Take Advantage of Fieldbus

Digital networks offer both technical and economic benefits.

By Craig McIntyre, Endress+Hauser

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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.

Second, a fieldbus system enables calibration and automated data collection on either a local or a remote basis. Instead of going to each field device with pen and paper, a technician can configure or service a number of devices electronically. This greatly improves productivity by saving time and by automatically documenting results.

Individual networked field devices can be linked with device data such as documentation, calibration and servicing history — and these data can be maintained in a web-based instrument lifecycle management system. This greatly reduces required personnel time and overhead costs compared to a facility currently manually maintaining its own device databases and support documentation.

Fieldbus systems also provide the diagnostic information required for predictive maintenance. With 4–20-ma analog hard wiring, a field device problem is communicated only when the signal drops below 4 ma. Operators then are forced to react in real time to manage the process affected by the lost information, and maintenance is pushed to fix the problem immediately.

Fieldbus gives plant personnel the tools to predict field device problems before they occur. Minor degradations in performance can be measured and addressed before catastrophic failure occurs, allowing repairs to be made or workarounds to be executed.
To sum up, fieldbus offers five key benefits:
1. Networked device configuration and health management saves money.
2. Networked device documentation saves money.
3. Predictive maintenance increases uptime.
4. Predictive maintenance improves performance.
5. Predictive maintenance cuts maintenance costs.
To realize these advantages, though, requires implementing a fieldbus system in a systematic manner.

Deploying Fieldbus
Implementation of fieldbus can pose a number of potential pitfalls. Common ones include:
• mismatch of fieldbus provider and user expectations;
• selection of the wrong fieldbus;
• incompatibility between the control system and field device information management systems;
• underestimation of fieldbus complexity; and
• underutilization or poor implementation of fieldbus capabilities.

Perhaps the best path is to initially deploy fieldbus on a small scale in an isolated area such as a lab or pilot plant. This methodology can reveal potential pitfalls that typically wouldn't be apparent during the design phase.

For example, each fieldbus claims to be open, meaning that instruments from one vendor can be connected in a seamless manner to a control system from another vendor. However, some features only may be available when a particular supplier's instruments and control system are used together.

A small-scale project can reveal these types of issues and many others. It also can be the best training ground for plant personnel. Maintaining and getting maximum benefit from a fieldbus system requires a rethinking of how field devices can be used most effectively — nothing sparks brainstorming like a working in-plant system.

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