Choosing an enlightened control path

           

Producing 20,000 tons/year, Wuxi Dong Tai Fine Chemicals Co., Ltd., is the largest fatty alcohol production facility in China and now site to some of the most advanced control technology in the country.

All of the raw materials are natural plant oils and animal fats, such as coconut oil, palm oil, palm kernel oil, suet, lard, cottonseed oil and soybean oil. The production equipment is composed of the host equipment and the auxiliary equipment. Auxiliary equipment includes water supply, power supply, heating, compressed air and nitrogen supply, and environmental protection treatment of wastewater.

Part of the production procedures were highly labor-intensive manual operations. Previously, fatty alcohol production was controlled by DDZ-III instruments. These are joined panel instruments composed of discrete single loop controllers with analog measurements and calculations. Mainly composed of discrete transistors, the instruments have only average accuracy and a high failure rate. To install and use the automation control instrument in the explosion prevention area (area 2), safety barriers must be added.

Unfettered by Legacy
Since we were not fettered by conventions or past experience with legacy control systems, we invited system suppliers with high-quality and cost-efficient products to bid. The bids we received were all for small to medium-scale solutions, such as ABB’s S900, Honeywell’s Plantscape system from Rosemount, and MTL Open System Technologies’ (Most) Matrix system.

We selected the Most Matrix system because of our fire and explosion prevention requirements, its distributed control capabilities, and the stringent requirements of the processes’ control targets. Wonderware InTouch software was selected for the operator interface application.

The new control system is new to China. Many people were also surprised that MTL produces and sells a DCS control system in the first place. Regardless, our main consideration was the performance of the hardware and software of the system. For the hardware, we believed that the control system was essentially an industrialized computer. The rapid development occurring in the computer field, especially in semiconductor and integrated circuits technology, gave us great confidence in choosing a digital control system.

From our point of view, the more recently developed the system, the better its performance and reliability would likely be. The hybrid controller of the new system uses the latest CPU technology, making it more advanced than other systems and one of the primary reasons we chose the system’s supplier.

MTL, one of the world’s largest manufacturers of safety barriers, has an excellent reputation in the automation industry. We thought that, because the system we purchased was a new product, it would be first-class. Moreover, besides the new functionality and good brand name, the system appealed to us because of its tight integration between the safety barrier and the I/O module. The I/O modules can be installed in a Division 2 area connecting directly to an intrinsic safety (IS) instrument without additional safety barriers. For users, this characteristic means a clean structure, low maintenance and cost-efficiency. It also saves a lot of cable and wiring costs as well.

A Dramatic Choice
To us, the process of choosing the system was dramatic. At the initial stage of the Natural Fatty Alcohol Project design, we had the idea of using distributed control technology. However, we could not be sure because we wanted to implement it in an explosion-prevention, Division 2 area. Conventional design wisdom usually prompts one to choose a combination of safety barriers plus I/O modules.

At the start MTL’s sales representatives proposed a safety barrier design to us. After they became aware we wanted to use distributed control technology, they recommended the remote I/O module of the MTL 8000 series. We felt that this was a new system structure, and it could satisfy our demand for distributed control. The problem was that not only did we need I/O modules, but we also needed controllers.

Conveniently, our chosen supplier offers controllers as well. Moreover, they are hybrid controllers. Other companies also have this type of I/O module. The designer, as well as the company’s project engineers, became interested in the remote I/O modules plus controllers plan that could be used in the Division 2 areas. After thoroughly understanding the Matrix system, and considering it with other aspects comprehensively, we chose the remote I/O modules plus on site controllers plan for the project.

Danger! Perfection Required
The control targets of fatty alcohol production are temperature, pressure, and liquid level. The highest requirements are for temperature and pressure control. The accuracy of temperature control is 0.3° C, and the accuracy of pressure control is ±50 Kpa. In addition, because the production of fatty alcohol is in a high temperature (200–250° C) and high pressure (20–25 Mpa) environment that involves methanol and hydrogen, the control system must have a set of perfect interlock, alarm and emergency stop systems. The Matrix and Wonderware Intouch control system conforms to these requirements.

Four local control panels, five controlroom operation workstations, and an engineering workstation constitute the 20,000 tons/year natural fatty alcohol production scheme. All panels and workstations are connected via Ethernet twisted-pair cables, and form a complete control system.

The entire system uses 13 power supply modules, four power supply monitoring modules, four CPU modules, and 167 I/O modules. In addition, in order to guarantee the convenience of equipment maintenance and failure rectification, there is one power supply module, one CPU module, and 10 I/O modules for cold backup (off-line backup).

The system uses Matrix Workbench 7.3.1 SP4 for programming configuration and has 189 control loops. It covers all the fatty alcohol production automation control process that includes PID (Proportional Integral and Derivative) control loop, ratio control loop, alarm interlock loop, calculation accumulation loop and motor control loop.

