LF364001_MNL - Overfill Protection SIS Design Manual

58 / 443

Local Fuel – The Shoreham Oil Terminal
Gasoline Import - Safety Instrument System
P & I Design Ltd DOCUMENT NO: LF364006_RPT
2 Reed Street, Thornaby, UK, TS17 7AF ISSUE: C DATE: 13.05.16
Tel: + 44 (0)1642 617444 PAGE 18 OF 23
Fax: + 44 (0)1642 616447
The proposed IEC61511 Edition 2 has a new requirement for hardware fault tolerance
Table 6 of the standard is reproduced below:
Minimum hardware fault tolerance
(unless clause 11.4.6 applies)
SIL 0 1 2
1 X
2 (low demand mode) X
2 (high demand mode) X
3 X
4 X
The Valve and actuator are classified as Type A devices operating in low demand mode and
clause 11.4.6 is not applied as the minimum HFT does not require reduction. Thus in this
instance, a hardware fault tolerance of 0 allows for the single final element to be used as a
1oo1 element for a SIL2 application.
6.3.3 Final Element (ESD Valves) Subsystem Summary
The combined PFD for the final element subsystem is 1.17 x 10-3 + 1.48 x 10-3 = 2.65 x 10-3
From the enclosed calculations and fault tolerance checks the sub-system meets the
requirements of >SIL 2 with a PFDG of 2.65 x 10-3
6.4 System PFD
System PFD = Sensor Sub-System PFD + Logic Solver PFD + Final Element Sub-System
PFD
System PFD = (8.57 x 10-4) + (4.20 x 10 -4) + (2.65 x 10-3)
System PFD = 3.93 x 10-3
From the enclosed calculations each sub-system individually satisfies the requirements of >
SIL 2 together with the combined PFD which meets the requirements of > SIL 2 with a
PFDSYS of 3.93 x 10-3 and meets the Risk reduction factor of at least 200.
Spurious trip rate calculated as 1 in 16 years which meets the target of less than 1 in 10 years.