filters are typically a good response to your problem.
Make sure to blow down regularly.
it is unusual to see the black powder you describe in high pressure lines steel lines, usually seen in low pressure and copper gas lines.
May want to check if company wants to start to use wire brush pigs...
the gas engineers handbook or the pipeline rule of thumb manual have reference information.
NFPA 54 has recommended sizes for various lengths for housepiping.
Formulas typically are used more for pressure class than lengths.
