Comparison of DCS and PLC in sequential control
Thread Content
There is a project that involves large boilers, and the control system used is DCS. I wonder whether the coal feeding system and the slag and ash transportation systems will also use DCS or PLC. It seems that these two systems mainly involve logical control, with little need for analog signals; therefore, PLC would be sufficient. It might be cheaper, but is communication with the DCS system guaranteed? Moreover, the PLC interface needs to be integrated within the DCS interface – is that compatible? If both of these systems are implemented using DCS, can DCS handle all of the sequential control logic that PLCs can handle? I’m used to working with PLCs, and I always feel that DCS is not as effective in terms of sequential control functions. Dear experts, please share your valuable opinions. Thank you!
DCS is not as useful as PLCs for sequential control functions
The sequential control capabilities of DCS are much stronger than those of PLC; in a real-world scenario, it’s simply used to control the sequential start and stop of a few conveyor belts. I could learn about the batch control implemented in DCS – it’s many times more complex than the conveyor sequence control you use. From a maintenance perspective, since there are two different systems, it is necessary to learn how to maintain *two different configurations, and separate spare parts are also required for each system. From a tendering perspective, separating the conveying system and using a PLC for it doesn’t seem to result in lower costs compared to integrating it into the boiler DCS; generally, the more I/O points there are, the lower the cost.
Don’t worry, OP. With the level of technological advancement we have today, if DCS isn’t capable of handling simple tasks like yours, it should have been phased out long ago. It’s better to use DCS uniformly; it’s also easier to make expansions on top of the existing system
It is recommended that the poster use DCS uniformly, as it facilitates maintenance, spare parts management, and debugging; DCS can fulfill all the functions of a PLC
If a DCS is necessary, then of course one set of DCS is sufficient; it just involves adding a few DI and DO cards. There will be no increase in software or engineering costs, and it will be easier to use and manage in the future. DCS is also very powerful in terms of logical control.
There is no right or wrong; different perspectives on a matter lead to different conclusions. It depends on the results of your company’s price comparison. The maintenance for this type of device is mostly carried out on-site.
There are large boilers, and the control system uses DCS. If there is spare capacity in the control system (spare capacity in terms of cups and cabinets), the coal conveying system and the slag and ash conveying system can be equipped with control points directly on the existing DCS model. This approach ensures compatibility with the existing system, saves costs, and also facilitates monitoring by operators.
A single dominant supplier does have advantages such as a limited range of spare parts and easier maintenance, but over time it can also lead to excessive dependence and biased expertise among the maintenance staff. PLCs and DCS systems are similar in function, though they often differ; technically, one is more geared toward customization while the other emphasizes flexibility. Only business considerations are the most important.
From the perspective of the end owners and maintenance personnel, it is appropriate to use one DCS across the entire plant; however, from the standpoint of design management, PLCs and complete sets of equipment can reduce the number of management interfaces, thereby facilitating coordination of the project schedule (usually, the progress of DCS configuration and integration is synchronized with that of larger equipment packages, and at that time, the relevant configuration files for those equipment packages are not yet available)
Since it has been decided to use a DCS system for the boiler, it is recommended to adopt the DCS system throughout the entire system. As mentioned above, it simply involves a few I/O cards; there’s no need to make things too complicated. If there are indeed not enough points, or if one wants to use a PLC to control part of the system, that’s no problem either. PROFIBUS communication cards can generally be installed as an addition in DCS systems, and there’s no issue at all if Siemens PLCs are used. Personally, I suggest using one system overall; is the belt section equipped with high-voltage power? Relays can be used to control high-voltage circuits with low-voltage signals, and in addition, switch signals can also be transmitted directly to the DCS. It’s possible that mixing strong and weak electrical currents could cause interference; use shielded cables, keep the electrical instrumentation trays separate, and there should be basically no problems~!