Dr. I.M. MishraProfessor, Dept. of Chemical Engineering

Dean, Saharanpur Campus

Indian Institute of Technology, Roorkee

Risk Assessment

By

Risk Assessment

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 ReleaseEmission-. vaporisation (if any) - air j entertainment-gas

dispersion ignition-flash fire or vapour cloud explosion.B. Toxic Release

Emission- vaporisation-air entrainment -gas dispersion.

Introduction

Risk Assessment of a Process Plant

Interaction Between Process Equipment/Instruments or Units

Series Systems

Parallel System

Availability and Unavailability

Probability of-Coincidence

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.

Fussel (1976) notes that the fault tree analysis is of major value in

Risk Assessment Methods

Fault Tree Method

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 RuleIf 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 RuleGate inputs should be properly defined fault events, and

gates should not be directly connected to other gates,

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.

Cause-Consequence Diagrams Hazards Estimation & Report Presentation

Forms of Presentation of Results of a Risk AssessmentA. Risk to workforce

Annual riskFatal Accident Rate (FAR)Osha incidence rate (IR)

B. Risk to PublicPhysical effects.

e.g. contours on the site mapIndividual Risk

e.g. contours on the site map, or annual risk at fixed location

Societal RiskFN TableFN curbeEquivalent annual fatalities

Acceptable Societal Risk

Fatality, Statistics for common industrial and non-industrial activities

General Risk Criteria Adopted