Pipeline-Scale Flow Measurement Standards for Natural Gas

NIST natural gas flow calibrations are performed offsite at the CEESI Iowa flow facility in Garner, Iowa under NIST’s metrological control. A bilateral key comparison done in 2010 between NIST and PTB demonstrates that NIST’s natural gas flow measurements are within their specified uncertainty and are equivalent with other national metrology institutes.

Operating conditions and uncertainties for natural gas flow calibrations:

• Line pressure: 5.5 MPa (800 psi) to 9.3 MPa (1350 psi) (varies seasonally)
• Pipeline scale: pipes with diameters ranging from 30.5 cm (12 inches) to 76 cm (30 inches)
• Flow Range: 0.125 m³/s (1.6 x 10⁴ acfh) to 9 m³/s (1.1 x 10⁶ acfh)
• Expanded Uncertainty (k = 2): increases from 0.22 % to 0.4 % with decreasing flow

The traceability of NIST’s natural gas flow calibration service is established using a bootstrap approach where an array of critical flow venturis (CFVs), each individually calibrated on NIST low pressure primary gas flow standards, are used in parallel to scale-up to high pressure natural gas flows. In particular, the CFV array is used to calibrate an array of turbine meters and ultrasonic flow meters, which in turn, are used to calibrate customer flow meters that buy and sell natural gas. NIST maintains metrological control of these offsite calibrations by ensuring all measurands necessary for flow calibrations (i.e., temperature, pressure, frequency, voltage, and gas composition) are traceable to NIST. All equipment used to perform natural gas calibrations (i.e., CFVs, turbine meters, ultrasonic flow meters, temperature transmitters, pressure transmitters, frequency counters, data acquisition, gas chromatograph, and environmental monitors) are routinely calibrated by NIST staff. All calibrations are performed via NIST protocol, and the raw calibration data is processed by NIST staff members who write the calibration report and determine the uncertainty of meter factor.

Future Directions:

NIST is working toward reducing the uncertainty to below 0.2 % and extending the low flow range to 0.0035 m³/s (400 acfh) with line sizes as small as 10 cm (4 inches).