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Proving Liquid Ultrasonic Flow Meters for Custody Transfer Measurement Technical Paper Issue/Rev

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Proving Liquid Ultrasonic Flow Meters for Custody Transfer Measurement Technical Paper Issue/Rev Smith Meter® Liquid Ultrasonic Meters Proving Liquid Ultrasonic Flow Meters for Custody Transfer Measurement Technical Paper Issue/Rev. 0.0 (1/10) Bulletin TPLS002 Introduction ity, temperature, pressure, piping configurations, etc. It is therefore important in the proving process to define Liquid ultrasonic flow meters (LUFMs) continue to gain both the operating conditions and the accuracy require- popularity in petroleum measurement with the promise ments because they are interdependent. A change in of high accuracy and low maintenance. These are favor- operating conditions can affect the meter’s accuracy. able features, but because of the high volume and value The greater the change in operating conditions, the of petroleum products, buyers and sellers must have a more uncertainty is introduced into the measurement. high level of confidence in the accuracy of measure- As a general rule, the meter must be proven at operat- ment. This assurance in custody transfer measurement ing conditions to validate its accuracy with the highest is gained by adherence to the standards, procedures degree of confidence. and methods that define the measurement process. The object of all custody transfer meters is the same: There are two fundamental ways that petroleum prod- highly accurate measurement, but the procedures to ucts are measured: statically and dynamically. The achieve the results may vary with the different meter measured volume determined by either method must technologies. This paper will focus on the standards, pro- be validated at operating conditions and traced to a cedures and methods used to prove ultrasonic meters. fundamental standard. In a static system the product is transferred to a tank or similar container. After the trans- Custody Transfer Requirements action is completed the volume can be measured and validated by a suitable method. In a dynamic system Custody transfer refers to the fiscal measurement used PD, turbine or ultrasonic meters provide instantaneous to determine the quantity and financial value of a petro- information on the rate and volume of the transfer. As leum product transaction (delivery). The custody transfer with static methods the measurement results must be requirements can be of two types: validated by a suitable method. Legal A key difference between a static and dynamic measure- ment is time. A static measurement is similar to a bank Defined by Weights & Measures (W&M) in the country transaction, where the cash draw can be pulled and veri- or jurisdiction in which the sale is conducted. The vari- fied at any time. When a static transaction is complete, ous W&M codes and regulations control the wholesale there is time to review the measured volume, and if an and retail trade requirements to facilitate fair trade. The error is suspected, the volume can be re-measured. regulations and accuracy requirements vary widely be- With a dynamic system, the measurement must be tween countries and commodities, but they all have one right the first time; there are no means to re-measure common characteristic – traceability. There is always a the volume. procedure that defines the validation process where the duty meter is compared to a standard that is traceable The proving system and process are essential in both to the legal metrology agency of the respective region. static and dynamic measurement systems to validate The meters for wholesale and retail trade are normally the accuracy of volume measured. There are various smaller (4” and under) to handle volume flow rates of worldwide standards and regulations that define the under 2,800 l/m (750 gpm). These applications are out- measurement system requirements, but they are fun- side the range of current liquid ultrasonic meters and damentally the same. Each is traceable to a national therefore will not be addressed in this paper. standard, which in turn, is traceable to the international standard. In dynamic measurement systems this link Contract is the prover volume. There are gravimetric and volu- A contract is a written agreement between buyers and metric methods where the prover volume is traceable sellers that defines the measurement requirements. the International Bureau of Legal Metrology (BIML). These are large-volume sales between operating com- The technical arm of the bureau is OIML (International panies where refined products and crude oils are trans- Organization for Legal Metrology). This is an intergov- ported by marine, pipeline or rail. Since even a small ernmental organization that deals with all aspects of error in measurement can amount to a large financial legal metrology. It has 59 member states and 50 cor- difference, custody transfer measurement must be at responding members. In this way all the countries are the highest level of accuracy possible. Because of the tied to the same weights and measurement standards, critical nature of these measurements, petroleum com- which facilitates world trade. panies around