SIL Approval Explained
The AccuTrak AT100 and AccuTrak AT200 high accuracy, magnetostrictive liquid level transmitters have been certified for operation in SIL 2 (Safety Integrity Level 2) environments.
The AccuTrak transmitters are the first magnetostrictive transmitters in the industry to be certified to the exacting
IEC 61508 safety standard.
They help reduce risk at the process measurement level when implementing Safety Instrumented Systems (SIS) in process
plant environments, such as chemical plants, refineries, and power plants.
What is a SIL?
• SIL ratings were established to define a metric for evaluating a system’s
(or function’s) level of operational reliability with regards to maintaining safety.
• SIL - stands for Safety Integrity Level
Background:
• Driving Standards for Industrial and Occupational Safety:
IEC 61508
IEC 61511
ISA 84.01
• Application of these standards occurs at the plant level with a Safety Instrumented System (SIS).
Definitions:
SIS
• A SIS is defined as “an instrumented system used to implement one or more Safety Instrumented Functions (SIF).” A SIS is composed of a combination of sensors, logic solvers, and final elements and is independent of the Basic Process Control System.
• The SIS is designed to :
Permit a process to continue when all specified conditions are satisfied.
Take a process to a safe state when any specified conditions have been violated.
Take action to mitigate the consequences of an industrial hazard.
SIF
• The SIF is defined as “a singular function to be implemented by a SIS which is intended to achieve or maintain a safe process state with respect to a specific hazardous event”.
• The SIL rating is applied at the SIF level, one function at a time.
• The SIF is also composed of the combination of sensors, logic solvers, and final elements related to a respective safety function.
• The SIL rating is related to the functional reliability of a SIF evaluated as a whole “system”.
Safety Integrating Levels:
| Safety Integrity Level | Probability of Failure on Demand per Year | Risk Reduction Factor |
|---|---|---|
| SIL 4 | <10-5 to <10-4 | 100,000 to 10,000 |
| SIL 3 | <10-4 to <10-3 | 10,000 to 1,000 |
| SIL 2 | <10-3to <10-2 | 1,000 to 100 |
| SIL 1 | <10-2 to <10-1 | 100 to 10 |
• Here, the probability of Failure on Demand per year looks at any type of failure and is an overall statistical reliability of the SIF.
FMEDA Analysis:
• FMEDA (Failure Modes, Effects, and Diagnostics Analysis)
• Defined as a component by component analysis of the discrete failure rates and the resulting effects (in terms of safety operation) while including the effects of internal diagnostics.
• Effects of a particular failure are categorized by:
λSD = Safe, detectable failure rate
λSU = Safe, un-detectable failure rate
λDD = Dangerous, detectable failure rate
λDU = Dangerous, un-detectable failure rate
• Safe Failure Fraction = (λSD + λSU + λDD) / (λSD + λSU + λDD + λDU)
Safe Failure Fractions and SIL Ratings
• Safe Failure Fraction for Type A - Simple Device
| Safe Failure Fraction | Hardware Fault Tolerance | ||
|---|---|---|---|
| 0 | 1 | 2 | |
| < 60% | SIL 1 | SIL 2 | SIL 3 |
| 60% - 90% | SIL 2 | SIL 3 | SIL 4 |
| 90% - 99% | SIL 3 | SIL 4 | SIL 4 |
| > 99% | SIL 3 | SIL 4 | SIL 4 |
• Safe Failure Fraction for Type B - Complex Device
| Safe Failure Fraction | Hardware Fault Tolerance | ||
|---|---|---|---|
| 0 | 1 | 2 | |
| < 60% | Not Allowed | 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 |
Certificates:
SIL2
FMEDA
AccuTrak Magnetostrictive Level Transmitters - technical details >>
Although utmost care has been taken in collecting the information given in these tables,
MeasurIT Technologies does not assume any responsibility as to their correctness and completeness.