Gas is assumed to come from reservoirs in a saturated state of water (that is, water is in a vapor form and the gas contains water molecules). Water content is normally removed or reduced substantially (and required in many pipeline tranport contracts to around 7 lb H20/mcf). Gas holds more water at higher temperatures and lower pressures (and the reverse). Thus, as gas is compressed, it heats up due to compression, and then is cooled down in heat exchangers or in the discharge line. Unfortunately, this drop in temperature (across pipeline friction, chokes, heat exchanges, etc..) causes water to drop out and will lead to corrosion (CO2 in the line makes carbonic acid when mixed with water). Another problem with water in the gas (if not removed) is the formation of hydrates (essentially ice plugs that can form above 32 deg Fahrenheit). Thus, Pipelines do not want their pipelines corroding, water forming liquid holdups in the pipelines (which increases pressure drop and thereby requires more compression at the next pump station), and do not wish to have their lines freezing up due to hydrates (even at 50 deg F). In addition, some gas is further processed in a Gas Plant to remove ethane, propane and heavier components from the gas stream (which is predominantly methane). These gas plants require the water removal to frigerate the gas to enable the separation of the hydrocarbon components to take place. More water has to be removed for LNG (Liquified natural gas), than it does for LPG (Liquified petroleum gases, which are heavier than methane).
