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Gas Chromatographs

Understanding Basic Field Operation and Their Critical Role in Natural Gas Revenue

Natural gas custody stations are equipped with gas chromatographs, which measure and report the composition of the natural gas at that point in the line. The value of the gas is based on its hydrocarbon composition; thus accurate reporting is critical to the company’s fiscal bottom line. In addition, correct installation and secure operation of the gas chromatograph (GC) is also key to the company revenue stream. Below explains how the gas chromatograph does its job and underscores the importance of the site installation for accurate performance. 

As gas field production has become automated, most gas chromatographs (GC’s) systems operate at remote metering stations. Ideally the system is housed in a protective shelter or enclosure as close to the sample point on the pipeline as is possible. The location should be easily accessible for maintenance technicians, provide for storage of calibration and carrier gas and have reliable power and remote communication connections.

► Gas Chromatograph Purpose & Operation

The purpose of the gas chromatograph (GC) is to measure the concentration of hydrocarbons in the natural gas and send the information to the control center for record keeping, billing and auditing. The GC does this in three distinct steps: take a sample of the flowing gas in the pipeline; separate the sample into its components; measure the concentration of the components as compared to the volume of the sample.

►Obtaining the Sample

A representative sample of the flowing gas is taken with a probe installed on the pipeline and transported through heated sample lines to the GC. The sample transport lines must be heated to a temperature 30*F above the expected hydrocarbon dew point (HCDP) to prevent hydrocarbon liquids from dropping out of the sample and compromising the accuracy of the measurement. Maintaining the sample temperature is critical through out the measurement process.

► Preparing the Sample

When the sample arrives at the GC, it goes to the Sample Handling Panel and passes through particulate and liquid filtration to remove contaminates. The sample enters a fixed volume sample loop, ensuring the samples all have the same volume.

► The Carrier Gas

A carrier gas is necessary to push the prepared sample gas through the GC process. The carrier gas is typically an ultra-pure gas like helium, hydrogen, nitrogen, or argon, which does not interact with the sample.

► Separating the Sample Gas

Next, the sample is injected into GC’s chromatographic columns. The chromatographic columns are housed in a heated compartment referred to as the oven. The stability of the temperature in the oven is critical to accurate sampling. Multiple columns and valves are used to speed up the separation process, with each column dedicated to separating out a specific set of components. 

The columns separate the sample gas into its components by physically retarding the flow, (some components will travel through the column faster than others) as well as in accordance to their boiling points, through several valve and loops.

The separation process is typically completed in 4 minutes.

► Measuring the Concentration of the Components in the Sample

Once the sample has been separated into its components, the components are sent to the detector. The most common type of detector used for pipeline gas analysis is a thermal conductivity detector or TDC. The TDC is preferred because it has good accuracy when the gases in the sample are known, and it is only the proportions of the content that must be determined.

► Analysis and Control

The controller is the brain of the GC, controlling the functions of the process from sample stream, valve operation and carrier gas injection, analysis of the data from the detector, calculations of the sample composition and communications to the control center. Through SCADA, the GC operation is remotely verified and calibrated daily and issues can be corrected, saving time for field technicians.

► Maintenance

Maintenance of the GC can be anticipated and scheduled by monitoring the performance diagnostics of the GC. Typical maintenance includes regularly replacing particle filters and moisture membranes on the sample-handling panel. Inside the GC, analytical valves periodically require overhaul and diaphragm replacement. Sample lines should also be flushed and dried.

► Solarcraft Gas Measurement Solutions

Solarcraft has been integrating  gas chromatographs in our enclosures for many years. We offer two enclosure solutions: a stand-alone gull-wing enclosure for installations where temperatures are mild year-round, and skid mounted insulated walk-in enclosure suitable for colder regions. Housing the gas analyzer in an enclosure close to the sample point is key to improving sample accuracy by reducing the distance and time to move the sample through the heated sample lines. Using a skid-mounted walk-in enclosure simplifies locating the GC close to the sample point by eliminating the headaches associated with digging or pouring a concrete pad.

The gull wing enclosure solution houses the gas analyzer, calibration standard, controller, internal heater and all tubing connections. Gull wing doors open up on both the front and back to allow a technician complete access to the GC. Carrier gas tanks are secured to the outside of the enclosure. Electrical connections are housed in an external explosion-proof box bolted to the outside of the enclosure. Option for skid mount is available.

For cold regions where the GC must be protected from sustained cold and snow accumulation, a skid mounted insulated walk-in enclosure protects s the entire system, sample gas, and the technician from cold temperatures, wind and precipitation. Solarcraft walk-in enclosures house the gas analyzer, calibration standard, controller, internal heater and all tubing connections, as well as communications, power and other field equipment. All systems are installed and tested for fast deployment when they reach the location. With an integrated forklift slot steel base, enclosures are easily transported and positioned on site.

In either solution, the GC and supporting systems are protected and secure for many years of accurate operation. When considering the role of the GC to the your company’s fiscal operations, investing in a quality well-designed field solution will pay for itself.

For more information on how Solarcraft can be of assistance contact a Technical Sales Associate today by calling at 877-340-1224.

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