Ultrasonic Flowmeters for Liquid Hydrocarbon Custody Transfer ...

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ARC Insights, Page 2 applications, such as allocation, check metering, and leak detection. Only spool-type meters having two or more paths with permanently affixed transducers are acceptable such that samples can be taken along multiple acoustic paths. Excluded are meters having clamp-on style transducers due to the potential ease with which the transducers could become dislodged from their original mounting position. Naturally this approval makes suppliers of such UFMs very happy, but what does it mean for manufacturers? Proving Meter Certainty The minimum requirement for fiscal measurement is a high degree of accuracy. If a regulatory body is involved, statutory requirements generally become much more stringent. An accepted practice of verifying meter measurement is comparing a known prover volume to an indicated meter volume. In addition to mandatory requirements for proving mandated by regulatory agencies and/or standards organizations, other valid reasons include: CAUSE Hydraulic distortion of flow profile Meter accuracy Variations in fluid properties i.e. viscosity Maintenance required EFFECT Removes effect of installation hydraulics on flow profile Provides verification Eliminates variation Provides indication based on results Reasons for Proving Custody Transfer Meters Proving is the recommended method for validating meter repeatability and proving to API standards is common throughout the world. To ensure an accurate picture of meter performance API requires a minimum of 10,000 meter pulses be obtained. Based on the assumption of a one pulse error each time a detector is passed, a possible error of 0.02 percent exists considering the proving system has a start and stop detector as in a pipe prover. The more traditional custody transfer measurement technologies such as turbines are mechanical in nature and can be proved in a relatively short number of proving runs, usually five, due to the inherent inertia of the rotor. The sampling methodology of UFMs produces a greater degree of data scatter due to the technology’s ability to measure minor variations in velocity thus producing a wider range of repeatability. Under these circumstances UFMs may require larger proof volumes and a greater num- ©2003 • ARC • 3 Allied Drive • Dedham, MA 02026 USA • 781-471-1000 • ARCweb.com
ARC Insights, Page 3 ber of proving runs to achieve the same uncertainty as a turbine or positive displacement meter. In actuality there is no difference in a repeatability range of 0.05% in 5 runs versus a range of 0.14% in 12 runs, meaning the operator can choose to increase the number of runs to account for the inherent proving variability of different suppliers. The standard is practical without affecting performance allowing operators to select up to 20 runs to meet repeatability standards. Volumetric Proving Methods There are three commonly accepted proving methods, the pipe or ball prover, the small volume or compact prover, and the master meter. All three methods have their own idiosyncrasies and it is up to the discretion of the user which method to use. Regardless of the selected method, good industry practice recommends proving be performed with the flowmeter in place to account for any hydraulic effects on the installation. Stationery Pipe Prover Pipe Provers A pipe prover is a closed system generally consisting of a section of pipe of constant size in which a piston is moved along by the liquid to be measured. Meter pulses are counted as the piston moves between the start and stop detectors embedded in the pipe run. Because the volume between the detectors is known, the pulses per liter of product can be easily deduced. The advantage of pipe provers is continuous flow through the meter being calibrated. Bi-directional provers offer long term stability while unidirectional provers offer minimal pressure loss. Despite their large size, pipe provers will likely be the system of choice although their larger proof volume is necessary for reliable proving of ultrasonic flowmeters. Trailer Mounted SVP Small Volume Prover As the name implies, small volume provers (SVPs) are small in size and are sometimes referred to as compact or piston provers. To facilitate field calibrations, they can be made portable by mounting on a truck or trailer. In this fashion the prover is more useful across the plant and it becomes more economical than requiring a separate prover for each meter. In an SVP the displacer does not move freely as it is a piston connected to a piston rod. SVPs use a faster test cycle which minimizes prob- ©2003 • ARC • 3 Allied Drive • Dedham, MA 02026 USA • 781-471-1000 • ARCweb.com
Page 1: ARC INSIGHTS By Paula Hollywood INS

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