Take Advantage of Fieldbus

Aug. 4, 2010
Digital networks offer both technical and economic benefits.

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.

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.

For example, multi-parameter transmitters are difficult to implement with 4–20-ma wiring but a natural fit for fieldbus systems. Employing transmitters that measure multiple variables saves money and improves performance by delivering additional information about the process to the control system.

Adding new field devices to a 4–20-ma system often is impractical when new wiring from the control room input/output (I/O) rack to the device is required. In contrast, fieldbus systems are designed from the ground up to accommodate expansion. Because new devices can be added easily, more parameters can be measured and controlled.

Many fieldbus systems not only can add instruments to existing installations in a hard-wired daisy chain fashion but also through tightly integrated wireless links. Wireless connections can save substantial amounts of money and enable measurement of parameters at remote locations such as tank farms.

While a small-scale system is a good way to familiarize your plant with fieldbus, larger-scale projects will deliver added benefits when properly executed. Whichever approach your plant takes, it's important to understand and follow best practices.

Best Practices
Fieldbus implementation and support require a different set of knowledge and tools than the traditional 4–20-ma infrastructure. Like an office network of PCs, a fieldbus installation is a network of connected computing devices — so its installation and support call for the services of fieldbus network engineers and technicians.

A fieldbus network must be modeled during the design phase to check and verify performance. The fieldbus network from device selection to cabling to integration must be carefully engineered. Attention must be paid to grounding and electrical noise issues as a typical fieldbus network will have more wires connected over a wider area than a point-to-point 4–20-ma installation.

A common mistake is improper distribution of nodes and spurs along with poor network design of connections, bricks, isolators and other components. After installation, users must ensure continuing engineering and support because changes and adjustments always arise after initial design and implementation.

Each field device should be recognized as compliant with the fieldbus based on testing and certification, preferably by a recognized third party or by the fieldbus support organization. It's also important to ensure that integration testing defined by the control system supplier has been performed and that that the instrument in question is specifically listed by the control system supplier as compliant.

For expert assistance on fieldbus, there are two main avenues.

The first is a supplier. It may provide extensive advice and support, often on a free or low-cost basis. But advice is likely to be skewed towards its platform, with limited visibility into alternative solutions. It's often best to consult with suppliers that support multiple fieldbus solutions as they can provide a view that may be broader in scope.

Service providers such as system integrators and engineering firms can be a more independent source for advice and support. If you opt for this route, make sure to engage a firm that has extensive experience not only with fieldbus but also with your plant's particular processes and control system.

Fieldbus systems can provide tremendous benefits in terms of lower costs, increased uptime and better overall plant performance. But take care when selecting and deploying the fieldbus solution; it's often best to start small.

Go Direct
A field device usually is first connected to the control system via a fieldbus. The control system then is connected to the IT system through some type of middleware such as a database server.

However, for some field devices, particularly those involved in process monitoring as opposed to process control, direct connection between the field device and an IT technology platform may be a better solution (see Figure 1). This approach avoids the deterministic delivery, data management, safety and security issues associated with more-demanding control system environments.

Connecting directly from the field device to the IT system cuts costs substantially for three reasons. First, a direct connection eliminates the need for intermediate hardware components and software systems. Second, the field device in question may not need to be part of the control system's overall validation and maintenance program. Third, field device access can be controlled through existing IT security systems.

Many environmental applications such as EPA reporting and enterprise applications such as inventory management don't require the millisecond update speeds or deterministic behavior inherent to most control systems. Some facilities have found that up to 70% of their field devices don't have any associated real-time control functions.

Information from these noncritical field devices can be conveyed directly to standard networks and associated databases via wired and wireless field gateways. Not only is the primary information delivered in fully defined engineering units but device status also can be continually monitored and communicated on an event-driven or periodic basis.

Remote servicing tools and asset management applications working at the network level also can be used to configure and manage connected devices. Standard IT security and data management tools can be used to control access.

Craig McIntyre is chemical industry manager at Endress+Hauser, Greenwood, Ind. E-mail him at [email protected].