Page 459 - Offshore Electrical Engineering Manual

446 CHAPTER 3 Notes on Safety Integrity Level Assessment

Two requirements must be defined to achieve ‘functional safety’:

• A clear definition of what the safety function has to achieve (i.e., what does it
do and how does it do it?) (derived from the hazard analysis)
• The safety integrity requirement (probability of success, or likelihood of func-
tion being performed satisfactorily) (derived from the risk assessment)

SAFETY INSTRUMENTED SYSTEM
A safety instrumented system (SIS) is an instrumented system used to implement
one or more SIFs. An SIS may contain any combination of sensors, logic solvers and
final elements.

THE SAFETY INTEGRITY LEVEL ASSESSMENT PROCESS

An SIL is a discrete level for specifying the safety integrity requirements of each SIF,
as allocated to an SIS as follows:

Low Demand Mode of Operation
Safety Integrity Level (SIL) Probability of Failure on Demand (PFD)
−1
1 10 –10 −2
−2
2 10 –10 −3
−3
3 10 –10 −4
−4
4 10 –10 −5
High Demand (or Continuous) Mode of Operation
Safety Integrity Level (SIL) Probability of Failure on Demand (PFD/h)
1 10 –10 −6
−5
2 10 –10 −7
−6
−7
3 10 –10 −8
4 10 –10 −9
−8
For the electrical/instrument engineer, SIL assessment involvement is almost
always associated with control loops such as pressure, temperature or level control
or monitoring such as fire and gas detection, known as SISs. During the assessment,
loops will be broken down into sensing elements (e.g., pressure transmitters), field
wiring, logic solver and final control elements. Power supplies and communications
interfaces may also play a part in the process, but devices should always fail to a safe
condition.
The SIL assessment process should follow the guidelines in ANSI/ISA S84.00.01-
2004/IEC 61511/61508.