Four criteria for selecting the right industrial gas chromatograph for gas applications

The global drive for decarbonized and renewable fuel sources has created a strong demand for a simple, robust, field-mounted gas chromatograph for control and custody transfer applications. Four criteria are highlighted.

Process sensor insights

  • The natural gas market is evolving due to global warming concerns, leading to a broader range of renewable sources and the need for comprehensive gas quality analysis.
  • Modern gas chromatographs (GCs) are designed to be robust, field-mounted, and provide real-time results, contrasting with traditional large, sensitive, and costly lab-based models.
  • Key factors for selecting industrial GCs include certifications, accuracy, design features, and ease of maintenance to ensure reliable and precise gas analysis in diverse conditions.

The natural gas market is undergoing a paradigm shift as the entire world focuses on global warming and greenhouse gases emissions. Natural gas is replacing coal as the primary fossil fuel for electrical power generation, renewable natural gas is being produced from biological feedstocks, and hydrogen is being blended with natural gas to reduce carbon dioxide emissions.

All these efforts are creating a need to analyze natural gas streams to assure gas quality for custody transfer, and to accurately measure and control impurities. Historically, this task has fallen to industrial gas chromatographs (GCs), which do an excellent job but tend to be quite costly. Fortunately, smaller but fully capable GC options are entering the market to better serve this growing demand. This article discusses the key features a user should consider when selecting an online GC for natural gas applications.

Changing application requirements

Until recently, the natural gas market was fairly straightforward. Gas was produced at the well, scrubbed of impurities — such as water, hydrogen sulfide, and carbon dioxide — and shipped to customers via pipelines. Gas analyzers, typically GCs, were normally used in the gas processing areas, and then with pipelines for custody transfers.

The process got much more complicated as worldwide efforts to address global warming have increased. Natural gas is now being produced from a broad range of renewable sources, including manure, food scraps, animal fats, landfill materials, and other resources. Each of these sources has the potential to inject a wide range of impurities into the gas, each of which must be detected and controlled.

This market shift has also increased the number of natural gas producers by an order of magnitude, suddenly creating a huge array of companies seeking to inject their gas into the natural gas transportation network. Each of these injection points requires a full analysis of the incoming gas to ensure it meets the gas quality specifications, as well as the BTU range for the network (Figure 1).

Figure 1: Field-mounted industrial gas chromatographs (Emerson Rosemount 470XA shown) are becoming indispensable for quality control and custody transfer in biogas, hydrogen blending, and renewable gas applications.
Figure 1: Field-mounted industrial gas chromatographs (Emerson Rosemount 470XA shown) are becoming indispensable for quality control and custody transfer in biogas, hydrogen blending, and renewable gas applications. Courtesy: Emerson

The best technology for this application is a GC, which directs a gas sample to diffuse through a long, packed column. Each gas component moves through the column at different rates depending upon its boiling point, and when it reaches the detector at the end, it creates a measurable electrical response, often referred to as a peak. By measuring the size and timing of the peaks, the GC can determine the presence and concentration of each sample gas constituent. Based on those results, the analyzer can calculate the energy value of the gas, typically expressed in BTU, and make a variety of other gas measurements.

For decades GCs were located inside laboratories. The equipment was very large, expensive, and quite sensitive. A trained lab technician was required to prepare and inject the sample and interpret the results, which might take hours. Today’s natural gas market demands a GC that is robust and field-mounted, with results generated in minutes and reported continuously. These needs have spurred a host of industrial chromatograph product introductions generally referred to as online GCs.

Four criteria for industrial GC selection

There are a number of key features required for successful industrial chromatograph installations, and candidate analyzers should be carefully evaluated against this list. These features generally fall in to four categories:

  • Certifications and measurements

  • Accuracy and performance

  • Design and functional capabilities

  • Reliability, operability, and ease of maintenance.

Specific requirements will depend on the application, but this article focuses on the needs associated with quality control and custody transfer of natural gas, biogas, and renewable natural gas.

1. Certifications and measurements:

The first place to start when evaluating a custody transfer application is by fully understanding the required measurements and certified calculations. Typically, these include calculations performed per these standards:

  • American Gas Association (AGA) 8

  • Gas Processors Association (GPA) 2172 using GPA 2145 physical properties table

  • International Organization for Standardization (ISO) 6976.

