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April 15, 2008
Uncertainty Projects in the PRCI Measurement Program
One of the more abstract issues that pipelines confront is the question of measurement uncertainty. Uncertainty is a complicated statistical concept that encompasses the fundamental design of a meter station, the primary measurement equipment (the meters), the associated data gathering and transmission systems, variation in pressure and flows around the original design point of a meter and station, as well as the ongoing physical changes to the equipment due to wear, fouling and periodic equipment calibrations. This can frustrate stakeholders who expect a simple answer to the question of how much energy is being delivered, but in reality must deal with an uncertainty band around that value. The costs to incrementally reduce this uncertainty can be extremely high as additional meters and supporting infrastructure are considered, and these costs must be borne by all parties so judgments must be made about the value of uncertainty reductions. Design and operating practices each have an impact on uncertainty, and two PRCI projects are underway that address this aspect of gas and liquid measurement.
Lower-Cost Liquid Meter Prover Calibration Method
This 2008 measurement project investigates the potential advantages of using a master meter calibration method over a water draw calibration method on on-shore stationary liquid meter provers. The study involves performing multiple field tests of each type of calibration on a newly installed BP Pipeline stationary prover in Pasadena, TX. The initial results from the water draw calibration tests indicated opportunities to improve the consistency of measured volume readings which is linked to the variation in the techniques used by the independent contractors who performed the recalibration. A second set of master meter calibration tests has been conducted and results are currently being analyzed. A primary cost saving element is that avoidance of the water draw method will eliminate the need to handle and process oil-tainted wastewater. The final report for this project will be available by September 1, 2008.
Natural Gas Meter Station Measurement Uncertainty Analysis Tool
This effort is focused on developing an uncertainty analysis tool (Excel based model) that allows end-users to assess the total measurement uncertainty of various natural gas meter station designs. The tool is being evaluated, and focuses on the impact of the number of meter runs on total station measurement uncertainty, a design capability that doesn’t currently exist in simple form. This will enable operators to better assess the impact of various station designs to optimize capital costs under a wide range of potential flow conditions. Another major accomplishment of the project is a literature search report to assess and validate current standards used as the basis for calculating uncertainty, such as the ISO “GUM” standard.
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