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Methodology ‘rules’
159 The use of this methodology requires the application of some simple rules:
No protective measures, which would protect against the hazardous event of concern, are
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considered at this stage. That is to say in this example, no alarms, trips or interlocks or
actions protecting against high level.
Thinking is not limited to the diagram boundary but is extended as required beyond what is
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on the diagram.
All modes of operation are considered: (a) normal operation, (b) start-up, (c) shutdown, etc.
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160 The hazardous event is put at the top of a page and the initiating events (demands) are then
developed in a systematic manner by asking the question ‘how?’ at each level of detail.
Mode of operation
161 When developing the demand tree and considering the question ‘how?’ it is important that
the different modes of operation are reviewed for failures that could lead to the hazardous event.
Table 11 may be used as a prompt to assist the systematic process.
Table 11
Modes of operation and initiating events
Mode of
operation
Class of initiating event
Equipment
failure
Failure of
services
Human failure External events
Normal operation
Start-up
Shutdown
Abnormal modes
Maintenance
162 In Table 11 services could include any or all of the following:
Loss of electrical power.
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Loss of steam.
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Loss of instrument air.
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Loss of cooling water.
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Other.
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Example demand tree
163 Figure 25 shows an example demand tree. The top of the demand tree is the hazardous
event of concern. This is expressed as clearly and precisely as possible to assist with
development of the rest of the tree.
164 The next level down may relate to modes of operation (eg start-up, shutdown, normal,
catalyst regeneration etc) or composition ranges (eg ‘high’ ethylene, ‘high’ methane, ‘high’
hydrogen concentration etc). The important requirement at this level is to keep the description as
generic as possible so that it can be developed in more detail further down the tree.