Natural gas quality is usually measured online by a process gas chromatograph, which separates the gas into its components and reports their mole fractions; calorific value, density, relative density and Wobbe index are calculated from that composition. Contaminants such as water, heavy hydrocarbons and sulfur compounds are tracked either through an extended chromatographic analysis or by dedicated analyzers. The results are passed to the flow computer, where they feed the compressibility calculation and the conversion of metered gas into energy.
This page covers the analytical side of a metering station; meter technologies, station layout and custody transfer governance have their own pages.
What gets measured, and what it is used for
Billing, compressibility calculation, pipeline specifications and equipment protection each depend on different properties:
Property | How it is obtained | Where it is used |
Composition (mole fractions) | Measured directly by a process gas chromatograph | Basis for every other calculated property |
Calorific value (gross and net) | Calculated from composition using a standard method | Energy measurement and energy-based billing |
Density and relative density | Calculated from composition | Volume conversion and some meter calculations |
Wobbe index | Derived from calorific value and relative density | Interchangeability of gases at burners and turbines |
Compressibility factor | Calculated from composition, pressure and temperature with an equation of state | Converting line volume to base conditions |
Water dew point or water content | Dedicated moisture analyzer; a standard correlation links the two | Hydrate and corrosion risk, pipeline specification checks |
Hydrocarbon dew point | Direct measurement or calculation from an extended GC analysis | Liquid dropout risk; protection of fuel gas consumers |
Hydrogen sulfide and sulfur compounds | GC with a sulfur-selective detector or a dedicated analyzer | Corrosion, specification compliance and odorant tracking |
How a process gas chromatograph works
At fixed intervals, the chromatograph injects a small sample into a carrier gas stream that sweeps it through separation columns. Each component leaves the columns at a characteristic time and is picked up by a detector: thermal conductivity detectors are usual for the main components, while other detector types can be specified for trace levels and sulfur species. Calorific value, density and Wobbe index are then derived from the reported composition using methods defined in international standards.
For billing, the main components plus a lumped heavy-hydrocarbon fraction are often enough; calculating hydrocarbon dew point calls for an extended analysis that resolves the heavier components individually. The chromatograph is calibrated and verified routinely against a calibration gas of known composition, and a separate standard method exists for assessing whether an analytical system is fit for purpose over the expected composition range and error limits.
Sampling and sample conditioning: where reliable results begin
An analyzer is only as good as its sample. The sample system must deliver gas without altering its composition and with an acceptable delay:
· Probe type and location: Placing the probe where the flow is representative and away from low points where liquids may collect.
· Pressure reduction without condensation: Gas cools as its pressure drops. If the sample falls below its dew point, heavier components condense and the measured composition shifts, so the let-down point and any heating are planned together.
· Lag time: Long, oversized sample lines delay results. A fast loop, short runs and properly sized tubing keep the delay down.
· Filtration and material compatibility: Particles and droplets are kept away from the analyzer. When sulfur compounds are measured, the wetted surfaces of the sample path should be chosen so they do not adsorb these species.
· Vents and returns: Where sample gas and carrier gas are routed after analysis is settled together with the hazardous area classification and site rules.
Moisture, dew point and sulfur analysis
Water dew point and water content describe the same condition, and a standardized correlation converts one into the other. Cooled-surface condensation hygrometers are one defined method; other sensor technologies are used online. Hydrocarbon dew point is a separate property altogether; it is watched closely on gas turbine and engine supplies, where liquid hydrocarbons can damage combustion hardware.
Sulfur compounds, including hydrogen sulfide, carbonyl sulfide, mercaptans and the tetrahydrothiophene added as an odorant in distribution networks, can be measured with chromatographs fitted with sulfur-selective detectors or with dedicated analyzers. Which compounds are monitored, and down to what level, depends on the gas supply specification and on how sensitive the downstream equipment is.
Analyzer shelters and layout
Analyzers usually sit in a climate-controlled shelter or enclosure close to the meter run, designed together with carrier and calibration gas cylinders, the sample conditioning panel, vent headers and gas detection. The hazardous area classification shapes the protection concept for electrical equipment inside the shelter and its ventilation.