Any candidate GC must be capable of performing the measurements required by the application or contract. For custody transfer applications a C6+ analysis is typical, with a three- or four-minute cycle time. In emerging carbon capture and storage applications, the ability to measure high levels of carbon dioxide is required, along with the ability to detect impurities that might adversely affect the compressibility of the gas. Renewable natural gas applications require the measurement of low BTU gas with high nitrogen and carbon dioxide content.

2. Accuracy and performance

The next area of evaluation should include the accuracy (repeatability in GC parlance1) and performance of the unit when subjected to the expected ambient conditions of the specific application. While field mounted GCs have been industrialized, the fact remains that the measurements analysis is still quite sensitive and must be performed under very tightly controlled conditions (Figure 2). A unit may be extremely accurate in lab conditions, yet perform far worse in the field when weather and other conditions vary.

Figure 2: Candidate analyzers must be able to maintain tight accuracy across a wide range of ambient conditions. This Rosemount 470XA Gas Chromatograph is being subjected to temperature cycling from 0 to 130 ℉. over 18 hours to confirm its performance under extreme ambient conditions.
Figure 2: Candidate analyzers must be able to maintain tight accuracy across a wide range of ambient conditions. This Rosemount 470XA Gas Chromatograph is being subjected to temperature cycling from 0 to 130 ℉. over 18 hours to confirm its performance under extreme ambient conditions. Courtesy: Emerson

When evaluating candidate GCs, end users should look at the repeatability under controlled and extreme ambient conditions. Some models may require installation in a temperature-controlled cabinet to meet their published performance specifications. Other units come in an industrially hardened cabinet rated for use in areas rated Class 1 Division 1, providing flexibility for mounting near the sampling point to provide highly repeatable measurements, despite extreme weather conditions. Typical energy value specifications for an online GC in custody transfer applications are +/- 0.025% calorific value, equivalent to +/- 0.25 BTU/1000 BTU, across a range of ambient temperatures from -4 to 140 ℉. To help guarantee the GC performance, it will often be subjected to environmental chamber testing during the manufacturing process.

3. Analyzer design and functional capabilities:

When all the candidate chromatographs meet the necessary certifications and repeatability measurements, the differentiator will usually be the design and features built into the unit. There are key features that make some models stand out for ease of configuration and operation.

End users should start by considering the basic design features of the analyzer. Some advanced applications may require multiple detectors and a very broad range of gas detection and analysis, and under those conditions, a top-tier GC will be required.

However, most natural gas and biogas analyzer applications have relatively straightforward requirements and can utilize a simpler and less costly single detector design. However, analysis time is important, so the time required to perform a measurement must be evaluated. A four-minute C6+ measurement is a typical requirement.

Probably the most useful and critical feature will be the local operator interface (Figure 3). A fully graphical interface is not often found on Class 1 Div 1 rated units, but having such an interface makes a significant difference for configuration, operation, calibration, and maintenance.

Figure 3: A well-designed gas chromatograph operator interface can be used to monitor measurements, access alarms and diagnostics, configure and calibrate the unit, and perform key maintenance tasks.
Figure 3: A well-designed gas chromatograph operator interface can be used to monitor measurements, access alarms and diagnostics, configure and calibrate the unit, and perform key maintenance tasks. Courtesy: Emerson

A fully functional operator interface should allow the user to access current and past analysis reports, check and acknowledge alarms, start and stop analysis, modify configurations, run calibration and validation cycles, and access maintenance and diagnostic data.

In addition to a local interface, the unit should include a software application, which can be used to configure, operate, and maintain the unit, and to access detailed measurement analysis details (Figure 4). The software should support multiple GCs, allow both local and remote operation and configuration, and be able to perform detailed analysis of the results.

Figure 4: The GC software should include the following features, as shown above and listed below, along with a letter designation: a) Simple drop-down menus, b) access to multiple GCs, c) full measurement display, d) response factors, e) timed events table, f) listing of components, g) overlay reading on archived readings, and h) ability to save to a hard drive.
Figure 4: The GC software should include the following features, as shown above and listed below, along with a letter designation: a) Simple drop-down menus, b) access to multiple GCs, c) full measurement display, d) response factors, e) timed events table, f) listing of components, g) overlay reading on archived readings, and h) ability to save to a hard drive. Courtesy: Emerson

The GC should contain an adequately sized memory to store thousands of analysis results. More capable units can store hundreds of days of analysis results at a four-minute cycle time, save a year of calibration and validation records, and provide a year or more of hourly, daily, and weekly averages of up to 250 variables.

