Designing a sample conditioning system for sampling natural gas in storage facilities can be challenging for several reasons, but some of the more prevalent challenges are as follows:
- The gas being stored is often a mix from different suppliers, so it is difficult to design for a specific composition because it will vary depending on which companies are storing gas at any given time.
- The source gas supplied to sample pressure regulators will not be constant, making it difficult for a single stage pressure regulator to control its output pressure. The pressures can vary between 600-3300 PSIG, depending on whether gas is being put into or removed from the cavern. The pressure will be at its lowest when the cavern is almost empty (there is a need to keep some gas pressure in the cavern to keep the liquids, like oil and water, at the bottom from flashing) and increases as more and more gas is put into the cavern. It is not uncommon for the gas in the cavern to be compressed above 3300 PSIG to maximize the volume of gas stored in the cavern.
- At higher pressures, the gas in the caverns tends to become supercritical in nature. Supercritical fluids are very dense. In supercritical fluids, gas becomes denser, like a liquid and liquid becomes less dense, more like a gas. The result is a homogeneous fluid that is neither gas nor liquid. The resulting fluid is very aggressive to sealing materials. When sampling a supercritical fluid, precautions should be taken during the sample extraction and pressure reduction process to ensure that the supercritical fluid is not allowed to go two phase (liquid & gas) at any time.
- It is a common practice to continue sampling when there is not gas coming into or flowing out of the caverns. This is not a good application for an insertion pressure regulator. A flowing gas stream is critical in the operation of insertion probe regulators because they utilize the flow of gas to offset Joule-Thomson cooling. Without gas flowing around the inserted probe regulator, heat transfer is minimized thereby allowing the inserted probe to cool below the surrounding gas temperature. Once this occurs, the likely result will be a frozen regulator. Minimizing the flow rate through the probe will reduce the cooling effect, but it is recommended that an external Genie® Heated Pressure Regulator or Genie® Joule-Thompson Pressure Regulator be used in conjunction with a non-regulated Genie® Membrane Probe. Doing so will ensure you get a representative sample while avoiding maintenance issues such a freezing regulators.
We hope this helped explain some issues you might see while sampling natural gas in storage facilities. Contact us at the factory or your local A+ Corporation distributor for help sampling such a challenging application.