It is necessary, before commissioning of gas pipelines to ensure that there is no residual moisture and in particular, no free water over its complete length of the pipeline. This means that before introduction of gas, the line must be dried to prevent hydrocarbon gas hydrate to build-up.
Another aspect is that the line should be dried immediately after bulk dewatering, to provide corrosion protection, as even with the best performance, residual water will remain in the line after dewatering, and cause corrosion.
Research has shown that by using the air-drying method of gas pipelines, which are not put into immediate operation, will protect the pipeline against corrosion. It is a known fact that corrosion is slowed considerably or even halted at dew-points below -10 ºC at atmospheric pressure.
Pipelines that have been dried by using the air-drying method after hydro-testing and have been left packed with dry air, for a period of 6 month or more showed no sign of corrosion. In cases where corrosion existed prior drying, no further spreading of corrosion could be detected.
Therefore it is also recommended that oil pipelines which are not put into immediate operation after hydrostatic test also should be dried to protect against corrosion.