Gas Composition Alters Specific-gravity Flow Rates

 Hutchinson Utilities found itself in an embarrassing situation. "Our utility owed a big gas bill—we owed our customers money," said John Webster, director of the natural gas division at Hutchinson Utilities in Hutchinson, Minnesota, at the Emerson Global Users Exchange in San Antonio.

The problem started with a 2016 local newspaper article headlined, “Utilities Owes Big Gas Bill,” that highlighted a billing issue identified by Hutchinson Utilities caused by measurement errors. "We had to refund approximately $1.8 million in overbilling of natural gas that occurred over a two-year period, and it was directly tied to specific gravity—nothing else," said Webster.

The overbilling was caused by differences in Hutchinson's own mass flow measurements and Hutchinson customers' volume flow measurements. "Testing showed our Micro Motion Coriolis mass flowmeters were perfect, but they needed a specific gravity or base density value to calculate the gas standard volume flow rate for the actual volume flow at the custody transfer points," said Webster. "Unfortunately, the calculation method being used was not able to account for changes in the natural gas composition."

The problem was the Coriolis output needs a specific gravity to convert to standard volume, said Webster. "I looked it up and used about a week’s worth of data to create an average, fixed factor specific gravity to convert from mass to volume," he said. "That worked well until the source of gas changed. The makeup of the gas was totally different. We found measurement differences from fixed factor to actual of 7% due to gas composition."

It was clear we needed to go to real-time specific-gravity measurement, continued Webster. "It was changing every second," he said. "Adjusting it every two months didn't work, and it eventually led to the need to refund our customers."

The problem is not unique to Hutchinson Utilities, noted Steve Eckhardt, a sales representative who has been with Emerson for 37 years. "It's happening all around the world every day," he said. "The significance of this real-time specific-gravity measurement in the industry is unknown or unreported, or it's unaware or unprepared. Are you underbilling or are you overbilling? Both can cost you money."

Hutchinson Utilities is the longest municipal utility pipeline company in Minnesota and serves approximately 5,500 natural gas distribution customers in the area. There are 93 miles of pipeline with six interconnect stations along the natural gas transmission line. Each interconnect station includes Micro Motion Coriolis and gas-specific gravity meters to accurately measure the gas standard volume flow. In 2017, 6.23 billion cubic feet of natural gas were transported.

Webster provided a definition for specific gravity: the ratio of the density of any substance to the density of some other substance taken as standard, water being the standard for liquids and solids, and hydrogen or air being the standard for gases. He also defined standard cubic feet of gas, which is a volume of gas at standard reference conditions, typically 60 °F and 14.7 psia.

"We get a daily report in the morning from Northern Borders climate graph data that provides the BTU value, the specific gravity and the gas constituents," said Webster. "The problem is the gas chromatograph (GC) data is taken in Iowa. It doesn't represent what we see well. As the gas flows from south to north, each incremental piece of gas has a different specific gravity. It can vary from 0.6 to 0.65. As gas is dropped at each interconnect station, it makes it impossible to predict, based on the GC, what will arrive at the end of the line. You cannot use the pipeline GC if you want to provide accurate measurement."

Coriolis meters were already providing the mass flow measurement at each interconnect station and were working excellently. "We added a Micro Motion gas specific gravity meter (SGM) to provide specific gravity measurement at each interconnect," said Webster. "It provides real-time measurement with updates every four seconds, enabling a real-time conversion from mass flow (lb/hour) to gas standard volume (cubic feet). The signals were combined through an Emerson FlowBoss flow computer that outputs the mcf/hour (standard volume)."

The Emerson SGM uses vibrating element technology, a similar concept to Coriolis meters, to measure density, but with fluid surrounding the vibrating element. It directly measures molecular weight and specific gravity for gases with up to 0.1% accuracy.

The SGM enabled real-time conversion of mass flow measurement to gas standard volume. "We are now able to account for changes in gas specific gravity as they occur at each interconnect station," said Webster. "I'm totally impressed with the new real-time specific-gravity measurement method."

After successful installation of Micro Motion gas specific gravity meters, the savings, based on the difference in average specific gravity verses actual, real-time specific gravity from April 2017 to March 2018 was about $119,000 at the Hutchinson interconnect station. Based on these actual results, for every 1 billion cubic feet of natural gas measured, approximately $61,000 can be saved. And future billing concerns are avoided.