• The ATG should have met the factory calibration tolerances specified in “API MPMS Chapter 3.1B”, prior to installation. • The ATG should meet the field verification tolerance for custody transfer as specified in “API MPMS Chapter 3.1B”, including the effects of installation methods and changes in operating conditions.
2.7 The Use of Automatic Shore Tank Gauges for Custody Transfer The use of automatic shore tank gauging systems for custody transfer measurement is becoming more frequent. The API MPMS standard that covers this is “Chapter 3.1B – Standard Practice for Level Measurement of Liquid Hydrocarbons in Stationary Tanks by Automatic Tank Gauging”. This standard separates the application of automatic tank gauges into two functions; inventory control and custody transfer. The use of automatic tank gauges [ATG] for custody transfer application requires a higher degree of accuracy than that of inventory control. There are numerous types of automatic gauging systems. While it requires little or no involvement from the inspector in obtaining a liquid level gauge there are several requirements that these units must comply with, if they are to be used for custody transfer, and verification of these requirements is part of the inspection process. These verification requirements are independent of the type of gauging system being used.
2.6 Meters
Terminal operators are responsible for the operation of their meters and meter provers. If meters are to be used for custody transfer, meters shall be proved in accordance with “API MPMS Chapters 4, 5, 12.2.4, and 12.2.5”, and this proving data should be provided to the Inspector. Prior to transfer, record the opening meter readings. Meter measurement tickets should be provided for each custody transfer and should include the information required in “API MPMS Chapter 12.2 - Calculation of Petroleum Quantities Using Dynamic Measurement Methods & Volumetric Correction Factors”. Terminal operators or inspectors who are aware of meter difficulties that could affect accuracy should report the problem immediately to all parties involved in the custody transfer. The incident and the resolution must be recorded in the inspection report. If manual and/or automatic shore tank measurements are taken, show a comparison with metered volumes. If volumes cannot be reconciled, recheck meter factors, shore tank measurements, and calculations. Report all results in the inspection report.
2.7.2 Factory Calibration Accuracy
Prior to installation, the reading of an ATG to be used for custody transfer should agree with a certified measurement instrument over the entire range of the ATG, traceable to a national standard, within ± 1 mm or ± 1/16-inch, and should be provided with a calibration correction table, with the exception of crude oil lease tank ATG’s used for custody transfer, which are required to agree within ± 3 mm or ± 1/8 inch. ATG’s for custody transfer applications installed prior to the effective date of the second edition of API MPMS Chapter 3.1B (2001), and crude oil lease tank ATG’s used for custody transfer which were installed prior to the effective date of the third edition of API MPMS Chapter 3.1B (2018) may not have factory calibration documentation.
2.7.1 Custody Transfer Usage
When used for custody transfer, an ATG must meet the following:
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