Measurement of natural gas flow with an orifice meter is a well-established methodology; however, orifice measurement accuracy is of concern when flow rates are low and the differential pressure (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 in 10-inch and 4-inch orifice meter runs at differential pressures approaching 1 inch of water column (1in H2O), simulating low flow transmission meter stations and depleted production well stations. The results were analyzed to characterize and...
Measurement of natural gas flow with an orifice meter is a well-established methodology; however, orifice measurement accuracy is of concern when flow rates are low and the differential pressure (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 in 10-inch and 4-inch orifice meter runs at differential pressures approaching 1 inch of water column (1in H2O), simulating low flow transmission meter stations and depleted production well stations. 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. Transmitters tested included typical DP transmitters with stated accuracies of 0.1% of full scale, DP transmitters with stated accuracies as a percent of reading; and high-frequency-response DP transmitters. Data acquisition methods, transmitter technologies, and various calibrated measurement spans were studied for their potential to improve orifice meter accuracy at low DP conditions.
Where low differential pressure measurements are required, it was found that measurement accuracy can be improved by the use of the high accuracy or high-frequency-response transmitters. Transmitters with stated accuracies as a percent of reading demonstrated the best repeatability of all three types studied. The tests described here suggest that data collected using an analog data acquisition protocol produced measurements with better repeatability than data gathered using the HART protocol. However, this may be attributable to a faster data acquisition rate in the analog system. Further work is suggested to identify the root cause of the improved repeatability and determine how it might be applied at field stations.
An uncertainty analysis was performed to quantify various sources of error in low DP flow measurements. Sources included measured orifice meter dimensions, measured temperature and pressure, computed gas properties, DP transmitter calibrations, and the reference mass flow rates of the flow facility. The uncertainty in the orifice meter discharge coefficient was found to dominate the overall orifice meter error; contributions of the transmitter uncertainty were a much smaller fraction of the overall uncertainty estimate.