1. Knowing basics of Process Safety
2. Having worked in process plants
3. Having a desire
In this part you become familiar risk reduction methods and how SIL can contribute to plant safety.
A Risk is the amount of harm that can be expected to occur during a given time period due to specific harm event. There is no such thing as zero risk. Therefore the concept of defining and accepting a tolerable risk for any particular activity prevails.
SIS achieves risk reduction by reducing the frequency/severity of the hazardous event. The amount of risk reduction achieved is indicated by the risk reduction factor (RRF):
RRF = probability of risk in state 1 / probability of risk in state 2
Layers of Protection
1. Basic Process Control System (BPCS).
2. Automated shutdown sequences in the process control system combined with operator intervention to shut down the process.
3. Safety Instrumented System (SIS). It is a safety system independent of the process control system. It has separate sensors, valves and logic system. No process control is performed in this system; its only role is safety.
4. Active protection layer such as valves or rupture disks designed to provide a relief point that prevents a rupture, large spill or other uncontrolled release that can cause an explosion or fire.
5. Passive protection layer like a dike or other passive barrier that serves to contain a fire or channel the energy of an explosion in a direction that minimizes the spread of damage.
6. Emergency Response Plan (ERP).
In this minute you get to know safety instumented system and its different components
The action of a Safety Instrumented System (SIS) is called a Safety Instrumented Function (SIF). More than one SIF may be assigned to a single SIS.
In this part you are familarized with Stages of SIL Study :
1.Target SIL Evaluation
What SIL should be allocated for the SIF?
Does SIS fulfill Target SIL requirements?
In this minute the trainer instructs 3 components should be taken into account in order to verify the selected SIL in a loop:
A. SIL capability stated in the certificate
B. Calculate PFD for each and then sum them and find the corresponding SIL
C. Check architectural constrains by checking first rout.
The probability that a system fails during a specified period of time.
Probability of Failure upon Demand (PFD) : equals to λ times TI divided by 2 if λ.TI<<1. It is assumed that after each time interval the equipment is as new as first day. Time interval is really important when regarding sil target.
HFT (Hardware Fault Tolerance): maximum number of failures that can be tolerated in a SIS component
SFF (Safe Failure Fraction): fraction of safe failures.
Subsystem type A: A subsystem can be regarded as type A if, for the components required to achieve the safety function
the failure modes of all constituent components are well defined; and
the behavior of the subsystem under fault conditions can be completely determined; and
there is sufficient dependable failure data from field experience to show that the claimed rates of failure for detected and undetected dangerous failures are met.
Subsystem type B: A subsystem shall be regarded as type B, if for the components required to achieve the safety function
the failure mode of at least one constituent component is not well defined; or
the behavior of the subsystem under fault conditions cannot be completely determined; or there is insufficient dependable failure data from field experience to support claims for rates of failure for detected and undetected dangerous failures.
Simplifying, one can say that as long as programmable or highly integrated electronic components are used, a subsystem must be considered as type B.
In this part the trainer investigates a real case example from his last project and shows how these 3 steps could be performed.