TECHNOLOGY Fieldbus technology passes beta tests at Texas plant

Fieldbus technology is successfully controlling the steam condensate section of Monsanto Co.'s Chocolate Bayou petrochemical complex in Alvin, Tex. (Fig. 1).

Fieldbus technology has completed beta plant testing at Monsanto Co.'s Chocolate Bayou petrochemical complex at Alvin, Tex.

The trial took place in the a steam condensate recovery section of the Chocolate Bayou plant, which produces acrylonitrile, linear alkylbenzene, and a number of other petrochemical derivatives.

Fieldbus is a plant communications network, or bus, that enables digital instruments to communicate with one another and with supervisory control systems. The fieldbus specification, written by the nonprofit organization Fieldbus Foundation, Austin, Tex., is called Foundation fieldbus.

The beta tests at Chocolate Bayou successfully demonstrated fieldbus performance in a process control application.

Fieldbus

Fieldbus Foundation's nearly 100 members comprise oil, chemical, engineering, and instrument companies, as well as universities and independent research institutes. The Foundation was formed in 1994 from the merger of WorldFIP and the Interoperable Systems Project.

The goal of the organization is to develop a single, international fieldbus standard that is compatible with the SP50 standards project of the Instrument Society of America (ISA) and with the International Electrotechnical Committee (IEC).

The Foundation fieldbus is based on the International Standards Organization's Open System Interconnect communications model. The technology comprises three main layers:

A physical layer (converts digital messages to an actual physical signal on a wire, and vice versa)
A data link layer (provides communication between devices on the bus)
An application layer (allows applications in the user layer to access needed communications services).

A user layer associated with the application layer allows control procedures to be carried out at the field device, rather than in a host device in the control room.

John Berra, president and chairman of Fieldbus Foundation, says this feature makes the technology not just a way of moving more information from the field to the control room, but rather a method of moving "intelligence" out into the field.

Fieldbus technology facilitates communication between field measurement devices, analytical devices, control systems, programmable logic controllers, and control valves. The Foundation fieldbus can hold as many as 12 powered devices (those that draw all their power requirements from the bus itself) or 32 devices that are powered separately.

The Foundation physical layer is 100% conformant with the ISA/IEC physical-layer standard.

At the operations terminal for the system, each data value from every device is accompanied by a status indication-either "good," "bad," or "uncertain." A substatus display provides information explaining why a value is bad or uncertain.

An essential aspect of the standard is interoperability among devices from different manufacturers. In a truly interoperable system, the user can simply connect a new device to the fieldbus wire and provide the control system with a device description. The device description provides necessary information about the device to the so-called "host."

Fieldbus reduces wiring, enables communication of multiple variables from a single instrument, and simplifies device integration and maintenance.

Monsanto trial

Fig. 1 shows the test site at Chocolate Bayou. Testing began in November 1995 using prototype fieldbus instruments on the steam plant's condensate recovery system.

The goal of beta testing, according to test manager Kurt Zech, was to demonstrate the functionality of the technology. The test also was designed to verify that the fieldbus specification is operationally correct and meets process automation requirements.

The beta test incorporates proportional integral derivative (PID) and cascade PID controls, system diagnostics, and network management systems. The test employed two fieldbus segments: a cascade loop that controls tank level and a PID loop that controls recirculation flow (Fig. 2 [83139 bytes]).

Thirteen vendors supplied the equipment necessary for the trial. Components included: field devices, such as transmitters and control valves; host devices; valves; power supplies and barriers; analyzers; and wiring.

The test procedures were designed to evaluate fieldbus operation under a number of process conditions. Experiments conducted during the beta test included:

Disconnecting and reconnecting devices during bus operation
Downloading new schedules on-line (The term "schedule" refers to the timing and sequence of messages between instruments, valves, control systems, etc., and the host device. Fieldbus is capable of accuracy as tight as 28 picoseconds.)
Adding new devices to the schedule to observe the effects on loops and other devices
Restarting the system after clearing all addresses in system management
Synchronizing function blocks and schedules
Overloading the bus
Checking the time stamp on events
Changing published parameters on-line.

Savings

Zech says the beta test demonstrated the benefits of fieldbus compared to traditional instrument wiring. Advantages include an 81% savings in wire, a 60% savings in screw terminals, and a 50% savings in input/output (I/O) cards and intrinsically safe (IS) barriers.

Table 1 [15189 bytes] shows a summary of the savings. Zech says he expects comparable results in other process loop applications.

Lester Davis, instrumentation and electrical engineering specialist for Monsanto, says fieldbus improves diagnostics and enables the plant's instruments to function as back-ups to other measurement devices. This enables Monsanto to perform predictive maintenance without waiting for a scheduled plant shutdown.

"With Foundation fieldbus in place," said Davis, "we now have access to a wide range of device status information for use in troubleshooting, as well as an enhanced view of device calibration and manufacturer information. This technology also frees our I/O and controllers."

Compatible devices

At least three independent vendors have completed testing of fieldbus "stacks" (software that facilitates fieldbus communications).

Last November, National Instruments, Austin, and Softing GmbH, Munich, both completed multivendor beta testing of hardware and software components. And in January, Smar International, Sertaozinho, Brazil, and a consortium comprising Honeywell Inc., Phoenix, Softing, and Ronan Engineering Co., Woodland Hills, Calif., passed both stand-alone and multivendor tests conducted by Fieldbus Foundation.

Fieldbus Foundation's 31.25 kbit/sec fieldbus specification was scheduled for release in late April. The technology is not owned or marketed by any one company; it is available to any company that wishes to use it.

A number of Foundation member companies are supplying fieldbus "starter kits" containing all the necessary hardware, software, training, and support for vendors to develop devices compatible with fieldbus technology.