When natural gas has to be analyzed for its composition, the sample can reach the gas chromatograph two ways: as a live line sample fed directly to an analyzer, or captured into a sample cylinder that is carried to a laboratory. The two methods answer the same question about gas quality but face different risks, mostly around purging, filling, and getting the gas to the analyzer without changing it. Choosing between them is a trade-off between immediacy and logistics. This guide compares the line sample and cylinder sample methods, the handling each demands, and where continuous analysis fits.
Line Sample vs Cylinder Sample in one line: A line sample is gas taken live from the pipeline and delivered straight to a gas chromatograph, giving an immediate composition of the flowing stream. A cylinder sample is gas captured into a sample cylinder that is transported to a lab for analysis later. The line sample avoids transport risks but requires an analyzer on site, while the cylinder sample is portable but must be filled and handled so its composition does not change on the way to the lab.
A line sample connects the analysis directly to the flowing gas. Gas is drawn from a sample probe in the pipeline, conditioned as needed, and fed live into a gas chromatograph that reports the composition on the spot. Because there is no container and no trip to a lab, the result reflects the stream as it is flowing at that moment, and there is no opportunity for the sample to change between capture and analysis. The cost is that an analyzer, whether a permanently installed online unit or a portable one brought to the site, has to be present to receive the gas.
A cylinder sample decouples capture from analysis. The operator fills a sample cylinder at the pipeline, closes it, and sends it to a laboratory where the chromatograph runs later, sometimes days later and often for many samples at once. The advantage is logistical: one lab with one analyzer can process cylinders from many locations, and no analyzer needs to sit at each remote sample point. The catch is that everything that happens to that cylinder between the field and the lab, the fill technique, the transport, the time, has to be controlled so the gas that is analyzed still matches the gas that was in the line.
Neither method is simply better; they suit different situations. A busy custody point or a stream that must be watched closely justifies an analyzer and line sampling or continuous online analysis. A scattered set of remote points that each need occasional composition data is often served more practically by cylinders sent to a central lab. The decision weighs how immediately the result is needed and how many points must be covered against the cost and reliability of putting analysis capability in the field.
Both methods live or die on purging, because gas standing in probes, tubing, and cylinder dead space is not the same as the flowing stream. For a line sample, the sample line from the probe to the analyzer must be purged of stale gas so the chromatograph sees fresh, live product rather than whatever was sitting in the tubing. For a cylinder sample, the cylinder and its connections must be purged before filling so the captured gas is not diluted or contaminated by residue from a previous sample or by air. Inadequate purging gives a confident analysis of the wrong gas.
Filling a cylinder correctly is its own discipline because gas composition can shift if the fill is done poorly. Filling too fast or in a way that lets heavier components condense, or lets lighter ones preferentially escape, biases the sample away from the true stream. A floating-piston cylinder helps here: a movable piston keeps the sample at line pressure so the gas is held as a representative single phase rather than being allowed to drop pressure and partially condense, which matters most for richer gas that carries heavier hydrocarbons prone to dropping out.
Transport is the risk a line sample never faces and a cylinder sample cannot avoid. A cylinder is exposed to temperature swings and time between the field and the lab, and if the gas cools enough for heavier components to condense inside the cylinder, the analysis run at the lab no longer represents the flowing stream. Maintaining pressure and, where needed, temperature during transport is why cylinder handling is prescribed carefully. The line sample trades away all of this transport uncertainty in exchange for needing analysis capability on site.
The line-versus-cylinder choice is really about how immediate and how continuous the gas quality data needs to be, and continuous online analysis pushes that toward the immediate end. An online gas chromatograph installed at the point analyzes the flowing stream repeatedly on its own, producing composition data without anyone drawing a sample at all. That eliminates both the transport risk of a cylinder and the site-visit labor of a periodic line sample, at the cost of the installed analyzer and its upkeep.
A cloud SCADA platform such as Merobix carries online analyzer output into a continuous, time-stamped record available from a browser, turning gas quality from an occasional lab result into a monitored trend. Composition, and the properties derived from it, can be watched over time and alarmed when they move, so a change in gas quality is seen as it happens rather than discovered when a cylinder's results come back. For streams where quality drives measurement or contract terms, that continuous visibility is a meaningful upgrade over spaced samples.
Cylinders and line samples still have their place, and physical samples remain necessary to verify an online analyzer and for streams that do not warrant a permanent instrument. The value of the SCADA-borne continuous data is that it fills the gap between samples and tells an engineer whether a sample-based result still reflects the stream today. The two field methods define how a gas sample reaches an analyzer; the platform is what makes the resulting quality data continuous, comparable, and available everywhere the gas is measured.
A line sample is gas fed live from the pipeline straight into a gas chromatograph, giving an immediate analysis of the flowing stream. A cylinder sample is gas captured into a sample cylinder and carried to a laboratory for analysis later. The line sample avoids transport risk but needs an analyzer on site, while the cylinder is portable but must be filled and handled so its composition does not change before it is analyzed.
Gas standing in probes, sample lines, and cylinder dead space is stagnant and no longer matches the flowing stream. Purging clears that stale gas so the analyzer, or the cylinder being filled, receives fresh live product. Whether taking a line sample or filling a cylinder, inadequate purging produces a precise analysis of the wrong gas.
A floating-piston cylinder is used when the gas is rich enough that heavier hydrocarbons could condense if the sample dropped in pressure. The piston keeps the captured gas at line pressure so it stays a representative single phase during transport, rather than partially condensing and biasing the analysis. It is favored where keeping the sample from dropping out its heavy ends is critical to an accurate composition.
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