Risk Assessment
(PROCESS SAFETY)
SUBMITTED BY:
Aguirre, Mariel Clyvi C.
Gucilatar, Reyziel E.
Logro, Tito T.
Masallo, Irish Ashley
SUBMITTED TO:
Engr. Jes Andre G. Trillana
PROBLEM SOLVING
1.) PROBLEM: A diagram of the safety systems in a certain chemical reactor is shown in Figure
12-5. 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.
Consider the alarm indicator and emergency shutdown system. Draw a fault tree for this
system.
Figure 12-5. A chemical reactor with an alarm and an inlet feed solenoid. The alarm and feed
shutdown systems are linked in parallel.
Solution:
The first step is to define the problem.
1. Top event: Damage to reactor as a result of overpressuring.
2. Existing event: High process pressure.
3. Unallowed events: Failure of mixer, electrical failures, wiring failures, tornadoes,
hurricanes, electrical storms.
4. Physical bounds: The equipment shown in Figure 12-5.
5. Equipment configuration: Solenoid valve open, reactor feed flowing.
6. Level of resolution: Equipment as shown in Figure 12-5.
Figure 12-14. Fault tree for Example 12-5
2.) Problem: The water flow to a chemical reactor cooling coil is controlled by the system shown
in Figure 12-4. 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.
Figure 12-4. Flow control system. The components of the control system are linked in
series.
Solution: These process components are related in series. Thus, if any one of the components
fails, the entire system fails. The reliability and failure probability are computed for each
component using Equations 12-1 and 12-2. The results are shown in the following table.
The failure rates are from Table 12-1.
The overall reliability for components in series is computed using Equation 12-8. The result is
3
R = ∏ Ri = ( 0.55)(0.75)(0.24) = 0.10
1
The failure probability is computed from
P = 1 – R = 1 – 0.10 = 0.90/yr.
The overall failure rate is computed using the definition of the reliability (Equation 12-1):
0.10 = e –μ
μ = –ln(0.10) = 2.30 failures/yr.
The MTBF is computed using Equation 12-5:
MTBF = 1/ μ = 0.43 yr.
This system is expected to fail, on average, once every 0.43 yr.
Answer: The overall failure rate: 2.30 failures/yr.
The unreliability:0.90/yr
The reliability : 0.10
The MTBF: 0.43yr
3.) Problem: For the reactor of Example 12-3 a high-pressure incident is expected once every 14
months. Compute the MTBC for a high-pressure excursion and a failure in the emergency shutdown
device. Assume that a maintenance inspection occurs every month.
Solution:
The frequency of process episodes is given by Equation 12-26:
𝜆=
𝑃𝑖
𝑇𝑖
𝜆 = 1 episode/[(14 months)(1 yr/12 months)] = 0.857/yr.
The unavailability is computed from Equation 12-25:
The average frequency of dangerous coincidences is given by Equation 12-27:
𝜆𝑑 = 𝜆𝑈 = (0.657)(0.023) = 0.020
The MTBC is (from Equation 12-29):
𝑀𝑇𝐵𝐶 =
1
1
=
= 50 𝑦𝑟.
𝜆𝑑
0.020
It is expected that a simultaneous high-pressure incident and failure of the emergency
shutdown device will occur once every 50 yr.
If the inspection interval 𝜏𝑖 , is halved, then U = 0.023, 𝜆𝑑 = 0.010, and the resulting MTBC
is 100 yr. This is a significant improvement and shows why a proper and timely maintenance
program is important.
4.) Problem: A PHA team has several major consequences with different initiative events and
frequencies. Develop a table to document the LOPA results for two major scenarios. The first
scenario is for a fire due to a tank rupture, and the second scenario is for a release from a reactor
because of control loop failure. Both scenarios have a vessel volume of 50,000lb of a flammable
above the BP, and the failures result in a six-month outage. The reactor is operated 100 days a
year.
Solution:
The LOPA results are shown in Table 12-6. A typical LOPA team develops a table with columns
for all significant events.
Table 12.6 General Format of LOPA
0
