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Local Fuel plc - Shoreham Oil Terminal
Safety Instrument System Functional Safety Assessment Stage 5
P & I Design Ltd
DOCUMENT NO: LF364100_RPT
2 Reed Street, Thornaby, UK, TS17 7AF
ISSUE: A DATE: 15.06.2015
Tel: + 44 (0)1642 617444
PAGE 20 OF 32
Fax: + 44 (0)1642 616447
www.pidesign.co.ukLogic Solver:
As stated in the above section that the sensor provides a 4 – 20mA signal which changes
relevant to its status. The safety manual for the sensor requires a logic solver which can
monitor and act upon this change of mA signal, it further states that it the signal should be
monitored for a 1mA change.
This is provided for within the logic solver by utilising a trip amplifier which is set at 15mA
falling and 17mA rising, mA signal outside this range will result in the de-energising of the
trip amplifiers relay output which is fed into the coil of a PILZ safety relay.
The pfd data on the Trip Amplifier is:
The calculation within the SIL Verification document Section 6.5 utilises the above data and
confirms that a PFD
S
= 3.98x10
-4
it is noted that the calculation refers to this pfd being for
perfect testing and imperfect testing provides a pfd of 3.94x10
-4
. The device has a SFF of 0.83
and as such would not satisfy HFT = 0 in accordance with BS EN 61508. However, the device
has been assessed and certified by Exida Prior Use against the requirements of BS EN 61511
and is certified as appropriate for a single device in a SIL 2 system.
The second part of the logic solver is a PILZ PNOZ s2 safety relay. The device has internal
architecture providing a 1oo2D with internal diagnostic coverage of 99% and providing a SFF
of 0.99. In accordance with BS EN 61508 the device can be utilised in a SIL 3 system.
However, in accordance with BS EN 61511 the device is limited to SIL 2 for a single device.
The pfd is certified as 2.13x10
-5
.