On the application of DCS and PLC in chemical enterprises

Today, PLCs and DCS systems are basically similar; in the past, PLCs were mainly used to handle digital signals, while DCS systems focused on analog signals, with PLCs having slightly weaker capabilities in dealing with digital signals. Which units are asking to switch to DCS these days?

This post was last edited by tiandiyi*an on 2017-10-26 09:24. PLC systems and DCS systems may be used in different applications. PLCs are usually randomly assigned or come built into the equipment; they have a very limited number of inputs, and often no manual operation is required. They are controlled by programs or sequencers, primarily for starting and stopping equipment ； DCS systems are typically used in installations where real-time monitoring and control are necessary, such as in PID control, cascade control, three-element control, etc. These systems involve a large number of points and significant amounts of data, and thus require a variety of functions. The safety aspects are the same; there is no SIL certification, except for safety PLCs.

PLCs mainly handle digital signals, while DCS systems focus on analog signals; the handling of digital signals is slightly inferior in DCS compared to PLCs

The DCS network is the central nervous system of the entire system, and the international standard protocol TCP/IP is typically used in DCS systems. It is a safe and reliable high-speed communication network with dual redundancy, offering better scalability and openness. Since PLCs are generally designed to operate as individual small systems, when communicating with other PLCs or higher-level computers, they usually employ a single-network architecture, and their network protocols often do not conform to international standards. All I/O modules in the DCS system are equipped with CPUs, which enable the evaluation of the quality of acquired and output signals as well as scalar transformations; faulty modules can be removed while the system is still operational, and replaced at will. The PLC module is merely a simple electrical conversion element; it lacks intelligent chips, and in the event of a failure, the entire corresponding unit becomes non-functional.

PLCs handle more digital signals, while DCSs have stronger capabilities in processing analog signals. Currently, safety requirements dictate that process control in chemical enterprises must be carried out using DCS systems. If the PLCs currently in use meet the control and safety requirements, I think there is no need for any modifications; you can simply state that DCS systems are being used.

If it is a requirement from the safety supervision department, consider it as equipment upgrading; in the case of DCS and PLC, the hardware differences don’t seem that obvious

It’s better to add SIS for extra safety

This post was last edited by tianya3047 on 2017-10-27 at 17:09. I haven’t seen any such requirements; I’ve never heard of them. In the case of our factory, PLCs are mainly small devices, part of complete systems. The main issue is that the systems are quite dispersed, with a large variety of models; the lower-level components use Siemens’ S7-400 series, while the upper-level components employ systems such as WINCC, Intouch, and IFIX. The disadvantages include difficulties in management, a large amount of knowledge that needs to be mastered, poor accessibility for maintenance, and heavy reliance on manufacturers. The DCS system is integrated, making management and spare parts handling easier. There is no difference in terms of safety. Moreover, when making changes to the PLC’s upper-level settings, each station has to be modified separately – for things like alarms and trend displays – which is very troublesome, and the clock times are not synchronized either.

Why are so many companies that originally used PLC systems being required to switch to DCS systems? Is there any difference in terms of safety between these two systems? In fact, the differences between DCS and PLCs are now very small; whatever can be done with one system can also be done with the other. In terms of safety, PLCs with safety certifications are called SIS; DCS doesn’t have such safety certifications, right? If you’re asking why companies are now required to replace PLCs with DCS, I think maybe there are **many fake experts around, haha; perhaps it’s to promote the development of automation. I’ve seen many small businesses use fake control systems, and nothing seems to be discovered as a result. I think repeated inspections can encourage these companies to conduct self-audits and make improvements; by stopping and making changes, the likelihood of problems occurring is reduced. Haha, as the other guys upstairs have said, using multiple PLCs is not conducive to the factory’s information management; it’s better to integrate a single DCS for smoother operation