Pascual Imec06

Optimization of Inspection Decisions of Safety Related Systems Rodrigo Pascual, Ph.D. Department of Mechanical Engineering Universidad de Chile Pressure vessel lu 1 lu 2 pt 1 pt 2 pt 3 v 1 v 2

Motivation ,[object Object],[object Object],[object Object],[object Object],Dormant systems Systems in storage Spares Weapons Standby equipment Safety systems

Moubray (1997) ,[object Object]

Motivation Medical Expose dose control in radiotherapy systems Air transport Flight-by-wire control systems Automotive Airbags Antilock brake systems Process Emergency shutdown systems Military/Law enforcement Gas detectors Railway Automatic train stop systems Applications

Motivation Safety Systems in Process Industry (34 incidents) Source: Bell (2003) http://www.iee.org/Oncomms/pn/emc/Paper03.pdf Conception Design Implementation Operation & maintenance

Balance Unavailability Inspection frequency Overall Unknown Known (inspections, repairs, replacements) Costs Reliability model

IEC 61508 (2004) Functional safety of electrical/electronic/programmable electronic safety-related systems ,[object Object],[object Object],4 3 1 1,E-05 1,E-04 1,E-03 1,E-02 1,E-01 2 Probability of failure on demand Safety integrity level IEC 61508 (2004) Nuclear IEC 61513 (2001) Machinery 62061 (2005) API 670 (2000) Generic ISO 14224 (2004) Railway EN 50126 (1999) EN 50128 (2001) Process IEC 61511 (2003) ANSI 84.01 (2004 )

Aim at the in-service phase ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Conception Design Implementation Operation & maintenance

Model considerations ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Sensor 1 Sensor 2 Sensor 3 Logic 1 Logic 2 2/3 1/2 1/2 Actuator 1 Actuator 2 Pressure Temperature Leak … Valve Break Fire extinguisher … Electronic unit Valve spring …

Epochs for functional assessment ,[object Object],[object Object],[object Object],[object Object]

Failures of safety systems IEC 61508 Detected at inspections Not detected at inspections Failure “ Random” Systematic Aging (component) Stress (subsystem) Design (specification) Interaction (human error)

Imperfect inspections ,[object Object],Goble, W.M., Periodic inspection and test: requirements and benefits, Hydrocarbon Processing, 81(6), 117, 2002.

Partial and full inspections instantaneous

Overall availability & safety availability * ✔ ✔ ? ? ? ✔

Simplified model Overall availability Partial inspections Full inspections Probability of failure on demand Safety availability overall safety

Practical consideration Component failure rate (standard databases) Safety system level failure rate (plant information system) Complex System model One component model Conception Design Implementation Operation & maintenance

Redundancy and common cause failures ,[object Object],[object Object],Common cause Independent ,[object Object],[object Object],[object Object],1- β 1- β β Comp. 1 Comp. 2    Conception Design Implementation Operation & maintenance

Example 2 out of 3 Pressure vessel lu 1 lu 2 pt 1 pt 2 pt 3 v 1 v 2

Example single component safety system ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object], Time Instantaneous availability Partial insp. Partial insp. Full inspection Overall Availability 0 2 4 6 8 10 No. of partial inspections before renewal

Complex system Failure rate (  10 6 hours) Common Cause Factor From:Hauge, S., et al., Reliability Prediction Method for safety Instrumented Systems; PDS Method Handbook, 2006 Edition.,SINTEF, Trondheim, Norway, 2006

Results Safety availability Partial coverage Full coverage  =0.7  =1.0

Number of partial inspections in a renewal cycle N

Sensitivity analysis Nr of partial inspections Period between Partial inspections Optimal overall availability Attained safety availability

Review ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Conception Design Implementation Operation & maintenance

Pascual Imec06

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