Regarding instrument redundancy issues in SIS
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This post was last edited by The project’s progress on 2021-9-26 at 11:43. The large tank is classified as SIS Level 2; the experts recommend level redundancy. May I ask if it’s possible to use the same principle for this? We used the differential pressure type before. Additionally, there are configurations with two units of redundancy or three units; it depends on what configuration is chosen. I realized that I had asked about this same type of issue before, and I saw everyone’s responses regarding redundancy. I originally thought that since experts recommend redundancy, I should add another unit, but the reply in the previous post reminded me that I need to consider how many units of redundancy to use – I just don’t know how to determine that. Are there any documents I can check?
In fact, there is no explicit document stating that sensors based on different measurement principles must be used; if it’s pressure, how can you use two sensors with different principles? Therefore, for redundant configurations, the sensors only need to be suitable for the measurements required in that process. Moreover, it’s not something that can be decided arbitrarily (i.e., one doesn’t just buy whatever they want); there is usually a complete set of procedures involved: HAZOP analysis, the preparation of reports, and then the design agency uses these reports to create detailed design specifications, including the selection of instruments. After that, there are tests and acceptance procedures, and many documents need to be collected. Having the equipment alone without the necessary documentation makes things uncertain in the future
This post was last edited by myface on 2021-9-26 at 13:32. The gb 50770 standard states this way; as for how to achieve redundancy, you should first come up with a proposal, but it still needs to be approved by the experts conducting the review.
1. Redundancy is determined according to the SIL level. Standards such as SHT 3018-2003 Code for Design of Safety Instrumented Systems in Petrochemical Industries and GBT 50770-2013 Code for Design of Safety Instrumented Systems in Petrochemical Industries both specify the following: 2. How should the measurement elements be redundantly configured for SIL2? At least two detection elements are required, with a logic of choosing one out of two to emphasize safety ； Emphasize two-out-of-two availability interlock ； That is, by emphasizing both reliability and usability, a majority vote is applied, resulting in the use of three detection elements. 3. To prevent common cause failures, it is recommended that redundant measuring instruments use different principles, especially for level measurement. GB 17681-1999 Technical Requirements for the Acceptance of Safety Monitoring and Early Warning Systems for Flammable and Explosive Tank Areas:
What we are doing now all requires different principles
I just heard that there’s a document with requirements based on this different principle, but I didn’t ask what that document is
The storage tank has an internal floating roof, and we have already installed two sets of differential pressure sensors. Now, if a radar is installed on the roof of the tank, it’s not clear whether that would be suitable for an internal floating roof; the owner is also concerned about this. I’d like to ask as well: do you consider using two or three sets for redundancy, is there any requirement specified in the documents?
Without conducting any analysis, the design institute won’t provide you with a design, as it has no basis for doing so. So, just like us, they installed a set of SIS systems, but without any design documents – it’s pointless; safety inspections won’t be passed either, and you’ll have to start over again.
For storage tanks classified as Level 2, it is advisable to have all the necessary safety facilities in place, as well as the appropriate safety interlocks for the DCS; this helps to reduce the SIL rating as much as reasonably possible. With the same measurement principle, common cause failures exist. Furthermore, this requires proper design, with a design institute issuing a series of documents. HAZOP /LOPA /etc
As for whether it is redundant or not, as long as the verification meets the requirements, for level two, it is acceptable even if it is not redundant.