Page 161 - Pipeline Rules of Thumb Handbook

148 Pipeline Rules of Thumb Handbook

Cleaning pipelines

It is believed there are no written specifications to define • Reduce abrasive damage to pipeline appurtenances such
the degree of cleaning for a pipeline since there is a question as valves and instruments
of the definition of a clean pipeline and furthermore, a • Facilitate pipeline drying
method for measuring the cleanliness has not been devel-
oped. It is known, however, that cleaning a pipeline does at Internal cleaning of the pipeline may be accomplished by
least four good things. Cleaning the pipeline will: any of or a combination of the following methods:

• Improve flow efficiency because of a smoother pipe • Running a brush pig with air, gas, or liquid
wall • Internal sand blasting
• Reduce product contamination and formation of • Chemical cleaning
hydrates • Purging with air or gas followed by a liquid flush

Brush pig run with gas

One brush pig pushed by gas or air will not usually remove of a good brush pig will more evenly distribute the loose
a significant amount of small debris from the pipeline, par- debris and leave a smoother pipe wall, which will improve
ticularly if the debris is wet. However, the scratching action pipeline flow efficiency.

Brush pig run with liquid

A brush pig displaced by water is a very efficient means for posal problems and will allow for higher velocities with lower
removing debris from the pipeline provided certain precau- pressures.
tions are taken. Experience has shown that a pig velocity Disposal of the dirty liquid may present a problem,
greater than three feet per second is desired. A pig, such as depending upon the location. As indicated earlier, environ-
the Power Brush Pig, that allows some of the liquid to bypass mental aspects should be investigated thoroughly during the
the pig to keep the bristles clean and move the loose debris pre-planning phase. Usually, rust will settle out of still water
into suspension in the liquid in front of the pig is also desir- in about one hour, leaving an almost clear liquid. If the
able. The debris must be free of any sticky material such as dumping of semi-clear water is acceptable, then the problem
oil, grease, or paraffin. may be solved by the use of holding tanks.
It is not necessary that the pipeline be completely filled The debris-holding ability of the propelling fluid is pro-
with liquid. Short slugs of liquid batched between two pigs is portional to the viscosity/density of the propelling fluid. The
usually preferable as long as there is enough water to keep greater the viscosity/density, the higher the holding ability. It
the pigs apart and maintain a velocity greater than three feet has been reported that gels have been developed and used
per second. Short slugs will minimize the water/debris dis- that are supposedly more effective than water.

Internal sand blasting

Internal sand blasting is an extremely effective method for natural gas was inexpensive and there were no environmen-
cleaning pipelines. This method was extensively used when tal controls. Some short, small-diameter pipelines are still