Master of Engineering: ME508 Safety Instrumented Systems Assessment 1

This document provides a detailed solution to the ME508 Safety Instrumented Systems Assessment 1, covering key concepts in safety engineering. It addresses questions on proof tests, Process Hazards Analysis (PHA), Mean Time Between Failure (MTBF), the IEC 61508 standard, Safety Requirement Specifications (SRS), failure modes, Layer of Protection Analysis (LOPA), and techniques to reduce spurious trip rates. The solution includes explanations of terms like Proof Test Coverage (PTC), dangerous undetected failures, and PFDavg, along with relevant formulas and examples. The document also discusses different demand modes, the contents of an SRS, and various failure modes as defined by IEC 61508. Furthermore, it outlines the prevention and mitigation layers in LOPA and defines tolerable and residual risk. Finally, the document provides techniques to reduce spurious trip rates, ensuring the safety and reliability of safety instrumented systems. The content is structured to provide a thorough understanding of the subject matter, making it a valuable resource for students studying industrial automation and safety engineering.

ASSESMENT GUIDE
NAME
INSTITUTION
COURSE
DATE

Question 1
The purpose of the proof test is to detect dangerous concealed or hidden defects and failures in
a safety-related system such that if necessary, the system may be restored to a "new" state or a
similar practical condition as quickly as possible through a repair.
Proof Test Coverage (PTC) is the term that refers to the percentage of potentially dangerous
undetected failures uncovered by the procedures undertaken during the proof test.
Question 2
Process Hazards Analysis(PHA) is a systematic method of identification and evaluation of
dangers of specific industrial processes associated with highly dangerous chemicals so as to
prevent or control any related potential hazards.
Process Hazard Analysis is conducted by PHA team which consists at a minimum of a process
engineer, an operator, and a facilitator.
Question 3
a) Mean Time Before failure (MTBF) is the predicted time that a system takes from one
previous failure to the next failure when the system is under normal operation limits.
b) Failure rate (λ) is the inverse of the MTBF.
Failure rate (λ)= 1
MTBF
c) Mean Time to Repair (MTTR) is the amount of time required to repair a failed system
and restoring it to full functionality
d) A Test interval is the period between two proof tests that access the operational safety
of systems to ensure that they are free from failures or any defects.
e) The formula for PFDavg under perfect test coverage and constant failure rate is;
PFDavg = λD ×tce
Where
tce= λDU
λD ( T 1
2 + MRT )+ λDD
λD
MTTR
λDU = Dangerous Undetected Failure Rate (per hour)
λDD = Dangerous Detected Failure Rate (per hour)

λD = λDD + λDU
T1 = Proof Testing Interval (hours)
MRT = Mean Repair Time (hours)
MTTR = Mean Time to Restoration (hours)
Question 4
a) Taking into assumption that the instrument is repairable
MTBF = MTTF+MTTR
MTBF=18000 hours + 24 hours =18024 hours
b) Percentage Availability
= upti me of system∨mac h ine
total h ours ( uptime+downtime ) × 100= MTBF
MTBF+ MTTR ×100= 18024
18024 +24 ×100
¿ 99.86 %availability
Question 5
On the basis of the IEC 61508 ;
a) A demand is a certain event which a safety instrumented system on a critical process
system responds to so as to achieve a safe state on the equipment under control.
b) The various demand modes recognized by the IEC 61508 include;
 Low –demand mode : Demands occur less than once per year.
 High-demand mode: Demands occur more than once per year.
 Continuous mode: demands are generally always present.
Question 6
The main contents of a Safety Requirement Specification(SRS) include:
i. The functional requirement specifications : This highlights what each safety function
will perform including ;
 Manual shutdown and bypass requirements
 Failure Mode and SIS response to failures
ii. The integrity requirement specification : It describes the capabilities of the safety
feature when in action including ;
 Spurious Trip Rates
 Resetting requirements

An SRS is developed once a need of a safety instrumented system (SIS) on the system is
identified and the target safety integrity levels (SIL) have been fully established for each safety
instrumented function (SIF).
Question 7
A failure mode is a cause of failure or a possible way in which failure of a system can occur.
Question 8
The different types of failure modes on the IEC 61508 include;
I. Fail safe undetected
II. Fail Dangerous detected
III. Fail Dangerous undetected
IV. No effect
V. Annunciation undetected
The most important failure mode in calculating PFDavg is Fail Dangerous which varies from
the fail dangerous detected to the fail dangerous undetected.
Question 9
a) In Layer of protection Analysis ,Prevention layer includes all the set of barriers or
actions that can be taken to prevent a hazardous control process from occurring for
example; , process control systems and safety systems, while, Mitigation layer involves
measures and actions taken to reduce the risk of an occurred or possible hazardous
catastrophe for example physical containment measures, plant evacuation procedures
and CCPS.
b) A tolerable risk is a degree of danger considered acceptable by community in order to
obtain a specific benefit or usefulness from it ,but in the understanding that the danger
has been evaluated and is being controlled.
c) A residual risk is the amount of dangers associated with an activity or incident that
remains after natural or intrinsic risk has been minimized by risk controls.
Question 10
Techniques to reduce spurious trip rate
 Fail-safe design on sensors and actuators to their likely failure modes and reviewing
spurious trip rates
 Ensuring physical and functional separation between BPCS and SIS

 Coming up with new diagnostics of checking performance of device so that maximum
performance is maintained.
Bibliography
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Instrumentation, Systems, and Automation Society, 2011.
[2]W. Goble, Safety Instrumented Systems Verification ' Practical Probabilistic Calculations.
[Place of publication not identified]: International Society of Automation, 2012.
[3]Safety instrumented systems for the process industry. Research Triangle Park, NC: ISA--The
Instrumentation, Systems, and Automation Society, 2003.
[4]"What Is IEC 61508? And Safety Integrity Level (SIL) Basics ...". [Online]. Available:
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[5]"MTBF, MTTR, MTTF & FIT Explanation of Terms". [Online]. Available: http://www.bb-
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