29 Questions
What is the primary purpose of a Safety Instrumented System (SIS)?
To bring the plant to a safe state from a hazardous state
In case the Basic Process Control System (BPCS) fails, what layer of protection is required?
Layer 2
What does IEC define a Safety Instrumented System (SIS) as?
A system used to implement Safety Instrumented Functions
Why are Safety Instrumented Systems (SIS) considered critical in industrial plants?
To respond to hazardous plant conditions
What is the main function of a Safety Instrumented System (SIS) in an industrial plant?
To bring the plant to a safe state from a hazardous state
Why are Safety Shutdown Systems categorized as part of a Safety Instrumented System (SIS)?
To protect equipment and personnel during hazardous situations
What is the purpose of a Logic Solver in a safety instrumented system?
Execute pre-programmed actions based on sensor information
Which characteristic is essential for a Logic Solver in a safety instrumented system?
Fast decision-making ability
What does a Final Controlling Element (FCE) do in a safety instrumented system?
Implements the action determined by the logic system
Why is it important for a Final Controlling Element (FCE) to comply with 'Fail Safe' requirement?
To ensure safe operation even in case of failure
What happens when a fail-closed valve vents all actuating air pressure?
The valve fails closed
Which component blocks air pressure and vents the actuating diaphragm when de-energized?
Solenoid valve
What is the purpose of Safety Instrumented Functions (SIF) in a Safety Instrumented System (SIS)?
To achieve or maintain a safe condition of the process
What does SIL stand for in the context of Safety Instrumented Systems?
Safety Integrity Level
Why does an Oil Refinery typically have a higher SIL requirement compared to a water treatment plant?
Oil Refineries have higher levels of risk associated with their processes
What is the main emphasis when determining the Safety Integrity Level (SIL) target for Safety Instrumented Functions (SIF)?
Achieving or maintaining a safe condition of the process
Which part of an Industrial Safety Instrumented System (SIS) is responsible for detecting abnormal process conditions?
Sensor & Transmitter
What do Safety Instrumented Systems (SIS) aim to achieve when referring to 'when demanded' performance?
Performance specifically when needed
What is the purpose of Layer 5 - Passive Protection Layer?
To reduce the impact on the process/plant environment during severe conditions
What are examples of structures that belong to the Passive Protection Layer?
Tank Bund Walls, Fire Walls
In the context of plant protection layers, what is the function of Layer 4?
Actively protecting against over-pressurization
Who primarily makes up the Plant Emergency Response Team (Layer 6)?
Different representatives from management, operations, and maintenance departments
When does the Emergency Response Team (Layer 6) come into action?
After all other protection layers fail
What is the purpose of a Common Alarm System in a system?
To provide audible and visual alarms for notification purposes
Which layer is responsible for bringing the process in a safe condition during severe plant conditions and failure of the Safety Instrumented System (SIS)?
Layer 4 - Active Protection Layer
How does Relay Logic differ from Electronic Logic in implementing a Safety Instrumented System (SIS)?
Relay Logic uses relays and contacts, while Electronic Logic performs logical operations on logic inputs.
Why is testing Safety Instrumented Systems (SIS) considered important?
To uncover any Functional, Potential, or Hidden failures
What does an Active Protection Layer mainly consist of?
Devices that bring the process into a safe condition during failures
What function does a Pressure Relief Valve serve in the Active Protection Layer?
To relieve moderate changes in tank pressure
Study Notes
Safety Instrumented System (SIS)
- SIS uses dedicated sensors and transmitters to detect abnormal process conditions, with response time and precision being crucial characteristics.
- SIS controllers (logic solvers) determine the action to be taken based on the information gathered from sensors and execute pre-programmed actions to prevent hazards.
Logic Solver (LS)
- A logic solver must be able to render a fast decision and comply with the "Fail-Safe" condition.
- It is typically a PLC or Relay Logic that reads signals from sensors and executes pre-programmed actions.
Final Controlling Element (FCE)
- A Final Controlling Element must have the correct Valve Flow Coefficient (Cv) and comply with the "Fail Safe" requirement.
- It implements the action determined by the logic system.
Support System
- Provides necessary utilities for the system to operate, such as 24VDC, A/S, H/S, etc.
Common Alarm System
- Provides an audible and visual alarm for notification purposes, which must be loud and bright to be heard and seen by control room personnel.
Methods of Implementing a SIS Logic Solver
- Using Relay Logic
- Using Electronic Logic
Importance of Testing SIS
- Testing is crucial to uncover any Functional, Potential, or Hidden type of failure.
- It affects the Safety Integrity Level (SIL) of the system.
Layers of Protection
- Layer 1: Basic Process Control System (BPCS)
- Layer 2: BPCS + Operator Intervention
- Layer 3: Safety Instrumented System (SIS)
- Layer 4: Active Protection Layer
- Layer 5: Passive Protection Layer
- Layer 6: Plant Emergency Response Layer
Layer 3: Safety Instrumented System (SIS)
- Defined by IEC as an Instrumented System used to implement one or more Safety Instrumented Functions (SIF).
- Examples: Critical Control Systems, Safety Shutdown Systems, Instrument Protective Function (IPF), Equipment Protection Systems, Emergency Shutdown Systems (ESD), Safety Critical Systems, and Safeguarding Systems.
Function of SIS
- Brings the plant to a safe state from a hazardous state.
- Designed to respond to industrial plant conditions that may be hazardous or have hazardous consequences.
Safety Instrumented Functions (SIF)
- A function implemented by SIS aimed at achieving or maintaining a safe condition of the process by referring to a specific hazardous event.
- Each SIF must be designed and tested to meet the SIL target.
Other Layers of Protection
- Layer 4: Active Protection Layer (e.g. Pressure Relief Valve, Pressure/Vacuum Valve)
- Layer 5: Passive Protection Layer (e.g. Tank Bund Walls, Fire Walls, Oil Contaminated Sewerage System)
- Layer 6: Plant Emergency Response Layer (e.g. Emergency Response Team)
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