Can DCS take signals from SIS and then adjust control valves (not on/off valves)?
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Can DCS take signals from SIS and then adjust control valves (not on/off valves)? If possible, compared to using a two-part safety barrier, which one has lower costs?
It should be possible to obtain feedback signals from the SIS, but it is not possible to adjust the control valves of the SIS
It’s possible to do it, but it can’t be done this way; they are two separate systems
By definition, SIS is a completely independent safety instrumented system that shares nothing with DCS. However, doing this in practice comes at a high cost; a compromise approach is to have the DCS provide some control interlocks for situations where the consequences are not too severe, while the SIS adds additional, independently important control interlocks on top of that. The additional interlocks are not shared with the DCS, and communication between the SIS and the DCS is limited to the transmission of signal data only. The above is what I have learned so far in the preliminary stages of the project; a detailed design has not yet been developed, and it is not clear whether it will be a completely independent SIS or merely a supplementary SIS. I hope someone with experience can provide a more accurate explanation.
DCS obtains signals from SIS in two ways: communication and hardwired points. If the maintenance soft switch for SIS is placed on the DCS screen, it is very convenient to access the communication data points. The drawback is that the SIS requires an AO output, while the DCS needs an AI input; the cost is higher than that of using signals from the field that are sent to a safety barrier and then divided into two streams to feed the SIS and the DCS respectively.
It’s possible to transfer SIS to a fully DCS system; there’s no rule preventing these two systems from being connected