the world have developed and adopted The various measurement standards either directly or standards to meet the industry’s needs. indirectly define the accuracy requirements over a range A typical contract may define a specific measurement of operating conditions which include: flow rate, viscos- standard such as the American Petroleum Industry The Most Trusted Name In Measurement (API) Petroleum Measurement Standards or an OIML Repeatability (of results of measurements) [VIM 3.6]: standard that is tied to the International Standards closeness of the agreement between the results of suc- Organization (ISO). These standards include all of the cessive measurements of the same measurand carried equipment, required and detail the process to achieve out under the same conditions of measurement which an acceptable level of measurement. include the same: measurement procedure, observer, Specifying a standard in a contract eliminates the need measuring instrument, conditions and location over a to enumerate many details concerning the equipment short period of time. and measurement process that are common industry API [1.0] adds “…the ability of a meter and prover practice. For example, API Standards are based on “Best system to repeat its registered volume during a series Practice..” They define the proper application parameters of consecutive proving runs under constant operating of a specific flow meter from field experience. OIML stan- conditions.” dards are “performance based.” They set quantitative Reproducibility (of results of measurements) conditions that any custody transfer meter must meet. [VIM 3.7]: closeness of the agreement between the To gain approval a specific size and type of meter must results of measurements of the same measurand car- be tested over the operating range by a notified body ried out under changed conditions of measurement. A (qualified testing agency) that issues a “pattern test valid statement of reproducibility requires specification report.” The “pattern test report” is then submitted to the of the conditions changed. The changed conditions may respective weights and measures authority as part of include: principle of measurement, method of measure- the application for approval. ment, observer, measuring instrument, reference stan- Even though the standards may appear to vary widely, dard, location, conditions of use and time. Results here they have two fundamental points in common: are usually understood to be corrected results. 1. They all strive to maintain a minimum measurement API [1.0] adds “…the ability of a meter and prover error for a specific application and system to reproduce results over a long period of time 2. They require traceability to national/international in service where the range of pressure temperature, metrology standards by proving the meter. flow rate, and physical properties of the metered liquid In terms of accuracy, which is expressed as uncertainty, is negligibly small.” a good API system would typically have a +/- 0.1% ex- Linearity of a Meter [API 1.0]: the ideal accuracy curve tended uncertainty. OIML requires a minimum system of volume meters is a straight line denoting a constant uncertainty of +/- 0.3% over a wide operating range. For meter factor. Meter linearity is expressed as the total large volume transfers, more precise measurement is range of deviation of the accuracy curve from a straight required, so the systems are proven over a more lim- line between the minimum and maximum recommended ited range of operating conditions, typically achieving a flow rates. +/- 0.1% extended uncertainty – the same as a system Bias [API 13] is any influence on a result that produces using API Standards. an incorrect approximation of the true value of the variable being measured. Bias is the result of systemic error. Measurement Definitions Error (of measurement) [VIM 3.10] is the result of a measurement minus the value of the measurand. Since The terminology associated with measurement can be the value of the measurand cannot be determined, in confusing, but is important in understanding the mea- practice a conventional value is used. surement process. Two good sources for measurement Types of Error [API 13 edited]: the difference between terminology are: the measured quantity and the true value of the quantity API Manual of Petroleum Measurement Standards includes all the errors associated with the measurement Chapter 1 – “Vocabulary” and Chapter 13 – “Statistical process – the person taking the measurement, the pro- Aspects of Measuring and Sampling” cess, the instruments and changes in conditions during VIM “International Vocabulary of Basic and General the period of measurement. In evaluating a system there Terms in Metrology,” commonly known as VIM, which are three (3) types of errors: is accepted by the international community of legal Spurious Error [API] is a gross mistake or blunder metrology that must be identified and eliminated. There are Included below is a summary of the key terminology statistical methods for testing for outliners (e.g., Dixon used in evaluating a meter’s performance. Definitions Outline Test) but data should not be discarded unless were selected from both sources.
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