Natural gas pipelines typically contain moisture between 20 ppm (1 lb/MMSCF) and 140 ppm (7 lb/MMSCF), which translates to a dew point of -20°C (-4°F) to 2°C (36°F) at a pressure of 50 bar (730 psi). (LNG, Liquified Natural Gas, has lower moisture content)
Natural gas is dried to these levels, to keep moisture from condensing in pipelines based on the local pipeline conditions (temperature and pressure of the pipeline).
Most global regulations specify acceptable moisture levels in terms of dew point at this pressure, with a common limit being -15°C (4°F). This translates to a moisture content of around 33 ppm.
However, the US and a few other countries use moisture content directly in their regulations. This historical difference stems from a lack of reliable dew point analyzers in the past.
While some companies still use moisture content, dew point is the more critical parameter for pipeline operations because:
Dew point is a direct measure of water's ability to condense. It directly affects potential problems like hydrate formation, which can block pipelines.
Dew point is pressure-dependent. The same gas can have different dew points at different pressures. This means a moisture content measurement needs additional pressure information to be truly useful. See the graph below.
Key Points:
Look for dew point measurement using a chilled mirror for the most accurate results.
Be wary of vendors claiming very low dew points based on moisture content calculations, especially if they don't specify the pressure used in the calculation. Pipelines don't operate at 1 bar!