Natural gas sampling is commonly used to secure a representative sample of produced gas for a specific period of time. To truly represent the varying composition stream, the more often sampling is required.
The sampling system commonly consists of:
- Sample probe
- Connecting tubing
- Sample containers
- Sample valve
- Heat tracing and insulation
- Injection valve to GC (If installed for onstream analysis)
Sampling is carried out by either of below three methods.
- Spot sampling: technician manually collects a sample directly from the stream at scheduled intervals or whenever require. The sampling frequency is generally monthly or longer basis.
- Composite sampling: are usually collected on a weekly or monthly basis. This sampling system grabs small natural gas samples and inject them into the composite sampling cylinder.
- Continuous sampling: provides a consistent flow of sample through a sample loop that passes near an onstream analyzer. In the case of an on-line chromatograph, the injection valve of GC is able to inject a sample from the flowing stream to the GC for analysis.
The natural gas sampling point is generally located downstream of the meter run. Moreover, it should be away from severe flow disturbances such as pressure control valve (PCV) and orifice plates by at least five nominal pipe diameters.
The sample probe can be positioned either upstream or downstream of the meter run when the flowing stream is away from its hydrocarbon dew point. However, the distance should be at least 5 pipe diameters in both cases to avoid flow disturbance due to elbows, headers, valves, and tees. The 8 pipe diameters to the downstream should be ensured when the stream is near its hydrocarbon dew point.
The sample point must not be installed within the engineering sections of the meter run because fitting and probe could produce disturbances in the flow profile and can affect the measurement. The sample probe must be as extend in the pipe as 1/3 of the internal diameter of the meter run. This ensures no contamination of samples due to heavy materials migrating along the pipe walls. However, for larger diameter pipelines, the probe length should not be greater than 10 inches. The probe should be equipped with an outlet valve to isolate the system for inspection and maintenance.
The tubing connection between probe and analyzer should be clean internally and as short as possible. Tubing can be made of nylon or stainless steel. Later material is generally preferred due to its high strength and flexibility.
The sampling system must be leakproof. Typically, if the leak occurs, smaller molecules tend to escape and create biases in the analysis. Moreover, leakage can cause cooling and eventually condensation in the sample.
If the ambient temperature is below the flowing temperature of the stream, the heat tracing should be installed to avoid condensation of the heavy molecules. Heat tracing should be properly and safely installed. Electrically limited tracing should follow the electrical codes of area classification.
Natural Gas Liquid Sampling
The NGL sample is collected in a pressurized sample cylinder. The sample pressure is maintained at 1.5 times the product’s maximum vapor pressure. The maximum vapor pressure is determined at the highest ambient or flowing temperature (whichever is highest). The pressure is maintained by a floating piston sample cylinder while mixing is carried out with an in-built mixing mechanism.
The floating piston cylinders are pre-charged with the inert gas at the 1.5-time product vapor pressure. The pressure prevents sample vaporization to avoid error in the analysis. Moreover, the sample cylinder also provides a compressible inert gas cushion to allow thermal expansion of the liquid. The pressure relieff valve should be installed on the cylinder. However, if operated, the sample integrity will be lost. The cylinder should not be filled more than 80 percent.