Natural gas meters sometimes operate with differential pressures (DPs) at the minimum acceptable level for proper measurement. In production applications and at production receipt points, depleted reservoirs can produce low DPs across custody transfer orifice meters, resulting in increased measurement uncertainty. A key component is the measurement uncertainty of the pressure transmitters, which are acceptable under typical flow conditions, but can interfere with accurate flow measurements in the lower DP range.
Measurement of natural gas flow with an orifice meter is a well-established practice; however, orifice measurement accuracy is impacted when flow rates are low and the DP across the orifice is at the extreme low end of common DP transmitter ranges. This research evaluated the performance of multiple types of transmitters at differential pressures simulating low flow transmission meter stations and depleted production wells. The results were analyzed to characterize and better understand the uncertainties and measurement errors associated with orifice meters operating with small bore diameters and low DPs.
Where low differential pressure measurements are required, it was found that measurement accuracy can be improved by the use of high accuracy or high-frequency-response transmitters. An uncertainty analysis was also performed to quantify various sources of error in low DP flow measurements.