28-12-2012, 06:07 PM
DOW FIRE AND EXPLOSION INDEX
EXPLOSION.pdf (Size: 234.28 KB / Downloads: 137)
INTRODUCTION
The hazard classification guide developed by the Dow Chemical Company and published
by the American Institute of Chemical Engineering, Dow (1994) (www.aiche.org), gives
a method of evaluating the potential risk from a process, and assessing the potential loss.
A numerical “Fire and explosion index” (F & EI) is calculated, based on the nature
of the process and the properties of the process materials. The larger the value of the
F & EI, the more hazardous the process, see Table 9.3.
To assess the potential hazard of a new plant, the index can be calculated after the
Piping and Instrumentation and equipment layout diagrams have been prepared. In earlier
versions of the guide the index was then used to determine what preventative and
protection measures were needed, see Dow (1973). In the current version the preventative
and protection measures, that have been incorporated in the plant design to reduce
the hazard are taken into account when assessing the potential loss; in the form of loss
control credit factors.
It is worthwhile estimating the F & EI index at an early stage in the process design, as
it will indicate whether alternative, less hazardous, process routes should be considered.
Only a brief outline of the method used to calculate the Dow F & EI will be given in
this section. The full guide should be studied before applying the technique to a particular
process. Judgement, based on experience with similar processes, is needed to decide the
magnitude of the various factors used in the calculation of the index, and the loss control
credit factors.
Calculation of the Dow F & EI
The procedure for calculating the index and the potential loss is set out in Figure 9.1.
The first step is to identify the units that would have the greatest impact on the
magnitude of any fire or explosion. The index is calculated for each of these units.
The basis of the F & EI is a Material Factor (MF). The MF is then multiplied by a Unit
Hazard Factor, F3, to determine the F & EI for the process unit. The Unit Hazard factor
is the product of two factors which take account of the hazards inherent in the operation
of the particular process unit: the general and special process hazards, see Figure 9.2.
Special process hazards
The special process hazards are factors that are known from experience to contribute to
the probability of an incident involving loss.
Twelve factors are listed on the calculation form, Figure 9.2.
A. Toxic materials: the presence of toxic substances after an incident will make the
task of the emergency personnel more difficult. The factor applied ranges from
0 for non-toxic materials, to 0.8 for substances that can cause death after short
exposure.
B. Sub-atmospheric pressure: allows for the hazard of air leakage into equipment. It is
only applied for pressure less than 500 mmHg (9.5 bara).
C. Operation in or near flammable range: covers for the possibility of air mixing with
material in equipment or storage tanks, under conditions where the mixture will be
within the explosive range.