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

5.27 Hardware Assessment
To evaluate the hardware design of the 3144P Temperature Tansmitter, a Failure Modes, Effects,
and Diagnostic Analysis was performed by exida. This is documented in [R2].
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. Table 1 lists these
failure rates as reported in the FMEDA report for the case where a single 4-wire RTD is used. The
failure rates are valid for the useful life of the devices. Based on Emerson endurance test data and
general field failure data a useful life period of approximately 50 years is expected for the 3144P.
This is listed in the FMEDA reports. Note that if the device is using a thermocouple rather than an
RTD or if more than one RTD’s are used, the failure rates will differ and the FMEDA should be
consulted to determine the correct failure rates.
Table 1 Failure rates according to IEC 61508 – 3144P Temperature Transmitter with single RTD
Failure Categories SD SU DD DU SFF
Low trip 265 FIT 112 FIT 20 FIT 40 FIT 90.8%
High trip 20 FIT 112 FIT 265 FIT 40 FIT 90.8%
An average Probability of Failure on Demand (PFD AVG ) calculation is performed for a single (1oo1)
3144P Temperature Transmitter with single 4-wire RTD. The failure rate data used in this
calculation is displayed in section 5.2.1 or [R2]Error! Reference source not found.. It is assumed
that the transmitter output is send low upon detection of failure and the safety function has a low
trip point.
The resulting PFD AVG values for a variety of proof test intervals are displayed in Figure 1. As shown
in the figure the PFD AVG value for a single 3144P Temperature Transmitter with single 4-wire RTD,
with a proof test interval of 1 year equals 2.38E-04.
© exida Certification rosemount 11-02-57 r002 iec 61508 assessment.docx, 5/14/2012
Michael Medoff Page 18 of 21