addition, there have been significant improvements made in instruments that measure combustible gas mixtures. This publication is n
6、ot an operating code, but is instead guide material consisting of background information and descriptions of various methods and procedures found by experienced operators to be effective in minimizing or controlling combustible mixtures. Applicable federal, state and local regulations must be observ
7、ed. The methods and procedures described within cannot be considered to have universal application because of various job conditions. The operator is cautioned that the material presented may not be adequate under all conditions encountered. We also wish to acknowledge the following for assistance i
8、n preparation of this manual. Safety 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, well- informed people. Chapters 1
9、, 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
10、 part, are no longer in use. This information is included for historical reference. 1.2 GLOSSARY OF TERMS Words and expressions commonly used in purging procedures are defined below: Aeration: Provision of a constant supply of air by mechanical means. Blanking: Insertion of a solid metal plate acros
11、s a pipe at fitting flanges. Channeling: The occurrence of lighter gases or fluids flowing over heavier gases or fluids during a pipeline purging process. Clear: See Purge Combustible: Capable of being ignited and rapidly oxidized when mixed with proper proportions of air. Combustible mixture: A gas
12、 and air mixture that can be ignited at ordinary temperature and pressure. (See Flammable limits) Concentration: Percent by volume unless otherwise noted. Dilution: A form of purging in which replacement of one substance by another is accomplished with appreciable mixing. Disking: See Blanking Displ
13、acement: A form of purging in which replacement of one substance by another is accomplished without appreciable mixing. End-point: Attainment of concentration (percent by volume) of inert substance in the closed system being purged that subsequent admission of air, if purging out of service, or admi
14、ssion of gas or vapor if purging into service, will not result in formation of a flammable mixture. Exhaust gas: The products of combustion gas (primarily carbon dioxide and nitrogen) from an inert gas generator that is used as an inert gas for purging. Explosive limits: See Flammable limits Explosi
15、ve mixtures: Gas and air mixtures that can be ignited at ordinary temperatures and pressure. (Synonymous: Flammable mixtures) 2 Flammable limits: The lowest (lower limit) and highest (upper limit) concentrations of a specific gas or vapor in mixture with air that can be ignited at ordinary temperatu
16、re and pressure. (Synonymous: explosive limits, limits of flammability and limits of flame propagation) Holding purge: The procedure of maintaining in a closed system during maintenance or repair an inert gas or liquid which has been introduced to replace the normal combustible content. Hot cutting:
17、 Cutting by oxy-acetylene torch or other means into any pipeline or vessel containing only combustible gas at slightly above atmospheric pressure. Hot tap: Cutting into a pipeline containing a combustible gas or liquid by use of a special machine. The machine is attached to suitable fittings, which
18、have been previously welded or otherwise secured to the loaded pipeline. The tapping machine and fittings are so constructed that the required size opening may be cut in the loaded pipeline and the machine may be safely removed without appreciable loss of combustibles or taking the pipeline out of s
19、ervice. Inert gas: A gas, noncombustible and incapable of supporting combustion, which contains less than two percent oxygen and combustible constituents of less than 50 percent of the lower explosive limit of the combustible being purged. Inert purge: The act of changing the contents of a pipe or c
20、ontainer by using an inert substance to displace the original content or to separate the two media being interchanged. Flammable mixtures are thus avoided. Isolation: Disconnection from all other equipment or piping of a chamber or space to be purged. Liquefied natural gas (LNG): (From NFPA, 59, 199
21、8) A fluid in the liquid state which is stored at cryogenic temperatures of - 200F to -260F and is composed predominantly of methane possibly containing minor quantities of ethane, propane, nitrogen or other components normally found in natural gas. Liquefied petroleum gas (LPG): Any liquid or lique
22、fiable hydrocarbon, or mixtures of hydrocarbons, which are completely gaseous at 60F and 14.74 psig. and whose vapor pressure at 105F does not exceed 225 psig. (“American Society of Testing Materials” designation) Liquid petroleum gases usually consist of propane, propylene, butanes and butylenes, o
23、r mixtures thereof. Mixed gas: A term generally applied to mixtures of natural and manufactured gases normally having a range of 600 to 1000 Btu per CU. ft. In local instances, the term may refer to mixtures of different heating value manufactured gases. For purging purposes, it should be treated as
24、 a manufactured gas, unless containing more than 75 to 80 percent of natural gas. Natural gas: A mixture of gases produced by nature in the oil and natural gas fields and consisting primarily of methane and ethane and low percentages of carbon dioxide, oxygen and nitrogen, normally having a heating
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