On the basis that you have not had a reply, the answer is that there is no "general" other than as low as possible. If you look up "gas pipeline black powder" from google, a number of interesting articles appear about how it is formed and the immense difficulty of getting rid of it when it does.
Some pipelines are plagued with it, others don't see it at all.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way