DCS (Distributed Control System) is an automation system used for industrial control and monitoring, which can effectively manage and control various equipment and production processes in factories. When choosing a DCS system, there are some differences between different manufacturers and systems. What are the differences between DCS systems from different manufacturers? What are the similarities and differences between DCS and PLC? DCS、SIS、GDS、 Can fire alarm systems and communication systems share UPS?

Q1: What are the differences between DCS from different manufacturers, such as configuration, human-machine interface, card components, anti-corrosion, SIL level?

There is indeed a trend of homogenization in software (such as configuration software, operator HMI), with differences, but not significant. Because most software tools nowadays are very sophisticated, but some products have unique features in software cloudification and usability, such as:

Cloud configuration, tag addressing, automatic dotting and other functions. For cards, the basic chip suppliers of each company may be similar, and the differences in cards are not significant. Specific specifications still need to be referred to. At present, most of them support G3 level anti-corrosion. DCS does not need to support SIL certification.

Q2: Due to safety reasons, the cabinet is not allowed to be placed inside the workshop, but it should be placed outside. The cable should exceed 150 meters, and the signal may be attenuated. How should we solve this problem?

1. If it is a traditional IO or wiring cabinet, it is indeed not suitable to install it in the workshop because there is a certain degree of danger in placing the cabinet in the workshop, and safety inspections in various places will not pass;

2. In my personal opinion, the current universal IO units, with 16 or even 48 points, cannot be called cabinets. However, some safety supervision units believe that those with IO functions are cabinets and are not allowed to be installed in workshops. As for me personally, if these universal IO units have a small number of points and the entire appearance, including IP protection level and explosion-proof, can meet the requirements for on-site installation (such as reaching IP66 and being able to be placed in Zone 2), I think they should not be used as cabinets and can be placed on site. (This viewpoint is only a personal opinion)

3. Shielded signal cable (1.0mm2) can support 24VDC signal for 800 meters.

Q3: How to configure digital content for DCS system selection to facilitate later management?

At present, DCS supports the following contents of smart factories:

OPC data interface provides data for factory management, advanced control, and optimization;

The instrument management system collects data from instruments such as HART to achieve intelligent instrument management.

Q4: What are the differences and similarities between DCS and PLC in terms of selection, functionality, configuration, and usage?

1. Both PLC and DCS can achieve factory control, with many similarities and a trend towards integration.

2. Differences:

• PLC is based on controller+IO, and the database for software configuration is limited to the controller range; DCS configuration is based on domains or projects, and software configuration has a factory level database. Therefore, PLC is mostly used for independent equipment control, while DCS is mostly used for factory control;

• The development paths are different. PLC evolved from single machine logic control with added analog control functionality. DCS was originally used for factory regulation and control.

• Different in application. DCS has advantages over PLC in the field of large-scale factory control. PLC has more advantages in single machine control.

Q5: What is the difference between domestic and imported DCS brands?

Both domestic DCS and imported DCS have high, medium, and low grades, making it difficult to simply answer the gap between the two.

But why is there still frequent discussion about the gap between domestic and imported brands? That's because in the 1980s and 1990s, there was indeed a gap between domestic and foreign products. The current situation is that in the field of DCS used in mainstream large factories, there is not much gap between DCS at home and abroad. Domestic DCS can also be applied to control large-scale refineries, chemical plants, thermal power plants, nuclear power plants, and other factories.

In the latest optical bus universal IO technology, domestic products are still leading. APL technology is on par both domestically and internationally.

Q6：DCS、SIS、GDS、 Can fire alarm systems and communication systems share UPS?

Sure. Strictly speaking, UPS is not a part of BPCS, it is just a device that provides reliable power for instrument control systems. BPCS faults can only cause the UPS to trip the distribution switch for BPCS at most, and will not affect the power supply of SIS.

According to Article 5.0.16 of GB/T50770, the power supply for the safety instrument system of petrochemical production equipment belongs to a particularly important load in the first level load, and should be powered by uninterruptible power supply (UPS). It is recommended to adopt a dual UPS power supply scheme, and the two power supply circuits should be independent of each other. The safety instrumented system can share the UPS power supply with the basic process control system, and the power circuit breaker should be set independently. The power supply for auxiliary facilities such as cabinet fans and lighting in the safety instrument system should use non UPS power supply.

Some experts have overinterpreted the general principle that "safety instrumented systems should be independent of basic process control systems", believing that "mutual independence" means that there should be no common parts. Some experts have suggested in the security review that a dedicated UPS should be configured for SIS, and the SIS power supply cannot be shared with DCS. This statement deviates from the standard and is not accurate. SIS and BPCS naturally need to be independent of each other, but the inherent meaning of independence is that "the failure (initial event) of BPCS cannot affect the safety protection function of SIS, and the target safety integrity level of safety instrument function should not be reduced". Without seeing the word "independent", it is assumed that SIS and BPCS cannot have any intersection. After all, the control and protection objects of SIS and BPCS are the same production equipment, and there are intricate relationships between them.

However, some provinces (such as Jiangsu Province) have already included the design of independent UPS power supplies for SIS in local policy documents. Although I personally think it is not entirely reasonable, please handle it carefully.

Q7: Is there a specification for the redundancy of DCS module configuration?

SH/T3092-2013 Design Specification for Distributed Control Systems in Petrochemical Industry

7.1.5.1 The number of IO slots reserved for each control station shall not be less than 20%

7.2.13.2 The quantity of each backup IO module should not be less than 20% of the actual quantity. (In practice, it is calculated based on the number of points).

Overall, choosing a suitable DCS system is crucial for the production efficiency and quality of a factory. When choosing a DCS system, many factors such as system reliability, flexibility, and scalability need to be considered. DCS systems from different manufacturers have certain differences in functionality, performance, and technical support, so sufficient comparison and evaluation are needed when selecting.

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