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Black powder has become an important factor in natural gas pipelines. Black powder mainly consists of iron compounds such as magnetite (mill scale) and iron sulfide and includes sand and clay, salt, weld spatter and even metallic iron. It is generated during gas production or in wet gas pipelines when hydrogen sulfide, carbon dioxide or oxygen are present in the gas, by bacterial corrosion of the steel, or from construction when lines are not cleaned adequately.
Once in a pipeline, black powder is transported through the pipeline by gas flow. The velocity required to move dry solids in a pipeline can be calculated and depends on pipeline diameter, gas pressure, particle size and particle density. Typical velocities required at 1,000 psi are 10 ft/sec for 8-inch lines, 13 ft/see in 24-inch lines and 14 ft/sec in 48-inch lines.
Black powder is an important operating parameter in wet and dry gas pipelines. It represents a threat to pipeline operation and natural gas's reputation as a clean fuel. Black powder is a catch-all term for solids in a gas pipeline, ranging from 100% iron sulfide to 100% iron oxide, also containing rouge, asphaltenes, salt, sand, clay, weld spatter and metallic iron. About the only thing these compounds have in common is that they are all black. Operators report that when black powder moves, it shatters and becomes very small in size, in the range of one micron or less, making it difficult to filter and possibly easier to move.
Deposition of black powder will occur if there are solids in the pipeline fluid and the velocity is not high enough to drag the particles along by viscous flow forces. Sediment deposits can lead to blockage of the line, especially during pigging, while flowing powder can damage compressors, plug filters and damage user equipment. One operator reported that when piping upstream of a compressor was inspected, the piping was half full of black powder, causing shutdown of the compressor and that 60 tons of black powder were subsequently removed from the piping.
The question of what is a clean pipeline is an interesting one, with many different answers. Engineers and operators have advised that they consider a pipeline clean when:
* The friction factor is less than a...