Minimizing Faults in Gas Measurement

The potential for accurate measurement of gas flow in the pipeline is crucial for custody transfer. Specifically, the measurement uncertainties could prove expensive to the sellers and therefore cannot be invoiced. A leading interstate pipeline operator of natural gas in the United States employed the RTC temperature calibrator to optimize temperature accuracy of gas flow measurement.

RTC temperature calibrator

Challenge

In recent years, the industry has started using highly precise ultrasonic flow meters with the advancement of technology and decreasing costs. Ultrasonic sensors are not affected by pressure loss, do not have moving parts, and offer virtually maintenance-free operation.

However, one aspect that is usually overlooked is that the lack of an equally dependable temperature reading would make it impossible to accurately determine the actual gas volume through the pipeline. In order to achieve improvements in accuracy by employing the new ultrasonic flow meters, the company required the right solution to validate the temperature sensor measurements.

Since the accuracy of the ultrasonic flow meters was ±0.3%, it was determined that the uncertainty of the temperature calibrator for the RTD probe verification should be less than 0.1 °F (0.06 °C).

For many years, low-cost temperature calibrators with a higher uncertainty were used. A number of calibrators were tested and it was found that they do not meet the new requirements.

Solution

The company tested and made decisions to standardize the Reference Temperature Calibrator type RTC-157 in combination with a reference probe. Thanks to its active dual-zone heating technology, the RTC-157 was able to make measurements with an accuracy of ±0.07 °F (0.04 °C), adequate for the verification.

The conservative assumptions used for the calculation on savings and ROI were as follows:

  • Gas flow velocity: 50 ft/s (15.24 m/s)
  • Pipeline diameter: 10 inch (0.254 m)
  • Static pressure: 1080 PSI (74.11 bar)
  • Temperature: ~69 °F (20.56 °C)
  • Gas price: $4.4/MMBTU ($0.16/m3)

Benefits

  • Calibrator accuracy provides more than 1000% Return On Investment
  • Reduced uncertainty enables invoicing of an additional $216.000 annually
  • 1% improvement in gas flow accountability

Conclusion

Thanks to an enhanced temperature accuracy of 0.2 °F (0.11 °C), the error in the calculated flow volume per hour is reduced by 5.7 MMBTU (161 m3), which corresponds to $25.83 per hour or $18,597 per month of previously unaccounted gas value.

This information has been sourced, reviewed and adapted from materials provided by AMETEK Sensors, Test & Calibration.

For more information on this source, please visit AMETEK Sensors, Test & Calibration.

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