IEC 61508 Functional Safety Assessment Emerson Process ... - Exida

5.2 Hardware Assessment
To evaluate the hardware design of the DVC6000 SIS Digital Valve Controller, a Failure Modes,
Effects, and Diagnostic Analysis was performed by exida Consulting for each component in the
system. This is documented in [R12], [R13], [R17], and [R18]. The FMEDA’s were verified using
Fault Injection Testing as part of the development, see [D45] to [D50], and as part of the IEC 61508
assessment.
A Failure Modes and Effects Analysis (FMEA) is a systematic way to identify and evaluate the
effects of different component failure modes, to determine what could eliminate or reduce the
chance of failure, and to document the system in consideration. An FMEDA (Failure Mode Effect
and Diagnostic Analysis) is an FMEA extension. It combines standard FMEA techniques with
extension to identify online diagnostics techniques and the failure modes relevant to safety
instrumented system design.
From the FMEDA failure rates are derived for each important failure category. The detailed failure
rates are listed in [R12], [R13], [R17] and [R18]. Table 2 lists these failure rates in IEC 61508
format as reported in the FMEDA reports. The failure rates are valid for the useful life of the
devices.
Table 2 Failure rates according to IEC 61508
Device λsd λsu 2 λdd λdu
Fisher Controls DVC6000, DETT SIS Digital
Valve Controller, 4 – 20 mA., Double or
Single-Acting, Normal
Fisher Controls DVC6000, DETT SIS Digital
Valve Controller, 4 – 20 mA., Double or
Single-Acting, w/PVST diagnostics
Fisher Controls DVC6000 DETT SIS Digital
Valve Controller, 0 – 20 mA., Double or
Single-Acting, Normal
Fisher Controls DVC6000 DETT SIS Digital
Valve Controller, 0 – 20 mA., Double or
Single-Acting, w/PVST diagnostics
0 1183 0 144
508 1004 97 47
0 974 0 91
543 786 62 29
Tables in the FMEDA reports [R17], and [R18] lists these failure rates for the DVC6000 SIS
including the optional LCP100 and LC340 under a variety of applications. The failure rates listed
are valid for the useful life of the devices. Based on general field failure data a useful life period of
approximately 10 years is expected for the DVC6000 SIS Digital Valve Controller. This is listed in
the FMEDA report. However, when plant experience indicates a shorter useful lifetime than
indicated in the FMEDA report, the number based on plant experience should be used. All other
assumptions are also listed in the reports.
2 It is important to realize that the “Residual Effect” failures are included in the “Safe Undetected” failure
category according to IEC 61508. Note that these failures on their own will not affect system reliability or
safety, and should not be included in spurious trip calculations
© exida Certification SA. EFC 07-07-41 R002 V1R2 IEC 61508Assessment.doc, March 13, 2008
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