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Safety and environmental standards for fuel storage sites
Final report
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Achieved requirement for SIL2 PFD
(avg)
1oo2 architecture
Figure 38
Typical tank overfill protection using 1oo2 architecture
49 Using the PFD
(avg)
calculations and the assumptions stated previously, the following values for
the PFD
(avg)
have been calculated for the 1oo2 architecture with a proof test interval of one year.
Sensor PFD
(1oo2)
3.82E-04
Logic Solver PFD
(1oo1)
7.11E-04
Valve PFD
(1oo2)
5.72E-06
Total loop PFD
(avg)
1.10E-03
50 These two worked examples show it is possible to achieve the requirement for SIL 2 PFD
(avg)
for both configurations. These are only two examples of the possible methods of achieving SIL 2
risk reduction, although other combination of architecture on the inputs and output elements may
also be equally valid.
51 It is worth noting that although the PFD
(avg)
requirement may have been achieved, architectural
constraints must also be satisfied and that may result in a more complex architecture – see Annex 2.
vent
Process
fluid
Storage tank
vent
LT
LT
Logic
solver