Accurate BTU determination crucial for SoCalGas metering.

Every year, billions of cubic feet of natural gas flow into and through the transmission and distribution system of Southern California Gas Co., Los Angeles. The biggest gas distribution utility in the U.S. provides clean-burning, versatile natural gas to more than 4.9 million residential, commercial

All gas purchased and sold by SoCalGas is done on an energy basis. Crucial to the company's measurement activities is determining the energy content of gas acquired from various pipelines and producers, and the energy content of the gas sold to millions of customers.

"When you buy and sell more than 900 Bcf a year, even a small percentage error in determining the BTU for a single day can mean a lot of money," said John R. Lansing, a process consultant in operations staff of SoCalGas's Energy Delivery Staff. "It is in the best interest of the gas company and the best interest of our customers to obtain the most accurate energy measurement we can."

Supply sources include offshore gas fields, onshore production and several pipeline suppliers, all with a varying degree of BTU content. The energy content of natural gas generally varies from the conventional 1,000 BTU per cubic foot. Keeping track of the energy content of the natural gas across SoCalGas' vast service territory is a complex task that relies on recent advances in gas sampling technology and the dedication of company employees.

The gas company uses sophisticated DynaPak 2010 on-line gas samplers from YZ Industries, Snyder, Texas, that acquire small natural gas samples on a regular basis from the pipeline stream. The energy content of the gas in the sample cylinders is determined with gas chromatographs, and the resulting information is coordinated with standard pressure measurements from volume meters to determine gas bills for customers.

BTU Districts

The SoCalGas service territory is divided into 47 BTU districts, based on the energy content of the pipeline system that feeds the distribution network. At least one sampler installation is located in every district to determine average BTU for billing.

Samplers are installed at custody transfer points wherever natural gas enters the SoCalGas system. Samplers are also located at the company's gas storage fields, where the energy content of the gas is measured on injection and withdrawal. At many locations the samplers are connected to nearby flow computers.

"About a third of our throughput delivered to 46 of our largest customers' facilities can real-time gas measurement data," Lansing said. "These large customers need more information about their purchases, and we provide this information with the system we have in place now." Except for these 45 sites, all gas sold requires the use of on-line gas samplers for BTU determination.

Volume information from more than 1,000 other non-core installations, generally large industrial and commercial customers, is collected daily with the use of automatic meter reading. The gas energy content from samplers is measured and then applied weekly at these locations. All other customers, residential, industrial and commercial, have energy content data applied monthly.

Field System

The DynaPak sampler removes a tiny sample of gas on a proportional-to-flow schedule, usually every 4 to 8 minutes depending on the pipeline pressure, and injects it into a 2,250-cc cylinder that is part of the installation. Pipeline pressures range from a low of 20 psig on distribution systems to more than 500 psig on some transmission systems.

Cylinders are collected weekly from all 200-plus locations by technicians and transported to a laboratory for analysis. A second cylinder, also assigned to the same sampler location, is then installed for the following week's gas sampling.

The cylinders are brought into one of the measurement gas analysis centers. The main location in Los Angeles processes approximately 100 cylinders per week and two satellite facilities, located at Ventura and Taft, each process approximately 50 cylinders per week. Each cylinder is measured twice, to minimize erroneous results, on a Daniel Industries' Model 500 four-minute gas chromatograph. Each analysis is reviewed and compared to the prior week's data to help ensure accurate information.

A computerized Gas Quality Measurement System collects all gas chromatography data and provides 3 previous years' of gas data on-line for review. The system developed by SoCalGas uses a Hewlett Packard Model 1000 computer.

The GQMS is the interface used by technicians to process all samples. Many features are included to minimize errors, speed production and increase system reliability. For example, bar coding is used to identify both the site and the cylinder being analyzed. This not only reduces keystrokes but minimizes errors and speeds analysis.

To ensure data integrity, a backup is made daily of all GQMS data before transmitting to a central computer for billing purposes.

Data analysis is only as good as the sample provided. Some unique features of the GQMS help ensure a quality sample is obtained.

Each cylinder is set with 2 psig remaining so that no air can enter the system. Cylinder pressure is recorded when it is removed the following week. The GQMS computer then calculates the actual gas quality injected during the week by considering the remaining gas from the prior week's analysis (the 2 psig volume).

"We have a very dedicated staff at the Gas Analysis Center," Lansing said. "Management stresses the importance of accurate, timely information. Technicians are very conscientious as they know their work directly impacts two-thirds of SoCalGas' revenue."

Sampler Benefits

A vital component of the SoCalGas measurement system is the YZ DynaPak sampler. The most recent in a long line of on-line pipeline samplers, the DynaPak uses microprocessor-based technology to provide reliable performance in a compact installation.

The DynaPak 2010 uses a DP-2000 pump designed for natural gas sampling. The pump, which has 30% fewer components than the unit it replaced, needs less field maintenance and any repairs are simple to accomplish.

"The old mechanical samplers required constant maintenance," Lansing said. "These new installations have been much more reliable. They also seem to be able to handle atmospheric conditions better as many sites are located near the ocean where corrosion has been a problem."

Conclusion

BTU measurement at SoCalGas has advanced during the past several years through the use of newer technology samplers and computer-controlled data collection systems. These new samplers not only provide a more accurate gas sample but reduce gas venting to atmosphere, are easier to maintain and permit volume-weighted sampling by accepting an output from flow computers.

To be competitive in today's energy environment, gas company must continue to implement more cost-effective equipment. SoCalGas' installation of more than 200 electronically controlled samplers is an example of modern technology being used to add customer value and increase a company's competitive position.