1、Manual of Petroleum Measurement StandardsChapter 14Natural Gas Fluids MeasurementSection 7Mass Measurement of Natural Gas Liquids and Other HydrocarbonsGPA 8182-18FIFTH EDITION, FEBRUARY 2018Special NotesAPI publications necessarily address problems of a general nature. With respect to particular ci
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6、 proven, sound engineering and operatingpractices. These publications are not intended to obviate the need for applying sound engineering judgmentregarding when and where these publications should be utilized. The formulation and publication of API publicationsis not intended in any way to inhibit a
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8、ts do in fact conform to the applicable API standard.Users of this recommended practice should not rely exclusively on the information contained in this document. Soundbusiness, scientific, engineering, and safety judgment should be used in employing the information contained herein. American Petrol
9、eum Institute and Gas Processors Association 2018. All rights reserved. No part of this work may be reproduced, translated, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or other-wise, without prior written permission from the publisher.
10、Contact the Publisher, API Publishing Services, 1220 L Street, N.W., Washington, D.C. 20005.ForewordMeasurement by mass is often preferred for chemical reactions and various processes where the mass ratios ofcomponents are of primary interest in effecting control of the operation.Since the 1970s, th
11、e importance of measuring mixed streams such as natural gas liquids (NGLs) using massmeasurement techniques has been recognized. The volume at standard conditions of each component of an NGLmixture may be accurately derived from the mass measurement process because, unlike volumetric measurement,the
12、 mass measurement process is not sensitive to the effect pressure, temperature, intermolecular adhesion, andsolution mixing have on the measured stream.Solution mixing and intermolecular adhesion occurs when smaller molecules fill in the spaces between the largermolecules in the solution. Temperatur
13、e and pressure also affect the amount of shrinkage caused by solution mixingand intermolecular adhesion. Due to these behaviors, the sum of the volumes of individual components in their purestate is greater than the volume of the mixture.Today, mass measurement systems are commonly used to measure N
14、GL mixtures and ethane-propane mixes, aswell as products such as specification ethane, ethylene, and propylene. On the other hand, many propane, isobutane,normal butane, and natural gasoline streams are measured using volumetric techniques. A number of industry-developed standards address the design
15、, construction, operation, and maintenance aspects of mass and volumetricmeasurement systems. Volumetric measurement depends on tables and correlations to correct the volume measuredat flowing conditions to a volume at base conditions. The actual stream composition is important to both mass andvolum
16、etric techniques.GPA Midstream publishes specifications for some of the products resulting from natural gas processing andfractionation, including commercial propane, HD-5 propane, commercial butane, and others. Many companies alsohave specifications describing, among other things, the compositional
17、 requirements of a particular product. Massmeasurement is the recommended method of measurement for these mixtures.These specification products rarely, if ever, are comprised of a single component. Instead, specification products arethemselves a mixture of several components, and the actual composit
18、ion may vary somewhat over time as a functionof plant operation. Solution mixing, therefore, occurs in specification products, as well as in NGL mixtures. Industry-developed tables and correlations address physical properties of certain specification products, within the limits of theresearch databa
19、se. Volumetrically measured streams are then adjusted using these tables and correlations fortemperature, pressure, and density effects. Errors may result when performing these volumetric measurementadjustments if the composition of the stream does not match the compositions for which the volume cor
20、rection tablesand correlations were derived.This standard was developed jointly by GPA Section H, Measurement, Calculations otherwise, the quantity derived using volumetric techniques for these streams will always be lower than the volume determined from mass measurement. Volumetric measurement may
21、be suitable for pure or essentially pure products as the aforementioned concerns may not be as significant. Volumetric measurement is often considered to be acceptable for specification LPG products of relatively high purity, such as HD-5 propane, isobutane, normal butane, and natural gasoline produ
22、cts, which are essentially free of very small molecules such as ethane. Solution-mixing errors for these products may range from as much as greater than 0.5 % for high-ethane HD-5 propane to negligible levels for heavy natural gasolines. Volumetric measurement has an additional uncertainty that is r
23、elated to the fact that the assumed compositions the algorithms or tables are based on may not be exactly the same as the stream being measured. 2 API STANDARD 14.7 Mass measurement is also useful for high purity ethane, ethylene, or propylene streams, and may, in fact, be used for any other specifi
24、cation product stream with excellent results. 4 Base Conditions Absolute density, often referred to simply as density, is defined as mass per unit volume. Absolute density is not affected by the buoyancy of the fluid in air. Mass is an absolute measure of the quantity of matter. Weight is the force
25、resulting from acceleration due to gravity acting upon a mass. Changes in the acceleration due to gravity from one locality to another will affect the resulting weight force observed. Therefore, quantities determined in this procedure shall be by mass rather than by weight. This should be accomplish
26、ed through the use of procedures in API MPMS Chapter 14.6 by referral of weighing devices used to calibrate density meters to test weights of known mass. This referral or calibration is done in the same locality (and gravitational force) as the density meter location, eliminating the need for furthe
27、r correlation for variations in local gravitational force. Weight observations to determine fluid density shall be corrected for air buoyancy (commonly called “weighed in vacuum“). Such observations can be used in conjunction with the calibration of density meters or for checking the performance of
28、equation of state correlations. Procedures are outlined in API MPMS Chapter 14.6. Volumes and densities shall be determined at the same operating temperatures and pressures for mass measurement to eliminate temperature and compressibility corrections. However, equivalent volumes of components are of
29、ten computed from the determined mass flow. These volumes will be stated at Standard Conditions as follows: 5 Standard Conditions Temperature: 15 C (or 60 F). NOTE These standard temperatures are not equal. 15 C is equal to 59 F and 60 F is equal to 15.56 C. Pressure: The greater of 101.325 kPa (14.
