A sample cylinder is a small, rated pressure vessel used to capture a portion of a process fluid and carry it, still at pressure, to a laboratory for analysis. It is the container that turns a fleeting reading in the field into a physical sample a gas chromatograph can measure, and getting the fill right matters just as much as the analysis that follows. Sample cylinders come in simple fixed-volume forms and in more sophisticated constant-pressure designs with a floating piston inside, each suited to different fluids and different questions about composition.
Sample cylinder in one line: A sample cylinder is a portable, pressure-rated vessel that captures a representative sample of process gas or liquid and holds it at pressure for transport to a laboratory, usually for gas chromatograph analysis. Constant-pressure or floating-piston cylinders keep the sample above its dew or bubble point so its composition does not change between the field and the lab.
The simplest sample cylinder is a fixed-volume vessel: a rated tube with a valve at each end that you fill, close, and carry to the lab. It works well for fluids that stay comfortably single-phase across the temperature and pressure changes of transport - a lean gas sampled well above its dew point, for instance. Its weakness shows up with richer streams. As a fixed-volume cylinder cools on the way to the lab, the pressure inside drops, and if that drop takes the fluid across its dew point, heavier components condense out. The gas the analyst later draws off the top no longer represents what was actually flowing in the line.
A constant-pressure cylinder solves that problem with a floating piston that divides the vessel into two chambers. The sample fills one side while an inert precharge gas or a hand pump maintains a set backpressure on the other side of the piston. As temperature changes, the piston slides to keep the sample chamber at a constant pressure, holding the fluid single-phase and stopping the heavy ends from dropping out. For rich gas, gas condensate, and live crude - anything near its phase boundary - a constant-pressure cylinder is what keeps the sample honest.
Choosing between the two comes down to how close the fluid is to changing phase over the conditions it will see between the field and the lab. A dry, lean gas can often travel safely in a fixed-volume cylinder. A rich or two-phase-prone fluid, or any sample destined for a detailed extended composition analysis where the heavy ends carry economic weight, calls for a constant-pressure design. The extra hardware and handling of a floating-piston cylinder is the price of a sample that still matches the pipeline when it reaches the bench.
A good sample starts with a clean, purged cylinder. Before filling, the cylinder and its connecting lines are purged of air and any residue from a previous sample, because trapped air adds oxygen and nitrogen that corrupt the analysis, and old residue biases the result. Many procedures purge and fill several times, or use a flow-through method where sample fluid runs continuously through the cylinder before both valves are closed, so the captured fluid genuinely represents the current stream rather than a stagnant pocket.
The pressure ratings and the physical hazards are not negotiable. A sample cylinder is a pressure vessel with a stamped maximum allowable working pressure and a service life, and it must never be filled beyond its rating or filled completely with a liquid that has no vapor space to absorb thermal expansion. A liquid-full cylinder that warms up can generate enormous hydraulic pressure, which is exactly the situation an outage device or a controlled vapor space is meant to prevent. Cylinders that carry sour or toxic fluids demand appropriate materials, and the person filling them needs the right gas detection and protective equipment for what is inside the line.
Traceability closes the loop between the field and the lab. Each cylinder is labeled with the sample point, the date and time, the operator, and the conditions at which it was taken, so the analyst knows exactly what they received and the result can be tied back to a specific well, stream, or custody point. A sample without that context is just a bottle of fluid; a properly documented cylinder is a defensible record. When the sample supports custody transfer or a contractual composition, that chain of documentation is what makes the number stand up if it is ever questioned.
A sample cylinder produces a snapshot: the composition of a stream at one instant, analyzed hours or days later on a laboratory gas chromatograph. That lab number is precise and defensible, but it is not continuous, and this is exactly where it complements the live monitoring a SCADA system provides. A cloud SCADA platform such as Merobix trends the continuous field measurements - flow, pressure, temperature, and any online analyzer output - between the discrete points where a cylinder was pulled and sent to the lab.
The two data sources check each other. When a field online analyzer or an inferred composition drifts, the lab result from a sample cylinder is the reference used to confirm or correct it, and recording the cylinder's fill conditions alongside the SCADA history lets an engineer line the grab sample up against exactly what the process was doing at that moment. A composition that the lab reports as off-spec becomes far more actionable when it can be placed on a trend of the pressures and temperatures the line was seeing when the cylinder was filled.
For remote sites, the practical workflow is that a technician pulls a cylinder on a scheduled visit, logs the fill conditions, and ships it to the lab, while the SCADA system carries the continuous picture in between. Bringing the lab results back into the same platform that holds the field trends turns a scattered set of grab samples into a composition record that can be tracked over months. The cylinder gives the accurate, traceable spot composition; the monitoring layer gives the continuity and the context that make each of those spot values useful.
A fixed-volume cylinder has a set internal volume, so its pressure falls as it cools, which can drop the fluid below its dew point and let heavy components condense. A constant-pressure cylinder uses a floating piston with a precharge gas to hold the sample at a steady pressure regardless of temperature, keeping it single-phase. Constant-pressure cylinders are used for rich gas, condensate, and live crude, where preserving the heavy ends is essential to an accurate composition.
A liquid is nearly incompressible, so a cylinder filled full with liquid and no vapor space has nowhere to absorb thermal expansion. If it warms up during transport, the trapped liquid can generate extreme hydraulic pressure and rupture the cylinder. Procedures leave a controlled vapor space or use an outage device, and constant-pressure cylinders manage this with the piston and precharge, so the fluid can expand safely.
Any air left in the cylinder or its connecting lines adds oxygen and nitrogen that were not in the process stream, and any residue from a previous sample biases the new one. Both corrupt the composition the laboratory measures. Purging - often by repeated fill-and-vent cycles or a continuous flow-through fill - ensures the captured fluid genuinely represents the current stream rather than a contaminated or stagnant mix.
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