Black powder in gas pipelines
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Black powder is a solid contaminant 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, equipment contamination, erosion wear in compressors, instrument and filter clogging, erosion and sealing problems for valves, and flow reduction.
The source of black powder is far from clear, with several possibilities existing:
- Mill scale (iron(II,III) oxide - Fe3O4) resulting from the pipe manufacturing process through high temperature oxidation of steel. These types of solids are very persistent, strongly adhere to the pipe wall, and are not easily removed;
- Flash rust (Fe2O33, FeOOH) from hydrotest water corrosion;
- Internal pipelines corrosion (microbially-induced 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 black powder forming at different rates. The treatment frequency in sour gas lines is determined for normal operation, but there is always a chance of plant upsets that might introduce water to the pipelines, initiating the formation of black powder. Another source of solids formation in sour gas lines is the mechanical mixing of a number of contaminants such as water, liquid hydrocarbons, salts, chlorides, sand, or dirt. If the pipelines are well supplied with corrosion inhibitors, then the quantity of solids is not significant. Nevertheless, in sweet gas pipelines, which are not so treated, any water condensation or any plant upset that could introduce water to the line will certainly lead to the formation of black powder.
Cleaning newly installed pipelines to remove mill scale and drying them to remove hydrotest water will delay the formation of black powder. Since some water always exists in the sales gas composition, even if only at the ppm level, any change in the atmospheric temperature or pressure can cause the water vapor to condense in the pipelines. Since water is an important factor supporting the environmental conditions necessary to form black powder, it is expected that the black powder problem in sales gas pipelines is not a one-time occurrence, but likely to be cyclic. Accurate water monitoring devices can help to determine the water dew point at strategic locations of the system and provide an alert when corrective action is required.
Water used to hydrotest pipelines contributes to the formation of black powder in the long run. Oxygen scavengers are always added to this water, but not corrosion inhibitors, although they would definitely reduce the chance of forming black powder. The reason is the unavailability of environmentally friendly corrosion inhibitors. Treating hydrotest water with corrosion inhibitor would introduce a major environmental concern, especially when large amounts of water are involved.
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 and dried completely prior to filter installation.
Although sandblasting pipe internals to remove mill scale is one way to remove mill scale, it exposes the line surface to the atmosphere, which increases the corrosion rate during the initial hydrotest. Therefore, chemical or mechanical cleaning after initial hydrotesting, might be the best option for removing mill scale and any corrosion products. As an alternative, a coating such as Leighs Paints Pipegard P100 could be applied 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 for minimizing the presence of 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 water containing a corrosion inhibitor right after initial hydrotesting.
- Run scrapers as proof of pipeline cleanliness prior to commissioning.
Once a line is contaminated with black powder, 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.
Liquified petroleum gas lines
Unlike wax deposits in crude oil, black powder removed from the pipeline inner wall does not re-dissolve in the fluid 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 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