To fieldbus or not to fieldbus

InTech, Oct 1997 by Ochsner, Marjorie, Schrier, Matthew

The benefits of using fieldbus must be quantified to determine if the additional information, wiring options, control options, and interoperability can offset the cost of its implementation.

Recent changes revolving around the various digital communication protocols for process control have confused end users. Where does fieldbus fit in? Is it time to upgrade, or will it cost more than it will save? This article compares fieldbus with other protocols typically used with transmitters as well as explores costs and other factors associated with transitioning an existing nonfieldbus protocol to a fieldbus solution.

Most fieldbus benefit comparisons contrast fieldbus with 4-20 milliamp standards. Such comparisons ignore the large number of installations now using digital communications. In reality, several digital communication protocols for field devices have emerged. Nearly every major transmitter vendor has introduced a protocol to communicate to smart transmitters.

Plants currently using a digital protocol to accomplish bidirectional communication between the transmitter and the control system are already realizing many of the benefits of fieldbus compared to analog technology. These protocols have already saved users money in installation, startup, process efficiency, maintenance, and troubleshooting costs. The key benefits of fieldbus over existing protocols are:

Interoperability

Wiring cost savings

Flexible implementation of control

Increased access to field information

The disadvantages of fieldbus are its higher installation and purchase costs for fieldbus devices. These costs are initially higher than for other digital protocols, but these early costs could be offset by increased process efficiency and decreased downtime. However, in a plant that already has digital communications, a careful cost-benefit analysis needs to be performed to justify a transition to fieldbus.

Interoperability is a key driving force for upgrading to fieldbus. Not only does fieldbus offer a wider choice of suppliers, it also allows a wider variety of devices than is currently available on other protocols.

Wiring costs could make the difference One area in which fieldbus could save money for end users is wiring. Wiring practices in plants that implemented digital field communications are not much different from analog implementations. They are point-to-point (i.e., a twisted pair of wires is run out to a junction box from each individual transmitter). With the recent introduction of multivariable devices, up to four process variables are now available from a single twisted pair. However, most wiring is still set up to use one twisted pair for each process variable. Fieldbus offers many different options; it can be connected point-to-point or in several other ways, including buses with spurs, daisy chains, trees, or a combination of these structures.

Figure 1 shows a bus with spurs topology with devices connected to a fieldbus through a junction box. Thus, the device "spurs" out of the junction box. This requires a junction box for each device, which can be even more costly than a point-to-point implementation. This topology would be used where individual devices are placed in remote locations. One factor to consider is the limited length of each spur based on the number of devices on the fieldbus.

When devices are daisy-chained together, as shown in Figure 2, the fieldbus is connected from device to device. This is similar to the bus with spurs topology without the junction boxes and spurs. Thus, it is less expensive than either the bus with spurs or point-to-point topologies. However, it also has disadvantages. The field devices must have special connectors to prevent disruption of the bus when an instrument is removed. Once this configuration is used, changes to the wiring are difficult. This topology is useful for connecting noncritical devices in close proximity.

The tree or chicken-foot topology shown in Figure 3 has multiple devices on a fieldbus connected via a single junction box. This is very similar to the practice used today with a home-run cable to a junction box and then to the individual devices. This option is especially useful in situations where many changes will be made or when upgrading a plant using existing wiring.

Figure 4 shows how various topologies can be mixed using fieldbus technology. A mixture of topologies can be practical. In most plants, a variety of topologies can be used in one area of the plant, For example, a plant area may contain process furnaces and reactors, a stabilizer, and a reboiler furnace. Each unit requires substantial instrumentation for overall control. The existing wiring is basically point-to-point, but it could run to a junction box with a home-run cable to the distributed control system (DCS). This existing wiring can be used with fieldbus.

Only a few loops will likely be put into each segment. However, each fieldbus segment could contain additional measurements that may not have been there originally to help tighten control. Currently, the primary instruments that control each process heater may be the furnace temperature and the fuel valve. To tighten control on the furnace, an additional temperature measurement may be put on the stack to measure the fuel header pressure and fuel flow. To save on wiring and installation costs, noncritical instruments may be daisy-chained to each other and then connected to an existing junction box. Since these instruments are noncritical, the impact of this segment failing while additional instruments are being added is minimal.