1、 AGA Report No. 4A Natural Gas Contract Measurement and Quality Clauses Sponsored by Transmission Measurement Committee i AGA Report No. 4A Natural Gas Contract Measurement and Quality Clauses Sponsored by Transmission Measurement Committee Copyright 2009 American Gas Association All Rights Reserved
2、 Catalog No. XQ0904 ii iii DISCLAIMER AND COPYRIGHT The American Gas Associations (AGA) Operations and Engineering Section provides a forum for industry experts to bring collective knowledge together to improve the state of the art in the areas of operating, engineering and technological aspects of
3、producing, gathering, transporting, storing, distributing, measuring and utilizing natural gas. Through its publications, of which this is one, AGA provides for the exchange of information within the gas industry and scientific, trade and governmental organizations. Each publication is prepared or s
4、ponsored by an AGA Operations and Engineering Section technical committee. While AGA may administer the process, neither AGA nor the technical committee independently tests, evaluates or verifies the accuracy of any information or the soundness of any judgments contained therein. AGA disclaims liabi
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9、ations for compliance with this document. Any certification or other statement of compliance is solely the responsibility of the certifier or maker of the statement. AGA does not take any position with respect to the validity of any patent rights asserted in connection with any items that are mentio
10、ned in or are the subject of AGA publications, and AGA disclaims liability for the infringement of any patent resulting from the use of or reliance on its publications. Users of these publications are expressly advised that determination of the validity of any such patent rights, and the risk of inf
11、ringement of such rights, is entirely their own responsibility. Users of this publication should consult applicable federal, state and local laws and regulations. AGA does not, through its publications intend to urge action that is not in compliance with applicable laws, and its publications may not
12、 be construed as doing so. This report is the cumulative result of years of experience of many individuals and organizations acquainted with the measurement of natural gas. However, changes to this report may become necessary from time to time. If changes to this report are believed appropriate by a
13、ny manufacturer, individual or organization, such suggested changes should be communicated to AGA by completing the last page of this report titled, “Form for Proposal on AGA Report No. 4A” and sending it to Operations typical ranges of constituent and parameter values found in tariffs and contracts
14、; definitions for most of the terms commonly found in tariff and contract measurement and gas quality clauses; and appendices containing general purpose checklists and sample language for tariff and contract measurement and gas quality clauses, pipeline tariff survey data and appropriate references.
15、 2 Gas Measurement and Analysis Considerations It may be necessary for parties negotiating gas custody transfer contracts or pipeline tariffs to determine whether the gas will be accounted for in units of energy, volume, or mass delivered or transported, as well as any data reporting protocols. In m
16、ost instances, natural gas custody transfer measurement takes place on an energy basis. However, measurement in volume or mass units is equally important. The energy and mass concepts combine the quantitative and qualitative parameters of gas volume measurement into a single unit of measure. It shou
17、ld be noted that AGA Report No. 5, Natural Gas Energy Measurement, provides additional energy measurement detail that specifically addresses assumptions and criteria relevant to the determination of heating value and quantity of energy transferred. 2.1 Terminology/Units Terms used in the gas industr
18、y, although not necessarily consistent with other currently accepted publications, have historical significance. Contracts generally refer to tariff provisions for addressing measurement and gas quality specifications. These terms are in many existing operating and service contracts that form the st
19、ructure of the gas market and cannot be easily 2 discarded. Section 6 includes definitions of terms used in this report. The definitions provided are intentionally limited in scope, specific to the content of this publication. 2.2 Contract Base Conditions Volume is measured at the operating pressure
20、 and temperature and then adjusted to a pressure and a temperature specified in a contract (i.e., “base conditions”), to establish a consistent commercial value associated with the measurement. The same adjustment must be applied to directly measured or calculated volume-based heating value. Contrac
21、t base conditions vary somewhat across the industry, requiring mathematical conversions to the base conditions stipulated in contract. The North American Energy Standards Board (NAESB) recommends “standard base condition” for standardizing the reporting basis for gas volumes as cubic feet at conditi
22、ons of 14.73 psia and 60F, with dry gas. The pressure base condition of 14.73 psia is used by NAESB, AGA, and through adjustment of tabular data also by Gas Processors Association (GPA) and American Petroleum Institute (API). International Standards Organization (ISO) uses 101.325 kPa (14.696 psia).
