"LPG [liquefied petroleum gas] as storage and transfer depots may have numerous aboveground containers clustered together. When pressure vessels like those pictured (known as ASME containers) have an individual water capacity of 12,000 gal (45 m3) or more and are installed for use in a single location, the number of containers in one group must be limited, and there must be adequate separation from other groups based on the type of fire protection provided."
Coffee Break Training - Fire Protection Series No. FP-2009-11
United States Fire Administration: http://www.usfa.fema.gov/