RISK ASSESSMENT LECTURE, Exams of Safety and Fire Engineering

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RISK ASSESSMENT

What is risk assessment? Risk assessment includes a determination of the events that can produce an accident, the probability of those events and the consequences. The consequences could include human injury or loss of life, damage to the environment or loss of production and capital equipment. Risk assessment is sometimes called hazard analysis. A risk assessment analysis procedure that determines probabilities is frequently called probabilistic risk assessment or PRA.



Gives organisations a more effective way of

managing hazards



Requirement of the law e.g.



OSHA 1994, CIMAH Regulations 1996 &

USECHH Regulations 2000



Requirement of OSH management system

standards :

 MS 1722:2011,

 OHSAS 18001 and



ISO 14001, etc.

4

WHY CARRY OUT RISK

ASSESSMENT?

Hazard Identification (HI)

Risk Assessment (RA)

Risk Control (RC)

Review

5

BASIC COMPONENTS OF

RISK MANAGEMENT

Average failure rate and reliability

Data is collected on the failure rate of a particular

hardware component. With adequate data it can be

shown that, on the average, the component fails after a

certain period of time. This is called the average failure

rate and is represented by μ with units of faults/time. The

probability the component will not fail during the time

interval (0, t) is given by a poisson distribution

Where, R called the reliability. This equation assumes a

constant failure rate μ.

Failure probability as t→ ∞ the reliability goes to 0. The speed at which this occurs is dependent on the value of the failure rate, μ. The higher rate failure rate, the faster the reliability decreases. The complement of the reliability is called the failure probability (or sometimes the unreliability), P and is given by

Mean time between failures (MTBF)

The time interval between two failures

of the component is called the mean

time between failures (MTBF) and is

given by the first moment of the failure

density function:

TYPICAL PLOTS OF THE FUNCTIONS Μ, F, P AND

R

Interactions between process units (process components in parallel) Process components interact in two different fashions. In some cases a process failure requires the simultaneous failure of a number of components in parallel. This parallel structure is represented by the logical AND function. This means that the failure probabilities for the individual components must be multiplied: where, n is the number of components and pi is the failure probability of each component. The total reliability for parallel units is given by where, ri is the reliability of an individual process component.

Interactions between process units (process components in series)

Revealed and unrevealed failures All failures in either the alarm or the shutdown system are immediately obvious to the operator and are fixed in a negligible amount of time. Emergency alarms and shutdown systems are used only when a dangerous situations occurs. It is possible for the equipment to fail without the operator being aware of the situation. This is called an unrevealed failure. Without regular and reliable equipment testing, alarm and emergency systems can fail without notice. Failures that are immediately obvious are called revealed failures.

COMPONENT CYCLES FOR REVEALED FAILURES