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International Society of AutomationColombia Section

SIL Ratings for Fire & Gas System Hardware –A Questionable Practice?

Standards

Certification

Education & Training

Publishing

Conferences & Exhibits Bogota 11

Paul Gruhn, P.E.

Where We’re Going…

1. The need for a new performance based fire & gas standard

2. What are SIL ratings?

3. How do fire & gas systems differ from safety instrumented systems?

4. Focusing on fire & gas system hardware alone is not the answeralone is not the answer

5. Attention must be paid to detector coverage and mitigation effectiveness

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Current Fire Standards

• NFPA 72: National Fire Alarm Code

• EN 54: Fire detection and fire alarmEN 54: Fire detection and fire alarm systems

• Both cover fire, not gas systems

• Both were developed for commercial applications

Both are prescripti e not• Both are prescriptive, not performance based

Current Gas Standards

• ANSI/ISA-12.13.01-2003

• ISA 92 0 01ISA 92.0.01

• Both cover gas, not fire systems

• Both were developed for industrial applications

• Both are performance based

– In terms of shock, vibration, response to humidity & temperature, drop test, etc., not SIL

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Performance Standards

• IEC 61508: Functional safety of electrical/electronic/programmable electronic safety-related systems

• IEC 61511: Functional Safety: Safety Instrumented Systems for the Process Industry Sector

Safety Integrity Levels

Safety Integrity

Safety Availability

Probability of Failure on

Risk Reduction Factor (1/PFD)

Level Demand (PFD)

4 99.99 – 99.999 .0001 - .00001 10,000 – 100,000

3 99.9 – 99.99 .001 - .0001 1,000 – 10,000

2 99 – 99.9 .01 - .001 100 – 1,000

1 90 99 1 01 10 1001 90 – 99 .1 – .01 10 – 100

0 Control (N/A)

For “Demand Mode” of operationFor “Demand Mode” of operation

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Multiple Layers of Protection

Community Emergency ResponseCommunity Emergency Response

Fire & Gas SystemsFire & Gas Systemsyy

Physical Protection (Dikes)Physical Protection (Dikes)

Physical Protection (Relief Devices)Physical Protection (Relief Devices)

Safety Instrumented SystemSafety Instrumented System

Alarms, Operator InterventionAlarms, Operator Intervention

Basic Process ControlBasic Process ControlBasic Process ControlBasic Process Control

ProcessProcess

IEC 61511 & Mitigation

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SIS and F&G Differences

• Prevention layers (keep it in the pipe) vs. mitigation layers

• For SIS: Does a SIL 2 rated sensor, SIL 2 rated logic box, and a SIL 2 rated final element provide a SIL 2 SIF?

• Can the same be said for a F&G system?

Event Tree

Detection Coverage

Hardware Response

Mitigation Effectiveness

Leak/Fire

Yes: P=.9

Yes: P=.99

No: P=.1

Yes: P=.9

X / year

No: P=.1

No: P=.01

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Detector Coverage

• Single sensor (1 out of N):

– Up to 98%Up to 98%

• Dual sensor (2 out of N):

– 20% to 90%

• Three or more sensors (3 out of N):

– < 60% or less

• A UK HSE report sites gas detector coverage of 76%

Detector Placement

A complete walkthrough 3D model of an offshore installation imported into the flame mapping software.

A view from a flame detector. The circular area represents the actual field of view of the detector.

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Expert vs. Computer

Expert layout using 9 detectors Computer layout using 5 detectors

Conclusions

1. Focusing on fire & gas system hardware alone is not the answer

2. Attention must be paid to detector coverage and mitigation effectiveness

3. Techniques are being developed to apply performance based

t f fi & tconcepts for fire & gas systems

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Thanks To…

• ISA 84 committee

• Fire & gas task team• Fire & gas task team

• Information presented by Shell, BP and Chevron

• Analysis work done by Kenexis