Both Process Safety Analysis (PHA) and Layer of Protection Analysis (LOPA) occur in the early stage of the Functional Safety Lifecycle, detailed in both IEC 61511 and IEC 61508. They both serve significant roles in the whole process safety management. Yet, as a facilitator for both PHA, LOPA and Alarm rationalization workshops, it seems to me many in the process industry regard PHA and LOPA as exercises for personnel safety only, without realizing how the results from PHA and LOPA are going to impact the SIS and SIF design, and even their process control design and philosophy.
Where are PHA and LOPA in the lifecycle?
Both PHA and LOPA belong to the very first phase of the lifecycle --- “Hazard and Risk Analysis”, which is in clause 8 of IEC 61511. The most important output of this phase is the Safety Requirements Specification (SRS). Like any design process, we start with a list of items that specify what are required of the product. The SRS details the requirements for the design of each of the Safety Instrumented Functions (SIF), which constitute the Safety Instrumented System (SIS) collectively. Hence, the SIFs will be designed up to the requirements for the prevention or mitigation of hazards.
What output information from the PHA and LOPA will be passed into the SRS?
The main objective of the PHA is to identify the hazards and their associated causes and consequences. If the methodology of Hazard and Operability Study (HAZOP) is adopted for PHA, safeguards will also be identified. LOPA is a quantitative study of the hazard frequencies. Using the information from PHA, LOPA is to further verify the safeguards and quantify the numbers, so that the Safety Integrity Level (SIL) for each of the SIFs can be calculated. Therefore, the most essential piece of information to be distilled into the SRS will be the target SIL.
Yet, the implication of SIL being affected transcends beyond the PFDavg. It is going to affect your voting on the sensors and final elements. It will also affect your choice for the SIL ratings of the involved instruments.
The relationship between PHA/LOPA and SIS is far more complicated than what it appears. I attempt to investigate a bit more on this topic in this webinar. Please drop by if you are interested.
As always, Stay safe.