This document discusses safety instrumented systems (SIS), which are designed to respond to hazardous conditions in industrial plants. An SIS monitors for conditions that could become hazardous and responds by taking actions to prevent or mitigate hazards. Examples provided include a furnace that shuts off fuel valves in response to high pressure and a reactor that opens a coolant valve when temperature rises too high. The document outlines standards for good engineering practices in designing, implementing, and maintaining SIS according to lifecycle phases from planning and design to operations and auditing. Key aspects covered are managing risks to people and procedures, assessing and mitigating risk through assigning safety integrity levels, and proving that SIS designs achieve the desired safety functionality.

1. Safety Instrumented Systems
ANGELA E. SUMMERS, PH.D., P.E.
SIS-TECH Solutions, LLC
We’re Proven-in-Use.
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2. Terminology
Critical Control Systems
Safety Shutdown Systems
Protective Instrumented Systems
Equipment Protection Systems
Emergency Shutdown Systems
Safety Critical Systems
Interlocks
Safety Instrumented Systems
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3. What is a Safety Instrumented System (SIS)?
An SIS is designed to:
respond to conditions in the plant which may be
hazardous in themselves or,
if no action was taken, could eventually give rise to a
hazard, and
to respond to these conditions by taking defined
actions that either prevent the hazard or mitigate the
hazard consequences.
Input ---- Logic Solver ---- Output
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4. Examples of Safety Instrumented Systems
High fuel gas pressure furnace initiates
shutdown of main fuel gas valves.
High reactor temperature initiates fail
open action of coolant valve.
High column pressure initiates fail open
action of pressure vent valve.
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5. OSHA
&
EPA
Process Safety Management
SIF included here
Written Internal
Guidelines
Emergency Shutdown
Systems, Control, Relief
Systems
Mentor/Engineering
Practices
Industry Codes
& Standards
GOOD ENGINEERING PRACTICE
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6. Standards Bodies that Define Good Engineering
Practice for Safety Instrumented Systems
ISA, Instrumentation Systems and
Automation Society
IEC, International Electrotechnical
Commission
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7. Other standards and guidelines must be
integrated with SIS standards!
NFPA
IEC 61508
Boiler Codes
API
ISA 84.01-2003
ASME
ISO
IEC 61511
AICHE
Books
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8. Safety Instrumented System Standards
IEC 61508 - “Functional Safety: Safety Related Systems”
Current version released 1999
Under revision for next release 2005
IEC 61511 - “Functional Safety: Safety Instrumented
Systems for the Process Industry Sector”
Published 2003
ISA 84.01-2003 - “Functional Safety: Safety
Instrumented Systems for the Process Industry
Sector”
Identical to IEC 61511 with inclusion of grandfather clause
To be published October 2003
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9. Planning
Front End
Engineering
Management
of Change
Safety
Lifecycle
Engineering
Design
Operations &
Maintenance
Commissioning
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10. Manage risk - People
Identify role of individual or departments
Who is assigned to project?
What are their roles?
Document competency of individuals and
departments
Does anyone need more training?
Who will mentor inexperienced engineers?
Who will review and approve?
This is good project management.
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11. Manage risk – Procedures
Define when the following will take place:
Verifications,
Audits, and
Validation.
Require procedures for evaluating the
performance of the SIS after it has been
installed
Performance audits and
Tracking failures rates.
This is good quality control.
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12. Assess risk and mitigate it
Will talk about the “how to” later
The assessment results in the
assignment of Safety Integrity Level
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13. Safety Integrity Level
SIL
PFDavg
Risk Reduction
Availability (%)
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10-4 to 10-5
10,000 to 100,000
99.99 to 99.999
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10-3 to 10-4
1,000 to 10,000
99.9 to 99.99
2
10-2 to 10-3
100 to 1,000
99 to 99.9
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10-1 to 10-2
10 to 100
90 to 99

14. Let’s talk about Risk.
What is your wager?
How many times do
you place the wager
each year?
What are your odds of
losing your wager?
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15. Design SIF
Justify selection of devices
Document the safety requirements
specification
Design SIFs to achieve Safety Integrity
Level.
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16. Prove it
Verify
Safety Integrity Level
Fault tolerance
Commissioning
Install SIFs per design documents
Functional safety assessment
Make sure all documents are in place and all
hazards analysis items are addressed.
Validation
Test SIFs to ensure that they have desired
functionality
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17. Maintain it
Operation, maintenance and
testing
Use diagnostics and testing to
maintain performance
Create and maintain procedure to
support these activities
Train personnel on procedures
Management of change
Monitor changes to SIS that might affect SIL
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18. Audit it
Includes design and procedures
Define frequency of audits
Determine the degree of independence
of auditing activity
Document audit
Define follow-up activities
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19. Planning
Front End
Engineering
Management
of Change
Safety
Lifecycle
Engineering
Design
Operations &
Maintenance
Commissioning
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