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Valtek ShearStream SB Ball Valves VLENDS4152-00-A4 05/15
3.3
Application limits
The materials of construction of a Shearstream SB Ball Valve are specified in the Shearstream SB Ball Valve datasheet.
It is especially important that the designer of the SIF checks for material compatibility considering on-site chemical
contaminants and air/hydraulic (as appropriate) supply conditions. If the Shearstream SB Ball Valve is used outside the
application limits or with incompatible materials, the reliability data and predicted SIL capability becomes invalid.
3.4
Design Verification
A detailed Failure Modes, Effects and Diagnostics Analysis (FMEDA) report is available from NAF AB for this product. This
report details all failure rates and failure modes as well as expected lifetime of the product.
The achieved Safety Integrity Level (SIL) of an entire Safety Instrumented Function (SIF) design must be verified by
the designer via a calculation of PFDAVG considering the architecture, proof test interval, proof test effectiveness,
any automatic diagnostics, average repair time and the specific failures rates of all equipment included in the SIF. Each
subsystem must be checked to assure compliance with minimum Hardware Fault Tolerance (HFT) requirements. The
exida exSILentia™ tool is recommended for this purpose as it contains accurate models for the Shearstream SB Ball
Valve and its failure rates.
When using the Shearstream SB Ball Valve in a redundant configuration, a common cause factor of at least 5% should be
included in the safety integrity calculations.
The failure rate data listed in the FMEDA report is only valid for the useful lifetime of the Shearstream SB Ball Valve. The
failure rates will increase after this useful lifetime period has expired. Reliability calculations based on the data listed in
the FMEDA report for mission times beyond the lifetime may yield results that are too optimistic, i.e. the calculated SIL
will not be achieved.
3.5
SIL Capability
3.5.1 Systematic Integrity
The Shearstream SB Ball Valve has met manufacturer design process requirements of Safety Integrity Level (SIL) 3.
These are intended to achieve sufficient integrity against systematic errors of design by the manufacturer. A Safety
Instrumented Function (SIF) designed with this product must not be used at a SIL higher than the statement without
“prior use” justification by the end user, or verification of diverse technology in the design.
3.5.2 Random Integrity
According to IEC 61508 the architectural constraints of an element must be determined. This can be done by following
the 1H approach according to 7.4.4.2 of IEC 61508 or the 2H approach according to 7.4.4.3 of IEC 61508.
The 1H approach involves calculating the SFF for the entire element.
The 2H approach involves assessment of the reliability data for the entire element according to 7.4.4.3.3 of IEC 61508.
The Shearstream SB Valve is classified as a device that is part of a Type A element according to IEC 61508, having a hardware
fault tolerance of 0.
The Shearstream SB Valve can be classified as a 2H device when the failure rates listed in the FMEDA report are used for
the Design Verification calculations. When 2H data is used for all of the devices in an element, then the element meets the
hardware architectural constraints up to SIL 2 at HFT=0 (or SIL 3 @ HFT=1) per Route 2H. If Route 2H is not applicable
for the entire final element, the architectural constraints will need to be evaluated per Route 1H.
When the final element assembly consists of several components additional to Shearstream SB Ball Valve, the SIL must
be verified for the entire assembly using the failure rates of all components. This analysis must account for architectural
constraints by comparing both SFF and HFT with IEC61508-2, Table 2 if following Route 1H
3.5.3 Safety Parameters
For detailed failure rate information refer to the FMEDA report for the Shearstream SB Ball Valve.
