The necessity to be able to take a representative sample of the hydrocarbon product known as natural gas is to ensure proper accounting for transactions and efficient product processing. The various sampling methods that are available and the most widely known options and limitations of these methods are discussed here. The appropriate equipment to use and the reasons for their use and correct installation of the equipment are also addressed.
In the last 25 years, the natural gas pipeline industry has transitioned from the supplier of clean, dry natural gas to the mover of billable gas energy; clean and dry or dirty and wet. 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. 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 resource becomes more expensive and plays a larger role in our energy needs worldwide.
From the Gas Processors Association publication GPA 2166-05 we read, “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 defined as, “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, which is currently under revision.