PRACTICAL CONSIDERATIONS OF GAS SAMPLING AND GAS SAMPLING SYSTEMS  (click on the title to view the complete article)

The amount of hydrocarbon product that is transported between producer, processor, distributor and user is significant. To be able to verify the exact composition of the product is important from an economic and product treatment standpoint. A small percentage savings made by correctly determining composition will quickly recoup
the investment made in the purchase of a system designed to obtain an optimum sample. In addition, if the best sampling procedures are followed, the potential for disputes between supplier and customer will be greatly reduced. The importance of properly determining hydrocarbon gas composition benefits all parties involved and will achieve greater significance as this precious commodity becomes less plentiful and more expensive.

From the Gas Processors Association publication GPA 2166-05, "The objective of the listed sampling procedures is to obtain a representative sample of the gas phase portion of the flowing stream under investigation. Any subsequent analysis of the sample regardless of the test, is inaccurate unless a representative sample is obtained.” And, from ISO-10715, a representative sample is, “A sample having the same composition as the material sampled, when the latter is considered as a homogeneous whole.” API 14.1 offers a similar statement in the latest revision, “a representative sample is compositionally identical or as near to identical as possible, to the sample source stream”, as does ASTM 5287-97. These standards are the most common referenced on Gas Sampling procedures, along with the AGA Gas Measurement Manual, Part No. 11, Section 11.3.
Proper sampling is fundamental to the correct determination of the product composition.

In a majority of cases, the sample is also the source for the determination of the specific gravity of the gas. This figure is a critical component of the flow formula, from which we derive the product quantity. An error in sampling effects both quality and quantity, and ultimately, profitability. Most current Gas Chromatographs boast an accuracy level of ½ of a BTU, but that should not be the comfort zone for the easurement department. A faulty sampling method or improperly installed and maintained equipment may alter the BTU content of the flowing stream by 25+ BTU. While the accuracy of the GC may be considered as a given, the properly executed technique for taking the sample is certainly not a given.

SPOT SAMPLING
While there are several methods for spot sampling natural gas, two common methods in use today are the fill and purge method detailed in GPA-2166-05 section 7.1 and the piston cylinder method detailed in section 7.7. Spot sampling was the primary method of acquiring a sample for analysis until the early 1970’s. This method is still widely used today. In today's world of growing trends toward therm-measurement and therm-billing, this method is increasingly expensive in analytical cost and man-hours, as well as a very questionable method of assessing an accurate heating value to volume sales. It is at best a "spot" sample of what was present at the moment the sample was taken. Minutes before and minutes after become unknown guesses. While this may be a reasonable risk if the gas source is known by a long historical data base, most gas being consumed today is a combined gas from several origins, or is switched from source to source by contractual updates; in some cases by daily or even hourly arrangements. Also, we find typically, that the older the well and the longer it stays in production, the higher the BTU value will become. Natural gas is an extremely fragile product and almost every step in the production, transportation and distribution of natural gas, will have an adverse effect on its quality.

COMPOSITE SAMPLING
Composite sampling is the proven middle ground between spot sampling and the continuous on-line analytical gas chromatographs. Composite or Grab sampling is the collection of the gas by direct introduction into a sample cylinder from a probe/valve combination or by means of a timed or proportional-to-flow sampler.

A composite gas sampler or gas sampling system consists of a probe, a sample collection pump, an instrumentation supply system, a timing system and a collection cylinder for sample transportation. Its sole objective is to collect and store a representative composite sample at line conditions, allowing it to be transported to the laboratory for repeatable analysis. This package will mount on a pipeline and collect samples over a desired sample period unattended.

A probe should be installed which extends into the middle 1/3 of the flowing stream. This location should be chosen to provide a representative sample of the gas stream, thus devoid of stagnant gas, i.e. blowdown stack, and devoid of free liquids and aerosols, i.e. downstream of piping elbows or orifice fittings which cause turbulent
flow. The probe should have a large ported outlet valve to prevent fractionation, resulting in compositional changes in the gas.