Safety Considerations Guide for Trident v2 Systems - TUV ...
Safety Considerations Guide for Trident v2 Systems - TUV ...
Safety Considerations Guide for Trident v2 Systems - TUV ...
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8 Chapter 1 <strong>Safety</strong> Concepts<br />
This table provides simplified equations <strong>for</strong> calculating the PFDavg <strong>for</strong> the key elements in an<br />
SIS. Once the PFDavg <strong>for</strong> each element is known, an SIL can be determined.<br />
Table 1 Simplified Equations <strong>for</strong> Calculating PFDavg<br />
Description Equation<br />
Sensors To calculate<br />
PFDavg <strong>for</strong><br />
sensors (2oo3)<br />
Block<br />
Valves<br />
To calculate<br />
PFDavg <strong>for</strong><br />
block valves<br />
(1oo2) in series<br />
(final elements)<br />
SIF To calculate<br />
PFDavg <strong>for</strong> a<br />
safety<br />
instrumented<br />
function<br />
Note Equations are approximate<br />
To determine the SIL, compare the calculated PFDavg to the figure on page 5. In this example,<br />
the system is acceptable as an SIS <strong>for</strong> use in SIL3 applications.<br />
For additional in<strong>for</strong>mation on SIL assignment and SIL verification, visit the Premier Consulting<br />
Services web site at www.premier-fs.com.<br />
<strong>Safety</strong> <strong>Considerations</strong> <strong>Guide</strong> <strong>for</strong> <strong>Trident</strong> <strong>v2</strong> <strong>Systems</strong><br />
PFDavg = (λDU*TI) 2<br />
+ 1/2*β*λDU*TI<br />
PFDavg = 1 /3(λDU*TI) 2<br />
+ 1/2*β*λDU*TI<br />
SIF PFDavg =<br />
Sensors PFDavg +<br />
Block Valves PFDavg +<br />
Controller PFDavg<br />
Table 2 Determining the SIL Using the Equations<br />
Variables<br />
(Supplied by the Manufacturer)<br />
λ = failure rate<br />
DU=dangerous, undetected failure rate<br />
TI= test interval in hours<br />
β = common cause factor<br />
λ = failure rate<br />
DU=dangerous, undetected failure rate<br />
TI= test interval in hours<br />
β = common cause factor<br />
β λDU TI PFD Result<br />
Pressure Transmitters (2oo3) .03 2.0E-06 4380 2.1E-04<br />
Temperature Transmitters (2oo3) .03 2.6E-06 4380 3.0E-04<br />
Total <strong>for</strong> Sensors 5.1E-04<br />
Block Valves (1oo2) .02 2.2E-06 4380 1.3E-04<br />
Total <strong>for</strong> Block Valves 1.3E-04<br />
<strong>Trident</strong> Controller 4380 1.0E-05 0.1E-04<br />
PFDavg <strong>for</strong> SIF 6.5E-04
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