As the natural gas industry strives to become more efficient and meet customer demands, we find that the old ways of doing business cannot meet today’s standards of performance.

One example is the method of purchasing and selling natural gas. The days of custody transfer based strictly on volume are quickly coming to a close. More and more, natural gas is bought and sold in terms of total heating volume (Hv), which is the energy measurement standard of the natural gas industry.

To determine total heating volume, two pieces of equipment are essential: the electronic gas meter (EGM) and the gas chromatograph (GC).

Gas chromatograph

A typical GC consists of three parts: the sample conditioning system, the chromatograph oven and the chromatograph controller.

The sample conditioning system ensures that a high-quality sample is obtained from the gas stream. A sample probe, which must extend to the center third of the gas stream, is used. The gas is regulated at the sample probe, typically to 15 psig.

Special regulators must be used in order to prevent a refrigeration effect that can cause the heavy hydrocarbons to drop out of the gas stream. A sample loop, sometimes referred to as a speed loop, is used to reduce lag times between the pipeline and the GC.

Stainless steel tubing is used to ensure the sample stream is not contaminated by contact with the sample line itself. It is also important that the temperature of the gas in the sample line be kept above the hydrocarbon dew point, as this will also help prevent the loss of the heavy hydrocarbons. The gas should also be filtered in order to protect the integrity of the analyzer columns.

The gas enters the chromatograph oven through a sample injection valve that measures a repeatable sample of gas. The sample flow is then blocked and equalizes to atmospheric pressure. This sample is then transported through a set of chromatographic separation columns by carrier gas and separated by components.

As the components leave the column, they come into contact with a thermal conductivity detector. This detector senses the thermal energy of each component. Heating of the oven is accomplished by conduction rather than convection.

The chromatograph controller is responsible for all timing functions, which includes sample injection, column reversals and auto calibration. The controller also is responsible for peak detection, peak identification, the opening and closing of integration gates and the assignment of response factors. The controller then calculates the mole fraction of each component and logs the results.

The cycle of analysis begins with the injection of a precise volume of gas into the separation columns. Zero grade helium is usually used as a carrier gas to transport the sample through the columns. The columns are packed with a material that selectively retards the passage of hydrocarbon molecules based on the number of carbon atoms in the molecule.

Components with fewer molecules travel very fast through the column; these are considered “light” hydrocarbons. “Heavy” hydrocarbons have many carbon atoms attached and travel through the columns much more slowly. The GC uses this difference of travel times to separate the components so that each can be properly measured. The components are identified by the time in which they escape the column.

A thermal conductivity detector is used to measure each component and sends a signal to the controller. The controller uses the signal from the detector to calculate the heating value of each component. A signal may also be sent to a strip recorder that creates a physical recording of the peaks known as a chromatogram.

Once the peaks have been identified and quantified, the total BTU of the gas can be computed. Typical GC cycle times vary from 4 minutes to 12 minutes. A typical sample of natural gas would contain the following components:

  • nitrogen – 2.5%;
  • carbon dioxide – 1.0%;
  • methane – 89.57%;
  • ethane – 5.0%;
  • propane – 1.0%;
  • iso-butane – 0.3%;
  • n-butane – 0.3%;
  • neo-pentane – 0.1%;
  • iso-pentane – 0.1%;
  • n-pentane – 0.1%; and
  • hexane (C6+) – 0.03%.

Electronic gas meter

The electronic gas meter, often referred to as a flow computer, provides the volume for the Hv calculation. The EGM is a low voltage DC instrument, usually powered by either a 6-v or 12-v battery. A solar panel is often used to provide a charging voltage, which greatly extends the life of the battery.

The EGM can be used for a variety of applications, but is most commonly used as either an orifice meter or a positive displacement meter. An EGM can be broken down into two parts: measurement inputs and the main electronics board.

Measurement inputs refers to the transducers, resistive temperature devices (RTD) or analog measurements unit (AMU). These devices send signals to the main electronics board through an analog-to-digital converter.

In a positive displacement meter, the first input is the pulse input, sent to the main electronics board by a pulsimatic transmitter. Other signals include atmospheric pressure, sent by a pressure transducer, and temperature, sent by a RTD. Once these inputs are received, the main electronics board performs volume calculations based on AGA 7 standards.

For an orifice meter, signals are sent from a differential transducer, a pressure transducer or an AMU, which combines both the differential and pressure function, to the main electronics board. A temperature signal is also sent via an RTD. As these inputs are received, volume is calculated based on either AGA 3 or AGA 8 standards.

The main electronics board is responsible for the volume calculation as well as the storage of hourly and daily flow data. A typical EGM can store up to 35 days worth of hourly and daily flow information. The main electronics board is also responsible for the monitoring and reporting of alarm conditions, such as high or low differentials, low battery or low charging voltage. Some flow computers can provide an output voltage to direct other instruments to perform a task, such as taking a sample or injecting an odorant.

Bringing it together

Once the GC and EGM have performed their assigned functions, the data must be brought together to determine the total heating volume. There are several methods used to complete this task.

The oldest and most time-consuming method is to have the data collected locally and sent to a gas measurement group. Upon receipt of the data, the group must correlate all of the information and then make the Hv calculation.

Another method is to have the GC and EGM connected to a SCADA system, which would then poll each instrument at assigned time intervals, usually each hour, and perform the Hv calculations. This information can then be retrieved as needed.

Yet another method involves the use of an analyzer interface unit (AIU). An AIU is capable of receiving data from the GC and transmitting it to up to 30 EGMs. Once the EGM receives the data, it can do the Hv calculation. The AIU also can be tied into a SCADA system to allow instantaneous retrieval of the GC data.

Conclusion

In order to remain competitive in today’s energy market, it is up to the natural gas industry to take advantage of new and existing technologies in order to satisfy customer demands.

The switch from gas sales based strictly on volume to one based on total heating volume is just one way of serving customers more efficiently. It is important to remember that technology only provides us with new and better tools; it is up to the industry to determine the best ways to utilize them.

References:

1. Kizer, “Energy Measurement Using On-line Chromatographs,” International School of Hydrocarbon Measurement, 1998.

2. Sheets, “Fundamentals of Gas Chromatographs,” International School of Hydrocarbon Measurement, 1998.

3. Price, “Energy Measurement Using Flow Computers and Chromatographs,” International School of Hydrocarbon Measurement, 1998.

4. Smith, “Flow Computers,” TXUTechnical Training Manual, 1995.

5. McGraw - Hill, “Dictionary of Chemical Terms,” 1985.