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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 6 OF 23
Fax: + 44 (0)1642 616447
www.pidesign.co.uk5
SPECIFICATION OF SAFETY INSTRUMENT SYSTEM
See Safety Requirement Specification LF364005_RPT and SIS Trip Matrix LF346001_SCH.
6
SAFETY INSTRUMENT FUNCTIONS
6.1
Sensor Sub-System
For the Gasoline tanks (tanks 1, 2, 3 & 4), the Sensor sub-system comprises of a Drexelbrook
RF admittance type level switch. Tag Numbers LE01, LE02, LE03 & LE04. The switch
operation is based upon a change in RF which senses the presence or absence of material, in
this case, the top of the floating deck. The level switch has a brass weight on the bottom to
prevent sparking and in the event of the weight falling off, the switch will sense the weight loss
and fail to the alarm position.
For the remaining, fixed roof distillate and additive tanks, the Sensor sub-system comprises of
a Drexelbrook RF admittance type level switch. Tag Numbers LE05, LE06, LE07, LE08,
LE09, LE10, LE11, LE18, LE18A, LE21 & LE22. The switch operation is based upon a change
in RF which senses the presence or absence of material, in this case, the liquid in the storage
tank. The switching point of the level switch is at the end of the switch.
The instruments are ATEX certified II 1G EEx ia IIC T5 (Nemko 03ATEX 1621X).
6.1.1 Sensor Suitability
The tanks are operated at atmospheric conditions. The materials of construction of the level
switch are 316L stainless steel and the process flange is carbon steel. For the floating deck
tanks, the switch element has a brass bottom weight and stainless steel cable, on which Gasoline
has no chemical effect, the brass bottom weight will not cause sparking on striking the floating
deck. For the fixed deck tanks, the switch element is stainless steel, on which none of the stored
products has any chemical effect. The level element is flanged ANSI 150 top mounted so there
is no risk of product release from the instrument tapping. During normal operation, the detector
gives a constant 16mA signal. On detection of a high level the milliamp signal reduces below
8 mA. On a fault the milliamp signal increases to 22mA. The signal is wired into a trip amplifier
which will be set to switch at 15mA falling and 17 mA rising.
6.1.2 Sensor Subsystem PFD
For 1oo1 Architecture - Max mode
PFD
avg
8.57 x 10
-4
Safe fail fraction 93.2%
This calculation has allowed for annual check and MTTR of 24 hours, with imperfect
testing.
Calculated value of PFD of 8.57 x 10
-4
equates to a Safety Integrity Level of > SIL2.