Genie® Membrane Probes & Probe Regulators™ have made it possible to extract a representative natural gas sample near the hydrocarbon dew point for decade(s). They can do this because of their ability to reject liquid contaminants at pipeline pressure and temperature conditions. Near the hydrocarbon dew point, even a small change of temperature or pressure can change the vapor- liquid equilibrium and gas phase composition. If the gas composition changes, the BTU value and the monetary value of the gas will change.
Membrane-tipped probes have been successfully used for many years in gas transmission and distribution systems. Due to this success, they are now being installed in gathering systems upstream of gas processing where the gas is wet. If the wet gas is on the saturated vapor end of the spectrum, a membrane tipped probe should work very well. If the wet gas is on the 80% gas volume fraction (GVF) end of the spectrum containing up to 20% liquid by volume entrained, it will very likely overwhelm a membrane-tipped probe. Unfortunately, current technology is not able to predict if a membrane-tipped probe will function properly in a specific wet gas service before installation.
In cases where a membrane-tipped probe is overwhelmed, it may still be possible to extract a gas sample by using a probe assembly with a membrane separator at the outlet of the probe and not at the tip. The Genie® Model 133 & 745 Probe Assemblies are two products that can be used in these types of applications. It is important to note that although these assemblies may be capable of providing a repeatable sample from a wet gas source, the accuracy of the composition is likely to be compromised if the sample temperature at the membrane separator differs from that of the source.
Accurate determination of BTU value requires a composition analysis of the hydrocarbon concentrations in the gas mixture. Sample accuracy is so critical to composition analysis that the potential gains from using a membrane-tipped probe make it worthwhile to spend the resources to prove whether it will work at locations where liquid is excessive. If errors in the hydrocarbon concentrations do not significantly impact the accurate measurement of a single component such as moisture, H2S or CO2, the probe assembly’s ability to tolerate wetter gas may be worth the compromised accuracy of the hydrocarbon concentrations and the higher initial cost. Whether to install a membrane-tipped probe or a probe assembly is a decision that must balance the need for accuracy against the need for higher liquids tolerance.