fuel must be kept a minimum safe distance from any potential source of ignition.

For example, here are the figures for liquid oxygen and its minimum safe distance from various potential sources of ignition:

  • Building structures
    • Wood frame construction 50 ft
    • Other than wood frame construction 1 ft
    • Confining areas 75ft/35ft
    • Wall openings 10 ft
  • All classes of flammable and combustible liquid storage
    • Above-ground storage
    • Below ground storage
      • Horizontal: Distance between tanks 15 ft
      • Distance between O2 and tank openings 25 ft

etc.
(source: http://www.airliquide.com/us/databook/)