Black powder in gas pipelines
Black powder is a solid contamination in finished product pipelines. The material may be wet and have a tar-like appearance, or dry and be a very fine powder, sometimes like smoke.
Black powder can cause a range of problems, including product contamination, erosion wear in compressors, instrument and filter clogging and equipment contamination for product consumer, erosion and sealing problems for valves, and flow reduction.
The source of black powder is far from clear, with several possibilities existing. Black powder could be generated from the following sources:
- Mill scale (iron(II,III) oxide - Fe3O4) that comes from pipe manufacturing process through high temperature oxidation of steel. These types of solids are very persistent and strongly adhere to pipe wall and are not easily removed.
- Flash rust (Fe2O33, FeOOH) from hydrotest water corrosion.
- Internal pipelines corrosion (microbiological influenced corrosion (MIC) or H2S reaction with steel.
- Carryover from gas gathering systems.
Black powder may be mechanically mixed or chemically combined with any number of contaminants such as water, liquid hydrocarbons, salts, chlorides, sand, or dirt. Chemical analyses of the material have revealed that it consists mainly of a mixture of iron oxides and iron sulfides.
Sour gas pipelines are always treated with corrosion inhibitors, while sweet gas pipelines are not. In both cases there is a chance of forming black powder at different rates. The rate of inhibition in sour gas lines is designed for normal operation, but there is always a chance of plant upsets that might introduce water to the pipelines. This water is not accounted for and will initiate the formation of black powder. Another source of solids formation in sour gas lines is the mechanical mixing of number of contaminants such as water, liquid hydrocarbons, salts, chlorides, sand, or dirt. If the lines are well inhibited, then the quantity of solids is not significant. Nevertheless, in sweet gas pipelines, which are not inhibited, any water condensation or any plant upset that could introduce water to the line will certainly lead to the formation of black powder.
Methods of preventing
Cleaning newly installed pipelines to remove mill scale and drying them to remove hydrotest water will delay the formation of black powder. Some water, even at ppm level, exists in the sales gas composition. Therefore, any change in the atmospheric temperature can cause the water vapor to condense in the pipelines. Since water is an important factor in the support of environmental conditions necessary to form black powder, it is expected that the black powder problem in sales gas pipelines is not going to be a one-time occurrence, but it is likely to be cyclic. Accurate water monitoring devices can help to monitor the water dew point at strategic locations on the system to provide an alert when corrective action is required.
Water used to hydrotest pipelines will contribute in the formation of black powder in the long run. Oxygen scavengers are always added to this water; however, no corrosion inhibitors are added. Adding corrosion inhibitors will definitely reduce the chance of forming black powder; however, due to the unavailability of environmentally friendly corrosion inhibitors, the hydrotest water is not treated with corrosion inhibitors. Treating the hydrotest water with corrosion inhibitor will introduce a major environmental concern, especially with this large amount of water.
The most common and historical means of dealing with black powder is to filter it just before it enters a compressor, station, or processing plant. Filters need to be installed in clean pipelines; therefore, newly installed pipelines as well as existing ones need to be cleaned prior and dried completely to filters instillation.
Although sandblasting pipe internals to remove mill scale is one way to remove mill scale, but it will expose the line surface to the atmosphere, and that will increase the corrosion rate during the initial hydrotest. Therefore, chemical or mechanical cleaning, after the initial hydrotesting, might be the best choice and time to remove mill scale and any corrosion products. However and as an alternative, a coating system such as Leighs Paints Pipegard P100 could be utilised after sandblasting. This and other such proprietary coatings not only prevent flash rusting and corrosion, but also promote or do not hinder flow efficiency through the gas line.
The following are recommendations to minimizing solids in newly constructed pipelines:
- Install end caps on the pipe after each day's construction.
- Enhance hydrotest drying operations and dew point monitoring.
- Chemically clean pipelines, using inhibited water, right after the initial hydrotesting.
- Run scrapers as proof of pipeline cleanliness prior to commissioning.
Methods for removal
Once a line is affected by black powder contamination, its removal becomes a real challenge.
Black powder in natural gas pipelines becomes extremely hard and thus difficult to remove from the pipeline inner wall. Current scraping technology can reduce the formation of black powder, but there are no documented cases where, once discovered, scraping has been successful in completely removing black powder from a line. Various companies offer methods for removing black powder, including gel scraping, chemical cleaning using diesel/surfactant mixtures/chelants, in-situ chemical cleaning and coating.
Unlike wax deposits in crude oil, black powder removed from the pipeline inner wall does not re-dissolve in the fluid (LPG) flow. This feature makes the evacuation of removed black powder out of the line extremely difficult. Especially if the pipeline is long and the amount of black powder significant, accumulation of deposit in front of a scraper pig can easily lead to pipeline blockage. This means that pipelines containing black powder should not be attempted to be cleaned in one pass unless progressive pigging, a by-pass pig, a jet pig, or gel pigs are used.
J. Smart adverts to a sufficient fluid velocity being extremely important for evacuation of removed black powder particles. Consequently, a technology applying enough cleaning force to (1) remove all black powder from the pipeline inner wall and (2) allowing for the required fluid velocity for evacuation is a good choice for cleaning. 
Hydraulically Activated Pipeline Pigging - Pipeline cleaning
- Hydraulically Activated Pipeline Pigging