Risk assesment Dr. I.M. Mishra Professor, Dept. of Chemical Engineering Dean, Saharanpur Campus Indian Institute of Technology, Roorkee

1. Risk Assessment
By
Dr. I.M. Mishra
Professor, Dept. of Chemical Engineering
Dean, Saharanpur Campus
Indian Institute of Technology, Roorkee

2. Risk Assessment
Introduction
 Risk assessment is the qualitative and quantitative assessment
of the hazards which could cause a high death toll.
 A full assessment involves the estimation of the frequency and
consequences of a range of hazards scenarios and the damages
expected.
 Damages include injury and loss of life, damage to the
environment and equipment, loss of work, and finally also
economic loss to the plant.
Typical Sequences of Events in a Risk assessment
A. Flammable Release
Emission-. vaporisation (if any) - air j entertainment-gas
dispersion ignition-flash fire or vapour cloud explosion.
B. Toxic Release
Emission- vaporisation-air entrainment -gas dispersion.

3. Risk Assessment of a Process Plant

5. Interaction Between Process Equipment/Instruments or Units
 Series Systems

6.  Parallel System

7.  Availability and Unavailability

8.  Probability of-Coincidence

9. Risk Assessment Methods
Fault Tree Method
Fussel (1976) notes that the fault tree analysis is of major value in
 Directing the analyst to ferret out failures deductively
 Pointing out the aspects of the system important in respect of the
failure of Interest
 Providing a graphical aid giving visibility to those in system
management who are removed from system design changes
 Providing options for qualitative on quantitative system reliability
analysis
 Allowing the analyst to concentrate on one particular system failure at
a time
 Providing the analyst with genuina insight into system behaviour.

10. Basic Rules For Fault Tree Construction
Ground Rule 1
Write the statements that are entered in the event boxes as
faults; state precisely what the fault is and when it occurs.
Ground rule 2
If the answer to the question, "Can this fault consist of a
component failure?" is "Yes, classify the event as a "stat-of-component
fault". If the answer is "No”. Classify the event as a
"state-of-system fault".
No Miracles Rule
If the normal functioning of a component propagates a fault
sequence, then it is assumed that the component functions
normally.
Complete-the-Gate rule: All inputs to a particular gate should
be completely defined before further 'analysis of anyone of
them is undertaken.
No Gate-to-Gate Rule
Gate inputs should be properly defined fault events, and
gates should not be directly connected to other gates,

11. Event Tree Method
Typical steps in an event tree analysis
 Identify an initiating event of interest.
 Identify the safety functions designed to deal with the
initiating event.
 Construct the event tree, and
 Describe the resulting accident event sequences.

12.  Cause-Consequence Diagrams
 Hazards Estimation & Report Presentation
Forms of Presentation of Results of a Risk
Assessment
A. Risk to workforce
Annual risk
Fatal Accident Rate (FAR)
Osha incidence rate (IR)
B. Risk to Public
Physical effects.
e.g. contours on the site map
Individual Risk
e.g. contours on the site map, or
annual risk at fixed location
Societal Risk
FN Table
FN curbe
Equivalent annual fatalities

13. Acceptable Societal Risk
Fatality, Statistics for common industrial and non-industrial activities

14. General Risk Criteria Adopted