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TiPS Newsletter - April 2006
The Operator as IPL
Following is an abstract of a paper recently written by Ian Nimmo, President of User Centered Design Services. You can access the full paper below. Read and post your feedback at www.alarmmanagement.com:
Plant designers endeavor to minimize risk using multiple independent
protection layers (IPLs). The first layer is the process design
and involves special designs, layouts, equipment, and management
systems such as procedures and training. The second layer
focuses on higher reliability and greater consequences in
the event of failure of the basic process control system (BPCS),
together with critical alarms, operator supervision, and manual
intervention. The next layer has even higher reliability but
should require no operator intervention: the automatic SIS.
Other layers exist but they are generally physical, such as
relief valves and containment dykes.
In recent years, operator supervision has declined and
become more of an administrative function, while alarms
have become an Achilles heel to this protective layer. Emphasis
is placed elsewhere as engineers are more comfortable designing
the physical or automated SIS layers. AIChE CPPS Guidelines
state that 'the BCPS...fails the test of specificity...should
not be considered an IPL'. If the BPCS is not considered
an IPL, the supervisor is not supervising, and the alarms
are not functioning as designed, then the IPL must be left
to operator intervention. This is highly variable due to
the reliability of people and the factors that impact human
performance.
Poor design, gaps in supervision, undetected manufacturing
defects or failures, unworkable procedures, clumsy automation,
shortfalls in training, and less than adequate tools and
equipment may be present for many years before they combine
with local circumstances and active failures to penetrate
the many layers of defense. Hence, people live with many
of these conditions and suffer the consequences. Why?
How do you feel about the operator being considered an
IPL?
Read the entire article here
Post your feedback here
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