GE Masoneilan 12420 - 8.3. Safety Requirements; 8.4. Safety Characteristics; 8.4.1. Assumptions

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8.3. SAFETY REQUIREMENTS
8.3.1. PROBABILITY OF FAILURE ON DEMAND (PFD
AVG
)
This table reflects the achievable Safety Integrity Level (SIL) depending on the mean provability of failure on demand. The
specified failure tolerances in this case apply to a safety function operated in the Low Demand mode.
Safety Integrity Level (SIL) PFD
avg
with Low Demand mode
4 ≥ 10
-5
to < 10
-4
3 ≥ 10
-4
to < 10
-3
2 ≥ 10
-3
to < 10
-2
1 ≥ 10
-2
to < 10
-1
8.3.2. SAFETY INTEGRITY OF THE HARDWARE
This table shows the achievable Safety Integrity Level (SIL) depending on the proportion of non-dangerous failures (SFF) and
the fault tolerance of the hardware (HFT) for safety-related type B subsystems.
Proportion of non-
dangerous failures (SFF)
Fault tolerance of the hardware (HFT)
0 1 2
< 60% Not permitted SIL 1 SIL 2
60% - < 90% SIL 1 SIL 2 SIL 3
90% - < 99% SIL 2 SIL 3 SIL 4
≥ 99% SIL 3 SIL 4 SIL 4
8.4. SAFETY CHARACTERISTICS
8.4.1. ASSUMPTIONS
The specified characteristics are applicable under the following assumptions that have been made during the FMEDA.
• The instrument is being used as a transmitter and the controller function is disabled (if applicable).
• The instrument must be configured as a SIL 2 device with failsafe position defined as Fail Low (<3.6 mA).
• In case of custom displacer, displacer weight must be lower the following rule:
Displacer Weight (gr) < 1600 x TT force x 4 / arm lengh
Torque tube force =1, 2 or 4
Arm length = 4", 8", 16" (side vessel) or other
i.e. 1600 x 1 x 4 / 4 = 1600 gr for a standard displacer and single force torque tube