Explanation of the Dow Fire and Explosion Index.
The Fire and Explosion Index (F&E!) calculation is a tool to help determine the areas of greatest
loss potential in a particular process. It also enables one to predict the physical damage that
would occur in the event of an incident.
The first step in making the F&EI calculation requires using an efficient and logical procedure to
determine which process units should be studied. A process unit is defined as any major item of
process equipment. The following process units could be identified in a typical plant.
Unloading facility
Storage tank
Reactor
Distillation Column
Quench Vessel
Storage Vessel
Loading facility
A designation of the Process Unit must be entered in the appropriate space on the F&EI
form. The Manufacturing Unit designation must also be entered on the F&EI form. A
Manufacturing Unit is the entire production facility including chemical processes,
mechanical processes, warehouse, packaging lines, etc.
It is quite clear that most manufacturing units have many process units. To calculate the Fire and
Explosion Index, however, only process units that could have an impact from a loss prevention
standpoint should be evaluated. These are known as Pertinent Process Units.
Important factors for selecting Pertinent Process Units include:
a. Chemical energy potential (Material Factor)
b. Quantity of hazardous material in the Process Unit
c. Process pressure and process temperature
d. Units critical to plant operation, e.g. Reactor
Important Considerations
A. The Fire and Explosion Index system assumes that a process unit handles a minimum of
2,500 kg of a flammable, combustible or reactive material. If less material is involved,
generally the risk will be overstated. However, F&EI calculations can provide meaningful
results for pilot plants if they handle at least 500 kg) of combustible or reactive material.
B. Careful consideration is needed when equipment is arranged in series and the items are not
effectively isolated from each other. An example would be a reaction train without an
intermediate pump. In such situations, the type of process determines whether several
vessels or just a single vessel should be considered as the Process Unit.
It should rarely be necessary to calculate the F&EI for more than three or four Process Units
in a single process area of a Manufacturing Unit. The number of Process Units will vary
according to the type of process and the configuration of the Manufacturing Unit.
A separate F&EI form must be completed for each process unit evaluated.
C. It is also important to give careful consideration to the state or point in time of the operation.
By their nature, such normal stages as startup, steady-state operation, shutdown, filling,
emptying, adding catalyst, etc., often create unique conditions having an impact on the
F&EI. Generally, good judgment will enable selection of the point in time of operation to
perform the F&EI calculation. Occasionally more than one point in time will have to be
studied to determine the significant risk.
DETERMINATION OF MATERIAL FACTOR
The Material Factor (MF) is the basic starting value in the computation of the F&EI and other
risk analysis values. The MF is a measure of the intrinsic rate of potential energy release from
fire or explosion produced by combustion or chemical reaction.
The MF is obtained from the flammability and instability rankings according to NFPA 704.
Generally, the flammability and instability rankings are for ambient temperatures. It is
recognized that the fire and reaction hazards of a material increase markedly with temperature.
The fire hazard from a combustible liquid at a temperature above its flash point is equivalent to
that from a flammable liquid at ambient temperature. Reaction rates also increase very markedly
with temperature. If the temperature of the material on which the MF is based is over 140 F (60
C), a certain adjustment may be required, as discussed below under C. ―Temperature
Adjustment of Material Factor.‖
Appendix A provides a listing of MFs for a number of chemical compounds and materials, and
these values will be used in most cases. If Appendix A does not list the material, the
flammability and instability rankings may possibly be found in NFPA 325M or NFPA 49
adjusted for temperature, if appropriate, and used with Table l to determine the MF. If the
material is a combustible dust, use the Dust Hazard Class Number (St number) rather than the
flammability ranking.
A. Unlisted Substances
If neither Appendix A, NFPA 49, nor NFPA 325M contains values for the substance,
mixture or compound in question, these values will have to be determined from the
flammability ranking or dust class (St) (see Table l). First, the parameters shown in the left
column of the table will have to be determined. The flammability ranking of liquids and
gases is obtained from flash point data, and the St of dusts or mists is determined by dust
explosion testing. The flammability ranking of combustible solids depends on the nature of
the material as categorized in the left column.
The instability ranking can be obtained from a qualitative description of the instability (or
reactivity with water) of the substance, mixture or compound at ambient temperature.
Definitions in National Fire Protection Association (NFPA) 704 should be used to assign hazard
ratings for materials which are not listed in the F&EI calculation tool in S2S.
PROCESS UNIT HAZARDS FACTORS
After the appropriate Material Factor has been determined, the next step is to calculate the
Process Unit Hazards Factor (F3), which is the term that is multiplied by the Material Factor to
obtain the F&EI.
The numerical value of the Process Unit Hazards Factor is determined by first determining the
General Process Hazards Factor and Special Process Hazards Factor listed on the F&EI form.
Each item which contributes to the Process Hazards Factors contributes to the development or
escalation of an incident that could cause a fire or an explosion.
When calculating the penalties comprising the Process Unit Hazards Factor, F3, pick a single
specific instant in time during which the material under consideration is in the most hazardous
normal operation state associated with the Process Unit. Startup, continuous operation and
shutdown are among the operational states that may be considered.
This rather strict definition is intended to prevent double or triple counting of hazards occurring
during the process. Since the MF is taken to be that of the most hazardous substance present in
the Process Unit, it can be certain that the Fire and Explosion analysis will really be based upon a
―worst case‖ when focus is placed on the most hazardous operational point involving the MF, and
this will be a realistic worst case — one that could actually occur.
In the F&EI system, only one hazard may be evaluated at a time. If the MF is based on a
flammable liquid present in the Process Unit, do not take penalties relating to combustible dusts,
even though dust may be present at a different time. A reasonable approach might be to evaluate
the Process Unit once using the MF of the flammable liquid and a second time using the MF of
the dust. Only the calculation resulting in the highest F&EI and Actual Maximum Probable
Property Damage need to be reported.
One important exception is the hybrid, described previously under ―Mixtures.‖ If a hybrid
mixture is selected as the most hazardous material present, it is penalized both as a dust and as a
flammable vapor in the Process Unit Hazards Factor sections of this manual.
Some items on the F&EI form have fixed penalty values. For those that do not, determine the
appropriate penalty by consulting the text that follows. Remember – analyze only one hazard at
a time, relating the analysis to a specific, most hazardous time (e.g., startup, normal operation or
shutdown). Keep the focus on the Process Unit and Material Factor selected for analysis and
keep in mind that the results of the final calculation are only as valid as the appropriateness of the
penalty assessments.
The entry of all the pertinent information to allow calculation of the Fire and Explosion Index and
the radius of exposure is made in the excel workbook F&EI Calculation workbook S2S July
2006.xls
When the indexes for all pertinent units in the plant have been calculated, the results give an indication of
the ranking of risk of each unit relative to another. This ranking can be used for screening out the lower risk
items and concentrating study on the higher ones.
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