After the controlled object is determined, what kind of control system is selected becomes an important issue. Mainly based on the project scale and investment budget, the digital system-based dcs system and the early analog instrumentation control system, from the implementation of the project, the essential difference is not big, mainly considering the project size and investment budget However, compared with analog instruments, DCS is more complicated and technically demanding. Let me talk about several problems in DCS system selection. In theory, DCS can be used with different processes, it is versatile, however, DCS manufacturers specialize in a certain field. For example, HOMEYWELL is mainly used in the petrochemical sector, BAILEY's N90 and INFI90 are mainly used in power systems, and ROSEMOUNT's RS3 and Δ-V are mostly used in chemical systems. However, it cannot be denied that there are some special requirements for different processes. For example, the power plant must have ESC equipment and SOE. The petrochemical department must have selective control. The cement industry must have large pure hysteresis control compensation, etc. Consider these factors.
The second point is economics, which should be considered from the perspective of DCS's own price and expected benefits. DCS is made in China and imported. For the same grade, the import control function is stronger. There are some advanced control algorithms, such as Smith estimation and 3D matrix calculation. The price of domestic DCS is much lower than that of imported ones. Meet the technical requirements. From the structural point of view, the controllers of foreign DCS controllers are not too far apart, the controller's preset algorithm is slightly different, and the connection mode between the controller and the I/O board is also different. The operation station is quite different. There are PC-based, small-computer-based, operating systems generally use UNIX-like systems. The price of a minicomputer is much higher than that of a PC. The price of an imported minicomputer operating station is higher than $40,000, and many models have been discontinued (such as DEC's VAX and alpha). The operating station of the PC is less than 30,000 US dollars, its operating system uses NT, and its stability is not as good as UNIX. The interface of the minicomputer is SCSI, and the transmission rate is 8 times that of serial. With the NT operating system as the operating station, the number of points should be less, or it will crash frequently. The domestic PC is much cheaper. With the PC operating station, the latest models can be used, and the operators can install and debug the software. Networking and development are also much more familiar. The monitoring software is dedicated or general-purpose, and the general-purpose monitoring software is more open. Users should choose according to the size of the project and the budget funds.
The reason why DCS is more expensive is that in addition to the above reasons, there is also a power supply system of the controller, which is usually powered by redundancy. The introduction and heat dissipation of the power supply are the main reasons for the high price, and the DCS system has a large difference in this respect.
The communication network that connects the controller to the operator station should also be considered. If a general-purpose Ethernet is used, the price of the network card is very low, and the dedicated network interface is expensive, and the most expensive can reach $20,000.
Therefore, economics is related to many factors. Sometimes foreign merchants offer you a lower price. Actually, they have changed some structures. They have not given you any discounts, and even his profits have increased. At this time, it is advisable to contact similar units in the country and consult experienced experts.
The third point is the technical strength of the contractor, that is, the contractor is familiar with which process and DCS itself. If the contractor of the chemical control system is often used as the rolling control system, he is not familiar with the control of the looper, the control of the curl and the control of the tension, and the engineering will not be too good. If the contractor is unfamiliar with the DCS itself, the controller will crash if it is too large. Many factories have purchased DCS systems from different manufacturers. We call them the "eight-nation coalition" state. At this time, we must select units with stronger technical strength to grasp the total, and not select a foreign manufacturer as a contractor.
The fourth point is after-sales service. Foreign manufacturers usually have high supply prices for spare parts and spare parts, and cannot provide them in time. DCS users should choose manufacturers with strong manufacturers, strong technical strength and good technical support within the territory. Computer technology is developing rapidly. DCS manufacturers will continue to update their products. The compatibility of new and old products is good. The old and new systems of individual manufacturers are not compatible, and the system upgrades cause great losses. The supply of domestic DCS manufacturers and spare parts is relatively timely, and the after-sales service is also better.
The fifth point is about the technological advancement of DCS, which means that the system adopts the latest proven technology, and has a promising future and vitality, including the opening and interconnection of DCS systems, the application of fieldbus, and the support of third-party software. One point to note here is that some manufacturers push some immature products to the third world in order to occupy the market. For example, DCS lacks small systems five or six years ago, and some manufacturers push less mature small systems to users. As a result, more than 270 systems have been retired in advance. As early as 1983, the DCS sold by a foreign company, after the power supply failed and re-powered, the output of the controller was arbitrary. At this time, the position of the valve was very dangerous. This product could not be shipped, but it was sold. In our country, the result of the purchase unit is basically not running.
In short, DCS has been applied for more than 20 years from the past seven to five years, and the reliability can basically meet the requirements. The selection of the system is a very important link. We not only consider the project scale and investment budget. Also consider a series of other factors, whether the selection is appropriate or not often determines the future fate of the system from the very beginning, and must not be taken lightly.
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