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Guideline – Demonstrating Prior Use v4
Page 5 of 30
2.
INTRODUCTION AND SCOPE
This document provides guidance to help duty holders prepare a demonstration of prior
use for a component of a Safety Instrumented Function (SIF). A simple SIF can be
described as having three elements, with each element having a number of components,
as follows:
Figure 1 Elements and Components of a Safety Instrumented Function
Sensor
: The components of the sensor element of the SIF that are used to detect an
event. A component may be, for example, a level/pressure/temperature instrument or a
barrier providing isolation.
Logic Solver:
The components of the logic solver element of the SIF that are used to
determine if an event has occurred. A component may be, for example, a simple
relay/logic circuit, or complex safety controller
Final element:
The components of the final element of the SIF that are used to maintain
the process in a controlled state should an event occur. A component may be, for
example, an isolation barrier, solenoid, valve, or pump.
When considering whether a component may be a suitable candidate for a
demonstration of prior use, reference can be made to ISA-TR84.00.04 ‘Guidelines for the
Implementation of IEC61511’, Annex L1, which provides a work process for the selection
of devices.
2.1
Safety Integrity Levels in BS EN 61511
Safety Instrumented Functions meeting the requirements of BS EN 61511 may be
assigned one of four Safety Integrity Levels (SIL): SIL 1, SIL 2, SIL 3, or SIL 4.
Within the requirements for SIL 1, SIL 2 and SIL 3 in BS EN 61511, there is a need to
achieve a specified degree of hardware fault tolerance (HFT)
1
. The hardware fault
1
See Section 2.2