How should one understand the phrase “utilize the spare circuit of the existing DCS power supply system” in the drawings from the technical renovation and improvement design institute?
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I work with DCS systems. When carrying out technical upgrades and modifications, I noticed that in the drawings provided by the design agency, the section related to power supply for the instruments indicated \"use the backup circuit of the existing DCS power supply system.\" I initially thought this meant using the backup circuit from the UPS system that powers the DCS system; based on that understanding, I assumed that these instruments did not require a DCS card to supply power to them. Some drawings state that \"the 24VDC power supply for the instruments is provided through the DCS circuit,\" which makes it clear that the card is used to supply power to the instruments. Is my understanding correct?
Because of the AI card, I need to determine whether the channel is active or passive. To put it simply, what concerns me is whether the card will supply power to the field instruments; if power supply is required, I will adjust the circuit inside the card to a 4-wire system, otherwise I will make it a 2-wire system. If there is anyone experienced who can help me understand this, I would like to ask: which instruments at a site generally do not require a DCS card to supply power to them, and which instruments typically need such a card? I’m not very familiar with on-site instruments. :)
As expected, if I just keep working on DCS without coming into contact with the field instruments, it’s becoming increasingly difficult for me to make further improvements.
As I understand it, your project is a type of technical upgrade project; in other words, it involves adding some instruments on-site, making use of the spare channels of the existing DCS system. The DCS cards I’m familiar with can generally provide 24VDC for two-wire instruments, eliminating the need to arrange separate power supply for those instruments; most of these cards can achieve this through jumpers. Therefore, the design institute no longer considers this option... However, if it is necessary to connect four-wire instruments such as electromagnetic flowmeters powered by 220VAC, then power distribution needs to be taken into account; most signals from such instruments are passive signals.
This backup circuit is powered; in other words, you can simply use any cards or terminal blocks that are not currently in use within your existing DCS system. There’s no such profound principle behind it.
You have a misunderstanding; the design institute won’t go into as much detail as you mentioned. To determine whether the AI circuit is powered by a separate supply, it depends on whether there are safety barriers present in the instruments at the site. It seems like you’re working with a Yokogawa DCS, right?
I am working on Yokogawa’s DCS. I also know that the AAI143 card allows the channel configuration to be set to 2-wire or 4-wire by adjusting internal jumpers, while the AAI135 requires wiring changes to be made on the terminal board or the safety barrier base plate. I need to know now: in the drawings provided by the design institute, does it usually specify whether the AI points they provide are intended to supply power to the instruments on site or not? And the statement in the design institute’s drawings, “Utilize the backup circuit of the existing DCS power supply system,” does it indicate what type of wiring system will be used for the AI points? As I understand it, the design office provided me with the drawings and indicated which AI points needed to be added. It was specified in the drawings whether those AI points should use a 2-wire system or a 4-wire system. So I would use a 2-wire system for those points that required it, and a 4-wire system for those that required it; I would make the necessary wire connections or adjustments as instructed. So, after reading the answers from some forum users above, does it mean that the phrase \"using the spare circuit of the existing DCS power supply system\" doesn’t imply that the field instruments don’t require cards to supply power to them?
After all, the drawings provided by the design institute are tailored to individual people. Those who work at the design institute and are diligent in their work produce drawings that are more detailed and accurate, which makes my job as a DCS engineer easier. The person who works at the design institute is careless; the drawings he produces are full of errors. When those drawings end up in my hands, it causes me a lot of trouble as well.
It’s not that complicated, right? As long as the power supply is connected properly and the capacity is sufficient, that’s fine; it just needs to have the correct signal channels
I understand now that it’s not enough to just focus on the drawings provided by the design institute. It is also necessary to communicate more with the field instrument operators to confirm whether the instruments to be installed are of 2-wire or 4-wire type. If they are 4-wire type, it needs to be determined whether to use the circuit breakers available in our DCS cabinet or to obtain power from another source. These settings vary slightly for different field instruments.