Thank you for your question regarding the minimum distances of down conductors from gas lines in a Lightning Protection System. It is our pleasure to help.
In the United States we use the NFPA 780 Standard for the Installation of Lightning Protection Systems. Annex C of this standard specifically addresses the issue of bonding distances and the reduction of Potential Difference within the Lightning Protection System (LPS). It is a fairly complex section that requires some knowledge of magnetic fields and the principles of induction to fully understand, and it would take a lengthy essay to discuss the impact of this section in full. If you are in Europe or are using international standards, the IEC 62305 / EN 62305 standards will have additional requirements in regards to the protection of flammable pipelines that are above and beyond the requirements found in the NFPA 780 standard.
Unfortunately, when it comes to the protection of flammable gases and other hazards, it becomes very difficult for us to legally provide any advice other than to get professional assistance. If your Lightning Protections System (LPS) is not properly bonded to the gas pipe lines, you could generate a lethal situation that could destroy life and property. We can tell you that the gas pipeline must be bonded to the Lightning Protection System (LPS) and that this must be done in such a way as to reduce the difference in potential between the metal components, without using the gas lines as an electrode or as a lightning conductor. You should coordinate any connections from your flammable pipelines to the LPS with your local gas company provider.
We are sorry for being so vague, but your situation really demands that a professional take a look at some very specific details of your project. E&S Grounding Solutions is of course more than happy to help you with this project, but if not us, please get someone with the proper expertise to help you with this important aspect of your lightning protection system. Feel free to contact one of our engineers directly, they will be happy to discuss the basics of your project with you free of charge.
The Engineering Team at E&S Grounding Solutions
Photo Credit: http://www.flickr.com/photos/upturnedface/3603443283/sizes/l/in/photostream/