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.
Contents
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.
Prevention
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:
Removal
Once a line is contaminated with black powder, its removal becomes a real challenge.
Gas lines
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.
An emerging and highly effective method to remove black powder is using powerful magnetic separation technology. Because black powder is ferrous (iron-based) it will be trapped, separated and removed. A leader in this field is Black Powder Solutions based out of Calgary, Alberta, Canada.
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.