Take this short quiz to check your knowledge of some key points in Layers of Protection Analysis.
1. Question: Which one of the following is an enabler?
A. Safety system is bypassed
B. Control valve fails closed
C. Transfer pump fails on
Answer: A) Safety system is bypassed
An enabler is an event or condition that must be present or active for a hazard scenario to proceed. Enablers do not, by themselves, initiate a scenario. Thus, a bypassed safety system may enable a scenario to occur because it is unavailable and will not function to prevent a scenario if a demand is placed on it. Failures of control valves and pumps may be initiating events for scenarios.
2. Question: Is the likelihood of dependent failures lower than independent failures?
Answer: No. Consider two independent failure events each with a failure probability of 0.1. The probability of both occurring is the product of their probabilities, i.e. 0.01. If they were dependent events, the probability of the first failure would be 0.1 but the probability of the second failure would be 1.0. Thus, the probability of the two dependent failures is 0.1 which is greater than the probability of the two independent failures.
3. Question: Which one of the following is a conditional modifier?
A. Disabled alarm
B. Extreme temperature
C. Possibility of ignition
Answer: C) Possibility of ignition
Any one of these may be an enabler. However, conditional modifiers are a special type of enabler that impact the scenario consequences directly. Probabilities of conditional modifiers are used to reduce the estimated probability of the consequences. They include the probability of ignition, the probability that a person will be exposed to a hazard, and the probability of harm if exposed.
4. Question: Is a process safety valve always an IPL?
Answer: No. In order to be considered an IPL, a device, system or action, at a minimum, must be effective, independent and auditable. A process safety valve may or may not meet these criteria depending on the circumstances.
5. Question: Do additional protection layers always result in less risk?
Answer: No. At a certain point, added protection layers increase the complexity of a process such that new hazard scenarios may be possible that go unrecognized thus potentially increasing risk.
6. Question: What percentage of CSB investigations found inadequate safeguards?
A. 10 - 20%
B. 50 - 60%
C. 80 - 90%
Answer: B) 50 - 60%
For incidents investigated by CSB through 2015, 56% of the incidents involved processes that were not adequately protected by safeguards.
If you have questions about Layers of Protection Analysis, or on any process safety topic, you may contact us via our website at request information; or by emailing info@primatech.com.
