FOUNDATION™ Fieldbus: A Simple, Reliable Solution for Boiler Control

By Paul Prescott, Seattle Steam Company

Seattle Steam is a district heating company located in Seattle, Washington. Founded in the late 1800s, the company maintains 18 miles of pipe buried under the streets of downtown Seattle in a one square mile area. Customers purchase steam for heating buildings and water, and in the case of hospitals, for use in sterilization processes.

Although Seattle Steam's steam generating operation had been modernized several times over its history, the company had not upgraded its plant automation equipment in many years. In 1996, management decided that a new boiler control system was in order. The project would involve replacing outdated pneumatic controls that were increasingly difficult for plant personnel to maintain.

Because of the lengthy period between upgrades, Seattle Steam had no preconceived notions about process automation technology. Control system selection began with a clean sheet of paper. However, plant engineers were most concerned about equipment obsolescence; as a small company, Seattle Steam could not afford to replace its control system every 5-10 years, and thus required a solution able to evolve with new technology. The company also viewed reliability as a key criteria, and wanted to utilize a PC-based computing platform rather a proprietary Distributed Control System (DCS).

After considering alternative approaches, Seattle Steam decided upon a fieldbus-based control system provided by Smar International (Houston, Texas). The new system initially implemented the specifications of the Interoperable Systems Project (ISP), but was easily migrated to conform with the FOUNDATION fieldbus standard.

At the heart of the fieldbus control system is the DFI302, a powerful, multifunction hardware component that is an integral part of Smar's SYSTEM302 enterprise automation solution. The DFI302 is a single, integrated unit that functions as an interface, linking device, bridge, controller, gateway, fieldbus power supply, and distributed I/O subsystem.

FOUNDATION Fieldbus: A Simple, Reliable Solution For Boiler Control

DFI302 integrates all of the hardware and software needed to manage, monitor, control, maintain and operate the steam plant boilers. Due to the use of open standards such as FOUNDATION fieldbus and OLE for Process Control (OPC), the unit provides tight integration of intelligent instruments, including those supplied by Smar and other instrumentation vendors, as well as the plant's Human-Machine Interface (HMI) software.

Employing the DFI302, Seattle Steam was able to configure a simple, reliable control system without the complexity and cost associated with a legacy DCS. A single DFI302 module implements four FOUNDATION fieldbus H1 (31.25 kbit/s) ports, Ethernet and serial Modbus ports directly on the controller without the need for separate interface modules. As such, the unit requires a fraction of the space of solutions using individual modules, and is easier to install, maintain and expand.

With the Smar fieldbus solution, continuous regulatory control for the boiler process is handled by field devices. This approach restores single-loop integrity and minimizes the chance of introducing errors into the process. Full redundancy and fault isolation also ensure increased safety and uninterrupted operation. For example, plant operators can turn off their computers, and as long as power is maintained to the fieldbus system, the boilers remain stable.

Most importantly, Seattle Steam realized significant hardware and installation cost savings by implementing a FOUNDATION fieldbus-based control system. Compared to traditional single-loop controllers, the fieldbus system reduced I/O requirements and the labor involved in wiring field instruments.

With a FOUNDATION fieldbus control solution now in place, Seattle Steam has a secure, affordable path forward to new technology. The fieldbus-based system has performed reliably during five years of continuous operation. That will help the company keep its customers warm well into the future!