Linking analysis data to custody measurement
Quality data enters the measuring system in two ways. For compressibility, the flow computer uses the chromatograph composition when converting line volume to base conditions. For energy, volume at base conditions, or mass, is multiplied by a representative calorific value. Legal metrology documents treat calorific value determining devices as part of the measuring system rather than as stand-alone instruments.
Before designing this link, decide whether one chromatograph can represent several meter runs, which fallback value applies during analyzer outages, and how results, alarms and calibration records are stored for audit. The contractual side is covered on the custody transfer page.
Selection criteria for a gas analysis system
Parameter | Why it matters in design | Effect on selection |
Purpose of measurement | Billing, specification monitoring and consumer protection call for different properties | Sets the analysis scope, redundancy and record-keeping needs |
Expected composition range | Calibration gas and column configuration are matched to this range | Drives analyzer configuration and the fitness-for-purpose assessment |
Variability of gas supply | Blending from several sources can change composition quickly | Influences cycle time and the choice of online analysis over fixed composition |
Contaminants to monitor | Water, heavy hydrocarbons and sulfur compounds need different techniques | Determines extra analyzers, detector types and sample path materials |
Line pressure and temperature | Creates pressure let-down and condensation risk in the sample system | Affects let-down staging, heating and probe type |
Number and layout of meter runs | It must be clear which runs the analyzer represents | Affects analyzer count, flow computer links and shelter location |
Hazardous area and site conditions | Shelter, cylinders and vents must be located safely | Determines shelter type, HVAC and protection method |
How TLY Enerji supports gas analysis projects
TLY Enerji provides engineering support from reviewing the measurement purpose and process conditions to selecting a suitable analysis technology; process gas chromatographs are part of its product portfolio. Depending on project scope, services can include supply of analyzers and auxiliary equipment, installation and testing of process analyzers, integration of data transfer to the flow computer and to PLC or SCADA systems, commissioning and ongoing technical support. Scope boundaries, such as which party provides the analyzer shelter, are agreed on the basis of the technical specification.
Information we need to scope an analysis system
· How the results will be used: energy billing, compressibility calculation, specification monitoring or consumer protection
· Existing gas analysis reports and information on how variable the gas sources are
· Properties to be measured: composition, calorific value, water or hydrocarbon dew point, sulfur compounds
· Pressure and temperature range and pipe size at the sampling point
· Number of meter runs and flow computers the analyzer will serve
· Space available for a shelter, site climate and hazardous area classification
· Target control system for the results and data logging expectations
Related solutions
· Custody transfer natural gas metering stations: How calorific value feeds energy-based billing and measurement governance.
· Fuel gas conditioning and metering skids: The role of hydrocarbon dew point and quality data at turbine and engine supplies.
· Natural gas metering stations: The meter run and volume conversion chain that uses analysis data.
· LNG send-out metering systems: Where the chromatograph fits in energy measurement of regasified gas.
FAQ: gas quality analysis
Can one chromatograph serve several meter runs?
Yes, provided the analyzed gas is representative of every run it serves, as is common on parallel runs fed from one source. Where gases from different sources are metered separately, or where the blend differs between runs, separate sample points or a multi-stream analyzer are needed. Whether a sample is representative should be agreed with the contracting parties and checked against applicable regulations.
What is the difference between water dew point and water content?
Water content expresses how much water vapor the gas carries, while water dew point is the temperature at which that water begins to condense at a given pressure. Both describe the same physical state, and one can be calculated from the other with a standard correlation. Which one is reported usually depends on how the gas supply specification is written.
Can a chromatograph determine hydrocarbon dew point?
It can, when an extended analysis resolves the heavy hydrocarbons in enough detail for the dew point to be calculated from composition. A standard billing analysis lumps the heavy components together, so it may not be sufficient for this purpose. Analyzers that measure hydrocarbon dew point directly are another option; the choice depends on the accuracy required and on why the dew point is being monitored.
How is energy calculated while the chromatograph is out of service?
The flow computer can be configured to continue with a predefined fallback composition or the last valid values during an analyzer fault or calibration. The parties should agree in advance which value applies, for how long, and how the period is flagged in the records. At critical points, a second analyzer or data from a neighboring station can be considered as backup.