23、 For gas volumes reported in cubic meters, the standard base conditions are 101.325 kPa (14.696 psia) at 15 C (59 F), with dry gas. Dry gas is defined in Section 6. Conversion methods are described in AGA Report No. 5 (AGA-5). The International Table British Thermal Unit (BTUIT) is the U.S. standard
24、 for natural gas contract energy measurement. A detailed treatment and discussion of the BTUITand joule relationship, including its dependencies, are included in AGA Report No. 5. Dry gas basis (dry BTU) is more common in contracts than a water-saturated basis since the required condition of natural
25、 gas transported to market in high-pressure pipelines is essentially dry. However, contracts may specify saturated, dry or actual water vapor content as the basis of custody transfer. It should be noted that heating value calculations may need to account for water vapor content if the gas is not ass
26、umed to be “dry” as specified within a tariff or contract. When energy is reported as the product of volume and volumetric heating value, it is imperative that the base conditions for both volume and volumetric heating value are identical. 2.3 Gas Measurement Natural gas measurement is a broad term
27、used to describe measurement of gas temperature, pressure, volume, mass and energy. AGA Report No.5 provides technical details for natural gas energy measurement and is referred to throughout this document. 2.3.1 Temperature Measurement Flowing gas temperature measurement is necessary so that deviat
28、ions from the base temperature can be monitored and accounted for in the adjustment and correction of the metered volumetric throughput. The gas volume adjustment for temperature is typically equated to a correction factor incorporating the ratio of the base temperature to the actual gas flowing tem
29、perature in degree Rankine or Kelvin (degree Rankine = 459.67 + temperature in degree Fahrenheit or Kelvin = 273.15 + temperature in degree Celsius). Such adjustments are important, especially when accounting for gas flowing temperatures significantly different than that of the base temperature, and
30、/or when gas volumes being measured are of large quantities, and/or when the value of measured gas is financially significant. 3 2.3.2 Pressure Measurement Like temperature measurement, flowing gas pressure measurement is also necessary to account for calculation of volumetric throughput adjusted to
31、 base pressure. The basis from which pressure is measured gives rise to the terminology of different types of pressure measurements common in contracts and tariffs, including absolute pressure and gauge pressure. A pressure above atmospheric pressure is termed “gauge pressure,”3when atmospheric pres
32、sure is taken as the reference point. Atmospheric pressure is applied in various methods across the industry, from using an assumed value to using an actual measurement in accordance with contract, tariff or company guidelines. When the reference point pressure is taken as zero, the pressure measure
33、d is called “absolute pressure.”4Absolute pressure is used in equations representing gas laws for calculation of volumes from one pressure base to another pressure base and flow rate. 2.3.3 Volume and Mass Measurement For natural gas custody transfer accounting, the quantity of natural gas exchanged
34、 is determined by either volumetric measurement or mass measurement. Volumetric measurement is more commonly used than mass measurement. For volumetric measurement, the measurements are typically made using various types of meters, which derive the adjusted and non-adjusted volumetric flow rate usin
35、g combinations of measurement data such as linear velocity, temperature, differential pressure, flow area, etc. Orifice, turbine, rotary and ultrasonic flow meters are some examples volumetric meters. Mass meters measure the mass of natural gas that passes through them without the need for conversio
36、n to base conditions. By measuring the mass, the total energy can be calculated by multiplying the measured mass by the heating value of the fuel per unit mass. Common mass meters include the Coriolis meter and the thermal mass meter. In contrast, the calculation of gas quantity using volumetric mea
37、surement requires several intermediate steps. First the gas volume, temperature and pressure must be measured. The volume is then corrected to the specified base condition of pressure and temperature by application of the laws of physics, such as Charles and Boyles laws. Finally, for a pressure high
38、er than atmospheric pressure, the volume must by adjusted by a gas compressibility factor in accordance with one of the procedures in AGA-5/AGA-8 to account for the non-ideal behavior of the real gas. Historically, most customer billing systems were set up based on volumetric measurement. With mass
39、meters, conversion from mass to volume may either be carried out within the meter onboard data processing electronics or within a remote terminal unit (RTU) or mainframe system. 2.3.4 Energy Measurement The total energy delivered and/or received in dekatherms or gigajoules is typically the basis on
40、which all financial accounting for custody transfer is performed. 3Gauge pressure can be specified as pounds per square inch gauge (psig) or kiloPascals gauge (kPa (gauge) 4Absolute pressure can be specified as pounds per square inch absolute (psia) or kiloPascals (absolute) 4 The successful determi
41、nation of the energy content of a given volume of natural gas depends largely on accurate measurement of the gas quantity, on either a volumetric or mass basis as described in section 2.3.3, and on either a compositional analysis or a direct measurement of heating value of the gas. Most often, the h
42、eating value is determined from compositional analysis where the heating value of a given quantity of the natural gas mixture is calculated at the base conditions by multiplying the mole fraction of each gas component by its respective heating value and then summing the component heating values for
43、the total mixture heating value. Alternatively, direct measurement of heating value can be made with calorimeters, thermal titrators, inferential/direct energy measurement devices spectroscopy, etc. Total gas energy measurement is achieved by two independent measurements: 1) gas volume in cubic feet
44、 or cubic meter converted to base conditions, and 2) the heating value reported as BTUs per cubic foot or megajoules per cubic meter at the base conditions. For example, 1,000 standard cubic feet (Mscf) of gas having a heating value of 1,000 BTU/scf contains 1 dekatherm of energy (1 million BTU or 1
45、 MMBTU) or 1,000 cubic meters of gas having a heating value of 38 megajoules per standard cubic meter contains 38,000 joules of energy. Similar calculations may be applied on a mass basis to determine energy, in which case the mass of gas in units of pounds or kilograms of gas is multiplied by the h
46、eating value of the gas mixture expressed in BTU per pound-mass or megajoules per kilogram. The energy equations and other useful information concerning energy measurement are provided in AGA Report No. 5. Table 1 in Appendix A of this report outlines various flow measurement references. 2.4 Gas Ana
47、lysis, Standard Methods however, other inerts include argon and helium. Diluents: include constituents that may be chemically reactive within the pipeline. The primary diluent is carbon dioxide; however, other diluents include oxygen and air and may include inerts such as nitrogen. It is important t
48、o note that some pipeline tariffs group all non-hydrocarbon gases together as “total inerts” but may also include specific constituent limits within the group (such as carbon dioxide, nitrogen and, in some cases, oxygen). Concentrations or combinations of inerts and/or diluents may impact gas facili
49、ties, pipeline operations and certain end-use applications. In some cases, inerts and diluents, such as nitrogen and air, are added to the gas stream in gas-conditioning applications to manage heating value and/or interchangeability. The reduction of heating value by total inerts is well understood. Caution should be exercised when considering specifications for total inerts as individual limits of carbon dioxide and/or nitrogen and oxygen may be necessary. 6The concentration of H2S and/or total sulfur may be restricted by local building departments, EPA definition of pipelin