All the loop controls and the interlock functions run individually in the local control panels (Panel No.1 to Panel No.4). The four local control panels communicate with the control room operation workstations via Ethernet. Operators interact with the local control panels from the five operation workstations and the engineering workstation via Ethernet. The operation workstations use the Chinese version of Intouch 7.11 software for human-machine interface configurations. The application is composed of 46 windows, and uses 2,975 tags.

Starting Up
The control system was configured, installed, and tuned from May to August 2003. The entire process took about 100 business days. Matrix’s Workbench Strategy Builder was used to configure the system and made the process very fast and convenient. The failure rate was minimal, too.

We found that the system we selected was comparatively cheaper than the DCSs from other companies especially in Division 2 areas. It saved investment cost because we did not have to purchase safety barriers in the danger/safe area.

Additionally, because the control panels could be installed at the control site, measurement instruments could simply be connected to the local control panels instead of connecting them to the central control room. A feature especially important for a control system that is widely dispersed and has to cover large areas—ultimately it saved us considerable signal cable and wiring costs.

The fatty alcohol production equipment occupies an area of 18,000 m2, and each of the four local control panels is 120-m away from the central control room. The operation workstations communicate with the control panels through redundant Ethernet. The signal transfer speed can be up to 100 Mbps. It sufficiently conforms to the data communication requirement of this system.

The DCS control system has been running well since it started operation from July 2003. During this period, it experienced two high temperature summers. During the high temperature summers, the internal temperature of the machine enclosure changed accordingly to the climate temperature. The highest temperature of the machine enclosure at 48° C was reached every afternoon between 4:00 and 5:00 pm. During summer, the lowest temperature of the machine enclosure was between 35° and 37° C. No failures, such as CPU shutdowns or restarts because of the high temperature, occurred.

I/O modules of the Most system are easy to change as well. After test, a skilled system maintenance attendant can change an I/O module within 30 seconds. The system’s hardware is simply structured and easy to install, which meant a low wiring failure rate and higher reliability.

The configuration of software is also convenient, is easy to maintain, and is intuitive. The graphical programming also helps users configure, debug, and maintain the system. Its online debugging function makes data monitoring clear at a glance, and makes it easy to find programming errors. The software’s function blocks conform to the requirements of the automation control system, and helps to simplify the configuration download.

After running for more than one year, the Most system has been a core component in the fatty alcohol production. It conforms to the control requirements of high temperature, high pressure, and dangerous media and we believe that the system provides us with a safe, reliable, and straightforward way to control our production processes.

Making the choiceIn the process of the plan designing, the bidding, and the decision making, as the user, we had the following primary considerations:The safety performance of the system: The Fatty Alcohol Project involved hydrogen and methyl alcohol medium, and some of the equipment was installed in the explosion prevention area, a Division 2 area. Therefore reliable isolation is most important. The Matrix system has the ability to be used in the Division 2 area. It can be mounted directly to the intrinsic safety instrument. Without doubt, it conformed to the requirement. The reliability of the system: Compared to traditional instruments, although the DCS system is a control station, it essentially is a large digital instrument. It centralizes the loop circuits, but also concentrates the risk. However, the rapid development of the semiconductor and computer technology makes digital instruments very reliable - even more reliable than analog instruments. The hardware quality between various big companies tends to be “homogenous.” MTL is a worldwide large corporation with a high reputation in the safety barrier field. We regarded MTL’s other products as the first-class in the world. It is a concern of psychological cognition. The cost-efficiency of the system: Currently, there are no absolutely advanced monopoly products in the world. Many companies have systems with the same performance. In the market competition, price is a major attribute. In the process of the DCS system bidding, Most’s Matrix system had a relatively lower price. After the thorough analysis, we believed that this low price did not sacrifice the quality of the product. MTL also wished to expand into the Chinese market, and offer new services. We welcomed this cost-effective plan.The maintenance and service of the system: The post-sale service of the system was one of our considerations. We desired the high quality hardware with low system maintenance of the Matrix system. On the other hand, the Matrix system hardware is produced overseas, and all hardware maintenance must be pursued overseas. This was one of our concerns. No matter how good the hardware is, it will have problems some day. It is a post-sale service issue whether the problem can be solved and the system can be restored in the shortest time when the breakdown occurs. Concerning this issue, we analyzed the situations of the bidding companies. Most of them were the same - only Honeywell has a factory in China. At last, we chose the rugged, redundant controllers and I/O modules plan. We hope after changing a hardware backup part, the crashed part will be fixed or replaced in a timely manner. We feel MTL China will pay attention to our needs to avoid or minimize any plant down situation.The upgrade service of the system: A system will not be perfect at the beginning. Users would like to continuously optimize their program while they are using the system. Therefore, if the system supplier can continuously help the user upgrade and optimize the system after the sale, it is welcomed without doubt. There is a “sentiment” factor between the user and the software. Users will not change, or give up software they are familiar with, especially for integrators. MTL commits to help users upgrade the software when supplying products. It helps promote the system application widely.