Choosing an enlightened control path

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.