Fires, particularly those involving hydrocarbons, can generate significant amounts of both radiant and convective heat. Except for the immediate area of the fire, radiant heat is of primary concern. For example, an 80 foot diameter pool fire of n-octane can generate sufficient radiant heat to warp steel 95 feet from the pool edge. As a result, protection against radiant heat from potential fires is often necessary for chemical plant facilities.
While protection against radiant heat can be provided by the passive fire protection methods discussed previously or by active fire protection systems, such as sprinklers, physical separation is a frequently used method and the most desirable since it also provides protection from explosions in adjacent areas. In some cases, however, physical space is limited and the appropriate separation distances cannot be provided; in such cases, other protective measures must be employed. In addition, it is not unusual for separation distances to be compromised as the result of subsequent plant expansions, process changes or other modifications. For this reason, it is essential that minimum separation distances be clearly defined and maintained if at all possible. If future plant modifications are anticipated which might impact separation distances, consideration should be given to employing larger initial separation distances and/or other protective means.
Adequate separation is often achieved by dividing up a plant into process blocks of similar or like hazards, for example, process units, tank farms, loading/unloading operations, utilities, waste treatment, and support areas, and then separating individual operations or hazards within each block. The block approach also serves to reduce the loss potential from catastrophic events, such as unconfined vapor cloud explosions, and to improve accessibility for emergency operations.
Two methods exist for determining minimum separation distances within chemical process plants. The first method is to use recommended separation distances for generic plant hazards. These distances are generally conservative and will cover most situations. Tables of recommended separation distances are available from several sources, including
API and some insurance companies.
The second method of determining minimum separation distances is by calculating the heat flux—the amount of heat received by an object—and the resulting surface temperatures that would be expected from a fire involving the actual hazards in question. While this method generally results in more realistic separation distances, the calculations are often complex and should only be performed by persons familiar with the concepts involved. In addition, the calculations should consider all possible scenarios. Space does not permit complete discussion of this subject; however, additional information can be found in the SFPE Handbook of Fire Protection Engineering (SPFE 1988) as well as various technical journal articles. Computer programs are also commercially available which can be used to estimate radiant heat from fires, although the sophistication and accuracy of these programs vary.
NFPA 30 also provides minimum separation distances, particularly with respect to storage tanks.
In addition to radiant heat exposure, other factors which should be considered in determining separation distances and plant layout include topography, prevailing winds (for normal and accidental vapor/gas releases), liquid drainage paths (for accidental liquid spills), location of fire protection equipment and accessibility for emergency vehicles.