All natural gas wellstreams contain water vapor as they leave the reservoir. In many instances, free water is produced along with the natural gas. Natural gas cools as it travels up the wellbore to the surface as a result of pressure reduction and conduction of heat through the pipe to cooler formations. Therefore, since the ability of gas to hold water vapor decreases as the gas temperature decreases, natural gas is nearly always saturated with respect to water vapor when it reaches surface equipment. Additional cooling of the saturated gas will cause the formation of free water. Should the natural gas further cool into the hydrate range, hydrates will form and serious equipment damage and stoppage of flow will occur. Thus it is understandable why it is important to remove water vapor from natural gas. The process for removal of water vapor from natural gas is known as DEHYDRATION. It is imperative that you use a coalescing separator (filter separator) downstream from compression and it is highly recommended that you have an upstream coalescing separator (filter separator) in all applications to protect the glycol from contamination.
Reasons for dehydrating:
There are three principal reasons for dehydrating natural gas. These are:
1. Prevention of line plugging due to formation of hydrates
2. Prevention of reduction of line capacity due to formation of free water (liquid)
3. Elimination or retarding of corrosion in the pipeline.
As stated before, two conditions must exist before hydrates can form - free water must be present in the gas stream, and the stream must be at or very near the hydrate temperature for the system pressure. By reducing the water content of natural gas with dehydration, the operators can be sure that no freewater with resulting hydrates will form in the pipeline until the gas in the line reaches its saturation temperature.
A more detailed discussion on hydrates
Freewater in the pipeline occupies volume and therefore reduces the line's gas carrying capacity. Any volume of water in the line means a loss in line capacity as the water will collect at low places in the line. Therefore, it is desirable to dehydrate the entering gas to a water vapor content that will prevent the formation of freewater in the pipeline. The third reason for dehydrating natural gas is to eliminate or retard corrosion. Not only will water moisture alone cause corrosion in a pipeline, but also liquid water in the presence of any acid in the stream will form even more corrosive acid solutions. By removing water vapor by dehydration to keep the gas in an undersaturated condition and eliminating the condensing of freewater in the pipeline, corrosion can be reduced.
Glycol Dehydration Unit
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