Page 160 - Pipeline Rules of Thumb Handbook

Pipeline Drying 147

PIPELINE DEWATERING, CLEANING, AND DRYING*

Newly constructed pipelines are typically hydrostatically requirement, the pipeline will need to be dewatered, cleaned,
tested, using water as the test medium, upon completion of and dried. Pipelines used to transport natural gas will need
construction. Older lines already in service may be re-tested some drying, depending on the operating pressure and the
either to confirm an earlier test or to qualify the line for a location of the line, to prevent the formation of hydrates.
higher operating pressure. Once the hydrostatic testing has Other pipelines may require drying to protect the pipe
been completed, it is necessary to remove the water from the from internal corrosion caused by the formation of corrosive
line and place the line in service. Dewatering can be a simple acids, such as carbonic acid in the case of carbon dioxide
process or, if the procedure is not properly planned, a diffi- pipelines.
cult one. Pipelines used to transport crude oil and/or refined
products will probably only require removal of the test water * Based on a paper entitled “The State of the Art of Drying Large
before the line is placed in service. If the pipeline will be used Diameter Gas Pipelines after Hydrotest,” by Marvin D. Powers, Pipeline
to transport materials which must meet a specified dryness Dehydrators, Inc., Houston, Texas.

Dewatering

Dewatering is considered to commence with the running • Air/gas may bypass the pig and create an air lock condi-
of the first pig after hydrostatic testing is completed and tion; and,
begins with the insertion of a displacer, commonly referred to • Other unknown reasons.
as a pig, in the pipeline. The dewatering pig may be pushed
through the pipeline with crude oil or other petroleum Air locks are more likely to occur in hilly country than in
product if no drying is required. If the pipeline is to be flat land. Air locks occur when the accumulated static heads
cleaned and/or dried, the pig will be pushed by either com- are greater than the available displacing pressure. In some
pressed air or gas. In either case, proper precautions must be cases, a pressure greater than the maximum allowable pipe-
taken to be sure the test water is properly disposed of and line pressure would be required to overcome the air lock.
that any required water discharge permits are obtained ahead Usually, the air lock occurs where air/gas has bypassed the
of the dewatering operation. pig and the downhill legs of the pipeline are filled with air
Several types of pigs may be used for the dewatering phase and the uphill legs of the pipeline are filled with water. The
and pig selection should depend upon the design character- air/gas can get in front of the dewatering pig in several ways
istics of the pipeline and the degree of cleaning, if any, that such as:
is desired. Ideally, the pig will form a perfect seal with the
inner periphery of the pipe and will not allow any of the • Poor filling techniques;
material behind the pig to leak past the pig and co-mingle • Poor dewatering procedure such as draining water from
with the test water ahead of the pig. the line at low points;
Normally, the pig will move through the pipeline with- • Air/gas bypassing the pig because the pig is too small for
out difficulty. However, there are many opportunities for the pipeline;
problems to develop, causing the pig to stick or even • Air/gas bypassing the pig before it is launched;
disintegrate. • Air/gas bypassing the pig in a fitting such as in a tee, or
Sticking may be caused by any or a combination of the a steel shaft pig in a short radius bend; and,
following things: • Other unknown reasons.

• Pig is incorrect length and cannot negotiate check valves, When an air lock condition occurs, it is necessary to either
tees, and bends; increase the displacing pressure, or remove air/gas through
• Pig is too large for heavy wall sections of the line; existing vents or other connections at high points in front of
• Pipeline may be equipped with reduced opening the pig.
valves; For these reasons, it is important that the dewatering
• Excess debris or construction material may be left in the process be carefully planned, especially for pipelines located
pipeline; in hilly country.