Humidity: Water Vapor in Natural Gas, Dew Point Control, and Glycol Dehydration Specs
Humidity is the quantity of water vapor held in a gaseous atmosphere, and although the term is most familiar from weather, it is a controlling variable throughout natural gas production, processing, and transportation. Relative humidity expresses the water vapor present as a percentage of the maximum the gas could hold at that temperature and pressure before condensation begins; at 100 percent relative humidity the gas is saturated and is exactly at its water dew point, the temperature at which liquid water first drops out. Warm gas holds far more vapor than cold gas, so any drop in temperature or rise in pressure pushes a gas stream toward saturation and free water. In oil and gas the concern is rarely the relative figure but the absolute water content, expressed in the field as pounds of water per million standard cubic feet (lb/MMscf) or, in SI units, milligrams per standard cubic metre (mg/Sm3), where the common pipeline target of 4 lb/MMscf is roughly 64 mg/Sm3. Controlling this water content is essential for three reasons. First, free liquid water combines with light hydrocarbons under the high-pressure, low-temperature conditions inside gathering lines and processing plants to form gas hydrates, ice-like solids that plug pipelines, valves, and instrument lines, a chronic winter hazard across the Western Canadian Sedimentary Basin. Second, water plus acid gases such as hydrogen sulfide or carbon dioxide forms corrosive acids that attack carbon-steel pipe from the inside, and sour WCSB gas makes this an integrity issue governed by CSA Z662. Third, sales-gas contracts and pipeline tariffs impose a firm maximum water content, typically 4 lb/MMscf in Canada, that a producer must meet to inject into a transmission system. Raw gas leaving a reservoir is usually saturated with water at reservoir temperature, so it must be dehydrated to hit spec. The dominant method is triethylene glycol (TEG) absorption: saturated gas rises through a contactor tower counter-current to lean TEG, which is strongly hygroscopic and strips the water vapor out, dropping the gas water dew point by tens of degrees. The rich, water-laden glycol is then boiled in a reboiler around 200 degrees C to drive off the absorbed water and regenerate lean glycol for another pass. A well-designed TEG unit reliably delivers 4 lb/MMscf or better, and enhanced regeneration with stripping gas can push outlet water below 1 lb/MMscf for deep-cold service. Humidity, then, is not a passive property; it is the design target that sizes dehydration equipment, sets hydrate-inhibition strategy, and determines whether gas can legally enter a pipeline.
Key Takeaways
- Relative Versus Absolute Water Content: Relative humidity is the vapor present as a percentage of saturation, reaching 100 percent at the water dew point. The gas industry works instead in absolute terms, pounds of water per million standard cubic feet (lb/MMscf) or milligrams per standard cubic metre, because the mass of water, not the percentage, drives hydrates, corrosion, and contract compliance.
- Water Dew Point Is the Key Metric: The water dew point is the temperature at which vapor begins to condense as free liquid. Dehydration lowers this dew point well below the coldest temperature the gas will see in the pipeline, guaranteeing no free water forms during transport regardless of ambient cooling or pressure change.
- Free Water Causes Hydrates and Corrosion: Liquid water with light hydrocarbons forms ice-like gas hydrates that plug lines and instruments, a serious winter hazard in the WCSB. Water combined with H2S or CO2 forms acids that corrode carbon steel from the inside, making water control an integrity requirement under CSA Z662.
- Pipeline Spec Drives the Target: Canadian sales-gas contracts typically cap water content at 4 lb/MMscf (about 64 mg/Sm3). Raw reservoir gas leaves the formation saturated, so it must be dehydrated to meet this firm tariff limit before it can be injected into a transmission system.
- TEG Absorption Is the Workhorse: Triethylene glycol contactors strip water from saturated gas counter-current, then a reboiler near 200 degrees C regenerates the glycol. Standard TEG units meet 4 lb/MMscf; stripping gas or vacuum regeneration pushes outlet moisture below 1 lb/MMscf for deep-cold or cryogenic-feed service.
Why Humidity Threatens Gas Systems in a Cold Basin
In the WCSB, gathering lines run through winters that reach minus 30 degrees C or colder, so any water carried in the gas is close to condensing. Where pressure is high and temperature drops, methane and water form hydrates that can fully block a line within hours, halting production and forcing methanol injection or line heating to clear. Saturated sour gas is worse: the same water that makes hydrates also carries H2S into an acid that pits pipe walls. Removing the water at the plant, rather than fighting its consequences downstream, is the only durable fix, which is why every WCSB gas plant runs dehydration ahead of the sales meter.
Sizing Dehydration to a Humidity Target
Designing a TEG unit starts from the required outlet water dew point, which is set below the minimum pipeline temperature plus a safety margin. Engineers use water-content correlations, such as the McKetta chart, to find the saturated inlet water load at contactor pressure and temperature, then select lean glycol purity (often 99 percent or higher) and contactor stages to reach the 4 lb/MMscf target. A shallow-cut Mannville gas battery might meet spec with a simple two-tray contactor, while a deep-cut Duvernay plant feeding a cryogenic turboexpander needs enhanced stripping to drive outlet moisture under 1 lb/MMscf and avoid ice in the cold box.
Fast Facts
The link between water and pipeline blockages was decoded by E.G. Hammerschmidt in the 1930s, who discovered that mysterious plugs stopping U.S. gas lines in winter were not ice but gas hydrates, solid clathrate cages of water trapping methane that can form well above the freezing point of water under pressure. His finding launched the entire discipline of hydrate inhibition and, indirectly, the modern glycol dehydration industry. Today a single field TEG unit can remove hundreds of kilograms of water per day from a saturated gas stream, quietly preventing the frozen plugs that once shut down winter gas delivery.
Related Terms
Humidity connects to the core gas-conditioning vocabulary. Dew point is the temperature at which the water vapor measured by humidity begins to condense, and it is the primary dehydration design target. Gas hydrate is the ice-like solid that free water and light hydrocarbons form, the main hazard humidity control is meant to prevent. Glycol dehydration is the TEG absorption process that removes water vapor to meet pipeline spec. Sour gas raises the stakes, because water combined with the H2S it contains produces the internal corrosion that dehydration and CSA Z662 integrity rules aim to stop.
Real-World WCSB Scenario: A Water-Spec Failure at a Grande Prairie Gas Plant
A gas plant near Grande Prairie processing rich Montney gas began failing its 4 lb/MMscf sales-water spec during a January cold snap, with the transmission operator flagging elevated water dew point at the custody meter. The cause was traced to a fouled TEG reboiler running below its 200 degree C regeneration temperature, so the lean glycol returned to the contactor still carrying water and could no longer strip the saturated inlet gas to spec. Free water then began forming hydrates in a downstream gathering segment, requiring methanol injection to keep the line open.
Operators cleaned the reboiler firetube, restored full regeneration temperature, and added stripping gas to lift lean TEG purity above 99 percent. Outlet water fell back to about 3 lb/MMscf, the pipeline accepted the gas without penalty, and the hydrate risk cleared, avoiding an estimated CAD 25,000 in deferred production and methanol costs over the event.