Module 9 - Redundancy Flashcards
What is redundancy?
More than a single means of performing a safety function
Explain hardware fault tolerance as applied to a MooN architecture
If N is the total number of channels available to perform a safety function, and M is the minimum number of channels, then HFT = N-M
In a 1oo2 architecture, how many devices must trip to trip the safety system?
1
In a 2oo2 architecture, how many devices must trip to trip the SIF?
Both devices must trip
How is SIF architecture typically documented?
Voting block diagrams
How is the probability of an AND gate calculated?
P = P(A)*P(B)
How is the probability of an OR gate calculated?
P = P(A) + P(B)
Where can the calculations for PFDavg with HFT be found?
The formula sheet
Is a higher HFT safer or more unsafe?
More safe
Does a higher HFT lead to a higher or lower spurious trip rate?
Higher spurious trip
What is a common cause failure?
Concurrent failure from a single event where the failures are not consequences of each other
Provide an example of common cause failure
Corrosion of a terminal strip
What are common mode failures?
Concurrent failures of devices via the same failure mode
What is a single point of failure?
A common mode failure due to a common device
What is the beta factor?
The probability that if one channel fails the others will fail the same way
What is the most important factor in a PFDavg calculator with HFT?
The beta factor
What is the default beta value for field devices?
2% for independent measurement devices, 10% for shared tapping point devices
What is a typical beta factor for final elements?
5%
What is a typical beta for electronics in equipment rooms?
1%
When should you determine beta more precisely rather than using a coarse value?
when PFD is close to the SIL boundary
How can you improve common mode failures?
Diversity
Does beta model systematic failures?
No, only common mode failures
Using IEC 61511 how can you achieve systematic capability?
- Independent certification
2. Prior use
When can a device be accepted on the basis of prior use under IEC 61511?
When the device has been supplied with sufficient evidence of low systematic faults, and the device must be used in similar operating environments. Evidence includes the manufacturers quality systems, and volume of operating experience.
