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ERT 322
SAFETY AND LOSS PREVENTION
RISK ASSESSMENT
Introduction
• Risk assessment includes:
1. Incident Identification
2. Consequence Analysis
1. Incident Identification
- describe how an accident occurs
- HAZOP
2. Consequence Analysis
- describes the expected damage
- Dow F&EI is a form of consequence analysis
Objectives
• To define the probability theory
• To discuss, analyze and evaluate
- Event Tree
- Fault Tree
- LOPA
Probability Theory
• Component failures or faults occur after a certain
period of time.
• Reliability, R – the probability that the component
will not fail:
• µ is a constant failure rate
(refer to Table 11-1 for selected component)
Probability Theory
• Unreliability, P – failure probability:
Probability Theory
• Mean time between failure (MTBF):
- time interval between two failures of the component
Interaction between process unit
• Accident in chemical/bioprocess plants are usually
the result of a complicated interaction of a number of
process components.
• Parallel or series interaction.
a) Parallel
- logical AND function
- Overall failure probability, P = multiply the P for
the individual components
- Overall reliability, R = 1 – P
Summary of Computation for Parallel
• Interaction between process unit:
b) Series
- logical OR function
- Overall Reliability, R = multiply the R for the
individual components
- Overall failure probability, P = 1 – R
Summary of Computation for Series
Example 11-1
The water flow to a chemical reactor cooling coil
is controlled by the system shown in Figure 11-4.
The flow is measured by a differential pressure
(DP) device, the controller decides on an
appropriate control strategy, and the control
valve manipulates the flow of coolant. Determine
the overall failure rate, the unreliability, the
reliability, and the MTBF for this system. Assume
a 1-yr period of operation.
• The process component are related in series.
• If any one of the components fail, the entire system
fails.
• Failure rates are from Table 11-1.
• Reliability – Eq. 11-1
• Failure probability – Eq. 11-2
• Overall reliability, R (Eq. 11-8)
• Failure probability, P
• Overall failure rate, µ
• MTBF
Example 11-2
A diagram of the safety systems in a certain
chemical reactor is shown in Figure 11-5. This
reactor contains a high-pressure alarm to alert the
operator in the event of dangerous reactor
pressures. It consists of a pressure switch within the
reactor connected to an alarm light indicator. For
additional safety an automatic high-pressure reactor
shutdown system is installed. This system is
activated at a pressure somewhat higher than the
alarm system and consists of a pressure switch
connected to a solenoid valve in the reactor feed
line. The automatic system stops the flow of
reactant in the event of dangerous pressures.
Assume a 1-yr period of operation.
Compute:
a) the overall failure rate,
b) the failure probability,
c) the reliability,
d) and the MTBF for a high-pressure condition.
Solution
• A dangerous high-pressure reactor situation occurs
only when both the alarm system and the shutdown
system fail.
• These two components are in parallel.
• For the alarm system the components are in series:
• For the shutdown system the components are also
in series:
• The two systems are combined using Equation 11-6
(parallel):
• For the alarm system alone a failure is expected once
every 5.5 yr.
• For a reactor with a high- pressure shutdown system
alone, a failure is expected once every 1.80 yr.
• However, with both systems in parallel the MTBF is
significantly improved and a combined failure is
expected every 13.7 yr.
Event Tree
• Begin with initiating event and work toward a final
result
• Consider the chemical reactor system shown in
Figure 11-8.
• This system is identical to the system shown in
Figure 10-6, except that a high-temperature alarm
has been installed to warn the operator of a high
temperature within the reactor.
• The event tree for a loss-of-coolant initiating event
is shown in Figure 11-9.
• Four safety functions are identified. These are
written across the top of the sheet.
1) The first safety function is the high-temperature
alarm.
2) The second safety function is the operator
noticing the high reactor temperature during
normal inspection.
3) The third safety function is the operator
reestablishing the coolant flow by correcting the
problem in time.
4) The final safety function is invoked by the
operator performing an emergency shutdown
of the reactor.
• Let us also assume that:
• The hardware safety function fail 1% of the time
they are placed in demand. This is a failure rate of
0.01 failure/demand.
• Assume that the operator will notice the high
reactor temperature 3 out of 4 times and that 3 out
of 4 times the operator will be successful at
reestablishing the coolant flow.
• Both of these cases represent a failure rate of 1
time out of 4, or 0.25 failure/demand.
• Finally, it is estimated that the operator successfully
shuts down the system 9 out of 10 times. This is a
failure rate of 0.10 failure/demand.
Fault Tree
• Method for identifying ways in which hazards can
lead to accidents.
• Identified top event and works backward toward the
various scenarios that can cause the accident.
• Top event: flat tire
• Cause can classified into:
i) Basic event – cannot be defined further
ii) Intermediate event – can be defined further
• Circle denotes basic event
• Rectangular denotes intermediate event
Example 11-5
Consider Example 11-2. Draw a fault tree for this
system.
Solution
• The top event is written at the top of the fault tree
and is indicated as the top event (see Figure 1114).
• Two events must occur for overpressuring: failure
of the alarm indicator and failure of the
emergency shutdown system.
• These events must occur together so they must
be connected by an AND function.
• The alarm indicator can fail by a failure of either
pressure switch 1 or the alarm indicator light.
These must be connected by OR functions.
• The emergency shutdown system can fail by a
failure of either pressure switch 2 or the solenoid
valve. These must also be connected by an OR
function.
Risk
• Usually describe graphically as shown in Fig
11-15.
Figure 11-15
General description of risk.
• Actual risk of a process or plant is determined
using
1. Quantitative risk analysis (QRA)
2. Layer of protection analysis (LOPA)
Layer of Protection Analysis (LOPA)
• LOPA is a semi-quantitative tool for analyzing and
assessing risk.
• To characterize the consequences and estimate
the frequencies.
• In order to lower the frequency of the undesired
consequences, various layers of protection are
added to a process.
• Figure 11-16 shows the concept of layers of
protection.
• The primary purpose of LOPA is to determine
whether there are sufficient layers of protection
against a specific accident scenario.