Low-budget, high-performance distributed control system

The Port Alice pulp mill, Canada, was faced with the problem of installing a new, modern wastewater treatment plant using sophisticated controls in an existing pulp mill with old, stand-alone controls and instrumentation. An added requirement was that the new system be suitable for future improvements in the production facility. In addition, the system had to be suitable for operation and maintenance in a remote location by a small staff. Complete replacement of controls with a modern distributed control system (DCS), though attractive technically, was out of the question financially. The system selected utilizes conventional programmable logic controllers (PLCs) with Ethernet and DH+ communications links, integrated with existing controls and designed for upgrades of the entire production facility. More than 4400 rungs of PLC code were written. To control the processes, 87 control screens and 13 trend pages were created. This paper describes the rationale used to approach the problem, compares alternative solutions, touches on significant details of installation and commissioning, and reports on system performance.