Safety Shutdown System for Oil Storage Tank - Desklib

CASE 1: SAFETY SHUTDOWN SYSTEM FOR OIL STORAGE TANK
The safety control system is used in the case of oil storage tanks for safe shutdown and
prevention of leakage of crude oil. The basic requirement for a control system is for
regulation and maintenance of the required flow rate and pressure of oil in the tank.
SOLUTIONS
a) Oversight modes of level sensor, control valve, BPCS and associated process.
The safety instrumented system (SIS) is developed to satisfy and comply the sets of rules
prescribed by the nation. These requirements sometimes get ignored and disaster occurs. To
determine the failures, all the equipment’s must be considered and evaluated by making a list
of all the failure modes and their effects. The failure modes should be selected as level of
severeness, low level, high level and medium level. The threats of failure which are so small
or negligible should be neglected and work must be followed on. There can be three types of
failure of the component; upside, downside and in position. The Level Control Valve (LCV-
001), Regulatory Control Loop (LIC-001) or Level Sensor and the basic control system could
face these three failures in context to their arrangements. Fails for LCV-001 if fully open
valve; upside fall controller output decelerate to zero, if fully closed valve; output rises to
maximum or if fails in position; output goes completely open if set variable is varied. In
Level Sensors failures could be either upside if valve is completely closed or downsides if
valve is completely open. All the failures of LCV-001 and LIC-001 are single variable
failure. To safeguard, controllers need to be precisely interlocked. [1]
Level sensors performance required by management is to reduce the near misses incidents, to
strike off the possibility of casualties due to hazardous liquid leakage or overflow. Level
sensors should encounter the difference between similarity and proximity. According to
MinHash algorithm, proximity is:
[Pa-b = {(Ka
∩Kb) ÷ (Ka
∪Kb)}]
where, Pa-b is proximity of events a and b;
Ka and Kb are sorted sentences, respectively in the events a and b sorted by search engine.[2]
BPCS (Basic process control system) uses layer of protection analysis (LOPA) for huge
pitfall situations. Various systems and levels of protection are used according to the situation.
IEC 61511 Edition 2 (2016); the new standards set by the process industries. The standard

analysing process for the risk reduction factor; more than 10000, LOPA is used with other
SIS to analyse the hazards. For example in hydrocracker reactor inside petroleum refineries
an uncontrolled exothermic reaction can cause severe casualties in lined with the exposure of
flammable hydrocarbons and hydrogen gas. The implementation of instrumented control
systems using BPCS protects the system from vulnerability. Concluding these analyses a
robust design of safeguard technology is achieved for requisite performance requirements. [3]
b) Reliability Block Diagram

c) Fault Tree Analysis diagram
Failure
Rate
Chances of safeguard from
failure
1
Alarm/response failure during
filling 0.088 0.912
2 Level sensor failure 0.41 0.59
3 Control valve shut-off failure 0.22 0.78
4 Level controller failure (BPCS) 0.088 0.912
5 Tank structural failure 0.0022 0.9978
6 Tank vessel failure 0.0022 0.9978
7 Corrosion 0.0044 0.9956
8 Insufficient tank repairs 0.0018 0.9982
9 Operator failure to check leakage 0.0018 0.9982
Total 0.8184
d) Target SIL of the new SIF
A new SIF is required for better safety from leakage of hazardous oil from tank, safe
shutdown before over flowing to stop any upcoming incident of measure casualty. Reduction
of risk by inducing new SIL measures in SIFs and BPCS using LOPA system is necessary. It
will change the criteria of risk estimation and removal with better control.
5 SIL 3 SIL 4 SIL 4
4 SIL 2 SIL 3 SIL 4 SIL 4
3 SIL 1 SIL 2 SIL 3 SIL 4 SIL 4
2 NIL SIL 1 SIL 2 SIL 3 SIL 4
1 NIL NIL SIL 1 SIL 2 SIL 3
1 2 3 4 5
RISK OF LOSS OF LIVES
RISK LEVEL OF FAILURE OF SAFETY SYSTEMS