What is dewatering and drying?

A pipeline that has passed its hydrostatic test is full of water, and most products cannot tolerate it. Dewatering removes the bulk of the water; drying removes what is left, down to a dew point set by the product and the owner’s specification. Together they are the last steps before a line is commissioned.

Dewatering

A train of pigs is launched behind the water and pushed through the line with compressed air or nitrogen, sweeping the water ahead of it to the receiver, where it is collected for treatment or disposal. Multiple pigs are used because each one leaves a film behind; the last pigs in the train remove what the first ones missed. Long lines with elevation changes are the difficult ones, because water collects in the low points and pigs can outrun it.

Drying

• Air drying pushes dry, heated, or dehumidified air through the line until the outlet dew point meets the target. It is slow but uses no consumable gas.
• Vacuum drying lowers the pressure in the line until the remaining water boils off at ambient temperature, then the vapour is drawn out. It suits long lines that can be sealed.
• Nitrogen drying uses dry nitrogen to carry moisture out and leaves the line inert at the end, which is why it is often chosen where the product is gas or where the line will wait before start-up.

What decides the method

The required dew point, the length and profile of the line, how long the schedule allows, and whether the line must be left under an inert gas. The dew point measurement at the outlet is the proof, and a crew’s instruments and records are what the owner signs off on.

Related terms: hydrostatic testing, nitrogen purging, pigging.