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Annex 3 Demand tree methodology for systematic identification of initiating causes
154 The purpose of this annex is to provide an example of an outline methodology for the
systematic identification of initiating events that can lead to hazardous events. This methodology
can be used with any SIL determination (such as LOPA, fault tree analysis) or other techniques
used for identification of the initiating events leading to a specific hazardous event.
Description of process example
155 Figure 24 shows the simplified schematic for part of a process sector plant. It has the incoming
flow from the left, with a flow controller (FIC210) setting the flow rate into the separator vessel shown.
Figure 24
Simplified process schematic
156 The incoming flow is separated in the vessel into two streams: a light vapour phase, which
exits the top of the vessel, and a liquid phase, which exits the bottom of the vessel. The liquid
level in the vessel is maintained by the level controller (LICA245) that adjusts the liquid flow out
of the vessel. The pressure in the vessel is maintained by a pressure controller (PIC214) in the
vapour line. Over-pressure protection is provided by a pressure relief valve on the top outlet from
the vessel.
157 Two instrumented protective measures are shown: (a) a low level trip (LZ246) protects against
loss of level in the vessel and vapour entering the liquid line and (b) a high level trip (LZ247) which
protects against liquid entering the vapour line.
158 The specific process concern in this example is associated with an uncontrolled high level in
the vessel and the consequences that would result from that. Detailed consequence analysis is
not necessary for illustration of the method for demand identification and so for the illustration the
hazardous event will be taken as ‘high level in the separator with flow into the vapour line’.
FT
210
FIC
210
LZ
246
LL
LZ
247
HH
XZV
247
XZV
246
FCV
210
LICA
245
LT
245
LCV
245
Liquid
PIC
214
PCV
214
Lights
Flare
Separator