API MPMS 14.7-2018 pdf downloadManual of Petroleum Measurement Standards Chapter 14—Natural Gas Fluids Measurement Section 7—Mass Measurement of Natural Gas Liquids and Other Hydrocarbons
1 Scope This standard describes the criteria for the equipment selection, design, installation, calculation processes, operation, and maintenance of single-phase dynamic liquid mass measurement systems that operate in the density range of 351 .7 kg/m 3 to 687.8 kg/m 3 at 1 5 °C and equilibrium vapor pressure. The mass measurement systems within the scope of this document include inferred mass measurement, where volume at flowing conditions is combined with density at similar conditions to result in measured mass, as well as direct mass measurement. 2 Application Liquids with densities below 351 .7 kg/m 3 or above 687.8 kg/m 3 at 1 5 °C and equilibrium vapor pressure and cryogenic fluids (colder than approximately –46 °C) are excluded from the scope of this document, but the principles described herein may apply to such streams. Mixed products, such as natural gas liquids (NGLs), should be analyzed to determine the mixture composition. This data should be considered in determining the measurement methods and equipment to be used. It is especially important to use mass measurement whenever the range of molecular sizes is great, such as in high ethane content (more than 2 % to 5 % ethane) NGL. It is less critical when molecular sizes are similar, such as in the case of mixed butanes. Sampling equipment and techniques are covered, including standards for analytical methods used to determine the composition of the sampled product. Equations of state and correlations used to calculate the density of the product are discussed. The standard used to convert mass to equivalent liquid volumes of components is also discussed.
Equipment exists that uses diverse principles for measuring volume, sampling the product, and determining the composition and density of the product. This standard does not advocate the preferential use of any particular type of equipment. It is not the intention of this standard to restrict future development or improvement of equipment. The contracting parties to any agreement should mutually agree on the equipment to be used. 3 Mass vs. Volumetric Measurement—Accuracy and Precision Implications Mass measurement techniques defined in this and other related industry standards eliminate the bias resulting from solution-mixing effects, as well as eliminating the uncertainty of the volumetric correction algorithms and tables. Mixtures containing multiple components of varying molecular sizes and NGL streams with high ethane concentrations should be measured using mass measurement techniques; otherwise, the quantity derived using volumetric techniques for these streams will always be lower than the volume determined from mass measurement. Volumetric measurement may 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 products, 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 related 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.
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 Measured Density Measured density of products shall be determined using density meters installed and calibrated in accordance with API MPMS Chapter 1 4.6, or as otherwise agreed between the contracting parties. 8.1.1 Density instruments or probes shall be installed as follows: 18.104.22.168 There shall be no interference between flow meter and density transducer or probe that would adversely affect either the flow or density measurement. 22.214.171.124 Temperature and pressure differences between the fluid in the flow meter, the density measuring device, and calibrating devices must be within specified limits for the fluid being measured and the mass measurement accuracy expected or required (see Figures 6 and 7, API MPMS Chapter 1 4.6). Insulation shall be provided when required. 126.96.36.199 Density meters may be installed either upstream or downstream of the primary flow device(s) in accordance with applicable 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).