A large installed base of orifice meters, many installed where newer technology meters are impractical, accounts for a significant fraction of the measured volume of natural gas in the United States. Ultrasonic meters are newer technology meters that provide integrated diagnostics to monitor the meter’s health and identify flow problems such as distorted velocity profiles and other operational issues. The need exists for similar diagnostics for orifice meters to identify operational causes of measurement errors (and possibly measure their magnitudes), allow users to perform orifice meter maintenance as needed, reduce maintenance costs, and reduce the fiscal impact of significant measurement errors. Research was conducted to identify and recommend candidate technologies for development into practical, cost-effective orifice meter diagnostic tools.
Work began with the task of identifying potential methods and technologies to diagnose operational causes of orifice meter measurement errors and then recommending candidate approaches for testing. The member companies identified and ranked operational issues of interest, and a literature review identified nine potential diagnostic technologies and data analysis methods for the chosen issues. Each technology and method was evaluated for its ease of implementation with existing equipment, the cost of implementation, and how much or how little modification to a typical orifice meter run would be needed to apply the approach. Three approaches identified in the evaluation matrix were then chosen for limited testing: analysis of sound spectra generated by the orifice plate and upstream disturbances, the use of a traversing probe inserted through the orifice pressure taps to measure velocity profiles, and the use of an infrared imaging camera to look for temperature gradients along the pipe walls that might be caused by internal flow disturbances.
The three diagnostic approaches were evaluated at Southwest Research Institute in the Metering Research Facility (MRF) High Pressure Loop (HPL). The diagnostic approaches were tested using a four-inch-diameter orifice meter run installed in a configuration meeting American Gas Association (AGA) specifications. Tests began with a baseline run to confirm that the orifice plate and meter run met AGA-3 requirements. Subsequent tests involved different arrangements of flow conditioner blockages and an upstream meter tube roughened by simulated grime. Data collected during tests included orifice meter measurements of flow rate, documentation of the abnormal conditions created in the meter run, and diagnostic data from the various test equipment.
Of the three methods, only the traversing probe showed an ability to identify abnormal flow conditions and their causes. A multiport Pitot probe, inserted through the flange taps of an unmodified orifice fitting, successfully detected changes in the axial velocity profile and swirl characteristics that could be attributed to the plugged flow conditioner and dirty meter tube upstream of the orifice plate. A device based on this approach, incorporated into a practical field instrument, could provide technicians with a tool to identify the presence of poor flow conditions and the cause of the conditions, expediting maintenance on meter stations by eliminating searches for abnormalities. The traversing Pitot probe also demonstrated a potential to estimate measurement errors by the orifice meter, which would provide data for correcting custody transfer measurement errors and associated fiscal errors. Improvements and additional tests were identified to develop the potential of this approach to diagnose orifice meter errors and their fiscal impacts in a practical and cost-effective manner.