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
25、value of 800 to 1300 Btu per CU. ft. and a specific gravity range of 0.59 to 0.75. Varying low percentages of propane, butane and gasoline may be present. It is not toxic but sufficient concentration in the atmosphere will produce oxygen deficiency. Ignition temperature is approximately 1600F. Flamm
26、able limits are approximately 4 percent to 14 percent gas in air. Operator: From DOT Pipeline Safety Regulations Part 192, a person who engages in the transportation of gas. Pig: A cylindrical, spherical, or barrel- shaped device that is moved through the pipe by gas or air or liquid introduced behi
27、nd it. When used for purging, the pig separates media being interchanged. It must be non-abrasive and non- sparking when flammable mixtures could be encountered. Pipe: See Pipeline Pipeline: Long cylindrical conduit or tubing used for transporting a gas or liquid. Purge: The act of removing the cont
28、ent of a pipe or container and replacing it with another gas or liquid. See Inert purge, Displacement, Dilution. (Synonym: Clear) Purge gas: Gas used to displace the contents of a container. To avoid flammable mixtures, the purge gas is usually inert except in certain instances where the relatively
29、small area of contact allows the amount of flammable mixture to be minimized and controlled satisfactorily. Purge into service: The act of replacing the air or inert gas in a closed system by a combustible gas, vapor, or liquid. Purge out of service: The act of repiacing the normal combustible conte
30、nt of a closed system by inert gas, air, or water. Pyrophoric: A substance or mixture that can ignite spontaneously. Slug: A quantity of inert gas interposed between combustible gas and air during purging. The slug does not fill the complete length of the 3 pipe but moves through the pipe as a separ
31、ate mass to prevent mixing of the gas and air. Slug shortening: Process that takes place as a slug of gas is mixed with the upstream and downstream gases as the slug travels the length of the pipeline. Supplemental Natural Gas (SNG): Also know as Substitute Natural Gas or Synthetic Gas (SG). A fuel
32、gas produced from coal, oil, oil shale, naphtha, LPG or waste, by processes that yield a gas that is generally interchangeable with natural gas. Stratification: Process of different gases settling into layers. Ventilation: The procedure in which doors, manholes, valves, etc. are opened to permit the
33、 ingress of air by natural circulation to replace gas contents. 1.3 FACTORS AFFECTING PURGING 1.3(a) DISPLACEMENT VS. DILUTION OR MIXING The replacement of one gas by another in an enclosed space or chamber takes place by means of two distinct actions: (1) displacement and (2) dilution or mixing. In
34、 a purge that is effected entirely by displacement, the gas or the air that is originally present is pushed out of the escape vents by the entering purge gas with little or no mixing of the purge gas and the original contents. Thus, the quantity of purge gas required for purging by displacement appr
35、oximates the quantity of gas or air being replaced. Frequently certain conditions, such as the size or shape of the chamber or the nature of the gases, cause the purge gas to mix with the original contents so that the purge tends to proceed by dilution. Purging by dilution can be accomplished in som
36、e situations by alternately pressurizing and depressurizing the facility. To accomplish a satisfactory purge by dilution or mixing requires a volume of inert purge gas that may be four or five times the free space of the chamber being purged. This occurs because as the purging proceeds, increasing a
37、mounts of purge gas are lost from the escape vents in mixture with original contents. Almost all purging operations are combinations of displacement and dilution actions. In actual practice it is impossible to avoid some mixing of the purge gas with the gas or air that is being replaced but, in gene
38、ral, the less the mixing or dilution, the more efficient the purge. Purging which proceeds with mixing or dilution such as occurs in tanks and holders should be accomplished with an inert purge medium to avoid flammable mixtures. Purging without the use of an inert medium should be limited to pipeli
39、nes where the amount of mixing can be controlled satisfactory by methods such as described in Chapter 5. 1.3(b) CAUSES OF DILUTION OR MIXING The factors affecting the relative proportions of displacement and mixing action in a purge should be understood thoroughly so that careful attention can be gi
40、ven to avoiding or minimizing those factors or conditions which promote mixing. Some of the more important causes of mixing during a purging are: (1) A large area of contact, promoting natural diffusion; (2) A long period of contact, permitting natural diffusion; (3) Agitation resulting from a high
41、input velocity; (4) Gravitational effects resulting from introduction of a heavy gas over a light gas or a light gas under a heavy gas; (5) Temperature changes and differentials causing convection currents. Failure to recognize the importance of such apparently insignificant things as the location o
42、f the purge gas input connection, the rate of input of the purge gas, or temperature differentials, can result in a purging operation being 80 to 85 percent dilution and only 15 to 20 percent displacement. 1.3(c)AREAOFCONTACT There is always some diffusion of the purge gas into the original gas and
43、of the latter into the purge gas at the surface of contact. The amount of mixing which results from this diffusion is dependent upon contact. The area of contact between the purge gas and the original contents 4 is dependent on the size, shape and internal construction of the chamber being purged. O
44、rdinarily little can be done to limit the mixing resulting from this factor. Nevertheless, contact area has a very great effect on the efficiency of a purge. When purging a tall, narrow tower, the area of contact between the gases is small compared to their volumes. Mixing is limited and the quantit
45、y of inert purge gas used may not be much greater than the volume of gas or air to be cleared out. The crown of a storage holder, in contrast, is a flat, shallow dome, having a height significantly less than the diameter. It is impossible to avoid having a very large area of contact in a chamber of
46、this shape. Consequently, it is usually necessary to use at least 1.5 to 2.5 volumes of inert gas per volume of free space in purging. When purging a pipeline, the area of contact may be so small that little mixing will occur. Advantage can be taken of this condition to conduct an inert purge by use
47、 of a quantity of inert gas that is only a fraction of the volume of combustible gas or air to be replaced. It is possible to introduce just enough inert gas to form a “slug” or piston between the original gas (or air) content and the entering air (or gas). This slug and the original gas or air ahea
48、d of it, is pushed along the pipe to the end of the section being purged by air or gas introduced after it. Recent research has greatly expanded the understanding of the slugging process, particularly for larger diameter pipelines. See Chapter 5 for more information. 1.3(d) TIME OF CONTACT The durat
49、ion of contact of the surfaces of the purge gas and the original gas or air should be as short as possible. Excessive mixing by natural diffusion will result if the purge gas input rate is too low. Interruptions and variations of the purge gas input rate should be avoided. 1.3(e) INPUT VELOCITIES The velocity of the entrance of the purge gas has an important effect on the nature of the purge. As a rule, the size of the purge gas inlet to containers other than pipe should be as large as practical, so that the input velocity will not exceed 2 or 3 feet