The ability to handle multiple switched sample streams in a single analyzer can be a useful feature in some applications, as is the ability to utilize a variety of carrier gases. Standard GCs can accept three switched streams and can use hydrogen, helium, and even some specialized carrier gas alternatives. Note that when utilizing switched gas streams, the time between measurements of any one stream will be extended, so fast measurement times may be critical when measuring multiple streams.

Local I/O points and multiple communication options are typically offered. The specific local I/O requirements will vary by application, but often at least one or two analog outputs may be required for local control and a digital output may be required for alarm purposes. Support for multiple serial and Ethernet communication options should be included.

One of the most important keys to success for any GC is the sample conditioning system that feeds it. Any GC will quickly become inoperable if the sample is not properly prepared for analysis. A properly designed sample conditioning system will provide a timely, clean, and dry sample that has not been chemically altered by the sample conditioning process. Some users have the knowledge and experience to design their own systems, but many must rely on outside help for this specialized design effort. The GC vendor or its partners should provide this critical service, and an easy means to incorporate the sample conditioning equipment into the GC module should be available.

4. Reliability, operability and ease of maintenance

Like any analyzer, GCs do require ongoing maintenance. However, modern GCs have incorporated a wide variety of design features and embedded software applications to allow the GC to self-perform most of this maintenance, and to help the technician perform the rest as easily and quickly as possible.

Key features of software embedded in the GC include configuration and start up commissioning support, as well as automated calibration and validation procedures. The GC software should also provide automatic valve timing adjustment to optimize performance, and it should provide guidance to ease routine maintenance, such as calibration gas changeouts.

A properly designed GC will minimize sample venting and carrier gas use by providing sample atmospheric venting flow of around 20cc/minute max. Typical GCs will use only one helium bottle every ten to eleven months, with typical calibration gas cylinders lasting two years, depending on size. If the application allows it, the ability to use hydrogen as a carrier gas, rather than helium, can reduce operating costs.

When the GC does have a problem, the software should include a wide variety of alarms and diagnostics to help the technician quickly identify and correct the issue. If a significant failure has occurred, GCs with a single, replaceable module that contains the oven and most of the major serviceable components enable quick and simple replacement (Figure 5). The entire module can be easily replaced in the field, and the software has an embedded application to perform an auto calibration of the new module upon startup to restore the GC to service. The module should also be user serviceable if repair, rather than replacement, is required.

Figure 5: Easy to maintain GCs offer a single, field-replaceable module that contains most of the serviceable components. This allows technicians to quickly get a GC back into service, with only a single replacement part of multiple units required for stocking.
Figure 5: Easy to maintain GCs offer a single, field-replaceable module that contains most of the serviceable components. This allows technicians to quickly get a GC back into service, with only a single replacement part of multiple units required for stocking. Courtesy: Emerson

These features enable a technician to perform a major repair on the GC and get it back into operation in a very short time, invaluable for mission-critical applications.

Training can also be an important part of long-term reliability, so the selected GC vendor should provide a range of remote or on-site training options covering the start-up, operation, and maintenance of the unit.

Conclusion

Field-mounted industrial GCs are quickly becoming an indispensable item in biogas and renewable gas applications. Historically, these devices have been very costly, but a range of more cost-effective options has entered the markets. A properly designed GC incorporating software to aid in configuration, operation, and maintenance will provide years of reliable and trouble-free service, while delivering a wealth of accurate process data.

To choose the right GC, end users must fully understand the requirements of their application and look for the best combination of design, capabilities, and features to meet their needs. Consultation with a knowledgeable GC vendor may also be valuable to better understand the alternatives and available features, and to choose the right equipment for the specific application.

Written by

Michael Palacios

Michael Palacios is a global product manager at Emerson responsible for the Rosemount natural gas chromatograph product line. In this role, he works to implement product solutions that improve the reliability, dependability, and application of GC products to existing and emerging natural gas analysis markets. Palacios earned his BS in electrical engineering at Texas A&M University and an MBA University of Texas at Dallas.