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Extended Low Flow Range Metering
Natural gas meters are often used to measure flows below their minimum design flow rate, resulting in significant errors. The objectives of this project were to examine parameters that contribute to measurement error at flow rates below 10% of a meter’s capacity, determine the expected range of error at these flow rates, and establish methods to reduce measurement error in this range. The project began with a literature search of prior studies of orifice, turbine, and ultrasonic meters for background information on their performance in low flows. Two conditions affecting multiple meter types were identified for study. Next, an analytical study focused on potential errors due to inaccurate temperature measurements. Numerical tools were used to model a pipeline with different thermowell and RTD geometries. The goals were to estimate temperature measurement errors under different low-flow conditions, and to identify approaches to minimize temperature and flow rate errors.
For the tests, an orifice meter, a Sensus Auto-Adjust turbine (AAT) meter, and a Sick Maihak ultrasonic meter were installed in AGA-recommended meter runs in a High Pressure Loop (HPL) metering facility. Test flow rates ranged from normal operating flow rates down to 6.25 actual cubic feet per minute (6.25 acfm) and a superficial velocity of 0.14 feet per second (0.14 ft/s). Tests were performed with heat transfer to the meter runs from the ambient air only, with heat trace simulating direct sunlight on the meter runs, and with pipe insulation blocking most heat transfer to the meter runs. Flow rate and diagnostic data were collected from the meters, and thermocouples were used to record pipe wall temperatures during tests. Approaches to reducing measurement errors were also tested experimentally.
Key Results
The Final Report is available for members on PRIME.