Page 151 - Dust Explosions in the Process Industries

124 Dust Explosions in the Process Industries

1.5.1.3
Hazard Analysis

Hazard analysis is a large subject in itself, coveredby a substantialquantity of published
information.The term hazard analysis comprises a number of different systematicmeth-
ods for identifying, and sometimes also quantifying,the hazards associated with a given
process or plant. In principle, such analyses can also be used as a basis for optimizing
the selection of means to prevent and mitigate dust explosions.
Cox (1986,1987) presented an informative summary of the various techniques in use
for hazard analysis, which is quoted more or less literally, under the following five
headings.

Hazard Surveys or Inventories These methods are essential preliminaries to many safety
studies. The survey consists of making an inventory of all stocks of hazardous material
or energy and noting relevant details of storage conditions.When carried out at the con-
ceptual stage of a project, such a survey can contribute to layout optimization and may
suggestprocess changes to reduce stored quantities.It generates information that can be
used in a preliminary risk assessment, but the hazard survey itself is little more than a
“screening” exercise designed to identify problem areas.

Hazard and Operability Studies (HAZOP) and Failure Modes and Effects Analysis (FMEA)
These two techniques have very similar objectives and methods of approach. The pur-
pose is to identify systematicallyall the possible ways in which the system investigated
could fail, evaluate these, and formulate recommendations for preventive and mitigat-
ing measures.
FMEA is the simpler of the two techniques. The procedure is to take each plant item
and component in turn, list all possible failure modes and consider the consequences of
each. The results are recorded in a standardformat in which recommendations for action
can be included. The weakness of FMEA is that there is no specified method for identi-
fying the failure modes and their effects. The engineer is expected to do this from first
principles or past experience, and the only discipline imposed on him or her is that of
the reporting format itself.
HAZOP overcomes this difficulty by introducing a systematicmethod for identifying
failure modes. This involves scrutiny of a large number of possible deviations from
normal operating conditions, which are generated by applying guide words such as
more, less, and reverse, to each parameter describing process conditions in each com-
ponent, plant item, or line in the plant. However, HAZOP in its original form has dis-
advantages, and some industrial companies have modified the way in which the results
of the study are handled. Instead of “recommendations,”the output is “identified prob-
lems,” which leaves more room for a coordinatedrational design revision, which is not
only cheaper but probably safer also.

Analysis of Systems Reliability by Fault Tree Analysis This method is applied to complex
systems, whether the complexity is due to the nature of the process itself or the instru-
ments required to run the process. In the basic technique, the fault tree analysis, the fail-
ure modes must first be identified, such as by HAZOP. These failure modes are named
top events. An example of a top event could be a dust explosion in a milling plant.