Take Advantage of Fieldbus

Most engineers undoubtedly have heard the terms "fieldbus" and "digital industrial networks" — but the reasons for using this technology in chemical plants and processes may not be clear. So, here, I'll briefly explain the basics of fieldbus and how it can improve performance, increase uptime and cut costs. I'll also mention potential pitfalls and best practices.

Fieldbus actually is a generic term for a number of digital industrial networks including, but not limited to, Foundation Fieldbus, HART, EtherNet/IP, Modbus TCP and Profibus. In this article, the term fieldbus refers to all these networks.

Fieldbuses are used to link field devices to control, monitoring and enterprise systems (Figure 1), which collectively I'll call the control system.

Field devices most typically are flow (Figure 2), level, temperature and pressure transmitters. Other common field devices are analyzers and valve actuators. Applications range from process control to environmental monitoring to supply chain management.

Connecting Field Devices

Figure 1. Delivering information directly to IT applications may provide a more-cost-effective alternative for some monitoring applications.

In the past field devices primarily were linked to control systems via 4–20-ma analog hard wiring. This hard-wired solution did a good job of transmitting process variable information from the field device to the control system. However, capabilities were limited — the only information that could be transmitted from the transmitter to the control system was a one-way 4–20-ma signal proportional to the measured process variable.

Enter HART, perhaps the first digital fieldbus in widespread use at process plants. HART has the advantage of being able to use existing 4–20-ma wiring as the carrier for its digital signal.

This digital signal establishes a two-way link between transmitters and the control system. This two-way link allows data exchange including instrument identification along with monitoring of multiple process variables, calibration parameters and field device health.

Although HART was a big step up from a one-way analog connection, its bandwidth is limited. So, dedicated digital fieldbus networks such as Profibus PA and Foundation Fieldbus H1 emerged, offering much higher speeds and additional capabilities.

Unlike HART, these new fieldbus systems require dedicated networks — necessitating new types of wiring practices, networking components and support tools. Ethernet-based fieldbuses such as Profisafe, HSE and Ethernet/IP then emerged — bringing even higher communication speeds. Wireless versions of these protocols such as WirelessHART were a logical next step to increase capabilities and reduce installation costs and complexity.

Today, different fieldbuses can be integrated together when needed to establish connectivity between field devices and the control system.

Doing More With Field Data

Figure 2. Field devices like these flow meters deliver more value when connected to control, asset-management and ERP systems via fieldbus.

The Benefits Of Fieldbus
Chemical plants, first and foremost, employ fieldbus to increase productivity through precise measurements of operating conditions. Two-way high-bandwidth digital communications with each field device provide the type of information needed by the control system to improve productivity and reliability.

Beyond executing initial device configuration and subsequent delivery of digital process variables, the most widespread use of fieldbus systems is to access field device diagnostic and health information. Following initial configuration, field devices require periodic calibration and health checks — a fieldbus system helps in two ways.

First, it provides access to parameters that can indicate when calibration or other action actually is needed. Calibrating field devices on a periodic basis isn't optimal because some devices will be calibrated too frequently and others not often enough. A fieldbus system along with National Institute of Standards and Technology (NIST) traceable verification tools can access information from field devices to set calibration or verification events on an as-needed basis, saving money and improving performance.