Safety Integrity Of The Digitrace Ngc-20 Hardware; Pfdavg Safety Function; Sil Related To Sff And Hft - Pentair DIGITRACE NGC-20-C-E Installations-, Betriebs- Und Wartungsanleitung

4.6.1 safety integrity of the DigiTrace nGC-20 hardware

4.6.2 PFDavg safety function

Type
DigiTrace NGC-20-CL-E
Table 4: Safety Integrity level

4.6.3 sIl related to sFF and hFT

NGC-20 INSTALLATION MANUAL
According to IEC 61508-2:2000, a distinction must be made between Type A systems and
type B systems. A sub-system can be viewed as a Type A system if, for the components
that are necessary in order to achieve the safety function:
1. The failure mode of all components that are used is adequately defined, and
2. The response of the sub-system in fault conditions can be completely determined,
3. Reliable failure data based on field experience are available for the sub-system,
to demonstrate that the assumed failure rates for recognized and unrecognized
hazardous failures can be achieved.
A sub-system can be viewed as a Type B system if, for the components that are necessary
in order to achieve the safety function:
1. The failure mode of at least one component that is used is not adequately defined, or
2. The response of the sub-system in fault conditions cannot be completely determined, or
3. No adequately reliable failure data based on field experience are available for the
sub-system, to support the assumed failure rates for recognized and unrecognized
hazardous failures.
The DigiTrace NGC-20-CL-E temperature control system with limiter corresponds to a
Type A system.
The limiter sensor, limiter electronics and the limiter relay together form the safety
related system that performs a safety function. The "average probability of the hazardous
failure of a safety function for the entire safety-related system" (PFDavg) is usually
divided among the subsystems. An external device e.g. an external power contactor
installed in a panel, is specific to the installation, and shall, in accordance with the
standards for the safety loop, be considered separately.
SIL level Architecture
SIL 2 1oo1D
MTTR = Mean time to repair
The following table presents the achievable safety integrity level (SIL), depending on the
safe failure fraction (SFF) and the hardware failure tolerance (HFT) for Type A safety-
related sub-systems.
Table 5 is valid for the DigiTrace NGC-20-CL-E:
Safe failure fraction (SFF)
SFF < 60%
60 < SFF < 90 %
90% < SFF < 99%
99% < SFF
Table 5: Relation SFF to HFT
Proof check MTTR (hrs)
interval
1 year 24
ZIEGLER ENGINEERING
PFD avg. HTF
3.017E-3 0 (1oo1)
Hardware fault tolerance (HFT) for Type A
0 1 2
SIL 1 SIL 2 SIL 3
SIL 2 SIL 3 SIL 4
SIL 3 SIL 4 SIL 4
SIL 3 SIL 4 SIL 4
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SFF
95.03%
19