30、696 psia) or product equilibrium vapor pressure at 15 C (or 60 F). 6 Abbreviations findicated density at operating conditions DMF density meter factor IMmindicated Coriolis meter mass IV indicated meter volume at operating conditions MFm meter factor when the Coriolis meter is configured to indicate
31、 mass MFmeter factor (volumetric) at operating conditions Qmtotal mass STANDARD FOR MASS MEASUREMENT OF NATURAL GAS LIQUIDS AND OTHER HYDROCARBONS 3 7 Mass Determination Mass measurement is frequently obtained by the following dynamic measurement methods: 7.1 Direct Mass Measurement In direct mass m
32、easurement processes, the meter factor is applied to the indicated mass measured by the meter. mmmMFIMQ = (1) 7.1.1 Coriolis Meters Coriolis meters used for mass measurement have an output in units of mass and do not require a density input to provide the mass quantity. Coriolis meters shall conform
33、 to API MPMS Chapter 5.6 for the service intended. 7.2 Inferred Mass Measurement Inferred mass measurement requires the volume at flowing conditions and the density at the same flowing conditions to be multiplied together, using appropriate and consistent units of measurement. vmfQ IV MF DMF= (2) Ca
34、re should be taken to ensure that the DMF is applied only once to the indicated density value. If the density is determined at a temperature and pressure different from the temperature and pressure at which the volume was determined, the flowing density shall be corrected to the temperature and pres
35、sure conditions at the volume meter. 7.2.1 Displacement or Turbine Meters When using displacement or turbine meters to determine volume at flowing conditions, refer to API MPMS Chapters 5.2 or 5.3. 7.2.2 Coriolis Meters Configured for Volumetric Measurement Coriolis meters may be configured to provi
36、de the flowing volume at operating conditions and determine the inferred mass flow rate by multiplying this volume by the density at flowing conditions from the Coriolis meter or a separate density device or calculation. NOTE This is not recommended since the measurement uncertainty is higher than u
37、sing the mass output directly from the Coriolis meter. 7.2.3 The density at flowing conditions may be measured by a density meter or may be calculated using an appropriate equation of state. Density meters conforming to API MPMS 14.6 have lower uncertainty and are preferred over calculated densities
38、. 4 API STANDARD 14.7 7.3 Orifice Meters 7.3.1 Use API MPMS Chapter 14.3 to determine the volume at flowing conditions for inferred mass measurement. 7.3.2 The mass measurement equations in API MPMS Chapter 14.3 may be used to determine mass flow rates through a system more directly than the volumet
39、ric method discussed in 7.3.1. These methods are typically used with ethylene and purity ethane streams. 8 Density Determination For inferred mass measurement, as discussed in Section 7, the density at operating conditions is required and may be determined by one of the following methods: 8.1 Measur
40、ed Density Measured density of products shall be determined using density meters installed and calibrated in accordance with API MPMS Chapter 14.6, or as otherwise agreed between the contracting parties. 8.1.1 Density instruments or probes shall be installed as follows: 8.1.1.1 There shall be no int
41、erference between flow meter and density transducer or probe that would adversely affect either the flow or density measurement. 8.1.1.2 Temperature and pressure differences between the fluid in the flow meter, the density measuring device, and calibrating devices must be within specified limits for
42、 the fluid being measured and the mass measurement accuracy expected or required (see Figures 6 and 7, API MPMS Chapter 14.6). Insulation shall be provided when required. 8.1.1.3 Density meters may be installed either upstream or downstream of the primary flow device(s) in accordance with applicable
43、 standards, but should not be located between flow straightening devices and meters, and shall not cause fluid to bypass the primary flow measurement device(s). 8.1.2 Density meter accuracy will be seriously affected by accumulation of foreign material from the flowing stream. This possibility shoul
44、d be considered in selecting density measurement equipment, determining mounting orientation, and determining the frequency of density equipment calibration and maintenance. Accuracy of the data recording, transmission, and computation equipment and/or methods should also be considered in system sel
45、ection. See API MPMS Chapter 14.6 for further comments. 8.2 Empirical Liquid density at flowing conditions may be calculated as a function of composition, temperature, and pressure when use of measured density is not practical. It is recommended that the calculated or measured density be applied in
46、real time to the volume measured at the flow meter. This provides for the maximum mass measurement precision; i.e., the incremental volume of liquid measured always corresponds in direct time relation to the density calculated or measured, and precludes errors caused by flow irregularities or stoppa
47、ges. However, it is common practice to use the composition of a sample taken continuously during the delivery period proportional to the volume delivered (flow proportional), and to use the average temperature and pressure for the delivery period. As compared with continuous density calculations, th
48、e uncertainty will be greater for density calculated from a composite sample at the average temperature and pressure measured during the delivery period. Calculations of density at flowing conditions may be made by means of empirical correlations or by generalized equations of state. The empirical c
49、orrelations are derived from fitting experimental data STANDARD FOR MASS MEASUREMENT OF NATURAL GAS LIQUIDS AND OTHER HYDROCARBONS 5 covering specific ranges of compositions, temperatures, and pressures, and can be inaccurate outside these ranges. GPA TP-1 for ethane-propane mixes and GPA TP-2 and TP-3 for high ethane raw make streams are examples of such correlations. Generalized equations of state have applications for a wide variety of systems and do not have strict limitations regarding composition and physical conditions; however, empirical correlations are more
