Dear Readers,
Let's discuss an example of SIS on a gas compressor driven by a gas engine. There are hundreds of instruments installed on a package these days. Sensors, transmitters, and switches are for monitoring as well as control purpose through control valves, SOVs, and valves. In addition to these monitoring and control applications; an additional system for safety instrumented is also installed.
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Typically following are the five tripping securities on a typical gas engine-driven compression package:
- High liquid level in fuel scrubber
It monitors the high liquid levels in the fuel scrubber of the compressor package. In case the level rises above the dangerous threshold, the system will achieve a safe state by isolating the compressor and its driver from all external energy sources.
- High liquid level in suction gas scrubber
It monitors the high liquid levels in the gas scrubber of the compressor package. In case the level rises above the dangerous threshold, the system will achieve a safe state by isolating the compressor and its driver from all external energy sources.
- High vibrations on the Engine side (Frame vibration)
The vibration sensor/switch is to monitor the vibration levels on the engine frame. In case vibration sensors detect excessive vibration, the system will achieve a safe state by isolating the compressor and its driver from all external energy sources.
- High Vibrations on Compressor Side(Frame vibration)
The vibration sensor/switch is to monitor the vibration levels on the compressor frame. In case vibration sensors detect excessive vibration, the system will achieve a safe state by isolating the compressor and its driver from all external energy sources.
- Emergency push button on the local control panel beside the package
The purpose of emergency pushbutton SIF is to provide manual shutdown of the compressor unit initiated by the operator. In case of manual shutdown activation, the system will lead the shaft line equipment into a safe state.
The activation of any of the above causes will trigger the compressor unit shutdown and bring the system into a fail-safe position.
BLOCK DIAGRAM HERE
The entire Safety Instrumented System should be tested including the sensor(s), the logic solver and the final element (s). The frequency of the proof tests should be followed as decided using the PFDAVG calculation. Different parts of the SIS may require different test intervals, for example, the logic solver may require a different test interval than the sensors or final elements. Each SIS shall be periodically visually inspected to ensure there are no unauthorized modifications and no observable deterioration (for example, missing bolts or instrument covers, rusted brackets, open wires, broken conduits, broken heat tracing, and missing insulation).
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4 Comments
WHAT IS SIL?
ReplyDeleteSIL is a relative level of risk reduction provided by a safety function.
SIL ratings correlate to frequency and severity of hazards. They determine the performance required
to maintain and achieve safety — and the probability of failure.
There are four SILs — SIL 1, SIL 2, SIL 3, and SIL 4.
The higher the SIL, the greater the risk of failure. And the greater the risk of failure, the stricter the
safety requirements.
HAZARD AND RISK ANALYSIS FOR DETERMINING SILs
ReplyDeleteEnsuring functional safety requires a hazard analysis and risk assessment of equipment under control (EUC). A hazard analysis identifies all possible hazards created by a product, process, or application. This determines the safety function requirements for the safety standard. For each hazard you identify, you’ll need to do a risk assessment. This assesses the frequency or likelihood
of a hazard occurring, as well as the severity of the consequences if it does occur. Risk assessments determine the safety integrity requirements for the safety standard. And they’re critical for determining the SIL required to reduce risk. You can use either qualitative or quantitative analysis
to assess risk. A specific method isn’t required. One way you can assess risk is to create a requirements traceability matrix and do a failure modes and effects
analysis (FMEA).
WHY SIL IS IMPORTANT
ReplyDeleteSIL ratings determine the functional safety
requirements you’ll need to fulfill. There are different
recommendations for software development and
design techniques based on SILs.
These recommendations are as follows:
• “HR” indicates that the method is highly
recommended.
• “R” indicates that the method is recommended.
• “---” indicates that the method has no
recommendation for or against being used.
For example, design and coding standards are
Deleterecommended for SIL 1 and highly recommended
for SILs 2, 3, and 4. And forward traceability
is recommended for SILs 1 and 2 — and highly
recommended for SILs 3 and 4.