Interlock changed to 2-out-of-2
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This post was last edited by liu530014416 on 2020-6-13 17:36. There is a gauge that often gives inaccurate readings and experiences fluctuations. The interlock mechanism originally worked on a 1-to-1 basis; now that another gauge has been added, the manager requires a 2-to-2 approach, meaning interlocking will only occur when both gauges give consistent readings. Is this workable? Are there any guidelines?
It seems that 1-out-of-1 interlocking wasn’t allowed a long time ago... At least it had to be 2-out-of-1. Based on your current situation, what you call 2 out of 2 isn’t very reliable; to put it more accurately, it’s rather like 1.5 out of 1. The original value isn’t trustworthy, and considering it as 0.5 is actually too high. From a safety perspective, there is either safety or no safety – there is no middle ground.
1. The instrument is unstable; it is necessary to identify the root cause and address it, as this is the fundamental solution； Instead of simply adding a table and changing the logic ； 2. Changing the logic is a type of modification (MOC); it requires evaluation and the necessary procedures for making such changes. More strictly, it needs to go through design by a design institute ； Ensure proper update of materials and evaluation of effectiveness. 3. As for the issue itself, changing the 1001 logic to 2002 logic increases spurious fault tolerance, which means that accidental shutdowns caused by faults in the instruments themselves are reduced; this achieves part of the goal. However, it is not possible to reduce shutdowns caused by process fluctuations and installation issues; therefore, it is also necessary to identify the causes of instrument fluctuations. In summary, the interlock logic is determined through HAZOP, LOPA, and SIL classification; the SIL level determines the instrument redundancy and the voting method for the logic. Don’t change easily.
Haha, this analysis is spot-on; experts, please take the time to guide us younger ones more often :handshake
It’s not possible; the loop PFD will increase
Taking 2 out of 2 increases availability compared to taking 1 out of 1, but it reduces security instead. . I’m a bit confused about how to define this safety fault margin.
Regarding this issue, I also consulted experts in China, and indeed there is such a concept: reduced practicality but increased safety... I’ll create a separate post to discuss this topic another time.
This post was last edited by HEJIYUER on 2020-6-14 at 20:50. If your interlock circuits have been classified, here are some suggestions for the input signals for your reference: SIL 1 can use a 2oo2 configuration; SIL 2 can use a 1oo2 configuration or a 2003 configuration. For SIL 3, a 2003 configuration is recommended. A 1oo1 configuration is not advisable for SIL circuits, as a single fault in the instruments could disrupt the entire process operation. It’s better to invest more money in providing insurance for those instruments.
In a two-out-of-two configuration, if one of them fails, the other will not trigger the interlock at all; hence, the safety level decreases. It only reduces the likelihood of malfunctions, so the usability improves. So it should be reduced security and increased availability, right?
Well, take a look at that chart at the top! Maybe my expression isn’t very clear.....