Pipeline Dewatering, Cleaning, and Drying

Newly constructed pipelines are typically hydrostatically tested, using water as the test medium, upon completion of construction.

Once the hydrostatic testing has been completed, it is necessary to remove the water from the line and place the line in service. Dewatering can be a simple process or, if the procedure is not properly planned, a difficult
one.


Pipelines used to transport natural gas will need some drying, depending on the operating pressure and the location of the line, to prevent the formation of hydrates.
Other pipelines may require drying to protect the pipe from internal corrosion caused by the formation of corrosive acids, such as carbonic acid in the case of carbon dioxide pipelines.

Dewatering
Several types of pigs may be used for the dewatering phase and pig selection should depend upon the design characteristics of the pipeline and the degree of cleaning, if any, that is desired. Ideally, the pig will form a perfect seal with the inner periphery of the pipe and will not allow any of the material behind the pig to leak past the pig and co-mingle with the test water ahead of the pig.

Normally, the pig will move through the pipeline without difficulty. However, there are many opportunities for problems to develop, causing the pig to stick or even disintegrate.
Sticking may be caused by any or a combination of the following things:

• Pig is incorrect length and cannot negotiate check valves,tees, and bends;
• Pig is too large for heavy wall sections of the line;
• Pipeline may be equipped with reduced opening valves;
• Excess debris or construction material may be left in the pipeline;
• Air/gas may bypass the pig and create an air lock condition; and,
• Other unknown reasons.

Cleaning pipelines



Cleaning the pipeline will:
• Improve flow efficiency because of a smoother pipe wall
• Reduce product contamination and formation of hydrates
• Reduce abrasive damage to pipeline appurtenances such as valves and instruments
• Facilitate pipeline drying

Internal cleaning of the pipeline may be accomplished by any of or a combination of the following methods:
• Running a brush pig with air, gas, or liquid
• Internal sand blasting
• Chemical cleaning
• Purging with air or gas followed by a liquid flush

Pipeline drying

Natural gas pipelines are usually dried to a lesser extent to prevent the formation of hydrates.

Drying with nitrogen is accomplished in much the same manner as when using super dry air. Nitrogen drying will cost more than super dry air by a factor of approximately 50%. Also, super dry air is plentiful and non-polluting.